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Karl Popper argued in 1974 that evolutionary theory contains no testable laws and is therefore a metaphysical research program. Four years later, he said that he had changed his mind. Here we seek to understand Popper’s initial position and his subsequent retraction. We argue, contrary to Popper’s own assessment, that he did not change his mind at all about the substance of his original claim. We also explore how Popper’s views have ramifications for contemporary discussion of the nature of laws and the structure of evolutionary theory.
POPPERS SHIFTING APPRAISAL
OF EVOLUTIONARY THEORY
Mehmet Elgin and Elliott Sober
Karl Popper argued in 1974 that evolutionary theory contains no testable laws and is
therefore a metaphysical research program. Four years later, he said that he had changed
his mind. Here we seek to understand Poppers initial position and his subsequent re-
traction. We argue, contrary to Poppers own assessment, that he did not change his
mind at all about the substance of his original claim. We also explore how Poppers
views have ramications for contemporary discussion of the nature of laws and the
structure of evolutionary theory.
1. Poppers Evolving Interest in Evolution
Although our main focus is on Poppers 1974 and 1978 papers, his interest in
evolutionary theory (ET) started considerably earlier, and a review of this his-
tory will be useful in locating Poppers two papers within his larger intellectual
trajectory. Poppers book The Poverty of Historicism was published in 1957, but
much of it (including the material on evolution) was given as lectures in the
1930s, and the book was rst published in three parts in Economica,in1944
45. Although Popper does not assert in The Poverty of Historicism that ET con-
tains no empirical laws, and this is the central issue in his 1974 and 1978 papers,
he advances philosophical theses about ET in that earlier book that helped shape
his subsequent thought. In fact, he there defends what seems like a logically stron-
ger thesisthat there are no laws of evolution, period.
Contact Mehmet Elgin at Muğla SıtkıKoçman Üniversitesi, Edebiyat Fakültesi, Felsefe Bölümü,
48170 Kötekli, Muğla, Turkey (melgin70@gmail.com). Contact Elliott Sober at the Philosophy De-
partment, University of WisconsinMadison, 53706 (ersober@wisc.edu).
We thank Dylan Beschoner, Richmond Campbell, Jonathan Hodge, Gregory Mayer, Alan Sidelle,
and the anonymous referees of this journal for useful comments. Mehmet Elgin acknowledges the sup-
port of TÜBİTAK BİDEB 2219. Elliott Sober acknowledges the support of the William F. Vilas Trust.
HOPOS: The Journal of the International Society for the History of Philosophy of Science, vol. 7 (Spring 2017).
2152-5188/2017/0701-0001$10.00. © 2017 by the International Society for the History of Philosophy of Science.
All rights reserved. Electronically published February 16, 2017.
31
In section 27 of The Poverty of Historicism (called Is There a Law of Evo-
lution? Laws and Trends), Popper argues that there are no general laws gov-
erning the evolution of societies, the evolution of organisms, or the evolution
of the whole universe. He claims this despite granting that many laws are used
in explaining the phenomena discussed in the social sciences, in evolutionary
biology, and in cosmology:
The crucial point is this: although we may assume that any actual suc-
cession of phenomena proceeds according to the laws of nature, it is im-
portant to realize that practically no sequence of, say, three or more causally
connected concrete events proceeds according to any single law of nature.If
the wind shakes a tree and Newtons apple falls to the ground, nobody
will deny that these events can be described in terms of causal laws.
But there is no single law, such as that of gravity, nor even a single def-
inite set of laws, to describe the actual or concrete succession of causally
connected events. ...The idea that any concrete sequence or succession
of events (apart from such examples as the movement of a pendulum or a
solar system) can be described or explained by any one law, or by any one
denite set of laws, is simply mistaken. There are neither laws of succes-
sion nor laws of evolution. (Popper 1957, 117)
In the same section of the book, Popper faults Comte and Mill for thinking
that there are laws of succession, each of which describes the sequence of states
through which a system of given type must move. Popper admits that there are
historical trends, but he says that trends cannot be characterized by universal
generalizations and so are not laws.
We have a few criticisms of these claims from The Poverty of Historicism.
Popper shifts between saying that there is no singlelaw of evolution and say-
ing that there is no one set of denite laws.The rst of these requires a prin-
ciple for individuating laws. If L
1
,L
2
,...,L
n
are each laws, why is their con-
junction not a single law? And conversely, a sentence that seems to express a
single law can easily be rewritten as a conjunction, each conjunct of which ex-
presses a law. For this reason, Poppers point is better put by using the second
formulation that there is no one set of denite laws.In any event, we do not
think that Popper gives a good reason for accepting this thesis. He grants that
there are laws of change for pendulums and solar systems but never makes it
clear why there are not laws of evolution for other objects. We also see a prob-
lem in Poppers expressing his thesis that there are no laws of evolution by as-
suming that laws must be deterministic. Why can a law not be probabilistic?
Popper is right to separate laws from mere trends. However, even if statements
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
32
like events of type T
1
are often followed by events of type T
2
are not law-like,
this leaves it open that probability statements sometimes are. After all, there are
objective interpretations of probability statements that do not equate probabil-
ities with actual frequencies. This is a point that Popper later took to heart when
he proposed a propensity interpretation of probability (Popper 1959b). Quan-
tum theory does more than describe de facto trends. Trends depend on initial
conditions, as Popper says, but this does not rule out the possibility that there
are probabilistic laws that are independent of initial conditions.
Poppers interest in ET broadened in the 1960s (Simkin 1993; Watkins 1995;
Munz 2006). There were three reasons why. Popper came to believe that there is a
close relationship between his method of trial-and-error elimination in science
and the process of natural selection (Ruse 1977; Nanay 2011). He also saw
new ways in which ET is relevant to his views on historicism. And nally, he be-
came convinced that ET is the only signicant scientic theory that bears on the
mind-body problem and on the problem of freedom in a physical universe.
1
As Popper delved deeper into ET, he came to believe that the principle of
natural selection (NS) is a priori. This was a conclusion that puzzled him. On
the one hand, he maintained that scientic theories are empirical, not a priori,
but on the other hand, he was convinced that ET is the most successful theory
in its domain and has no serious competitors. Popper (1974, 1978) attempted
to reconcile these ideas, although he reached different conclusions in the two
papers. His solution in the 1974 paper was to suggest that ET is not a theory
of forces like Newtonian theory but rather involves what he calls situational
logic.This formed part of his thesis that ET provides in-principle explanations
without supplying actual explanations of particular phenomena, since actual
explanations of a given event must cite empirical laws. In the 1978 paper, Pop-
per additionally suggested that there is an alternative formulation of the prin-
ciple of NS that is not a priori. The 1974 a priori principle and the 1978 em-
pirical reformulation are these:
(PNS) If traits T
1
and T
2
are found in a population, T
1
is tter than T
2
,
the two traits are heritable, and no other evolutionary causes impinge on
the population, then T
1
will increase in frequency.
2
1. Clear statements of his interest in ETregarding the relationship between ET and his evolutionary
epistemology can be found in Popper (1957, 133; 1959a, 91; 1972, 261; 1974, 133; 1994a, 12). For
Poppers view about how ET bears on the mind-body problem and on freedom in a physical universe,
see Popper (1972, 1978, 1994a).
2. Popper (1974) does not explicitly indicate which statement about NS he thinks is a priori, but he
does say that given the statistical denition of tness, the principle he has in mind is a priori. This is our
justication for saying that Popper thinks that PNS is a priori.
Elgin and Sober |SPRING 2017
33
(PNSR) All traits in all organisms evolved because of natural selection.
3
Popper (1978) says that the distinguished evolutionary biologists C. H. Wad-
dington, G. G. Simpson, J. B. S. Haldane, and R. A. Fisher all attribute great
explanatory power to PNS. Popper replies to this pantheon by pointing out
that PNS is a tautology and that the explanatory power of a tautology is zero.
PNSs tautological status stems from the fact an organismstness is dened as
its degree of actual reproductive success. By proposing the PNSR reformula-
tion, Popper hoped to increase the empirical content of ET by turning its main
principle into an empirically testable statement. As we explain in what follows,
Popper (1978) thought that PNSR is false. Given this, it is questionable how
much ET is improved by replacing the trivial claim PNS with the false claim
PNSR.
2. Difculties concerning A Priori Formulations of ET
In his 1961 Herbert Spencer Lecture (which appeared in print as Popper
[1972]), Popper says that it is a mistake to compare ET to Newtonian physics:
Darwins discovery of the theory of natural selection has often been compared
to Newtons discovery of the theory of gravitation. This is a mistake. Newton
formulated a set of universal laws intended to describe the interaction, and
consequent behavior, of the physical universe. Darwins theory of evolution
proposed no such universal laws(267). Popper claims that Darwinisms suc-
cess resides in its providing in-principle explanations: Nevertheless, Darwins
revolutionary inuence upon our picture of the world around us was at least as
great, though not as deep, as Newtons. For Darwins theory of natural selection
showed that it is in principle possible to reduce teleology to causation by explaining,
in purely physical terms, the existence of design and purpose in the world (267).
4
Two paragraphs later, Popper says: Although this was a great achievement, we
have to add that the phrase in principle is a very important restriction. Neither
Darwin nor any Darwinian has so far given an actual causal explanation of the
adaptive evolution of any single organism or any single organ. All that has been
shown ...is that such explanations might exist (that is to say, they are not log-
ically impossible).Thus, the reason Popper thinks it is a mistake to liken ET
to Newtonian theory is that ET does not postulate empirical laws; it offers a
3. This is how we interpret Poppers (1978, 345) claim that all organisms and especially all those
highly complex organs whose existence might be interpreted as evidence of design and, in addition, all
forms of animal behavior have evolved as the result of natural selection.
4. In this quotation, the use of italics is Poppers. The same holds for italics in subsequent quota-
tions, unless otherwise noted.
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
34
priori mathematical models, which at best can show how it is possible in prin-
ciple to explain adaptations in causal terms.
How, according to Popper, did evolutionary biology lapse into tautology? In
his 1961 Herbert Spencer Lecture, he alludes to the source of this problem:
What I regard as Darwinscentralideahis attempt to explain genetic changes
which led to better adaptation in the sense of better chances for the individual an-
imal or plant to survivehas recently suffered an eclipse. This is due very largely
to the fashionable pursuit of mathematical exactness, and to the attempt to de-
ne survival value statistically, in terms of actual survival (of a gene, or some
other genetic unit, in a population)(Popper 1972, 271). In the next two par-
agraphs, Popper argues that such mathematically exact theories cannot explain
complex adaptations. Popper addresses this topic in his Science: Problems,
Aims, Responsibilities,initially presented at the Plenary Session of the 47th An-
nual Meeting of the Federation of American Societies for Experimental Biol-
ogy and published in the Federation Proceedings in 1963 but subsequently re-
published as chapter 4 of his The Myth of the Framework:There is a difculty
with Darwinism. While Lamarckism appears to be not only refutable but ac-
tually refuted (because the kind of acquired adaptations which Lamarck envis-
aged do not appear to be hereditary), it is far from clear what we should con-
sider a possible refutation of the theory of natural selection. If, more especially,
we accept the statistical denition of tness which denes tness by actual sur-
vival, then the theory of the survival of the ttest becomes tautological, and
irrefutable(1994b, 94).
Although Popper talks about the difculty of formulating empirical laws
about NS that use a statistical denition of tness in his 1961 Herbert Spencer
Lecture, it is only in his 1963 talk that he claims that this way of describing
NS is tautological. However, Popper does not give a full treatment of the is-
sue in that talk. He just adds to the above quoted remarks that we should avoid
the mistake of thinking that NS is the only mechanism that can lead to appar-
ently goal-directed adaptations. The way forward, he suggests, is to conceptu-
alize alternative mechanisms and then design crucial experiments to decide be-
tween them and NS. We note that this is and has been common practice in
evolutionary biology, where drift is often taken to be an important competitor
to NS.
The issue of tautology reappears in Poppers 1965 Arthur Compton Memo-
rial Lecture: Quite apart from evolutionary philosophies, the trouble about
evolutionary theory is its tautological, or almost tautological character: the dif-
culty is that Darwinism and natural selection, though extremely important,
explain evolution by the survival of the ttest(a term due to Herbert Spen-
cer). Yet there does not seem to be much difference, if any, between the asser-
Elgin and Sober |SPRING 2017
35
tion those that survive are the ttestand the tautology those that survive are
those that survive’” (Popper 1972, 24142). Popper raises the same issue in his
1969 Emory University Lectures:
Biologists (especially Fisher) felt compelled to dene as more tthose
which more often survive. Thus, what once looked like a promising ex-
planatory theory becomes quite empty. The statement Evolution tends
to produce higher forms because only the ttest survivemay sound like
an explanation. But if we substitute here for the ttestits dening phrase,
we get: Evolution tends to produce higher forms because those forms
which more often survive more often survive.So our becausephrase
has degenerated into a tautology. But tautology cannot explain anything.
All tautologies are equivalent to All tables are tablesor Those who live long
are those who live long.(Popper 1994a, 54, emphasis added)
5
The tautology issue comes up again in chapter 2 of PoppersObjective Knowl-
edge, but this time he goes a little further in his treatment of the subject: A
central problem of evolutionary theory is the following: according to this the-
ory, animals which are not well adapted to their changing environment per-
ish; consequently those which survive (up to a certain moment) must be well
adapted. This formula is little short of tautological, because for the moment
well adaptedmeans much the same as has those qualities which made it sur-
vive so far.In other words, a considerable part of Darwinism is not of the na-
ture of an empirical theory, but it is a logical truism(Popper 1972, 69).
After this paragraph, Popper attempts to separate, to our knowledge for the
rst time, the empirical and the a priori parts of the theory. He says that the
empirical claims are these: the environmental conditions in which organisms
live change slowly, organisms are sensitive to such changes, the only way organ-
isms can survive such changes is by producing mutations some of which re-
spond to these changes, and useful mutations sometimes occur. Popper then
describes what he thinks the a priori part of this theory is: If the process of
adjustment has gone on long enough, then the speed, nesse, and complexity
of the adjustment may strike us as miraculous. And yet, the method of trial and
of the elimination of errors, which leads to all this, can be said not to be an
empirical method but to belong to the logic of the situation. This, I think, ex-
plains (perhaps a little too briey) the logical or a priori components in Dar-
winism(1972, 70). Popper is here constructing a conditional statement whose
5. We see things differently; although all mathematical truths are mathematically equivalent, some
help explain empirical phenomena while others do not (Sober 1984, 2011a).
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
36
antecedent describes properties of organisms and environment and whose con-
sequent is about the emergence of novel adaptations. The conditional, he says,
is a priori, although the antecedent and the consequent are both empirical.
Logically speaking, Poppers conditional resembles if someone is a bachelor,
then he is unmarried.Popper does not take the discussion any further in this
work.
Poppersrst thorough treatment of the tautology issue and ET was in sec-
tion 37 of his 1974 autobiography (entitled Darwinism as a Metaphysical Re-
search Program), which appeared in the Schilpp volume on Popper. Here Pop-
per announces that ET is not a testable scientic theory, but a metaphysical
research programa possible framework for testable scientic theories.He
claries what he means by a metaphysical research program in footnote 242:
The term metaphysical research programmewas in my lectures from about
1949 on, if not earlier; but it did not get into print until 1958, though clearly
in evidence in the last chapter of the Postscript (in galley proofs since 1957). I
made the Postscript available to my colleagues, and Professor Lakatos acknowl-
edges that what he calls scientic research programmesare in the tradition of
what I described as metaphysical research programmes(metaphysicalbe-
cause nonfalsiable)(Popper 1974, 175).
This remark requires clarication. Although Popper thought that ET is a
metaphysical research program,he did not think that Newtons theory or
Einsteins theory falls in this category. Lakatos, in contrast, held that all three
are scientic research programsbecause all three contain hard-core assump-
tions that are unfalsiable because of the Duhem-Quine problem. For this rea-
son, Lakatos proposed that such programs should be evaluated by their overall
ability to solve theoretical and empirical problems. The latter part of what
Lakatos says in fact coincides with what Popper says about metaphysical re-
search programs. Popper decides whether a theory is metaphysical on the basis
of whether its purported laws are empirical (Stamos 2007); he does not deny
that genuine physical theories may also deploy metaphysical assumptions. It is
in this connection that Popper sees a difference between ET on the one hand
and Newtons and Einstein
s theories on the other.
It is important to recognize that just because Popper labeled ET as meta-
physical, we should not jump to the conclusion that he took ET to be a pseu-
doscience (Hull 1999). For Popper (1978, 344), the doctrine of natural selec-
tion is a most successful metaphysical research programme.Psychoanalysis
and astrology are not in that league; Popper thinks they are pseudosciences.
Popper then explains Darwinisms success as a metaphysical research program
in terms of situational logic:I think that there is more to say for Darwinism
than that it is just one metaphysical research programme among others. In-
Elgin and Sober |SPRING 2017
37
deed, its close resemblance to situational logic may account for its great success,
in spite of the almost tautological character inherent in the Darwinian formu-
lation of it, and for the fact that so far no serious competitor has come forward
(1974, 135). The a priori character of ETs main principle does not lead Pop-
per to dismiss the theory; rather, he tries to provide a philosophical explanation
of how ET could be scientically successful.
Popper then clearly states that by Darwinian theory or Darwinism he is re-
ferring to the modern synthesis theory of evolution. He says that this theory
involves two main theses:
(1) The great variety of the forms of life on earth originate from very few
forms, perhaps even from a single organism: there is an evolutionary
tree, an evolutionary history.
(2) There is an evolutionary theory which explains this. It consists in the
main of the following hypotheses. (Popper 1974, 135)
Popper then lists four hypotheses that esh out this second component. The
rst says that heredity is faithful. The second says that small variations arise
in populations caused by accidental mutations. The third says that NS is a
mechanism that eliminates unt variants. The fourth says that the variability
(what Popper calls the scope of variation) found in a population is subject
to NS. This last hypothesis basically says that the hereditary mechanisms that
give rise both to mutations and to mostly faithful reproduction are themselves
the products of NS. It is in this sense that NS is said to determine the scope of
variation.
It should be clear that the two main theses of ET are not tautologies, and
neither are the rst, second, and fourth hypotheses that Popper mentions. Pop-
per thought that unrestricted existential claims are not falsiable and therefore
not empirically testable; however, he also thought that existential statements
are falsiable if they are restricted to specic space-time regions (Popper 1963,
19596).
6
This is how common ancestry and the three hypotheses should be
understoodthey each pertain to life on planet Earth.
7
It follows that there
are important parts of ET that are empirically testable. The only element that
is a candidate for being a tautology is the third hypothesis. If this analysis of
what Popper says is correct, then the only reason Popper has for thinking that
ET is not a testable scientic theory is that the third hypothesis is a tautology.
6. Others do not require much in the way of background theories; there are ve apples in the bas-
ket nowcan be refuted just by counting them apples.
7. The idea that common ancestry is a historical hypothesis, not a general law, is already in The
Poverty of Historicism; see Popper (1957, 1067).
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
38
The problem, he thinks, is engendered by the denition of tness that biolo-
gists use: Adaptation or tness is dened by modern evolutionists as survival
value, and can be measured by actual success in survival: there is hardly any
possibility of testing a theory as feeble as this(Popper 1974, 137).
Although Popper speaks of the synthetic theory of evolution,he evidently
is not thinking of all the propositions in that theorywhen he says that the
theory is feeble or tautologous. We suggest that when he here considers what
evolutionary theoryis, he is looking for laws, so all the singular empirical
statements in the theoryare set to one side. What remains, he says, is a lonely
tautology. We emphasize that the fact that PNS is a tautology does not show
that there are no laws in ET. Otherwise, Newtons theory would fall by the
wayside if F5maturned out to be a denition of force. Popper thinks that
PNS is the main principle of ETand the only candidate law that ET offers. For
Popper, PNS is the common thread of ET that connects all the other evolu-
tionary claims he lists; if it fails to be empirical, what we are left with is a bunch
of unconnected claims.
As already noted, Popper thinks that the way to understand how ET can be
successful despite its lacking empirical laws is by understanding its deployment
of situational logic. To get a grip on what Popper has in mind here, consider
the following remark: Models, as here understood, may be called theories,
or be said to incorporate theories, since they are attempts to solve problems
problems of explanation. But the opposite is far from true. Not all theories are
models. Models represent typical initial conditions rather than universal laws.
And they therefore need to be supplemented by animatinguniversal laws of in-
teractionby theories which are not modelsin the sense here indicated(Popper
1994b, 165).
8
Popper thinks that in the natural sciences there are two kinds of
explanandatoken events and types of events. Token events are explained or pre-
dicted by the conjunction of universal laws and initial conditions. However, types
of events are explained by constructing models that represent typical initial con-
ditions,which are always idealizations (164). Poppers example of a token event
is the occurrence of a particular lunar eclipse, and his example of a type of event is
the periodic recurrence of lunar eclipses. The idea is that if you want to either
explain or predict a single occurrence of a lunar eclipse, you just write down some
laws and relevant initial conditions and then show that the event in question fol-
lows from them. However, in explaining why lunar eclipses have a periodic cycle,
you need to build a model representing typical initial conditions and show that
under these typical initial conditions lunar eclipses periodically recur. In the sec-
8. This quotation is from The Myth of the Framework. The chapter is based on a talk Popper gave in
1963.
Elgin and Sober |SPRING 2017
39
ond case, laws enter into explanations only indirectly in the sense that the models
are built or constructed in accordance with those laws.
Although Popper says in the passage previously quoted that a model-based
explanation must always have universal laws in the background, he says a page
later that model-based explanations can go forward in the social sciences even
though they are not backed up by universal laws: I wish to propose the thesis
that what I have said about the signicance of models in the natural sciences
also holds for models in the social sciences. In fact, models are even more im-
portant here because the Newtonian method of explaining and predicting sin-
gular events by universal laws and initial conditions is hardly ever applicable in
the theoretical social sciences. They operate almost always by the method of
constructing typical situations or conditionsthat is, by the method of con-
structing models(1994b, 166). Constructing a model in the social sciences
proceeds by describing a typical situation, the problem in that situation, the
parts of the typical situation that are relevant to the problem at hand, and
the aims of an agent. Popper thinks that these models do not require the spec-
ication of any psychological laws. All you need to assume, according to Pop-
per, is a so-called rationality principle. This principle says that agents act in ac-
cordance with the requirements of the situation for achieving their aims.
Although Popper seems to think that the only candidate for a law in the so-
cial sciences that resembles Newtonian laws is the rationality principle, he says
that this principle is not a law; he thinks it is almost empty. This is because he
holds that the rationality principle is just an assumption about how a typical
agent would behave in a given typical situation, and in this sense it does not
necessitate an outcome in the way that Newtons laws together with initial con-
ditions have entailments about Uranuss orbit.
Right after he asserts that there are no testable laws in ET, Popper (1974,
135) says that Darwinism is an application of situational logic. His point is this:
First you describe a population of objects that is somewhat stable, but not ex-
actly, and that contains small variations where some of those variations help
their possessors to better cope with the conditions in which they nd them-
selves. Then you add to this the assumption of the existence of a special frame-
worka set of perhaps rare and highly individual conditionsin which there
can be life, or more especially, self-reproducing but nevertheless variable bodies.
Then a situation is given in which the idea of trial and error-elimination, or of
Darwinism, becomes not merely applicable, but almost logically necessary
(134). Here again, Popper is formulating a conditional in which the assump-
tions of the model are the antecedent and statements about resulting adapta-
tions are the consequent; the whole conditional is a priori. In accordance with
this general framework, you can construct specic models for a specic situa-
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
40
tion; the specic case is an instance of a general type of event. The models, then,
are testable but not the framework itself. All you can say about this framework is
how often it is realized in nature.
9
In contrast, Newtonian theory as a whole is
empirically testable and so are its applications in specic models.
For Popper, what holds things together in a model in the social sciences is
the so-called rationality principle, while in ET what holds things together is
PNS. However, in the case of the social sciences, the rationality principle is ob-
viously false if you interpret it as an empirical claim; therefore, this principle is
better interpreted as a regulative principle in the construction of models.
10
In
the case of ET, the situation is different, in that the PNS lawis true a priori.
Even so, there is something that ET and the social sciences have in common;
for Popper, the point of building models in both is not to test these general
lawsbut to test singular models constructed in accordance with those laws.
Poppers demand that a genuine scientic theory must include empirical
laws shaped his view of what a scientic explanation is. For him, a causal ex-
planation of a token event must cite at least one empirical law: To give a causal
explanation of an event means to deduce a statement which describes it, using
as premises of the deduction one or more universal laws, together with certain
singular statements, the initial conditions(Popper 1959a, 38). Popper was
therefore driven to conclude, not just that ET contains no empirical laws but
that it cannot provide causal explanations of the adaptive features that organ-
isms actually possess.
It is interesting that Popper thinks that a model-based explanation is about
the typical situationsthat a kind of system occupies. Does this mean that the
system usually obeys the models postulates? If so, Poppers usage of modelis
at variance with what scientists and philosophers now often mean by that term.
They have in mind the idea that models contain idealizations that render them
false. Far being true of what a system is like in the kinds of situations it usually
occupies, the model is true only in situations that the system never occupies.
3. A Change of Heart or the Old View in Disguise?
We now return to Poppers famous talk delivered in 1977 at Darwin College,
Cambridge, which was published in Dialectica in 1978. Although Popper says
9. Brandon (1990) endorses this position.
10. Popper (1994b, 169) says that the rationality principle is a consequence of a methodological
rule: The adoption of the rationality principle can therefore be regarded as a by-product of a method-
ological postulate. It does not play the role of an empirical postulate.PNS in ET, however, cannot be
interpreted as a methodological or regulative rule since it is an a priori truth. Yet, according to Popper,
their functions in the construction of models are the same.
Elgin and Sober |SPRING 2017
41
that he has changed his mind about the absence of testable laws in ET, we will
argue that Popper in fact did not change his mind at all (see also Hull 1999,
485). He begins by describing an evolutionary claim that is empirical: In its most
daring and sweeping form, the theory of natural selection would assert that all
organisms, and especially all those highly complex organs whose existence might
be interpreted as evidence of design and, in addition, all forms of animal be-
haviour, have evolved as the result of natural selection. ...If formulated in this
sweeping way, the theory is not only refutable but actually refuted(Popper
1978, 345). Popper mentions two reasons for thinking that this PNSR formu-
lation is falsiablesexual selection and drift. To discuss sexual selection, Pop-
per uses Darwins example of the peacocks tail. Peacocks evolved gaudy tails
because peahens preferred to mate with gaudy males. Popper sees this as a coun-
terexample to PNSR because he accepts Darwins assumption that sexual se-
lection is not a species of NS. Many biologists would now disagree. They would
say that when males evolve gaudy tails in response to the mating preferences of
females, this is just like polar bears evolving thicker fur in response to the weath-
ers getting colder. Sexual selection is a special kind of NS wherein the selective
environment experienced by one sex is created by properties of the other. As for
Poppers idea that drift is a counterexample to PNSR, he does not mention any
actual examples of drift; rather, he notes that an organ may look adaptive and
that this may lead us to mistakenly conclude that it was produced by NS. He
says this can be a mistake because it is possible for an organ to be a product
of drift and become adaptive only later. However, the mere possibility of this sce-
nario is not enough to show that PNSR is false; one needs an actual case. Given
the assumption that there are such cases, Popper can conclude that some organs
and behaviors have evolved without NSs being the cause.
Poppers point about PNSR is fair enough; it is falsiable. However, that
does not show that PNS is empirical or that ET contains an empirical law. Pop-
per has not changed his mind at all; he has merely changed the subject, from
PNS to PNSR. This is also evident from another remark in the 1977 Darwin
lecture: The fact that the theory of natural selection is difcult to test has led
some people, anti-Darwinists and even some great Darwinists, to claim that it
is a tautology. ...I mention this because I too belong among the culprits. ...
My solution was that the doctrine of natural selection is a most successful
metaphysical research programme. ...I still believe that natural selection works
in this way as a research programme. Nevertheless I have changed my mind
about the testability and the logical status of the theory of natural selection
(Popper 1978, 34445). When Popper says that the theory of natural selec-
tionis testable, he presumably has PNSR in mind. He grants that this principle
is not true. If laws must be true, then PNSR is not a law. But is it law-like? That
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
42
is, if it were true, would it be a law? Maybe not; perhaps PNSR resembles the
claim that all cancers are caused by cigarette smoke. If this statement were true,
would it be a law? We think not. It could be an accidental generalization. It
violates no law of nature to suppose that some cancers have other causes. As
noted, Popper thinks there actually are other causes of trait evolution apart from
NS. But what is actual is also nomologically possible. This suggests that even if
PNSR were true, it would not be a law.
Although Popper (1974) is grappling with a genuine philosophical puzzle
concerning the conceptual structure of ET, Popper (1978) achieves something
that is rather trivial; he simply constructs a statement about NS that is testable
and (let us grant) false. Similar constructions can be carried out in phrenology
and psychoanalysis, but Popper would never dream of viewing those construc-
tions as vindicating those two pseudosciences. Popper steadfastly maintained
that a scientic theory must contain one or more empirical laws; his picture
of ET, both in 1974 and in 1978, is that the theory lacks this crucial element.
The thought occurs to us that Popper may have seen that point quite clearly.
4. Evaluating Previous Interpretations of Poppers Views
about Laws in ET
Hull (1999), citing Popper (1978), says that Popper was misled by Fisher, Hal-
dane, Simpson, and other evolutionary biologists. Hull claims that the deni-
tion of tness as actual survival is an idealization introduced to make the math-
ematics work well; for this reason, it should not be considered an essential part
of ET. Hull further submits that biologists are aware that such idealizations are
needed for the theory to be operationalized. We do not think that this reply to
Popper is successful. For one thing, the mathematics of ET does not require
that one equate an organismstness with its actual degree of reproductive suc-
cess. This is something that the propensity interpretation of tness (Brandon
1978; Mills and Beatty 1979) made clear. Second, operationalizinga theory
that contains probabilistic parameters requires estimates of those parameters
values; it does not require that one dene probability to mean actual frequency.
And third, even if we reject the statistical denition of tness, the question re-
mains of what should replace it. Defenders of the propensity interpretation of
tness have suggested the following replacement:
(PNSP) If traits T
1
and T
2
are found in a population, T
1
is tter than T
2
,
the two traits are heritable, and no other evolutionary causes impinge on
the population, then T
1
will probably increase in frequency.
Elgin and Sober |SPRING 2017
43
PNSP is just like PNS, except that probablyhas been inserted. Poppers prob-
lem remains, since PNSP appears to be a priori, given the denition of tness
as a probabilistic propensity.
11
Nanay (2011) claims that Popper changed his account of the scientic pro-
cess of conjectures and refutations after the 1960s under the inuence of Laka-
tos, Zahar, and Worrall and that he came to think that new scientic conjec-
tures are not generated at random; rather, they are inuenced by scientists
beliefs concerning which earlier conjectures have been refuted and which have
been left standing. According to Nanay, since Popper thought that there was a
strong analogy between ET and his theory of the growth of knowledge, apply-
ing his new views to ET led Popper to offer an alternative theory of evolution.
Ruse (1977) also claims that Popper was misled by his attempt to understand
ET from the perspective of his theory of the growth of knowledge, since there
are important differences between the two. We claim that there is a deeper
philosophical reason behind Poppers proposing an alternative theory of evolu-
tion: he believed that ET provides in-principle explanations for typical events
but can never provide actual explanations for particular phenomena, and he
thought this because he believed that the main principle of ET is a priori.
Stamos (1996) argues that it is a consequence of Poppers philosophy that
ET is not a genuine empirical science. Central to Stamoss argument is the idea
that Popper held that a genuine scientic theory must postulate universal laws
and that they and only they are falsiable and empirical in the strict sense; see
also Stamos (2007). Starting with Poppers discussion of whether there are laws
of evolution in The Poverty of Historicism, Stamos claims that it is a conse-
quence of Poppers position that the hypothesis of common ancestry is not a
genuinely scientic claim. However, as we explained in section 1, Popper does
not make any claim about the scientic status of ET in The Poverty of Histor-
icism (Ruse 1977; Hull 1999). Popper rightly denies that the Darwinian hy-
pothesis of common ancestry is a law, since that hypothesis is about the living
things on Earth; it is a singular statement about that token biota. Popper does
not deny that such spatiotemporally restricted existence claims are falsiable
(Hull 1999).
In his later treatments of ET, Popper clearly states that there are testable em-
pirical parts of ET. And even Popper (1959a) is clear that only whole theoret-
ical systems are falsiable, not the parts thereof.
12
Therefore, we object to
11. Given that Popper (1959b) proposed a propensity interpretation of probability, it is curious that
he never saw that there could be a propensity interpretation of tness.
12. This holistic stance on testability encounters the well-known tacking problem.If U is untest-
able and T is testable, then U&T will often be testable.
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
44
Stamoss (1996) claim that Popper thinks ET is not a genuine scientic theory
because it contains untestable existential statements. Poppers problem with
ET is that PNS is a priori. As Hull (1999) pointed out, this sort of misunder-
standing of Poppers views stems from the false dichotomy in which the only
alternatives for Popper are genuine science and pseudoscience. Popper thought
that some theories in the social sciences and in psychology are pseudoscientic;
however, he did not think that all sciences that fail to postulate universal laws
of nature are thereby pseudoscientic.
In Poppers view, there are three categories into which you can place a the-
oretical system: sciences that generate empirical knowledge and understanding
through the postulation of universal laws of nature, sciences that do not pos-
tulate suchlaws but nonetheless generate knowledge and understanding through
the proposal of methodological rules and the construction of models, and pseu-
dosciences that do not generate empirical knowledge and understanding at all.
Popper is placing ET in the second category, not in the third. His position co-
incides with the positions taken by several other subsequent philosophers who
claim that evolutionary biology or biology in general lacks universal empirical
laws of nature in the strict sense; this position is defended in different ways by
Smart (1963), Rosenberg (1994), and Beatty (1995).
13
For criticisms of Ro-
senbergs and Beattys arguments, see Sober (1997).
With respect to the issue of tautology, Stamos cites Gould as an authority
and claims that Popper failed to see that tness does not need to be dened in
terms of actual survival. Consequently, when Popper thought that NS is a tau-
tology, he was being misled by biologists who failed to understand how tness
should be dened. Our reply, noted before, is that dening tness as a prob-
abilistic propensity gives rise to tautologies of its own. In addition, it is well
understood today that Fishers fundamental theorem of NS really is an a priori
mathematical truth (Price 1972; Ewens 1989; Lessard 1997). Thus, Popper
did not make a mistake in thinking that there is a tautology problem that needs
to be addressed. His question was how a science like this can be valuable de-
spite the fact that it does not postulate universal empirical laws. This in itself is
an interesting philosophical problem regardless of the issue of what the best
understanding of tness is.
Ruse (1977) argues that what Popper said in The Poverty of Historicism
agrees with the received view of evolution, rather than being a criticism of
it. Ruse thinks that Poppers mistake lies elsewhere, especially in his Intellectual
Autobiography, where Popper claims that Darwinism is not a testable scientic
13. Mitchell (1997, 2000) and Woodward (2000) argue that biology can do without strict universal
laws. This, of course, involves no commitment as to whether there are such laws.
Elgin and Sober |SPRING 2017
45
theory. After citing Poppers claim that Darwinism does not predict the variety
of life forms, Ruse explains in detail under what sort of initial conditions ET
would issue in this type of prediction. Ruse lists the following conditions: there
is a mainland with large populations and there are isolated areas with varying
conditions, populations vary genetically and phenotypically, and organisms in
the mainland sometimes get to those isolated areas. Ruse then writes: Had one
reason to believe that life on the planet was fairly old (e.g., through the fossil
record or general complexity of structure), yet were one to nd that absolutely
no speciation at all had occurred, then I suggest that, contra Popper, modern
evolutionists would be worried. Their theory, parts of it at least, would have been
falsied. The claims that they make about speciation would seem not to hold
(1977, 645, emphasis ours).
The rst question to ask about Ruses argument is whether speciation has
never occurredis an observation. If it is not, the fact that it is incompatible
with ET does not show that ET is falsiable. But which part of ET conicts
with speciation has never occurred? It conicts with the hypothesis of com-
mon ancestry, but that hypothesis is not law-like, as it describes the history of
life on Earth. Popper grants that there are empirical elements in evolutionary
biology. What he denies is that there are empirical laws. Similar comments
apply to Hodges (1987, 233) point that Popper grants that spatiotemporally
restricted statements are both falsiable and veriable, and so are scientic.
Hodge correctly notes that such statements are to be found in evolutionary bi-
ology. However, this point does not touch Poppers contention that ET lacks
universal empirical laws.
Evolutionary biologist John Maynard Smith (1986, 5) says that the theory of
evolution is not falsiable in the sense required by Popper.He says that the the-
ory is a logical deductionand that Popper is demanding something morethat
a scientic theory must say something about the world, and not merely about
logical necessity.Maynard Smith goes on to say that Darwinism as a testable
scientic theory can take various forms. I will give it rst in the form in which
Darwin himself proposed it, and then in a neo-Darwinistform in which most
biologists hold it today.Maynard Smith then says that the hypothesis of com-
mon ancestry is clearly falsiable ...as a single fossil rabbit in Cambrian rocks
would be sufcient.This is a double mistake. The discovery that MaynardSmith
describes would require an important reworking of when various taxa rst ap-
peared and how different taxa are genealogically related, but neither of these
changes shows that the hypothesis of common ancestry is false. And even if it
did, that does nothing to establish that there are falsiable universal laws of evo-
lution. Maynard Smithnext turns to whathe regards as the second component of
Darwinian theory, that NS was the major, but not the only, cause of evolution.
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
46
He says it would be falsied if one could show that organisms lack one of the
three necessary properties of multiplication, variation, and heredity.Here again,
what is falsied is not a law. When Maynard Smith then turns to the modernthe-
ory of evolution (67), he notes three innovations, but says that they are changes
of emphasis rather than substance.He does not say whether the modern theory
satises Poppersrequirements.
More than a decade later, the biologist Ernst Mayr reacted to Popper in a
lecture Mayr delivered in Stockholm when Mayr received the Crafoord Prize
from the Royal Swedish Academy of Science: Despite the initial resistance by
physicists and philosophers, the role of contingency and chance in natural pro-
cesses is now almost universally acknowledged. Many biologists and philoso-
phers deny the existence of universal laws in biology and suggest that all regu-
larities be stated in probabilistic terms, as nearly all so-called biological laws have
exceptions. Philosopher of science Karl Poppers famous test of falsication
therefore cannot be applied in these cases(Mayr 2000, 82). It is ironic that
Mayr saw physicists and philosophers as devotees of determinism. By 1999,
quantum mechanics had been around for a long time. It is a probabilistic the-
ory, and many physicists and philosophers regarded it as making a strong case
for indeterminism. However, Mayr is right that probability claims are unfalsi-
able under Poppers (1959a) strict denition of that concept; if a coin lands
heads in each of a thousand tosses, this does not falsify the claim that the coins
probability of heads on each toss is 1/2. Even so, Popper (19097) supple-
mented his strict denition of falsiability with something more liberal. This
relaxed denition of falsiability says that the hypothesis that the coin is fair is
refuted by the thousand heads, since the hypothesis says that that outcome is
sufcientlyimprobable. It should be noted, however, that Popper thinks that
it is a matter of convention how improbable an outcome needs to be if it is
to falsifya hypothesis. In any event, the deeper question raised by Popper
(1974, 1978) lies elsewhere. The question is not whether laws can be proba-
bilistic but whether they must be empirical. Quantum theory is empirical,
but PNSP is not.
14
5. Popperian Themes in Current Philosophy of Biology
In this section, we relate Poppers views on whether ET contains empirical laws
to some more recent literature. Popper claims that ET cannot be interpreted as
14. Although we have said something about the reception of Popper (1974, 1978) among philos-
ophers and scientists, there is another story to be told concerning the papersreception among cre-
ationists. For some discussion of this, see Numbers (2006).
Elgin and Sober |SPRING 2017
47
a theory of forces, but we disagree. The different evolutionary causes that im-
pinge on a population (e.g., selection, mutation, migration) are represented in
the theory as vectors that combine to produce a resultant. Indeed, the Hardy-
Weinberg law functions as a zero force law in population genetics, and, as such,
it resembles the law of inertia in Newtonian physics. And PNS (or better,
PNSP) functions like the law of gravitation in that it describes how a system
will behave when a single force acts on it (Sober 1984).
15
Even if Popper had
known about these analogies, we think he would have dismissed them on the
grounds that these evolutionary principles are a priori statements rather than
empirical laws. Our response (to be eshed out more in what follows) is that
laws need not be empirical. Matthen and Ariew (2002) agree with Popper that
ETshould not be interpreted as a theory of forces, although their reasons differ
from Poppers.
As discussed earlier, Popper claims that ET can provide in-principle causal
explanations but cannot provide actual causal explanations of single events. Al-
though the distinction between how-possibly and how-actually explanations is
now standard in philosophy of biology (Brandon 1990; Plutynski 2004, 2005;
Lange and Rosenberg 2011),
16
it is a further step to claim that a priori models
furnish only the former and that empirical laws are needed to furnish the latter.
We think that a priori models can do both (Sober 2011a). We further suggest
that a priori dynamical models deserve to be called laws, since they do the same
explanatory and predictive work that empirical laws perform (Elgin 2003).
And while empirical causal laws are hard to nd in evolutionary biology, they
obviously exist in physics. This marks an interesting difference between the
two sciences.
The package of ideas just described has come in for criticism. Lange and
Rosenberg (2011) criticize Sober (2011a) by arguing that explanations built
by using a priori models of NS are not causal explanations. Our response is that
Lange and Rosenbergs thesis about explanation is perfectly compatible with
the claim that there are a priori causal models of NS, a claim that Lange and
Rosenberg do not dispute. We also think that Lange and Rosenberg fail to pro-
vide a convincing criterion for when an explanation is causal (Elgin and Sober
2015). Díez and Lorenzano (2015) develop a different line of criticism; they do
not contest the claim that there are a priori causal models in evolutionary biol-
ogy but argue that the same is true in physics. Our reply is that Sober (2011a)
already noted that a priori models can be constructed in any science that con-
15. See Hitchcock and Velasco (2014) for further discussion of whether the force idea applies to
ET.
16. Ewens (2004) thinks that there is a difference between models in physics and models in evo-
lutionary biology in that the former are more realistic and less idealized.
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
48
tains empirical laws. The fact remains that there are few if any empirical laws in
evolutionary biology but that such laws obviously exist in physics.
17
Poppers idea of situational logic resembles two ideas that continue to be
well received in philosophy of science. The rst is Gieres (1979, 1999) ap-
proach to the structure of scientic theories. According to Giere, a theory (re-
gardless of whether it is physical or biological or social) can be divided into two
components. First, there is an a priori denition of a kind of theoretical system;
for example, Newtons three laws of motion plus his law of gravitation dene
what it means to be a Newtonian system. The second component is empirical.
It involves the claim that this or that token physical system is a Newtonian sys-
tem. Notice that neither of Gieres components is both general and empirical.
Poppers analysis of ET as a situational logic is similar. For Popper, ET provides
a collection of statements that dene a certain kind of object; lets call it a Dar-
winian system. We then apply this denition to an actual case by saying that it
is an instance of a Darwinian system. Although the denition of a Darwinian
system is not empirically testable, specic instances of models constructed by
using this denition are. When some of these specic instances are refuted, we
learn something about the worldnamely, that the assumptions that dene a
Darwinian system are not satised in those cases. However, this does not show
that the denition of a Darwinian system is wrong. For Giere, this pattern ap-
plies both to physics and to biology, but for Popper there is an important dif-
ference because in physics at least sometimes the background laws of a theory
can be empirically refuted. Popper can grant that Gieres formulation applies to
both sciences and still maintain that there is an important philosophical differ-
ence between them.
The second point of contact between Popperssituational logicand more
recent ideas in philosophy of science is the semantic view of theories. Beatty
(1981), Thompson (1989), and Lloyd (1994) argue that ET should be inter-
preted by using this framework; Ereshefsky (1991) disagrees. These authors are
responding to the same problem that exercised Popper; they all are trying to
provide a philosophical explanation of ETs success despite the apparent fact
that ETs only lawis a priori. Popper was against conventionalism(his term
for the view that so-called laws are mere denitions) in the context of physical
theories, although he thought that conventionalism is a coherent and consis-
tent philosophical view. His position on the social and biological sciences is
close to the semantic view of theories, and he embraces conventionalism about
the status of social and biological laws.
17. We do not claim that there cannot be empirical laws in evolutionary biology; our thesis con-
cerns the science as it now is constituted.
Elgin and Sober |SPRING 2017
49
6. Are There Any Empirical Laws in ET?
Popper argues that PNS is a priori, but he claims something more; he addition-
ally claims that there are no empirical laws at all in ET. What is his argument
for this stronger thesis? The same question arises if you think that PNSP is a
better rendering of the probabilistic concept of tness. Popper is neglecting the
possibility that ET contains both source laws and consequence laws (Sober
1984).
18
PNS and PNSP are consequence laws; they purport to describe the
changes that will occur (deterministically, or with some degree of probability)
if certain initial conditions obtain. It is a separate task to describe the circum-
stances that make one trait tter than another. These sources of tness differ-
ence can be found in the physical environment that a population inhabits, in
the biotic environment outside of the population (e.g., the prey, predators, par-
asites, and diseases that the population has to deal with), and in the population
itself (as described by theories of sexual selection and of sex ratio evolution).
Source laws specify the synchronic supervenience bases for variation in tness;
they have the form
If a population contains traits T
1
and T
2
at time t, and its environment
has properties E, and the organisms in the population have other traits
O
1
,O
2
,...,O
n
with frequencies p
1
,p
2
,...,p
n
at time t, then T
1
will
be tter than T
2
at time t.
Some propositions of this form, like the models of sex ratio evolution con-
structed by Carl Dū
̈sing, R. A. Fisher, and W. D. Hamilton, are mathematical
truths (Sober 2011b), but are there any empirical laws that have this form?
There is another way to pursue the question of whether there are empirical
laws of evolution while granting that PNS and PNSP are not examples. Instead
of trying to determine whether there are empirical source laws, one might con-
sider something like the converse of PNS and PNSP. PNS and PNSP are both
forward-directedconditionals; they say (roughly) that if there is variation in
tness at a given time, then you should expect change in trait frequency some-
time later. Campbell and Robert (2005) propose a backward-directedcondi-
tional that approximates the converse of PNS and PNSP, namely,
(D*) For almost every pair of variant organisms aand bthat occupy the
same environment, if ais complex and is generally more reproductively
successful than b, then there exist heritable traits of aand bwhich, in
18. Popper is not the only philosopher who ignores the possibility that the theory of NS contains
source laws; Fodor and Piattelli-Palmarini (2010) make the same mistake.
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
50
interaction with aand bs environment, cause ato be more reproductively
successful than b.
This is an English paraphrase of the D* statement that Campbell and Robert
put in logical notation. The word generallyin D* leads us to think that aand
bare types of organisms, not tokens. The same word also indicates that D* is
not about some brief period of time in which ahappens to out-reproduce bbut
rather concerns a sufciently long stretch of time. Campbell and Robert (2005,
673) say that they prefer to call D* a model rather than a law.
19
The existential quantier in D* is meant to mark the fact that the selection
process that makes trait T
1
tter than trait T
2
in environment E
1
need not be
the same as the selection process that makes T
3
tter than T
4
in environment
E
2
. Perhaps T
1
and T
2
are traits of a buttery species and T
1
is tter than T
2
because of predators, whereas T
3
and T
4
are traits of an elm species and T
3
is
tter than T
4
because of diseases. Here Campbell and Robert are taking on
board the fact that the property of tness is multiply realizable (Brandon
1978; Rosenberg 1978; Mills and Beatty 1979; Sober 1984); this has the con-
sequence that the property a population has of undergoing a selection process is
also multiply realizable. Campbell and Robert emphasize that their D* has the
quantier order for each pair of traits, there exists a story about natural selec-
tionnot the quantier order there exists a single story about natural selection
that applies to all traits.We hope that our English rendering of D* captures this
point.
Richmond and Robert argue that D* is testable, and we agree. Our question
is whether D* would be a law of nature if it were true. Richmond and Robert
do not consider this question, but the question is worth asking in the context
of the current article. What we said earlier about Poppers PNSR reformulation
of PNS is relevant here. Popper said that PNSR is false because of sexual selec-
tion and drift. We likened PNSR to the claim that all cancers are caused by
cigarette smoke. Our point was that PNSR is not law-like; even if it were true,
it would not be a law. We suggest that the same point applies to D*. For D* to
be a law, it must be nomologically impossible for there to be a process other
than NS that could often bring it about that one trait generally is more repro-
ductively successful than another. Here is a reason to doubt that this is so: for
any nite temporal interval, if the two variants aand bare equally t and thus
19. Campbell and Robert formulate their D* in reply to Brandons (1978) contention that expla-
nations of specic evolutionary outcomes that appeal to NS are empirical, but that a general principle
of natural selectionwill be a tautology or will be so vague that it cannot be tested or will be empirically
false.
Elgin and Sober |SPRING 2017
51
evolve by the nonselective process of pure drift, there is a nonzero probability
that awill generally out-reproduce bduring that interval. This claim about
a single pair of traits applies to nsuch pairs, for any nite n.
20
Even if D* is not
an empirical law of nature, the question remains whether there are other
backward-directed propositions about evolution that are.
7. Concluding Comments
We have argued that Poppers (1974) reason for holding that ET is not a test-
able scientic theory is that he thinks that ET lacks empirical laws. He thinks
this even though he grants that there are testable statements in ET. As Stamos
(2007) points out, postulating testable empirical laws is, for Popper, a necessary
condition for a theory to be scientically testable. This is why Popper con-
cludes that ET is a metaphysical research program. Popper does not conclude
that ET is a pseudoscience; instead, he unsuccessfully attempts to reformulate
PNS and also tries to explain ETs success in terms of his idea of situational
logic.
Those who agree with Popper that laws of nature must be empirical may
want to resist Poppers conclusion by insisting that a theory can be testable
without its containing testable laws. Indeed, this suggestion nds a happy
home in Poppers own concept of falsiability; if S is a falsiable statement,
and S and A are logically compatible, then S&A is falsiable. The fact that
S is not a law and A is a tautology does not change that fact. We suspect that
Popper was misled by the examples he found in physics; Newtons theory, rel-
ativity theory, and quantum mechanics are testable theories, and the laws they
propose are testable. As noted earlier, Popper does not hesitate to speak of evo-
lutionary theory,but he does demand that it conform to the pattern he found
in physics. Finding that it does not, Popper concludes that ET is a metaphys-
ical research program,not a testable scientic theory. However, if testable the-
ories need not contain testable laws, there is no need to draw Poppers conclu-
sion.
This is a good reply to Popper, but we take issue with the assumption that
laws of nature must be empirical. Modern evolutionary biology is replete with
mathematical models that are mathematical truths. They describe the proba-
20. There is an additional point: even if D* is an empirical law, it does not do much to explain why
one trait generally out-reproduces another. In this respect, D* resembles adaptationism,at least on
some interpretations of that ism. Both D* and adaptationism are generalizations that can be tested
by case-by-case analyses of different traits in different populations, and if you believe these generaliza-
tions, they will play a heuristic role, guiding your search for specic explanations (Orzack and Sober
1994).
HOPOS |Poppers Shifting Appraisal of Evolutionary Theory
52
bilities that populations have of changing their trait frequencies, given the pop-
ulations initial and boundary conditions. These dynamical models explain and
predict in the same way that empirical laws do; this is why we want to drop the
requirement that scientic laws must be empirical. We are not suggesting that
bachelors are unmarriedis a scientic law; to say that some laws are a priori is
not to say that all a priori statements are laws. Popper was right that ET is dif-
ferent from theories in physics, but that does not mean that ET is untestable or
that it is not a genuine scientic theory.
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... It is worth noting, too, that Karl R. Popper, one of the most influential philosophers of science of the twentieth century, as late as the 1970s still had trouble recognizing the empirical and scientific nature of the theory of evolution, albeit for very different methodological reasons from the empiricism of the nineteenth century (cf. Elgin and Sober 2017). 45 It is worth noting that the Academy was created by Pope Pius XI in 1936, thus renewing the Academy of the Lynx (Accademia dei Lincei) and the Pontifical Academy of the New Lynx, which dated back to the 16th and 19th centuries respectively (see PAS, official website https://www.pas.va/en/about/history.html (accessed on 23 January 2025)). ...
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Described by the philosopher A.J. Ayer as a work of ‘great originality and power’, this book revolutionized contemporary thinking on science and knowledge. Ideas such as the now legendary doctrine of ‘falsificationism’ electrified the scientific community, influencing even working scientists, as well as post-war philosophy. This astonishing work ranks alongside The Open Society and Its Enemies as one of Popper’s most enduring books and contains insights and arguments that demand to be read to this day. © 1959, 1968, 1972, 1980 Karl Popper and 1999, 2002 The Estate of Karl Popper. All rights reserved.
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For the past century and a half no issue has dominated discussions of science and religion more than evolution. Indeed, many people see the creation-evolution debates as the central issue in the continuing controversy. And for good reason. More than a century after the scientific community had embraced organic evolution, many laypersons continued to scorn the notion of common descent. In the United States, where polls since the early 1980s have shown a steady 44-47 per cent of Americans subscribing to the statement that 'God created human beings pretty much in their present form at one time within the last 10,000 years or so', nearly two-thirds (65.5 per cent), including 63 per cent of college graduates, according to a 2005 Gallup poll, regarded creationism as definitely or probably true. As we shall see, such ideas have been spreading around the world. /Creation and Creationism /In 1929 an obscure biology teacher at a small church college in northern California self-published a book entitled Back to Creationism. This brief work, appearing just as the American anti-evolution movement of the 1920s was winding down, attracted little attention.