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Turtles, monsters and the revised evolutionary theory

  • Senckenberg Centre for Human Evolution and Palaeoenvironment an der Universität Tübingen
DOI: 10.1111/ede.12226
Turtles, monsters and the revised evolutionary theory
Evolutionary theory is incomplete. That claim has been made
several times since Darwins 1859 publication of On the
Origin of Speciesspecially between those studying paleon-
tology and development (e.g., Alpheus Hyatt, Richard Owen,
and more recently Stephen J. Gould; Amundson, 2005;
Gould, 1977; Pfeifer, 1965)but recently a growing
movement has gained space on scientific and general public
forums calling for an Extended Evolutionary Synthesis of
Evolution (Laland et al., 2015; Pigliucci & Finkelman, 2014).
Its supporters argue that the Modern Synthesisdeveloped
during the first half of the 20th centuryleaves out of the
picture certain processes related to ontogenetic development
(but not only those; for a summary see Laland et al., 2015),
that have the potential to causally explain some evolutionary
events poorly addressed by the microevolution centered view
of the Modern Synthesis.
In this context, Olivier RieppelsTurtles as Hopeful
Monsters joins the movement to increase those claims. The
title may be slightly misleading: the book is not about turtles
although they play a very important role in the argument,
only the last two chapters are turtle-focused”—but about the
last concept, the Hopeful Monsters.The author provides a
very detailed historical and philosophical account of how
early critics of Darwins theory saw its two basic processes,
mutation and natural selection, as insufficient to explain all
evolutionary events.
The term Hopeful Monster was coined by Richard
Goldschmidt, a GermanJewish geneticist who spent the
last part of his career at the University of California,
Berkeley, studying development, genetics, and evolution. A
monstrosity,according to Goldschmidt (1940), is a
phenotypic alteration that develops in a mutant individual,
such as a Manx cat with truncated tail vertebrae. For this
individual, the mutation does not generate an advantage
(although it may not be disadvantageous, also), hence, it is
just a monster. However, the same monstrosity in a non-bird
dinosaur, say an Archaeopteryx mutant, may result in a
rearrangement of the tail feathers, improving its flight
capacity and its fitness. The mutant Archaeopteryx is, thus,
a hopeful monster.
This kind of mutation should occur early in ontogeny,
reprogramming the developmental pathway while the embryo
is not well-differentiated yet, resulting in a distinct adult
morphology. As such, when comparing adults they appear to
emerge in a single step,that is, shortened tail offspring
generated by normal tailed parents. This creates a gap
between the ancestral”—long tailand the descendant”—
short tailforms. The explanation of the emergence of novel
structures by reprogramming developmental pathways is
called emergentist paradigmby Rieppel (p. 124). In
contrast, the Modern Synthesis embraces the so-called
transformationist paradigm, which explains morphological
evolution in a gradual transition between small steps. There
can be no gaps in this process.
These contrasting views recall the distinction between
structuralists and functionalists well developed by Ron
Amundson (2005). In Rieppels book, the emergentist
paradigm sees structures coming beforeand, hence,
determiningfunctions, whereas in for the transformation-
ists the opposite is true: function shapes structures. On
Amundsons account, those supposedly incommensurable
views were contrasted several times before DarwinsOrigin,
and succeeded each other in dominating the predominant
view of nature. Along the exciting and easy reading 216
pages, Rieppel presents his view on how this transforma-
tionist versus emergentist explanations also shaped the debate
about morphological evolution after Darwins publication,
using the turtle shell as the prime example.
Briefly, two competing hypotheses have been raised to
explain the unique body plan of turtles (Rieppel, 2001). The
first one considers the carapace (the dorsal portion of the
shell) as an endoskeletal structure, derived from the
cartilaginous precursors of the dorsal ribs and vertebrae.
The second posits an exoskeletal nature for it, in which the
ribs and vertebrae would fuse to overlying dermal
ossifications, the osteoderms. The latter view is related to
what Rieppel calls a Polka Dot Turtle Ancestor. This would
be a hypothetical ancestor, that would explain the transition
from a shell-less reptile to a turtle, through a gradual
accumulation of osteoderms on its back. This hypothetical
ancestor has been called to bridge the gap in the fossil
record a couple of times during the last three decades (e.g.,
Joyce et al., 2009; Lee, 1996).
Present address of Gabriel S. Ferreira is Hölderlinstraße 12, room: 407,
D-72074 Tübingen, Germany.
Evolution & Development. 2017;19:227228. © 2017 Wiley Periodicals, Inc.
The big problem is that, since at least the end of the 19th
century, studies on the development of the carapace (e.g.,
Rathke, 1848) show that the ribs and vertebrae in the turtle
trunk never fuse with dermal ossifications, supporting the first
hypothesis. That means that, even with evidence to the
contrary, several researchers have favored the more main-
stream transformationist view.
Rieppels new book presents a view of science factsas
social constructs, building on Ludwik Flecks concept of
thought style to explain why sometimes scientists favor one
view in spite of another and how it refrains the community of
thinking outside this box of knowledge. The Modern
Synthesis can be viewed as one of these boxes (or circles,
in Flecks terminology), limiting our thoughts about how
morphological evolution is realized through small and
gradual steps, even when the sudden emergence of a new
structurelike the mutant Manx cat short tails or the turtle
carapaceseems more likely. According to Rieppel (p. 161)
there is no question that Darwinian evolution through
variation and natural selection [...] does occur, but the
question remains whether this is the only possible way of
evolutionary transformation.The book highlights that it may
be time to embrace the hopeful monsters out there and revisit
the dogmasof Evolutionary Theory.
Gabriel S. Ferreira
Biology Department, Faculty of Philosophy, Science, and
Letters at Ribeirão Preto, University of São Paulo,
Ribeirão Preto, Brazil
Senckenberg Center for Human Evolution and
Palaeoenvironment (HEP) at Eberhard Karls Universität,
Tübingen, Germany
Fachbereich Geowissenschaften, Eberhard Karls
Universität, Tübingen, Germany
Amundson, R. (2005). The changing role of the embryo in evolutionary
thought: roots of evo-devo. Cambridge, UK: Cambridge University Press.
Goldschmidt, R. (1940). The material basis of evolution. New Haven,
USA: Yale: University Press.
Gould, S. J. (1977). Ontogeny and phylogeny. Cambridge, USA:
Harvard University Press.
Joyce, W. G., Lucas, S. G., Scheyer, T. M., & Heckert, A. B. (2009).
A thin-shelled reptile from the Late Triassic of North America and
the origin of the turtle shell. Proceedings of the Royal Society of
London B 276, 507513.
Laland, K. N., Uller, T., Feldman, M. W., Sterelny, K., Müller, G. B.,
Moczek, A., ...Odling-Smee, J. (2015). The extended evolutionary
synthesis: its structure, assumptions and predictions. Proceedings of
the Royal Society of London B 282, 20151019.
Lee, M. S. Y. (1996). Correlated progression and the origin of turtles.
Nature 379, 811815.
Pfeifer, E. J. (1965). The genesis of American Neo-Lamarckism. Isis
56(2), 156167.
Pigliucci, M., & Finkelman, L. (2014). The extended (evolutionary)
synthesis debate: where science meets philosophy. BioScience 64(6),
Rathke, H. (1848). Über die Entwickelung der Schildkröten. Braunsch-
weig, Germany: Friedrich Vieweg und Sohn.
Rieppel, O. (2001). Turtles as hopeful monsters. Bioessays 12, 987991.
ResearchGate has not been able to resolve any citations for this publication.
Full-text available
Scientific activities take place within the structured sets of ideas and assumptions that define a field and its practices. The conceptual framework of evolutionary biology emerged with the Modern Synthesis in the early twentieth century and has since expanded into a highly successful research program to explore the processes of diversification and adaptation. Nonetheless, the ability of that framework satisfactorily to accommodate the rapid advances in developmental biology, genomics and ecology has been questioned. We review some of these arguments, focusing on literatures (evo-devo, developmental plasticity, inclusive inheritance and niche construction) whose implications for evolution can be interpreted in two ways-one that preserves the internal structure of contemporary evolutionary theory and one that points towards an alternative conceptual framework. The latter, which we label the 'extended evolutionary synthesis' (EES), retains the fundaments of evolutionary theory, but differs in its emphasis on the role of constructive processes in development and evolution, and reciprocal portrayals of causation. In the EES, developmental processes, operating through developmental bias, inclusive inheritance and niche construction, share responsibility for the direction and rate of evolution, the origin of character variation and organism-environment complementarity. We spell out the structure, core assumptions and novel predictions of the EES, and show how it can be deployed to stimulate and advance research in those fields that study or use evolutionary biology. © 2015 The Author(s).
A new, thin-shelled fossil from the Upper Triassic (Revueltian: Norian) Chinle Group of New Mexico, Chinlechelys tenertesta, is one of the most primitive known unambiguous members of the turtle stem lineage. The thin-shelled nature of the new turtle combined with its likely terrestrial habitat preference hint at taphonomic filters that basal turtles had to overcome before entering the fossil record. Chinlechelys tenertesta possesses neck spines formed by multiple osteoderms, indicating that the earliest known turtles were covered with rows of dermal armour. More importantly, the primitive, vertically oriented dorsal ribs of the new turtle are only poorly associated with the overlying costal bones, indicating that these two structures are independent ossifications in basal turtles. These novel observations lend support to the hypothesis that the turtle shell was originally a complex composite in which dermal armour fused with the endoskeletal ribs and vertebrae of an ancestral lineage instead of forming de novo. The critical shell elements (i.e. costals and neurals) are thus not simple outgrowths of the bone of the endoskeletal elements as has been hypothesized from some embryological observations.
Recent debates between proponents of the modern evolutionary synthesis (the standard model in evolutionary biology) and those of a possible extended synthesis are a good example of the fascinating tangle among empirical, theoretical, and conceptual or philosophical matters that is the practice of evolutionary biology. In this essay, we briefly discuss two case studies from this debate, highlighting the relevance of philosophical thinking to evolutionary biologists in the hope of spurring further constructive cross-pollination between the two fields.
TURTLES exhibit some of the most extreme postcranial modifications found in vertebrates. The dorsal vertebrae and ribs have fused with dermal armour, forming a totally rigid box-like trunk region1,2 Our understanding of chelonian origins has been restricted by a paucity of information on intermediate forms3,4, however, and it is often assumed that they must have evolved saltationally5. It has been suggested that pareiasaurs, a group of large herbivorous anapsid reptiles, are the sister-group of turtles6. Here I show that certain pareiasaurs-dwarf, heavily armoured forms such a Nanoparia-approach the chelonian morphology even more closely than previously thought. Evolutionary trends within pareiasaurs, such as the elaboration of the dermal armour, shortening and stiffening of the presacral region, and increased reliance on limb-driven as opposed to axial-driven locomotion, suggest that the rigid armoured body of turtles evolved gradually, through 'correlated progression'7.
In this book Ron Amundson examines 200 years of scientific views on the evolution-development relationship from the perspective of evolutionary developmental biology (evo-devo). This new perspective challenges several popular views about the history of evolutionary thought by claiming that many earlier authors made history come out right for the Evolutionary Synthesis. The book starts with a revised history of nineteenth-century evolutionary thought. It then investigates how development became irrelevant to evolution with the Evolutionary Synthesis. It concludes with an examination of the contrasts that persist between mainstream evolutionary theory and evo-devo.