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Biological Theory
https://doi.org/10.1007/s13752-021-00387-6
ORIGINAL ARTICLE
Scaffolding Natural Selection
WalterVeit1
Received: 21 September 2020 / Accepted: 9 September 2021
© Konrad Lorenz Institute for Evolution and Cognition Research 2021
Abstract
Darwin provided us with a powerful theoretical framework to explain the evolution of living systems. Natural selection
alone, however, has sometimes been seen as insufficient to explain the emergence of new levels of selection. The problem
is one of “circularity” for evolutionary explanations: how to explain the origins of Darwinian properties without already
invoking their presence at the level they emerge. That is, how does evolution by natural selection commence in the first
place? Recent results in experimental evolution suggest a way forward: Paul Rainey and his collaborators have shown that
Darwinian properties could be exogenously imposed via what they call “ecological scaffolding.” This could solve the “black
box” dilemma faced by Darwinian explanations of new levels of organization. Yet, despite “scaffolding” recently becoming
a popular term in the study of cognition, culture, and evolution, the concept has suffered from vagueness and ambiguity.
This article aims to show that scaffolding can be turned into a proper scientific concept able to do explanatory work within
the context of the major evolutionary transitions. Doing so will allow us to once again extend the scope of the Darwinian
model of evolution by natural selection.
Keywords Ecological scaffolding· Ecology· Group selection· Hierarchy· Individuality· Major transitions·
Multicellularity· Multilevel selection· Natural selection· Scaffolding
Introduction
Darwin (1859) provided us with a breathtakingly simple
yet powerful tool to explain the evolution of living systems,
that is, the theory of evolution by natural selection. Out
of all the principles in biology, natural selection appears
to be the closest thing biologists have to something resem-
bling a general ordering principle in physics, subsuming a
vast range of diverse phenomena under a single theoretical
framework (Okasha 2018). Externalist approaches relying on
natural selection that explain organismal properties through
recourse to properties external to the organism,1 however,
have often been considered insufficient to explain how levels
of selection—i.e. the major transitions in evolution such
as the origins of life and the evolution of multicellularity
(Smith and Szathmáry 1995)—arise in their own right. That
is, how does evolution by natural selection get started in the
first place?
This problem has long been a major challenge for evolu-
tionary biology. As James Griesemer (2000) argued early on,
the very reason it has been so hard to explain the major tran-
sitions has been something like a problem of “circularity.”
Since Darwinian explanations invoking natural selection
typically already assume the preexistence of so-called “Dar-
winian properties” at a level of spatial organization—such
as Lewontin’s (1970) preferred triad of variation, fitness dif-
ferences, and heritability—there is an inherent challenge in
offering hypotheses that explain the origins of these proper-
ties without recourse to the very properties we intended to
explain in the first place. This is the most obvious for the
origins of life, where the emergence of Darwinian entities
is typically explained through non-Darwinian means (e.g.,
through appeal to principles of biochemistry and physics).
While there is now a very extensive literature (both theo-
retically and empirically) on such evolutionary transitions in
individuality towards a new level of organization (so-called
evolutionary transitions in individuality), the dominant view
* Walter Veit
wrwveit@gmail.com
1 School ofHistory andPhilosophy ofScience, University
ofSydney, Camperdown, NSW, Australia
1 Externalist approaches should be conceived as the opposite on a
spectrum to internalist approaches that explain organismal features
through recourse to other properties of the organism. In practice, the
differences between these two modes of explanation is admittedly
often only one of emphasis.
W.Veit
1 3
has simply assumed the existence of some higher-level Dar-
winian properties, without realizing that they cannot just
be taken for granted.2 Despite significant progress having
been made, the literature remains far from reaching con-
sensus regarding both general theoretical debates and in
its mechanistic hypotheses. The black box dilemma forced
upon us by the recognition that Darwinian properties are
themselves derived traits in need of Darwinian explanations
has been largely unaddressed. Recent work by the biologist
Paul Rainey, however, suggests a new, exciting path forward
that could simultaneously offer both a general theoretical
framework for the major transitions and a testable empiri-
cal program that may break the circularity problem entirely
by employing natural selection itself in the emergence of
Darwinian properties.
A New Proposal: Ecological Scaffolding
In his recent collaborative work, Rainey has developed his
idea that Darwinian properties could be exogenously rather
than endogenously imposed, through a process that he calls
“ecological scaffolding,” thus allowing natural selection
to commence in something like an eco-evo feedback loop
(Rainey etal. 2017).3 While a further mathematical expli-
cation of the idea by the physicist Andrew Black involved
the philosopher of biology Pierrick Bourrat (see Black etal.
2020), there has yet been no discussion of the idea in philo-
sophical circles, save for a brief mention in Veit (2019a).
However, the idea of ecological scaffolding is already being
applied and further developed elsewhere, such as in Doulcier
etal. (2020) where it is explicated using mechanistic models
in the context of egalitarian transitions (where lower-level
entities are alike) to explain the origins of heredity.
This lack of attention among philosophers of biology is
unfortunate for two reasons. Firstly, Rainey’s idea of ecolog-
ical scaffolds presents a unique merger of theoretical biol-
ogy, experimental work, and philosophy of biology, in order
to address the problem of major transitions that researchers
in all three fields have long been wrestling with. Philoso-
phers of biology may thus be interested in seeing an elegant
case demonstrating how ideas from multiple disciplines
can be turned into a new research program. Secondly, and
perhaps more importantly, philosophers of biology are in a
unique position to analyze and further explicate the notion
of “ecological scaffolding” in order to help it flourish into a
novel research program on the major transitions. This idea is
intriguing because it solves this circularity dilemma faced by
Darwinian explanations relying on natural selection. In this
article, I will take the opportunity to further develop the idea
in order to make progress in our understanding of the major
transitions within an evolutionary framework.
Despite the fact that the term “scaffolding” has recently
become a popular theme in the study of cognition, culture,
evolution, and even science itself (see Sterelny 2010; Capo-
rael etal. 2014; Chapman and Wylie 2015; Currie 2015;
Walsh 2018 for a variety of interesting applications), the
notion has generally suffered from vagueness, imprecision,
and ambiguity. If one hears the term used in different con-
texts or by different scientists, it is often unclear whether
there is any shared understanding. After all, there is no gen-
erally agreed-upon definition, even within particular disci-
plines. This provides ample opportunity for philosophers
of science to step in, as two of the most beneficial roles
philosophers of science can play in the sciences are those
of conceptual clarification and theoretical incubation. This
article aims to draw on the work of Rainey to show that the
notion of “scaffolding” can successfully be developed from
a mere metaphor that is used in a vague sense describing
environmental support, into a proper scientific concept able
to do explanatory work within the study of the major transi-
tions. By developing a general definition of scaffolding I
aim to help Rainey’s project to incubate a new set of ideas
in the study of evolution. Doing so will allow us to address
old philosophical questions on the scope and explanatory
power of natural selection and open an array of research
topics at the intersection of experimental biology, theoretical
modeling, and the philosophy of biology.
Article Outline
In order to achieve these goals, the article is structured as
follows: in the next section, I briefly illustrate the circularity
problem that threatens the status and generality of natural
selection as the unifying principle of the biological sciences.
In the third section, I analyze Rainey’s proposal of “ecologi-
cal scaffolding” as a solution to this problem and a potential
explanation for the major transitions. In the fourth section, I
analyze the concept of scaffolding and its role in evolution-
ary thinking by linking it more closely to recent ideas in the
philosophy of biology, such as Samir Okasha’s work on the
strategy of endogenization, Peter Godfrey-Smith’s idea of
3 The idea was inspired by earlier experimental work on Pseu-
domonas fluorescens by the same group (Hammerschmidt et al.
2014), which I have discussed here: Veit (2019a).
2 While this is far from an exhaustive list, see Buss (1987), Michod
(1999), Griesemer (2000), Queller (2000), Michod and Roze (2001),
Okasha (2006), Grosberg and Strathmann (2007), Michod (2007),
Koonin (2007), Hochberg etal. (2008), Godfrey-Smith (2009), Folse
III and Roughgarden (2010), Clarke (2010), Calcott and Sterelny
(2011), Bourke (2011), Ratcliff etal. (2012), Bouchard and Huneman
(2013), Tarnita etal. (2013), Queller and Strassmann (2013), Niklas
and Newman (2013), Fisher etal. (2013), Ratcliff etal. (2013), Ham-
merschmidt et al. (2014), Libby and Ratcliff (2014), Fisher (2015),
Ratcliff et al. (2015), Pichugin (2015), West et al. (2015), Birch
(2017), Van Gestel and Tarnita (2017), Bourrat (2019), Veit (2019a),
Staps etal. (2019), Rose etal. (2020), Niklas and Newman (2020).
Scaffolding Natural Selection
1 3
scaffolded reproducers, and the scaffolding literature more
generally. Finally, I conclude the discussion in the fifth sec-
tion and suggest further avenues for future research.
A Circularity Problem fortheGenerality
ofNatural Selection
Since Darwin’s (1859) The Origin of Species by Means of
Natural Selection, his concept of natural selection has fun-
damentally changed biology and our understanding of the
world. Previously, no one had been able to provide a sat-
isfying explanation of the design and apparent fine-tuning
of organisms to their environment. His perhaps most often
quoted formulation of natural selection is offered in the latter
part of the Origin:
These laws, taken in the largest sense, being Growth
with Reproduction; Inheritance which is almost
implied by reproduction; Variability from the indirect
and direct action of the external conditions of life, and
from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to
Natural Selection, entailing Divergence of Character
and the Extinction of less-improved forms. (Darwin
1859, pp. 489–490)
As several authors have noted, Darwin emphasizes two spe-
cial features of natural selection (see Dennett 1995; Godfrey-
Smith 2009), the first one being the almost lawlike form of
natural selection: if the conditions for natural selection are
satisfied, natural selection operates. This, importantly, does
not require that evolutionary change or adaptation occur,
as other evolutionary factors such as drift may undermine
the effect of natural selection. The second special feature of
natural selection that Darwin emphasizes is the abstractness
of the process. This feature allows for selection on different
levels of organization than just the obvious case of organ-
isms, leading to the fairly heated group selection debate on
the proper level of selection (Wilson and Sober 1994; Oka-
sha 2001, 2006; Leigh Jr 2010; Veit 2019a; Lloyd 2020).
As I indicated in the introduction, the principles of natu-
ral selection are often simply stated as the satisfaction of
three conditions: (1) variation, (2) heritability,4 and (3) dif-
ferential reproductive success. In his paper “The Units of
Selection,” Lewontin (1970) has provided the most cited
formulation of these three ingredients and the Darwinian
“algorithm” of natural selection:
As seen by present-day evolutionists, Darwin’s scheme
embodies three principles [...]:
1. Different individuals in a population have different
morphologies, physiologies, and behaviors (phe-
notypic variation).
2. Different phenotypes have different rates of sur-
vival and reproduction in different environments
(differential fitness).
3. There is a correlation between parents and off-
spring in the contribution of each to future genera-
tions (fitness is heritable).
These three principles embody the principle of evolu-
tion by natural selection. While they hold, a population
will undergo evolutionary change. (Lewontin 1970, p.
1)
Naturally, it has been recognized that these three criteria can
be satisfied at different levels of biological hierarchy, though
there have been many disputes regarding the status of levels
other than the organism, such as the group or the gene. Far
from being solely tied to biology, many thinkers have also
since used the tools of evolutionary biology to explain cul-
tural evolution, giving these conditions cultural interpreta-
tions. As Godfrey-Smith emphasizes, the very “idea of an
extension of Darwinism beyond its original domain is almost
as old as Darwinism itself” (2009, p. 18).
But explanations employing natural selection will inevita-
bly be faced with a problem of circularity that the Darwinian
project has to address. Regardless of how the specific condi-
tions for natural selection are explicated, these Darwinian
explanations will still be insufficient to explain how these
conditions themselves came into existence (especially in
the case of the origin of life). The problem here is not that
the conditions for natural selection we specified have been
stated too strictly and would require a more relaxed form,
though such an approach might be useful to provide a better
understanding of natural selection itself (see, for instance,
Godfrey-Smith 2009; Rainey and De Monte 2014) and the
very first selection processes in the hierarchy of life. There
is a deeper conceptual problem here that cannot be overcome
by such an analysis since it lacks a mechanistic explanation
for how this process comes to operate. In a previous paper,
I have summarized the problem Rainey tries to address as
follows:
[M]odels already assuming a level in the ‘hierarchy
of life’, the evolution of which we want to explain
must conceptually prove insufficient for the purpose
of explaining the evolution and persistence of new
Darwinian individuals on a higher level. After all, one
needs to explain the emergence of a mechanism of
4 Or at least correlation between the phenotypes of parent and off-
spring (Godfrey-Smith 2009).
W.Veit
1 3
group reproduction, without such a mechanism already
being present. (Veit 2019a, pp. 4–5)
Similar points have been made by Griesemer (2000), who
urges us to go into mechanistic detail in order to address
this circularity problem. Hierarchical organization cannot be
simply taken for granted as if it was just exogenously given,
and yet, this fact is typically already assumed in evolutionary
models (see Okasha 2006, 2018). The levels-of-selection
question must address the origins of biological organiza-
tion. So far, there have been numerous ingenious attempts
to address the evolution of reproduction at a new level (often
within a kin-selection framework), but this work has largely
focused on the internal side of things—with Darwinian
properties emerging as a result of interaction among parts—
with relatively little attention to the ecological context as one
might expect from the externalist model of natural selection
(see Queller 2000; Michod and Roze 2001; Michod 2007;
Hochberg etal. 2008; Fisher etal. 2013; Ratcliff etal. 2012,
2013, 2015).
I emphasize the term “externalist” as important here
more so than any other field, because evolutionary biology
is dominated by an externalist mode of explanation in which
features of a system (i.e., the organism) are explained in
terms of properties of features external to it (i.e., their adap-
tive fit to their environment).5 As the evolutionary biologist
Arlin Stoltzfus (2019) elegantly puts it: “[t]he suspicion of
internal causes in the dominant neo-Darwinian culture ran
so deep that every internalist idea, no matter how reason-
able, was treated as an appeal to vitalism” (p. 46). Yet, to
explain the evolution of Darwinian properties in terms of
external selective pressures has been precisely the source of
the circularity problem, because it apparently presupposed
the existence of the level of organization we are supposed to
explain in the first place. It is thus unsurprising that some,
such as the cell biologist Stuart Newman, have emphasized
the role of physical laws and constraints over the generality
of natural selection and adaptive explanations in thinking
about the evolutionary transition to multicellularity, despite
the resistance to non-Darwinian explanations (Newman and
Bhat 2008; Niklas and Newman 2013, 2020).
This is certainly one way to circumvent the circularity
problem: employ a non-Darwinian explanation of the emer-
gence of Darwinian properties. My goal is not to claim that
this approach does not work. In fact, there may very well
have been too little attention given to the possibility that
multicellularity itself is not an adaptation, and I would be
pleased to see a more pluralist approach to modeling the
origins of multicellularity (see Veit 2019b, 2020, 2021c).
Rather, my goal here is to demonstrate that the idea that
natural selection may itself be scaffolded has the potential
to once again extend the scope of natural selection as a gen-
eral ordering principle in biology, by providing a noncircu-
lar Darwinian explanation of the emergence of Darwinian
properties. Here, I explore Rainey’s notion of “ecological
scaffolding” as an externalist and mechanistic solution to
recover a Darwinian approach to the circularity problem.
In contrast, the prevailing paradigm has been to explain
the emergence of these properties in virtue of the interaction
of lower-level particles (see Buss 1987; Smith and Szath-
máry 1995; Okasha 2006; Godfrey-Smith 2009). Reproduc-
tion, for instance, is often considered as a simple byproduct
of natural selection acting on particles at the lower level.6
However, a mere assembly of single-cell organisms does not
have a special group-level mechanism of group selection. So
where does it come from?
What ecological scaffolding allows is for us to fill in these
gaps and provide Darwinian explanations for the origin of
Darwinian properties, i.e., the conditions under which natu-
ral selection acts. As Griesemer (2000) urged early on, we
should recognize that reproduction is not a simple process,
instead often being composed of the complex interactions
between various parts (see also Wade and Griesemer 1998;
Griesemer and Wade 2000; Bourrat 2014; Griesemer 2014,
2016, 2018, 2019; Wilkins and Bourrat 2020). Collective-
level reproduction is not the same as reproduction at the level
of individual cells, but something that requires an explana-
tion of its own, a fact that is unfortunately often overlooked
or idealized away. But to explain the evolution of multicel-
lularity is, as Black etal. (2020) note, intimately linked to
explaining the emergence of life cycles, development, and a
soma/germ line distinction (see also Coelho etal. 2007; De
Monte and Rainey 2014; Rainey and De Monte 2014; Staps
etal. 2019; Miele and De Monte 2021).
It is nowadays not uncommon for people to think that
natural selection is an important force once its condition
are satisfied, but then to hold that these Darwinian proper-
ties could only have been arranged through something like
a higher power (Nagel 2012). Recent work on scaffolding,
however, may be able to undermine this leftover of pre-
Darwinian creationist thought: from the origins of life to
the evolution of multicellularity, ideas stemming from work
in experimental evolution have offered a novel way to solve
this omission in evolutionary biology. Though I suspect it
unlikely that a scientific concept of scaffolding will turn cre-
ationists, it has the potential to at least remove this force of
their argument, providing us with a simultaneously general,
5 See Endler (1986), Godfrey-Smith (1996, 2001, 2002), Chiu
(2019), Lewontin (1983, 2000), Mayr (1982, 1994), Gould and
Lewontin (1979), Gould (1977, 2002), Walsh (2015), Williams
(1966), Veit (2021a).
6 Though see Michod (2007) for a more sophisticated attempt at
solving the problem.
Scaffolding Natural Selection
1 3
yet testable mechanistic framework for thinking about the
major transitions. Let me thus now introduce Rainey’s con-
cept of “ecological scaffolding” as an externalist solution to
the circularity problem, one that does not rely on an internal-
ist explanation seeking the emergence of Darwinian proper-
ties as an interaction between its parts.
Rainey’s Solution: Ecological Scaolding
Rainey has introduced his notion of “ecological scaffold-
ing” in three recent publications (see Rainey etal. 2017;
Black etal. 2020; Doulcier etal. 2020), though precursors
of the idea can be found in his earlier work (e.g., Rainey
2007). Above, I briefly sketched the idea as the exogenous
imposition of Darwinian properties by the environment.
And this is largely how Rainey etal. (2017) define the con-
cept—seeing the environment as a crucial and neglected
factor in the major transitions by “exogenously imposing
Darwinian properties on otherwise ‘unwitting’ particles”
(p. 104). Rainey himself sees his contribution as involving
both a succinct statement of the essence of the circularity
dilemma which he thinks has been ignored by many in the
field and a straightforward solution, i.e., ecological scaffold-
ing, which simply follows from the former clarification of
the problem.7 A more precise definition of this solution has
been given in Black etal. (2020), where they maintain that
“Darwinian properties can be […] exogenously imposed in
such a way as to cause lower-level entities (for example,
cells) to become passive participants in a selective process
that occurs over a longer timescale than the timescale over
which cell-level selection occurs and as part of a larger (col-
lective-level) entity” (p. 426). As I see it, Rainey’s notion of
ecological scaffolding is essentially a combination of three
separate ideas: (1) an older suggestion by Wilson and Sober
(1989) in their “Reviving the superorganism” that selection
on groups could take place in virtue of the environment
rather than properties internal to the group;8 (2) a general
growing awareness of the importance of ecology, beyond
population structure, within the study of major evolution-
ary transitions (see Wade 2016; Tarnita 2017; Veit 2019a);
and (3) the exploding usage of the term “scaffolding” across
the sciences to explain complex phenomena (see Caporael
etal. 2014).
But the idea of ecological scaffolding is not merely
intended as a theoretical contribution to debates in the phi-
losophy of biology. While his career began in experimental
biology, Rainey developed an early interest in the more theo-
retical questions in biology. His research (Rainey and Rainey
2003; Rainey 2007; Rainey and Kerr 2010; Rainey and De
Monte 2014; Rainey etal. 2017; Black etal. 2020) has long
been concerned with the question of how multicellular
organisms evolved from free-living single-celled organisms,
serving as a paradigm example for an experimental biolo-
gist becoming engaged in a debate that was once the almost
exclusive domain of evolutionary theoreticians and philoso-
phers of biology. Rainey’s introduction of the term ecologi-
cal scaffolding builds on decades of (ongoing) experimen-
tal and theoretical work on the evolution of multicellularity
and was explicitly intended to be able to be operationalized
and to guide future empirical work.9 The primary example
used for ecological scaffolding is an experiment by Ham-
merschmidt etal. (2014). In this experiment they attempt
to test Rainey’s hypothesis of cheats as propagules or—in
terms of the new terminology—a scaffold for group-level
reproduction, variation, and heredity through a single-cell
bottleneck.10 If the bacterium Pseudomonas fluorescens is
propagated in test tubes with a nutritious broth, mutations
quickly arise that lead to adhesive cell-glue production that
bind cells together. The cost of glue production is twofold:
firstly, the costs of producing the glue itself, and secondly
the costs of life in close proximity, as daughter cells cannot
detach themselves from their parent. However, some such
mutant assemblies are able to survive by taking over the
interface between air and broth, attaching themselves to the
walls of the “microcosm,” without which these non-buoyant
mats would sink. They occupy a new niche rich in oxygen, a
benefit that outweighs the cost of glue production. In virtue
of their access to oxygen these “cooperative” groups achieve
access to a limiting resource, taking over the entire surface.
At some point, however, mutations lead to cheating cells
living within the mat without producing glue themselves.
These cheats prosper, as they have access to oxygen without
the costs of glue production. The mat becomes heavier and
loses structural integrity until the lack of cell–cell glue leads
to the doom of the mat, i.e., the extinction of the “proto-
group” organisms.11
Of course, it is not meaningful to speak here of a Dar-
winian individual in its own right, unless it has some means
of reproduction (see Godfrey-Smith 2009). But Rainey and
Kerr (2010) suggest that the very cheats that spell doom for
the mat could also be its savior, by functioning as a sort of
propagule—forming a new mat and thereby the beginning
7 From personal communication (June 2021).
8 Wilson and Sober (1989) do not, however, apply this idea to the
major transitions. (From personal communication with Rainey; June
2021).
9 From personal communication (June 2021).
10 First articulated in Rainey (2007), Rainey and Kerr (2010).
11 The slime mold Dictyostelia may provide a similarly useful model
system for the origins of multicellularity (Queller and Strassmann
2013; Kawabe etal. 2019).
W.Veit
1 3
of a new life cycle. Hammerschmidt etal. (2014) facilitated
a grand-scale experiment showing that a cheat-embracing
regime in which a new life cycle begins with a cheating
cell12 is able to decouple the fitness of groups from the
fitness of individual cells. The lower-level units become
de-Darwinized, as Godfrey-Smith (2009) might put it, by
becoming subservient to the life cycle of the collective unit.
The first steps towards multicellular organisms have been
taken in the lab, successfully testing hitherto philosophical
ideas.13
As I have previously argued, some may legitimately crit-
icize the artificial nature of this experimental setup (Veit
2019a). The point here is not to claim that there is some
deep metaphysical divide between artificially induced selec-
tion and “real” ecological scaffolding in nature, but rather,
that whereas it is comparatively easy to exogenously induce
Darwinian properties in the lab, aggregates of lower-level
units in the world outside the lab rarely meet the conditions
of evolution by natural selection, because they lack heritable
fitness differences (Lewontin 1970; Godfrey-Smith 2009;
Doulcier etal. 2020). One might thus criticize the Ham-
merschmidt etal. (2014) work for lack of a plausible story
of how their model could map onto the real world, but this
would be to misunderstand the paper. Similar to mathemati-
cal modelers, experimental biologists such as Rainey are
often asked to provide us with a detailed story that maps the
artificially induced selection onto natural selection operat-
ing in the real world. Such a requirement forces the abstract
and generalized explanations presented in papers reporting
the data of an experiment or model, to narrow to particu-
lar target systems. This then misinterprets the very point of
the original paper, which was to provide a very general and
idealized framework that would have to be filled out for the
particular contexts we are interested in. When examples and
narratives for such general explanations are provided, they
are mostly suggestive, providing one possible way in which
their explanation could be realized, not how they must be or
how it actually happened. The lack of specific cases is thus
not a flaw but a feature quite familiar from modeling in biol-
ogy. The economist Robert Sugden (2011) has called such
models, fittingly, “explanations in search of observations.” I
suggest the same applies to laboratory experiments explor-
ing general mechanisms operating in living systems. Rainey
etal. (2017) offer a supporting analysis: “the overarching
goal of much experimental evolution […] is to simplify in
order to understand processes too complex to fathom in real
world situations” (p. 106). Indeed, in Black etal. (2020) they
further idealize from their already fairly abstract example
to provide a general and simple model that highlights the
important role of ecological factors as possibility proof for
the imposition of Darwinian properties through properties
of the environment, which has been corroborated in their
experiments. Nevertheless, even if it is possible to scaffold
Darwinian properties, we will still require plausible scenar-
ios of how this could have happened outside the laboratory.
In order to illustrate the importance of ecological scaffold-
ing it is necessary to provide examples of how something
analogous to the artificial scaffolds, such as beakers in the
Hammerschmidt etal. (2014) experiment, could have been
“highjacked” by natural selection.
In Rainey etal. (2017) they provide one plausible story of
how multicellularity could have evolved in the evolutionary
past, or future for that matter. It is here that they introduce
the idea of “ecological scaffolding” using the case of reeds
as an example for the imposition of Darwinian properties:
Consider a pond with randomly placed reeds, each of
which constitutes a scaffold around which a mat can
form. Mats eventually collapse and go extinct due to
their increasing mass, but provided a mat can reestab-
lish at the original reed, or around one or more new
reeds, then a process akin to collective-level reproduc-
tion occurs. With this comes the possibility of a Dar-
winian process at the level of mat collectives. (Rainey
etal. 2017, p. 104)
Instead of their “artificial” set-up this scenario depicts a
plausible scenario for how all three Lewontin conditions
can be scaffolded at the collective level through patchily-
distributed resources at reeds and means of dispersal that
allow for competition between mats. This provides an
interesting analogy to natural selection. Darwin’s theory of
natural selection has provided an explanation that drew on
artificial selection to explain the appearance of purpose in
the natural world, the diversity of living beings, and their fit
to the environment. Much the same applies for ecological
scaffolding by externally inducing Darwinian properties to
“unwitting” particles. Instead of intentionally placed scaf-
folds for a specific purpose, ecological scaffolds are blind
and random occurrences in the natural world that can nev-
ertheless serve a “purpose” in the evolution of Darwinian
properties. In an illustration (see Fig.1), Rainey etal. (2017)
argue that ponds could constitute a plausible scenario for
ecological scaffolding outside the lab.
As Fig.1 illustrates, reeds could possibly enable the crea-
tion of a sort of proto life cycle by exogenously imposing the
necessary and sufficient conditions for evolution by natural
selection. Given the right ecological conditions, mats then
become subject to selection themselves. Take these reeds
away, and the mats no longer have the relevant Darwinian
properties for natural selection to occur. Now it should be
12 As opposed to a cooperating cell.
13 For a more detailed and thorough analysis of the Hammerschmidt
etal. (2014) experiment, its philosophical implications, and the role
of cooperation in the evolution of multicellularity, see Veit (2019a).
Scaffolding Natural Selection
1 3
clear why Rainey and colleagues don’t just use the widely
used and imprecise metaphor of “environmental scaffolding”
to refer to any sort of environmental supporting relation-
ship (Caporael etal. 2014), instead opting for the previously
vacant term “ecological scaffolding” which they intend to
have a more precise meaning.
Reproduction can take place via biotic and abiotic
means. While reeds are biotic components of the envi-
ronments, nothing of course prohibits the occurrence of
something roughly analogous to reeds as an abiotic com-
ponent of the environment playing the same role. In the
experiment this was the beaker, or rather the surface of the
beaker at the interface between the broth and air. Consider,
for instance, the case of stones in a pond peeking out of the
water surface, as an abiotic scaffold. Though perhaps a less
realistic scenario, we keep in mind that the evolution of
multicellularity has independently taken place at least 25
times (see Smith and Szathmáry 1995). Hence, we should
not be surprised that the actual evolution of multicellu-
larity in eukaryotes may have been a fortunate interplay
between extremely unlikely environmental conditions and
the right mutations occurring in the required order. Proto
life cycles acting as marginal Darwinian individuals, far
from being stable, are constantly threatened by extinction
and this is the fate we should expect almost all proto life
cycles in evolutionary history to have suffered. The mul-
ticellular lineages we observe today thus represent only
those stable enough to persist.
The creation of such proto life cycles in the lab—able to
increase their stability before they eventually go extinct—is
already quite the achievement. These experiments suggest
ways forward, illuminating the ecological scaffolds that may
have played important roles during the evolution of mul-
ticellular organisms. As Rainey etal. (2017) note: “ecol-
ogy is everything: the structure of the environment permits
realisation of Darwinian properties at the collective level
even in the absence of these properties being endogenously
determined” (pp. 104–105). Use of the term “ecological”
rather than “environmental” scaffolding also highlights the
importance of eco-evo feedback loops in ecological scaffold-
ing that can lead to the evolution of genuine life cycles. Only
in virtue of a cheat-embracing regime, which scaffolds mat
reproduction, do we get a germ-soma distinction with cheat-
ing cells playing the role of proto germ cells—a proto life
cycle that can plausibly become a unit of selection. And with
selection coming to act at the level of the mat, these exog-
enously imposed Darwinian properties can plausibly come
under developmental control, i.e., become endogenized. The
Fig. 1 “Pond scum acquires Darwinian properties via ecological scaf-
folding. The illustration shows six reeds in a pond. Surrounding each
reed is a set of different microbial mat-forming types. Reeds are suf-
ficiently widely spaced as to prevent confluent growth of mats, thus
ensuring variation at the level of mats. Consider that the yellow mat
occupying the reed marked with the solid arrow collapses. Death of
this mat provides opportunity for birth of a new mat, provided there
exists a means of dispersal (by biotic or abiotic means) between
reeds. In this example, cells from the red mat recolonise the vacant
reed. The dispersal and recolonisation event is akin to mat-level
reproduction and, because the cells founding the new mat came from
the old mat, the offspring mat resembles the parental mat (there is
heredity). Mats begin to take part in the process of evolution by natu-
ral selection by virtue of Darwinian properties that are exogenously
determined. Additionally, selection sees two time scales: the doubling
time of individual cells and the doubling time of mats. Continued
selection under such ecological conditions allows the possibility that
Darwinian properties become endogenised, that is, they come to be
determined by the activity of the collectives themselves with no need
for scaffolding. An early stage might be the evolution of a develop-
mentally determined life cycle.” (Figure and description reprinted
from Rainey etal. (2017, p. 105), with permission from Elsevier; full-
color figure available in electronic version)
W.Veit
1 3
second example Rainey etal. (2017) offer is perhaps less
obvious (see Fig.2).
Unlike the previous thought experiment this one does
not have a straightforward real-world analogy to the role
of reeds in the development of mats and the construction
of a building via scaffolds. The focus here is the general
phenomenon of the exogenous imposition of Darwinian
properties. Nevertheless, it could be made into a model for
the evolution of multicellularity if each droplet was founded
by a single cell, which could be instantiated through pores
in alkaline vents.14 Doulcier etal. (2020) have developed a
mechanistic model with this motivation in mind and show
that egalitarian transitions can occur through ecological
scaffolding. Consider the standard conditions for natural
selection listed in the second section: (1) phenotypic varia-
tion, (2) differential fitness, and (3) heritability. Upon closer
inspection it turns out that all conditions for natural selec-
tion are here themselves being exogenously imposed by the
experimenter. Without the experimental scaffolds, the Dar-
winian properties at the level of drops would cease to exist.
In order to get distinct groups of individual cells, the setting
above depends on “enclosure within a droplet surrounded
by oil” (Rainey etal. 2017, p. 105), enabling the “evolu-
tion” of group-level variation. The oil can here be seen as a
natural border for multicellular organisms, one that is abi-
otically scaffolded. If individual cells can freely move from
one “group” to the “other,” group-level properties become
diluted. While not strictly necessary for selection to occur
at the level of groups, some form of boundedness is required
to have natural selection “conquer,” or as Godfrey-Smith
(2009) might put it, “de-Darwinize” the lower level units
in favor of the group. Boundedness strengthens the role of
higher-level selection since phenotypes have a higher cor-
relation between generations, thus opening the possibility for
group-level beneficial adaptations at the level of individual
cells. The scenario explored in Fig.1 similarly requires that
the reeds be placed sufficiently far from each other for mats
not to overlap, but close enough to enable differential repro-
duction. In addition, differential fitness of groups is here
introduced by the introduction of an extinction process of
darker groups, while the brightest groups are allowed to take
over vacant “niches” of now extinct groups. Both differential
fitness and heredity at the group level are here imposed via
the experimenter. But as Black etal. (2020) demonstrate in
their follow-up study, these properties could nevertheless
become endogenized.
I hope that this detailed analysis of Rainey’s proposal,
and the examples of its application, have given a good indi-
cation of the great potential it has for the interdisciplinary
research among philosophers, experimentalists, and model-
ers. It’s also an elegant example of how cooperation among
these groups can lead to progress on a problem we have
grappled with for decades. Philosophers of biology would
be well-advised to help practicing biologists to develop this
research program further, and I hope that the next part of this
article will set the first stones for this exciting new avenue
of research by attempting to make the notion of ecological
scaffolding more precise and once again extend the scope
of the Darwinian model of evolution by natural selection.
Fig. 2 “Ecological scaffolding in a droplet-based evolution machine
ensures droplets are units of selection. Parallel horizontal lines are
walls of Teflon tubes containing an emulsion of oil and regularly
spaced droplets harbouring bacterial cells. a At the start of opera-
tion all droplets are founded by identical types. b A period of cellu-
lar growth occurs within droplets. During this stage mutations within
individual cells arise that affect the colour of each droplet. Droplets
whose colour is not sufficiently bright are marked for extinction
allowing the possibility that when the contents of the droplets are
diluted in order to establish a new round of selection, the brightest
droplets are split into two offspring droplets. c Selection thus works
over two time scales—the doubling time of cells, and the dou-
bling time of droplets. As in [this figure], Darwinian properties are
imposed (scaffolded) on droplets causing droplets to function as units
of selection in their own right. (Figure and description reprinted from
Rainey etal. (2017, p. 106), with permission from Elsevier;full-color
figure available in electronic version)
14 I thank Rainey for this example.
Scaffolding Natural Selection
1 3
Scaolding andNatural Selection
Black etal. (2020) exemplify the idea that “[e]volu-
tion is the control of development by ecology,” an idea
famously endorsed by the American evolutionary biolo-
gist Leigh Van Valen (1976, p. 180). Ecological scaffolds
are exogenously imposed and can subsequently become
endogenized—thus eliminating the circularity problem for
the application of natural selection to explain the emer-
gence of Darwinian individuals at a new level. This is
an exciting proposal with compelling empirical evidence
for the early stages of the endogenization of Darwinian
properties, i.e., development, which deserves further phil-
osophical analysis. In this section, I will draw on recent
work in the philosophy of biology do to just that: offer
a general, yet nevertheless simple definition of scaffold-
ing as a distinctive phenomenon in nature and clarify its
importance within evolutionary biology. This will help
us to conceive of natural selection itself as a scaffolded
process.
A Simple Definition ofScaffolding
To begin with, we should get a firmer understanding of what
scaffolding is. After all, the title of this article is “Scaffold-
ing Natural Selection.” Yet, this question is far from easy
to answer, precisely because the term is often used in a
deliberately vague and metaphorical way. I have noted
that it has had an explosive growth in recent years, but its
usage has varied widely due to the different areas in which
it is employed. Nevertheless, substantial collaborative work
across fields has already culminated in some progress, such
as the volume Developing Scaffolds in Evolution, Culture
and Cognition (Caporael etal. 2014), which arose from the
23rd Altenberg Workshop “Scaffolding in Evolution, Culture
and Cognition” at the Konrad Lorenz Institute for Evolu-
tion and Cognition Research (KLI). The prefix “Develop-
ing” was deliberately added to emphasize the tentativeness
of the project in attempting to make more sense of scaffolds
across fields. The volume explores conceptual and empiri-
cal questions on scaffolding across different disciplines and
can here only be recommended for anyone seeking a broader
overview of this topic in fields beyond evolutionary biology.
Caporael etal. (2014, p. 2) argue that:
The word scaffolding is both a noun and a verb; it
names a structure and a process. The common mean-
ing of scaffolding refers to a temporary structure
of platforms and poles erected so that workers can
build, repair, clean, or decorate a building. [...] Scaf-
folding is widespread, so widespread that its “attach-
ment” to discourses in biology, culture, evolution,
and human development indicates its centrality to
processes of support and change of many kinds.
Indeed, scaffolding processes understood in this sense are
ubiquitous in nature. We can distinguish “scaffolds” as
structures from “scaffolding” as a process, although the
distinction can admittedly get blurred. But the way they
are loosely described here in terms of an environmental
supporting relationship provides us with hardly any pur-
chase on how to make them useful scientific concepts. It
would turn a coffee cup into a scaffold in the same way
as a beaver uses logs as a scaffold to build a dam. That
the environment always plays a role in evolution would
hardly constitute an interesting or novel thesis; it is almost
universally accepted. Nevertheless, the ubiquity of scaf-
folds will remain an important feature for several reasons,
once we turn to the relationship between scaffolding and
natural selection.
The most interesting property of scaffolding in rela-
tion to natural selection is undoubtedly, as the example
of Rainey’s work illustrates, its alleged usefulness for
explaining “black boxes” in traditional research para-
digms. Caporael etal. (2014, p. 2) likewise point out that:
[S]caffolding is a “missing concept,” perhaps
because its primary virtue is that it is commonly
temporary: it is often external, and either falls away
or becomes assimilated to or part of the scaffolded
relation.
Now, this is precisely the sort of relationship we discussed
with Rainey’s thought experiment concerning reeds at which
mats could form and his experimental work on the origins
of endogenization. Here, Darwinian properties are in an
important sense, induced from the outside. While one might
expect scaffolding to differ widely from one context or target
system to another, there is a straightforward analogy here to
familiar cases in engineering where the term originated. At
construction sites, scaffolds serve as temporary structures
to enable or at least aid the creation or maintenance of a
physical structure such as cranes and metal frames used to
construct buildings. The common language use allows for a
straightforward definition, once any intentional notions such
as the purpose of the scaffold is eliminated. I propose the
following working definition for scaffolding:
X is a scaffold iff:
1. X exogenously induces or supports the realization
of property Y in process/system Z.
2. X vanishes from or becomes part of the system,
while property Y in process/system Z becomes
endogenized.
W.Veit
1 3
The important notions in my definition of scaffolds are (1)
the exogenous realization of properties relevant to a pro-
cess of system—such as Rainey’s external imposition of
Darwinian properties—and (2) the endogenization of these
properties, i.e., when they become part of the system itself
or fall away without the important properties or effects
thereby being lost. It is this second part of the definition that
is unfortunately sometimes neglected, with the mere modal
possibility of endogenization seen as sufficient for treating
something as a scaffold. But as I shall demonstrate below,
this would make the notion too permissive to be used as an
explanatory concept in its own right, making it an almost
trivial consequence of the fact that few if any processes
occur independently from their environment.
Given the excessive use of the terms “scaffold” and “scaf-
folding,” which are perhaps not always justifiably applied in
multiple diverse domains, one may worry that my attempt
to provide a general definition is doomed to failure and that
the only unifying usage might be a vague metaphorical one.
But if this were true, the vigor with which philosophers have
joined the numerous debates embracing the position would
appear quite strange. Doubly so, because unlike traditional
philosophers of science, many of these philosophers have
actually used these terms to probe new research programs,
an undertaking that can only serve as exemplary.15 But there
is a tension in recent work that tries to (1) highlight the ubiq-
uity of scaffolds in nature, and (2) use it as an explanatory
concept. Scaffolding can either be understood broadly as any
kind of entity standing in a supporting relationship, or as a
more precise concept that relies on the eventual endogeni-
zation or elimination of the scaffold. My definition rests on
the motivation that it is only the latter understanding that
can do genuine explanatory work. The former has far less
explanatory value, as supporting relationships can be found
almost anywhere, without thereby providing any further
explanatory understanding of a target system. And since I
am here primarily interested in naturalist conceptual engi-
neering (see Veit and Browning 2020), rather than a mere
conceptual analysis of the term, my focus is on endogeniza-
tion. Unfortunately, many famous proponents of scaffolding
seem to defend the much broader former notion, including
Rainey. Here, one may object that my reservations against a
broad usage of the term scaffolding are biased by my focus
on major transitions.16 But as I shall argue, such an uncon-
strained usage of scaffolding fails to distinguish it from other
processes and obscures a unique role that scaffolds play in
nature. Kim Sterelny, for instance, has been one of the most
fervent users of the term scaffolding, but that very feature
may also explain his apparently very broad definition of the
term. Consider his example of Canberra’s road signs:
These cue names, directions to important locations,
speed limits and rights of way. They are not deceptive;
they are regularly present; their content is highly reli-
able. This set of resources scaffolds navigation around
Canberra’s confusing street network. (Sterelny 2010,
p. 475)
Road signs, of course, are here to stay. When Sterelny speaks
of environmental scaffolding he uses the term interchangea-
bly with environmental supports. Perhaps, scaffolding is the
more attractive name for seemingly providing a novel posi-
tion, but this can also serve to create an illusion, applying a
new term for a point that would otherwise appear somewhat
trivial. My point here is not that the term scaffolding can-
not be used in these circumstances, but rather that it would
make the notion explanatorily hollow and mix it up with a
scientifically useful, but more restrictive application of the
term. After all, the claim that many processes in the natural
world, such as navigation, perception, cognition, and the
development of organisms are supported by the environment
is far from novel nor particularly interesting. In fact, it is
doubtful that anyone would object to such a weak version
of scaffolding. More so, it raises the question of why the
term was used to begin with if all it states is the existence of
supporting relationships. Scaffolding, rather than a scientific
concept in its own right, seems to be used as a mere analogy
to strengthen the idea of niche construction, i.e., a process in
which the organism alters its own selective environment to
its own benefit (Odling-Smee etal. 2013; Scott-Phillips etal.
2014; Laland etal. 2015), in both the biological and cultural
realm. While this metaphorical use is not problematic in its
own right, indeed, can play an important role in science (see
Veit and Ney 2021), my definition above is intended to pro-
vide a simple account that has the following triad of useful
features: (1) it is generalizable across different contexts and
domains, (2) it can be used in actual scientific work, and (3)
it closely tracks common language use of the term.
Caporael etal.’s volume did not attempt to provide us
with a general definition, with its contributors using the
terms in widely different ways, sometimes solely meta-
phorically, sometimes with a precise and narrow meaning,
fitted to the particular target of their investigation—and this
was certainly admissible in the first collected volume on the
topic. The goal was to let a hundred different flowers bloom
and a deliberately vague use of the term was the ideal scaf-
fold to accomplish just that. But in order to make the notion
a precise scientific concept, there is an eventual need to pro-
vide a more precise and general definition that is applicable
to the myriad scaffolding phenomena in nature, with their
different timescales and levels of organization—even if such
an attempt at “policing” the usage of the term will strike
15 See again Caporael etal. (2014) for an overview.
16 I thank an anonymous reviewer for pressing me on this point.
Scaffolding Natural Selection
1 3
some as unwelcome. Admittedly, my definition may very
well fail in due course and be replaced with a better one.
Perhaps it will be demonstrated that the various phenom-
ena conceived as scaffolding processes are too disunified to
allow for a useful general scientific concept. This is not too
much of a problem, as it is beyond the scope of any single
stand-alone paper to survey an entire literature and prove
that the definition stretches across all important instances of
scaffolding processes. Nevertheless, if such an objection can
be brought forth against my constrained definition of “scaf-
folding” as a process of endogenization, it is even less likely
that a much broader definition can play a useful role in sci-
ence. For the present purposes of turning “scaffolding” from
a metaphor into an explanatory concept, my general defini-
tion will enable us to think about natural selection itself as a
scaffolded process, enabling us to explain the major transi-
tions in a noncircular, yet nevertheless Darwinian manner.
Darwinizing Natural Selection
Previously, I have mentioned that the idea of ecological
scaffolds comes in the form of an externalist explanation,
forming a break from the usual internalist explanations of
Darwinian properties arising at a higher level in virtue of
the interactions of parts (cells). As I have emphasized in
the second section, despite the general reluctance among
contemporary neo-Darwinian evolutionary biologists to
consider internal causes as a source of evolutionary change,
many have nonetheless drawn on internalist patterns of
explanations to avoid the circularity problem of explaining
the emergence of Darwinian properties.
One popular example for an internalist explanation of a
transition in Darwinian individuality is the idea of co-option.
In co-option, preexisting traits of particles become relevant
for the collective entity, turning it into a Darwinian individ-
ual in its own right. Black etal. (2020) give the example of
the colonial volvocine green algae, where groups can form
by co-opting the retinoblastoma cell cycle regulatory path-
way in the unicellular Chalydomonas (see Hanschen etal.
2016). Similarly, they point to exciting work of the Ratcliff
lab on snowflake yeast (Saccharomyces cerevisiae) able to
generate collective-level reproduction through co-option of
the programmed cell death (apoptosis) to create propagules,
thus showing that Darwinian properties can emerge from the
interaction of parts (Ratcliff etal. 2012).
Nothing about this need be vitalist or mysterious. The
point of Rainey’s idea of ecological scaffolds is decidedly
not to say that it is a competitor to co-option explanations.
It will be exciting to see where these experimental research
programs on possible co-option processes will lead us in the
next decades. We may start to think the actual evolution of
multicellularity could have been a result of both co-option
and ecological scaffolding, resisting a neat fit into either
category. Here, it would be a mistake to restrict ourselves
to only one model. As I have previously argued: “for almost
any aspect x of phenomenon y, scientists require multiple
models to achieve scientific goal z” (Veit 2019b, p. 91).17
But to restrict ourselves to co-option may force us to miss
the important role of ecology. As Rainey would put it, we
attempt to strip the problem to its bare bones, taking nothing
for granted, and see how far we can get.18
Unfortunately, the debate often reflects a higher-order
commitment to kin selection in the co-option of almost
clonal cells and to multilevel selection in the scaffolding
approach. But these shouldn’t be seen as mutually exclu-
sive, such that co-option necessarily “beats” ecological scaf-
folding. This is easily recognized when we look at the very
emergence of life, when there are no Darwinian units whose
properties could become co-opted, and why Rainey’s idea
is even more useful outside of his work on multicellular-
ity. It is at the origin of life—as a process—that ecological
scaffolding provides us with a Darwinian view of how Dar-
winian properties emerge. It Darwinizes natural selection.
Ecological scaffolding may provide us with a highly fruit-
ful framework to develop plausible explanatory sketches of
how natural selection could itself have gradually emerged
in an eco-devo feedback loop. While Black etal. (2020)
only talk in a few places about the transition to life from
nonliving matter, the idea of scaffolds finds a natural place
within recent work on the origins of life. I will not go into
any empirical detail on this issue here, since I am treating
ecological scaffolding at a fairly general level as something
that can explain the emergence of Darwinian properties in a
noncircular, yet Darwinian way.19
In order to Darwinize natural selection in this way, I will
introduce a concept that I shall dub “evolutionary scaffold”
as a broader conceptual framework in which to understand
Rainey’s notion of ecological scaffolding. Drawing on our
general definition of scaffolding, we can define “evolution-
ary scaffolds” straightforwardly as follows:
X is an evolutionary scaffold iff:
1. X exogenously induces or supports the evolution of
property Y in process/system Z.
17 See also Winther etal. (2013).
18 From personal communication (June 2021).
19 Damer and Deamer (2020), however, may provide a useful hypoth-
esis about the origins of life that may be explicated in terms of eco-
logical scaffolding.
W.Veit
1 3
2. X vanishes from or becomes part of the system,
while property Y in process/system Z becomes
endogenized.
Importantly, this is not how Rainey sees ecological scaffold-
ing. For Rainey, it is scaffolding regardless of whether the
properties induced by the environment become engodenized
or not.20 He effectively gives up the second part of my defi-
nition since endogenization need not be realized. But this
is then returning to the overly expansive usage of the term
“scaffold” as a mere environmental supporting relation-
ship, a criterion that is notably weak due to its broadness.
Whereas simple models of the origin of life and multicel-
lularity may idealize all environmental factors away, merely
consisting of the replication of units, almost all actual bio-
logical processes involve feedback between organism and
environment. The real power of Rainey’s idea of ecological
scaffolding, I suggest, is thus not this environmental role per
se, but rather the process of endogenization.
To make this clear, it will be helpful to elaborate further
in an evolutionary context what is meant by “endogeniza-
tion.” Borrowing the term from the economic modeling
literature—where exogenous and endogenous variables are
typically distinguished and the former are treated as given
inputs, whereas the latter as outputs determined by the
model—Okasha (2018) suggests that a similar distinction
can be drawn in evolutionary models. Applied mathemati-
cians switching from economics to biology may be surprised
to learn that the role of endogenization has been given lit-
tle attention in evolutionary models. Yet this omission can
simply be seen as an artefact of the modeling choices inher-
ent to any evolutionary model: some factors are taken as
mere inputs, whereas others can be influenced by the model
itself. In the second section, we saw that natural selection
is often treated as a lawlike ordering principle of biology.
This is largely due to the procedural subsumption of more
and more phenomena under the general explanatory scheme
of evolutionary theory. Okasha (2018, p. 2) describes this
historical trend as a strategy of endogenization: “[i]t involves
devising evolutionary explanations for biological features
that were originally part of the background conditions, or
scaffolding, against which such explanations took place.”
In the language of modeling, we can see this trend as the
subsumption of phenomena under a more general model of
evolutionary change. There are thus two senses of the term
endogenization: one about a phenomenon being subsumed/
integrated into an explanatory program, the other about a
phenomenon being endogenized over evolutionary time into
the development of the organism.
Both Wimsatt and Griesemer (2007, p. 245) and Caporael
etal. (2014, p. 377) briefly talk about the “internalization”
of scaffolds as an important factor involved in the evolution
of complex forms of organization (both in biology and cul-
ture), but unfortunately treat this as optional. Whereas these
authors see such a process as an important role for scaffolds,
I see endogenization of Darwinian properties at the very
heart of what makes something an evolutionary scaffold.
This distinction provides us with an interesting further ques-
tion of how well the endogenization of variables into an
explanatory model can serve as an adequate representation
for endogenization of scaffolds in nature, such as Rainey’s
reeds, that I will further address in this section.21 Despite
this ambiguity in the usage of the term endogenization, I
expect that it will nevertheless be useful to think about our
attempts at modeling the origins of Darwinian units and
exogenous variables becoming endogenized. The explana-
tory reach of evolutionary theory has constantly pushed its
limits, though this has often been criticized as too “adap-
tationist.” However, Okasha (2018) elegantly demonstrates
that evolutionary theory has been extended to explain many
phenomena formerly considered to be outside the purview
of—or at least obstacles to—Darwinian explanations such
as the origin of variation, biological sex or anisogamy,
altruism and population structure, niche construction, the
genotype–phenotype map, and the origins of hierarchical
organization. This is not to say that these phenomena have
been fully explained in Darwinian terms, but rather that we
are developing evolutionary models where these phenomena
are treated as outputs, rather than just take them for granted.
These points help to illustrate what Rainey has in mind when
he speaks of the endogenization of Darwinian properties.
Internal factors (development) become more important, with
life becoming more “autonomous” from external forces in
the process.
In 2006, Okasha distinguished between the “old”
approaches to the levels-of-selection question as “syn-
chronic” approaches that treated hierarchical levels as given
in the explanation of adaptations on that level, whereas he
considered the growing evolutionary transitions literature
to constitute a “diachronic” approach to the levels-of-selec-
tion question. Here, the level of selection itself is explained
through recourse to Darwinian explanation, which as Oka-
sha (2018) points out in his recent paper, can be understood
as the endogenization of the “hierarchical organization
itself” (p. 12). Hierarchy is no longer seen as a necessar-
ily exogenous factor in evolutionary models. This binary
way of describing the growing major transitions literature
can be misleading, however, since it may be interpreted as
suggested that we have already subsumed these phenomena
20 From personal communication (June 2021). 21 I thank an anonymous reviewer for pressing me on this point.
Scaffolding Natural Selection
1 3
under the scope of the principle of natural selection. Par-
tially, this is due to the origin of the endogenous/exogenous
distinction that Okasha draws on from the modeling litera-
ture in economics. In economics, either a variable is an input
of the model or it is an output. But in nature we find grada-
tions, and endogenization as a natural phenomenon should
not merely be understood as a feature of our models and
explanations. It is a real phenomenon in nature, and it is the
very origins of this endogenization process, i.e., the origin
of development, that we should pay attention to since it is
here that natural selection emerges in an eco-devo feedback
loop. It is because of this that ecological scaffolding ought to
include endogenization in the sense of ecological properties
becoming organismal ones—i.e., the second component of
my scaffolding definition—in order to turn it from a mere
metaphorical notion into an explanatory concept.
Much in the above discussion of Rainey’s work has
focused on reproduction, but other Darwinian proper-
ties face the same problems (for a discussion of heredity,
see Doulcier etal. 2020). Nevertheless, as Okasha (2018)
describes it, Darwin’s worries about the origins of variability
in populations have been solved, and to some extent this is
certainly true. We no longer treat variation as a black box.
We have come up with both short-term and long-term mac-
roevolutionary explanations for why variability itself can
be explained as an adaptation. Indeed, this work elegantly
overlaps with work on evolvability (see Pigliucci 2008).
But debates about the role of evolution, or rather natural
selection, in the emergence of a phenomenon rarely focus
on the question of whether or not natural selection is part of
the story. The question is how important natural selection is
in the explanation of a particular phenomenon as opposed to
other factors such as physical or chemical constraints on the
possibility of “perfect” replication. This is why the abbrevi-
ated Van Valen quote in Black etal. (2020) about develop-
ment being controlled by ecology must not be misunderstood
as suggesting that we need either a developmental or an
evolutionary explanation. Rather, Rainey defends the idea
that development and evolution are two sides of the same
coin. They are both in control of each other.
There is no interesting thesis here about one beating the
other, and we need to pay more attention to the evolution of
development. As Griesemer (2000, p. 29) noted, develop-
ment has long been a weak link in evolutionary theorizing
and ought to be integrated in our thinking rather than ideal-
ized away. Similarly, Van Valen maintained that despite the
possibility of thinking about evolution in terms of ecology
controlling development, “neither area has figured impor-
tantly in evolutionary theory since Darwin, who contributed
much to each” (1973, p. 48). And it is this more important
point by Van Valen that makes Rainey’s ecological scaffold-
ing framework such an interesting proposal, through inte-
grating development, evolution, and ecology in an eco-devo
feedback loop. Even at the longest possible timescales, it
suggests that development is a factor that cannot be idealized
away. Rather, it is a process that underlies the very endogeni-
zation of Darwinian properties in their own right. Ecological
scaffolding explains the very origins of internalist develop-
mental explanations, without taking anything for granted.
I am thus here not interested in claiming that development
controls evolution through the endogenization of Darwinian
properties. There is as much truth to this statement as there
is to Van Valen’s.22 Indeed, Okasha warns us not to confuse
the widespread prevalence of endogenization in evolution-
ary biology with a reductionist view of natural selection as
a “universal acid” (see Dennett 1995). Even if we want to
treat natural selection as something like a “first principle of
science,” we can defend a more nuanced view that likewise
makes use of the idea of endogenization.
The alternative is this. It is not the core Darwinian
principles themselves that bear the explanatory bur-
den in evolutionary biology, but rather those principles
as they operate in specific biological settings, in the
presence of additional contingent biological features.
(Okasha 2018, p. 18)
Naturally, there is a myriad of ways background features can
play a role in specific explanations that make use of natural
selection. If these can in turn be explained through natural
selection it is tempting to buy into Van Valen’s dictum. But
Okasha (2018, p. 19) rightfully maintains that even if we
can provide an evolutionary explanation for the background
features of a particular phenomenon we want to explain,
this does not entail that the phenomenon can be explained
without an explicit appeal to the background features. What
is important for the purposes of this article is simply to rec-
ognize that background assumptions often serve as a scaffold
in biological theorizing, only to later become part of a Dar-
winian explanation. But in ecological scaffolding, as I aim
to more narrowly definite it here, it is not these background
conditions of an explanation that become endogenized, but
the phenomenon itself.
22 Consider an egalitarian marriage of the 21st century. Historically,
marriages were sometimes jokingly described as a man being chained
by a woman, suggesting that women are in control of their hus-
bands. But this would be a gross misrepresentation of the patriarchal
and sexist power structures we found in marriages in the past (and
unfortunately in many in the present). Such statements have largely
been abandoned or at least come to be scrutinized due to their sexist
nature. In an egalitarian marriage, however, it need not be the case
that either partner has power over the other. Unless we want to define
the term “control” as whoever has more causal power, we can legiti-
mately say that each partner has control over the other. And so it is
with development and natural selection. We do not have to defend a
view in which one side must “win.”
W.Veit
1 3
By understanding ecological scaffolding as an externalist
attempt to endogenize Darwinian properties, we can tease
apart this duality in Rainey’s discussion of ecological scaf-
folding between what can be understood as an explanatory
scaffold and what I have called an evolutionary scaffold.
This distinction is subtle, but important. On the one hand,
ecological scaffolding can be understood as an externalist—
as opposed to internalist—mode of explanation, in which
Darwinian properties are induced by Darwinian means in a
noncircular fashion. Here, we rely on natural selection (envi-
ronmental filtration) as an explanatory scaffold for explain-
ing the origin of Darwinian properties. We subsume these
phenomena in a Darwinian model of life. On the other hand,
the environment itself is an evolutionary scaffold, a genuine
scaffold in nature that induces Darwinian properties that are
then able to become endogenized across evolutionary time.
One is a tool for coming up with new explanations for Dar-
winian individuals, the other is a natural phenomenon that
we are trying to make sense of here. In trying to understand
the role of scaffolds in nature, we can also recognize some-
thing of a “reverse” process of evolutionary scaffolding in
which Darwinian properties become exogenized rather than
endogenized, with organisms losing autonomy and becom-
ing more dependent on special ecological conditions. Such
an idea has been hinted at in Godfrey-Smith’s discussions of
viruses, which while brief can be elegantly co-opted:
Simple reproducers need not be the lowest-level repro-
ducing entities in a hierarchy, however. A third cat-
egory I will call scaffolded reproducers. They might
even be called reproducees, or at least many of them
could. These are entities which get reproduced as part
of the reproduction of some larger unit (a simple repro-
ducer), or that are reproduced by some other entity.
Their reproduction is dependent on an elaborate scaf-
folding of some kind that is external to them. However,
these entities do have parent–offspring relationships,
hence they form lineages or family trees. (Godfrey-
Smith 2009, p. 88)
Viruses straightforwardly use other organisms for the
continuation of their own life cycle. They depend on the
reproduction of other living entities, in order to facilitate
their own reproduction. Without this scaffold the life cycle
is “stuck”—an evolutionary dead end. It is not, however,
evolutionary scaffolding as I constrained the term. While
viruses did evolve via other reproducers they did not dis-
card or endogenize them. Only if a virus were to evolve the
ability to reproduce without the need to hijack the reproduc-
tive capacities of other organisms would there be a case of
evolutionary scaffolding. It may thus be worthwhile to draw
a distinction with developmental scaffolding, which better
describes Godfrey-Smith’s notion of scaffolded reproducers.
Nevertheless, in thinking about evolution something very
much like a reverse ecological scaffolding process must
have been at the origins of virus lifestyle (and perhaps more
generally at the evolution of parasitism), whence autonomy
was lost.
Hence, we should recognize that the role of scaffolds in
evolution can go in two ways: endogenization and exog-
enization. In the former, scaffolds become endogenized or
discarded (e.g., the origin of life and plausibly the origin
of multicellularity), and in the latter, organisms introduce
scaffolds into their life cycles that make it evolutionarily
viable to exogenize Darwinian properties such as reproduc-
tion (e.g., a virus). These two should not be confused, and it
is for this reason why the scaffolding metaphor in the latter is
bound to be misleading, since there are two quite distinctive
processes going on. Perhaps a better way of distinguishing
the two processes would be between endogenized scaffold-
ing as a process of autonomization (from external forces)
and the exogenization of development as a loss of autonomy
(reliance on external factors).23 Nevertheless, in both it is
the role of the ecology that is the crucial key to understand-
ing the evolutionary dynamics these systems take, and we
can see my narrower concept of evolutionary scaffolding as
enabling us to distinguish these two directions evolution can
take organisms. I hope that my refinement of Rainey’s idea
of ecological scaffolding as a process of endogenization will
help to highlight the importance of this simple fact: natural
selection is an ecological process and should therefore be
studied in terms of the external ecological scaffolds that gave
rise to the eco-devo feedback loops of life. Before we move
to the conclusion, however, I hope that this section has pro-
vided something of a proof that Caporael etal. (2014) were
right in their prediction for the usefulness of a developed
concept of scaffolding:
A more highly analyzed and developed concept of
scaffolding will highlight the role of temporal and tem-
porary resources to development, broadly conceived,
across concepts of culture, cognition, and evolution.
(Caporael etal. 2014, p. 3)
Conclusion andFurther Avenues
forResearch
The theory of natural selection is the most impressive the-
oretical development in the biological sciences. No other
theoretical framework has achieved the striking explanatory
breadth and growing list of successes of this basic Darwin-
ian idea. However, the major transitions have continued to
23 The relation of these processes to niche construction and the role
of agency in evolution would be a further interesting topic to explore;
see Veit (2021b) for some initial thoughts on the subject.
Scaffolding Natural Selection
1 3
resist this Darwinian imperialism. The aim of this article has
been to offer the first philosophical analysis of Rainey’s idea
of ecological scaffolding as an ambitious attempt to once
more push the boundaries of externalist Darwinian theoriz-
ing to encompass the major transitions, including the very
origins of life.
Firstly, I have argued that Rainey’s work constitutes a
beautiful example for the possible fruitfulness of collabo-
ration between philosophers of biology, theoreticians, and
experimental biologists, since it provides both a general
theoretical framework and an empirically testable hypoth-
esis that can be implemented in the lab with plenty of pos-
sibilities for feedback between both. Collaboration between
philosophers and evolutionary biologists is, of course, not
a new phenomenon (see Lloyd etal. 2008 for a particularly
elegant case); they have worked together on the nature of
evodevo (Wagner etal. 2000; Love and Raff 2003; Love and
Travisano 2013) and on how theoretical, field, and lab work
interacts in practice (Winther etal. 2015). Naturally, this is
far from an exhaustive list and many more examples could
be given once we extend our view to experimental evolution-
ary ecologists such as Robert Brandon and Janis Antonovics.
But what I hope this article has shown is that philosophers of
biology would do well to keep up to date with this exciting
new work in experimental biology, addressing old philo-
sophical problems with exciting new experimental tools. In
this, I can only echo a recent critique by Pradeu (2017):
Clearly evolution offers one unifying framework for
all biology, and some aspects of evolution are highly
theoretical, but this should not hide the fact that evolu-
tion has also a key experimental component. Recent
major advances in evolution have come from studies
in “experimental evolution”, such as those of Richard
Lenski, Michael Travisano, and several others (Lenski
etal. 1991; Lenski and Travisano 1994; Sniegowski
etal. 1997). An exclusive focus on the theoretical
dimension of evolution might lead philosophers of
biology to miss the importance of these experimental
approaches to evolution. (Pradeu 2017, pp. 159–160)
Secondly, I have tried to offer an analysis and explication
of ecological scaffolding, an idea that while highly useful
and fruitful for the illumination of old biological problems
in the major transitions literature, could benefit from further
refinements by philosophers of biology. While usage of the
term “scaffolding” has exploded in recent years, it has often
been used in rather imprecise, vague, and merely metaphori-
cal ways. Here, I have attempted to give both a precise, yet
general definition of scaffolding and evolutionary scaffold-
ing that restricts Rainey’s idea of ecological scaffolding to
only those circumstances in which exogenously imposed
properties come to be endogenized, since it is here that we
come to observe the dawn of development. In addition, I
have argued that this emphasis on endogenization allows us
to distinguish a reverse “scaffolding” process, with organ-
isms becoming less autonomous, that must have occurred
during the evolution of viruses and other parasitic life cycles.
Future work may attempt to model the adaptive dynamics
leading to exogenization or endogenization. The greatest
strength of the inherently Darwinian explanatory strategy of
scaffolding may indeed lie outside of its original application,
i.e., at the origin of life, when there are no lower-level units
with Darwinian properties that could be co-opted. It is in
this sense that the very origins of natural selection may very
well have required something like a complex ecological scaf-
fold. Indeed, this recognition may help us to explain why the
origins of life research has failed to make much progress: a
partial neglect of ecology. While one may legitimately reply
that the study of the chemical conditions for life has been
about the ecological conditions of protolife forms, this work
does seem to have been dominated by an internalist mode
of explanation which focused on the minimal conditions of
living systems without a role for ecological scaffolding. This
is why I have titled this paper “Scaffolding Natural Selec-
tion.” The origins of life may require appropriate ecological
conditions in order to scaffold Darwinian properties onto
nonliving systems, in something like an original eco-devo
feedback loop in which natural selection itself gradually
emerged alongside the process of life. However, I have also
argued that we should resist the temptation to label any envi-
ronmental support found in nature as environmental scaf-
folds, as this would make the explanatory concept hollow.
Scaffolding can indeed be developed into a genuine scientific
concept with explanatory power, but only if we restrict it as
I have argued to processes that contain the very origins of
the endogenization of Darwinian properties.
Further directions that deserve exploration are the con-
nections between development, ecological scaffolding, and
natural selection. While this article has aimed at addressing
these issues, I have only taken some of the first steps, and
there is much further work that needs to be done. Using the
framework suggested here, many puzzling problems about
the evolution of multicellular organisms and even the ori-
gin of life may come to be illuminated. The scaffolding of
natural selection offers an exciting new research area within
experimental evolution, and it is likely to play an important
role in future research on the major transitions. Unfortu-
nately, it has been common among evolutionary biologists to
ignore the role of development at the timescale of the major
transitions. But this is a mistake. In evolutionary scaffold-
ing processes (whether endogenizing or reverse exogenizing)
much hinges on the development of complex life cycles. I
expect that future work inspired by Rainey’s experiments
will show the need to distinguish between what we may call
developmental scaffolding as a process within a life cycle
and evolutionary scaffolding as a process across life cycles,
W.Veit
1 3
and how these forces can pull in opposite directions in the
cases of multilevel selection. Unfortunately, a proper treat-
ment of these further problems will be a task for another day
(though see Griesemer (2016, 2018, 2019) for a set of inter-
esting ideas). Nevertheless, the need to distinguish different
scaffolding processes only further emphasizes the need for
greater conceptual clarity.
Finally, I hope that there is at least some kernel of truth
to be found in the analysis I have offered, and that it will be
useful to philosophers, experimentalists, and theoreticians
alike in our joint goal to further progress our understanding
of evolution. To conclude: scaffolding is a natural phenom-
enon, one that plays a myriad of underexplored yet important
roles in evolution. We can be Darwinians about the Darwin-
ian process of natural selection itself, thus once again broad-
ening the scope of natural selection as a “first principle” of
biology as expressed in Theodosius Dobzhansky’s famous
dictum that, “nothing in biology makes sense except in the
light of evolution” (Dobzhansky 1973).
Acknowledgements Early versions of this article were presented at
talks in Krakow, Istanbul, Ankara, and Oslo. Thanks to the audience
members for discussions and Paul Rainey, Samir Okasha, Pierrick
Bourrat, Heather Browning, and two anonymous reviewers for their
feedback on draft versions.
Funding This research was supported under Australian Research Coun-
cil's Discovery Projects funding scheme (Grant No. FL170100160).
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