Philosophy of Science, 76 (December 2009) pp. 797–808. 0031-8248/2009/7605-0016$10.00
Copyright 2009 by the Philosophy of Science Association. All rights reserved.
797
The Paradox of Stasis and the Nature
of Explanations in Evolutionary Biology
Jonathan Michael Kaplan
†‡
Recently, Estes and Arnold claimed to have “solved” the paradox of evolutionary
stasis; they claim that stabilizing selection, and only stabilizing selection, can explain
the patterns of evolutionary divergence observed over “all timescales.” While Estes
and Arnold clearly think that they have identified the processes that produce evolu-
tionary stasis, they have not. Instead, Estes and Arnold identify a particular evolu-
tionary pattern but not the processes that produce that pattern. This mistake is im-
portant; the slippage between pattern and process is common in population and
quantitative genetics and contributes to a persistent misunderstanding of the nature
of explanations in evolutionary biology.
1. Introduction: Patterns, Processes, and the Paradox of Stasis. In “Re-
solving the Paradox of Stasis: Models with Stabilizing Selection Explain
Evolutionary Divergence on All Timescales” (2007), Estes and Arnold
claim to solve a long-standing problem: given the substantial amount of
heritable phenotypic variation in most populations at most times, why
do so many populations show so little phenotypic change over evolu-
tionary time periods? More precisely, for many of the populations for
which sufficient data for evaluation exist, why is there only a very slight
trend toward increased phenotypic divergence over vastly increasing num-
bers of generations? Using data from Gingerich (2001), Estes and Arnold
note that average phenotypic divergence increases with time only very
slowly—by approximately 0.84 phenotypic standard deviations per mil-
†To contact the author, please write to: Philosophy Department, 208 Hovland Hall,
Oregon State University, Corvallis, OR 97331-3902; e-mail: kaplan@onid.orst.edu.
‡Audience members at the International Society for the History, Philosophy, and Social
Studies of Biology conference in Exeter, England, a colloquium at the Department of
Ecology and Evolutionary Biology at State University of New York at Stony Brook,
and the PSA meeting in Pittsburgh provided extensive useful feedback on this material.
Kim Sterelny in particular made several useful suggestions, and Massimo Pigliucci
provided helpful feedback on various earlier drafts of this paper. All remainingmistakes
are of course my own.

798 JONATHAN MICHAEL KAPLAN
lion generations in the sample studied! After considering a variety of
proposals for addressing this problem that have appeared in the literature,
Estes and Arnold argue that, on the basis of fairly straightforward quan-
titative-genetics models, only stabilizing selection can explain the observed
data.
But have they in fact produced an explanation for evolutionary stasis?
Estes and Arnold certainly seem to think so; in fact, they claim that “most
studies of stasis have focused on the evolutionary pattern without inves-
tigating the processes that produce that pattern” and that it is only through
confronting quantitative genetic models with data from the real world
that one can “explicitly test alternative explanations of stasis” (2007, 228).
This move is, however, made too quickly. The models Estes and Arnold
deploy cannot identify processes at all. Rather, these models serve to
provide a formal description of the phenomenon to be explained. As such,
Estes and Arnold can reject certain kinds of scenarios and hence reject
certain classes of causal processes. However, the set of processes that are
not rejected do not form anything like an explanatory kind: any number
of causally quite different processes can lead to the statistical pattern that
Estes and Arnold identify as “stabilizing selection.”
The difficulty is that while “stabilizing selection” may sound like a causal
process, as the term is used here, it is really nothing of the sort. The debate
surrounding possible resolutions to the paradox of stasis therefore pro-
vides an excellent entry into the different ways in which the term “selec-
tion” gets used (see Matthen and Ariew 2002), the difficulties with treating
the models of population genetics as explanatory (see Glymour 2006),
and the nature of explanations in evolutionary biology more generally.
2. The Paradox of Stasis and Estes and Arnold’s “Resolution.” Estes and
Arnold note that there have been a number of proposed solutions to the
paradox of stasis, including at least
1. protracted periods of stabilizing selection,
2. genetic and developmental constraints,
3. selective constraints due to coevolution,
4. canceling of “positive” and “negative” evolutionary trajectories over
time,
5. mathematical artifact,
6. habitat selection, and
7. complexities involved with evolution in metapopulations. (Estes and
Arnold 2007, 227; numbering added)
Estes and Arnold produce a variety of quantitative genetics models and
compare the outputs of those models to the data set fromGingerich (2001).
Selection, in these models, is about the relationship between the optimum