Pluralism, Entwinement, and the Levels of Selection


531

*Received June 2002; revised March 2003.

†To contact the author write to Department of Philosophy, 4–115 Humanities, University
of Alberta, Edmonton, T6G 2E5, Alberta, Canada; e-mail: rob.wilson@ualberta.ca.

‡An early and brief version of this paper was presented at the Western Canadian Phil-
osophical Association meeting in Edmonton in October 2000, and at the Canadian
Philosophical Association meeting in Quebec City in May 2001. I thank my respective
commentators, Jim Brown and David Castle, and audiences for their feedback on these
occasions. Some of the ideas in the final sections of the paper were informally bruited
at a meeting of the Science and Humanities Circle at the University of Alberta in
January 2002, and I thank participants there, especially Alex Rueger, for some feed-
back. Special thanks also to Michael Wade for some valuable written comments on a
draft, and for drawing my attention to some of the recent biological literature; and to
two referees for Philosophy of Science, one of whom drew my attention to the recent
work of Kerr and Godfrey-Smith, for their particularly astute comments.

Philosophy of Science, 70 (July 2003) pp. 531–552. 0031-8248/2003/7003-0005$10.00
Copyright 2003 by the Philosophy of Science Association. All rights reserved.

Pluralism, Entwinement, and the Levels
of Selection*

Robert A. Wilson†‡

This paper distinguishes and critiques several forms of pluralism about the levels of
selection, and introduces a novel way of thinking about the biological properties and
processes typically conceptualized in terms of distinct levels. In particular, “levels”
should be thought of as being entwined or fused. Since the pluralism discussed is held
by divergent theorists, the argument has implications for many positions in the debate
over the units of selection. And since the key points on which the paper turns apply
beyond this specific issue, the paper may prove of general interest in thinking about the
metaphysics of science

1. Introduction. The “return of the group” has focussed the attention of
philosophers and biologists on the relationships among various putative
levels at which natural selection operates, particularly on those between
group, individual and genic selection. Both unbridled enthusiasts and
more circumspect critics of the form of group selection that has received
most attention—that of David Sloan Wilson and Elliott Sober (Sober and
Wilson 1998; Wilson and Sober 1994)—have endorsed pluralism about the

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 . 532

units of selection. In fact, one or another form of pluralism about the units
of selection constitutes the current orthodoxy in the philosophy of biology.
The two goals of this paper are to critically examine this pluralistic con-
sensus, and to introduce an alternative to it, one that questions the ade-
quacy of the underlying conception of natural selection as operating at
distinct “levels”.

I shall distinguish several forms that such pluralism has taken (Section
2), elaborate on the most widely endorsed form of pluralism and the cen-
tral argument given for it (Section 3), and then critique this argument
(Section 4). Pluralism is motivated by the intuition that there may not be
a determinate answer to the question of just which level is the level at
which selection occurs in any particular case. I propose a way to spell out
this intuition that breaks from the pluralist consensus (Section 5).

The key positive idea is that various levels of selection are often en-
twined or fused, not just in the sense that they co-occur, or operate in the
same direction, but in that they are reliably coinstantiated and do not
make isolatable, distinct contributions to the ultimate evolutionary cur-
rency, fitness. This idea calls into question the view that the levels at which
selection operates are sufficiently separable for it to make sense to invoke
criteria for preferring one level to others, either in particular cases or in
general. I want to suggest not so much the epistemic point that it is some-
times difficult to determine whether individual or group selection is the
level at which selection is operating in a particular case as to undermine
the claim that there is always a determinate answer to this question. Life,
unfortunately, isn’t that simple.

This challenge to the idea that the biological world is neatly segmented
into layers or levels has broader implications for how we might think
about the metaphysics of science. Talk of the various levels at which ob-
jects, events, properties, states, and processes exist or can be described is
pervasive in discussions within the general philosophy of science and the
philosophy of particular sciences (e.g., of physics, biology, psychology,
economics). The metaphorical status of such talk has seldom been rec-
ognized. While there are reasons to expect entwinement to be pervasive in
the biological world, and so for thinking “entwinement talk” to be more
revealing than simple “levels talk” in biology, the argument of this paper
has implications for these other areas of the philosophy of science. In
closing I briefly raise three issues for anyone wishing to explore the notion
of entwinement more fully (Section 6).

2. Two Kinds of Pluralism about the Levels of Selection. Consider the label
“pluralism”. The multilevel theory of selection proposed by Wilson and
Sober has been called pluralistic, and it may pay to begin with the contrast

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between two forms that their own pluralism takes (cf. also Griffiths 2002;
Sober and Wilson 1998, 331).

The first and most obvious sense in which Wilson and Sober’s view is
pluralistic is that it denies that there is any single level at which selection
operates; rather, selection can, and often does, operate at multiple levels,
levels that often pull in opposite directions, with no one of these levels in
general trumping the others. That is, there is a plurality of units of selec-
tion—gene, individual, group—with no single one being more fundamen-
tal in general than any of the others. As such, this form of pluralism
suggests realism about the existence of the unit(s) of selection: there are,
independent of our particular perspectives, distinct units of selection in
the natural world, with their own distinctive properties and subject to
particular processes. And it is incompatible with what we might call fun-
damentalism, the idea that one unit of selection is more fundamental (the-
oretically or ontologically), than the others. I shall call this form of plu-
ralism unit pluralism, since at its core is the idea that there is a plurality
of units of selection in the biological world itself.

A second and less obvious form of pluralism also exists in Wilson and
Sober’s multilevel selection theory. It arises in their discussion of the re-
lationship between the theory of group selection and a variety of supposed
alternatives to it. According to Sober and Wilson,

In science as in everyday life, it often helps to view complex problems
from different perspectives. Inclusive fitness theory, evolutionary
game theory, and selfish gene theory function this way in evolutionary
biology. They are not regarded as competing theories that invoke dif-
ferent processes, such that one can be right and the others wrong.
They are simply different ways of looking at the same world. When
one theory achieves an insight by virtue of its perspective, the same
insight can usually be explained in retrospect by the other theories.
(1998, 98)

Wilson and Sober go on to argue that the exclusion of the theory of group
selection from this “happy pluralistic family” of alternative perspectives
“reflects a massive confusion between process and perspective. The theo-
ries that were launched as alternatives to group selection are merely dif-
ferent ways of looking at evolution in group structured populations”
(1998, 98).

This version of pluralism holds that various prima facie distinct models
of natural selection—for example, selfish gene theory and group selection
theory—are actually non-competing accounts of one and the same pro-
cess. I shall refer to this view as model pluralism, since it identifies a plu-
rality in the models that evolutionary biologists adopt, rather than in the
reality that those models depict. Model pluralism holds that while there

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 . 534

may be strategic or pragmatic advantages to using one rather than another
model in a particular case, these models do not compete for the truth about
the nature of natural selection.

Here I am particularly interested in model pluralism, for this form of
pluralism has also been advocated by those with a more sanguine view of
the return of the group, such as Sterelny (1996) and Dugatkin and Reeve
(1994). Model pluralism has also been elegantly articulated and defended
more recently by Kerr and Godfrey-Smith (2002a). According to these
authors, there is no fact of the matter as to which of two (or more) putative
vehicles or interactors is the level at which selection operates. In particular,
descriptions cast in terms of groups and group selection are equivalent to
those cast in terms of individuals and what Dugatkin and Reeve call broad-
sense individualism, the view that “most evolution arises from selfish re-
productive competition among individuals within a breeding population”
(1994, 107).

Model pluralism has proven popular recently amongst biologists work-
ing on the levels of selection (Dugatkin and Reeve 1994, Grafen 1984,
Reeve 2000, Dugatkin 2002), the social insects and the origins of euso-
ciality (Bourke and Franks 1995, Crozier and Pamilo 1996), and the evo-
lution of multicellular and social life from simpler forms (Frank 1998,
Michod 1999). In general terms, these theorists adopt the view that models
positing higher-level processes, such as group selection, do not differ sig-
nificantly or fundamentally from models positing lower-level processes,
such as genic or kin selection. As Bourke and Franks say in summarizing
a chapter devoted to this topic, “ . . . colony-level, group, individual, and
kin selection are all aspects of gene selection. This means that the practice
of attributing traits to, say, either colony-level selection or kin selection is
illogical” (1995, 67). As this quotation illustrates, model pluralism is some-
times combined with the denial of unit pluralism in that it suggests that
there is a sense in which one of these levels—typically, that of the individ-
ual or the gene—is more fundamental than the others.

On Sterelny’s construal of broad-sense individualism, groups simply
form part of the social environment of individuals (cf. Dawkins 1989, 258;
Sterelny and Kitcher 1988). Here the form that individual selection takes
is frequency-dependent selection, where the fitness of a given trait varies as
a function of the frequency of either that trait or some other trait in the
group or population. Sterelny (1996, 570–573) suggests that this form of
pluralism is applicable to a range of cases, including ant colonies/nests and
the elaborate chemical and behavioral warning systems that its members
have, kin selection in general, honey bee altruism, and trematode altruism
(from Dawkins 1982).

Positions similar to model pluralism have been endorsed more generally
in debates over the nature of the biological world, particularly when those

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debates have seemingly either reached a stalement, or when defended po-
sitions have led to clashes with widely shared intuitions. For example,
faced with the many definitions of biological species, some have concluded
that species are “unreal”, or that there are simply different species concepts
applicable to different explanatory contexts or projects. And to take an
example closer to the model pluralism that is my focus here, Dawkins
(1982, ch.1) famously replied to the putative departure that his selfish gene
theory made from traditional, individual-centered Darwinian theory by
suggesting that the two views were simply different but non-competing
perspectives that one could adopt to the unit of selection (see also Dawkins
1989, preface). Shifting between the gene’s-eye view and the individual’s-
eye view was like undergoing a gestalt shift in looking at a Necker cube:
two views each equally anchored in the reality they reflect (cf. also Grafen
1984 on inclusive fitness). This Neckerphilia, as we might call it, has been
enthusiastically endorsed most recently by Kitcher (2002) and Kerr and
Godfrey-Smith (2002a).

I am suspicious of these ways of deflating prima facie ontological dis-
agreement (see also Wilson 1996, 1999 on species). I shall focus my critique
on the model pluralism of Dugatkin and Reeve (1994) and Kerr and
Godfrey-Smith (2002a) and the key argument they give for it. But first I
lay out the model pluralist view more fully.

3. A Closer Look at Model Pluralism. I have characterized unit pluralism
as realist and anti-fundamentalist about the units of selection. By contrast,
model pluralism is either conventionalist or fundamentalist about these,
views I find objectionable. Dugatkin and Reeve express conventionalism
in claiming that “individual and trait-group selection are not alternative
evolutionary mechanisms; rather, they are alternative pictures of the same
underlying mechanism” (1994, 108). As Sterelny says in glossing Dugatkin
and Reeve’s broad-sense individualism, on this view there are only “heu-
ristic differences between the two approaches [broad-sense individualism
and group selection theory] . . . the vehicles we should recognize depends
on our explanatory and predictive interests” (Sterelny 1996, 572).

Central to the argument of both Dugatkin and Reeve (1994) and Kerr
and Godfrey-Smith (2002a) is the claim that there are mathematical, log-
ical, and explanatory equivalences between broad-sense individualism and
multilevel selection theory. These are taken to imply that while one might
adopt one or the other of these views of natural selection for pragmatic
or heuristic reasons, they do not correspond to alternative processes in
nature. Rather, the labels “broad-sense individual selection” and “group
selection” pick out one and the same process of natural selection, and
function in intertranslatable frameworks. Hence, the differences here be-
tween broad-sense individual and group selection perspectives are in the

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 . 536

eye of the beholder, not in the reality they purport to describe. As Reeve
says elsewhere,

the ‘new’ group selection models . . . are not mathematically different
from the broad-sense individual selection (e.g., inclusive fitness) mod-
els at all. Rather, the group selection models are generated from a
fitness-accounting scheme that merely produces an alternative picture
of the same selective processes described by the inclusive fitness mod-
els. (Reeve 2000, 65–66)

Much the same view is expressed by Sober and Wilson in the passage
quoted in the previous section.

Sterelny’s form of model pluralism shifts between this type of conven-
tionalism and fundamentalism. Sterelny’s view is fundamentalist in that it
claims not only that groups can be regarded simply as parts of an individ-
ual’s environment, and so group selection is recast as frequency-dependent,
individual selection, but also that this implies that the latter is more fun-
damental than the former. Such fundamentalism derives from Sterelny’s
skepticism about the status of trait groups in general as units of selection.
While he holds that trait groups that form superorganisms are (objec-
tively) vehicles, those that are mere trait groups, i.e., the vast majority of
them, are not vehicles (Sterelny 1996, 583) and so there can be no level of
selection that operates on them. Thus, a translation scheme that allows
one to move from descriptions of group selection to those of individual
selection should be taken to imply the eliminability of the former in favor
of the latter. With the exception of organism-like groups, trait groups do
not form a part of the objective fabric of the biological world subject to
natural selection. So alternative descriptions of the level at which selection
operates, such as that provided by frequency-dependent, individual selec-
tion, are to be preferred. This skepticism about the reality of groups as
“vehicles” or “interactors” is widespread, particularly amongst biologists
who adopt model pluralism, and introduces a crucial asymmetry between
individual (and genic) selection and group selection.

Common to both of these versions of model pluralism is the claim that
there are empirically adequate models of evolutionary change that posit
different, alternative units of selection. But to derive the specific forms of
model pluralism outlined above we need one or the other of two additional
assumptions. For Dugatkin and Reeve, this assumption is one about the
translatability of claims made in either framework into those of the other;
they take this to imply conventionalism about the unit or units of selection.
For Sterelny, the missing assumption concerns the unreality of trait groups
in general; for him this implies fundamentalism about lower-level units of
selection (perhaps excepting the special case of organism-like groups). Al-
though the shared assumption itself warrants further discussion, as does

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the view of groups that Sterelny articulates, I shall concentrate primarily
on the translatability assumption and its putative significance for model
pluralism. Consider this line of reasoning, as found in Dugatkin and Reeve
(1994) and Kerr and Godfrey-Smith (2002a).

Dugatkin and Reeve claim that “the mathematics of the gene-, indi-
vidual-, kin- and new group-selection approaches are equivalent” and
that “this must be the case and that individual and trait-group selection
are not alternative evolutionary mechanisms; rather, they are alternative
pictures of the same underlying mechanism” (1994, 108). Their claims of
the “logical equivalence” and “logical translatability of one picture into
the other” (108) are accompanied by an acknowledgement of the heuristic
differences between individual and trait group selection. They take these
claims together to entail that claims that pit these two forms of selection
against one another are “empirically empty” (109).

Kerr and Godfrey-Smith reinforce this view by showing how the basic
parameters in what they call the individualist or contextual approach and
the multi-level or collective approach to “dealing with group structure in
a population” (2002a, 481) can be defined in terms of one another. This
allows them to show the parallels between the fitness structures that each
of these models generates, and, ultimately, to derive a dynamically suffi-
cient representation of phenotypic change using either model (2002a, 482–
484). Since they emphasize the virtues of being able to move between both
models, they call their view gestalt-switching pluralism. They are Necker-
philiacs.

Underlying this sort of argument is the idea that if alternative models
of natural selection can be represented either in a common mathematical
framework or by the very same equations, then these models differ at most
heuristically, not on some deeper level. Dugatkin and Reeve illustrate this
by showing how to translate between equations that state when a trait of
an individual will evolve by natural selection, and when an allele will be
naturally selected. A shared mathematical structure provides the basis for
translating between these different models of selection. As they conclude,

If broad-sense individual selection, genic selection, and trait-group se-
lection all can be represented by a single condition based only on allele
frequencies, then they cannot fundamentally differ from one another
(1994, 109, their emphasis; cf. also Bourke and Franks 1995, 45–49,
Box 2.1).

This conditional, however, is false, and the more general argument that
it reflects, fallacious. The sense in which various models and forms of
selection can be represented by a shared framework, and the significance
of this, needs to be explored.

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4. Representing, Translation, and Levels. To begin, note that having a
shared mathematical framework, or being represented by the very same
equations, does not itself entail that two or more processes “cannot fun-
damentally differ from one another.” This is because mathematical models
capture just some aspects of the dynamics or kinematics of the processes
they model, and they serve as models of those aspects only given further
assumptions not represented in the models themselves. Indeed, one should
expect at least some mathematical models to be applicable to fundamen-
tally different processes. Two processes represented in the same mathe-
matical framework or by the same mathematical equations can funda-
mentally differ from one another because the mathematics (i) is an attempt
to selectively represent just some aspects of the phenomenon being mod-
eled, and (ii) requires further, interpretative assumptions in order to serve
as a model of that phenomenon.

Consider some examples in areas other than biology. Decision theory
is concerned with how agents ought to make choices, and its formal side
provides various rules that constrain such choices, including those speci-
fied by utility theory, Bayes’s account of probabilistic reasoning, and game
theory. Such theories can be applied to a range of agents, including human
agents, organizations, insect societies, or anything that can be construed
as having preferences and means of attaining them. Applying them in each
of these cases will require different auxiliary assumptions, but this “plu-
ralism” implies nothing about whether these various domains of applica-
tion differ fundamentally or not. The backpropagation algorithm familiar
from connectionist cognitive modeling in cognitive science can be used to
model not only various cognitive processes (verb acquisition, memory,
attention, character recognition), but non-cognitive systems that can be
construed as networks with basic units whose activation is a function of
both past and desired levels of activation. Its application within the cog-
nitive realm does not show that the various cognitive processes “cannot
fundamentally differ from one another.” Likewise, its application to cog-
nitive and non-cognitive domains carries no implication concerning
whether these are fundamentally similar or not. Cellular automata have
also been used to model cognition as well as the operation of the immune
system (Bernardes and Zorzenon Dos Santos 1997), and again the exis-
tence of shared mathematical frameworks across domains does little to
show just how similar those domains are.

This objection—to the general inference suggested by Dugatkin and
Reeve and by model pluralists more generally—does not take us very far,
however, for two reasons. First, it begs one question at issue by neglecting
(or denying) the unifying potential that shared formalisms introduce.
Showing how to apply, say, game theory to previously disparate phenom-
ena itself allows those phenomena to be at least partially viewed as in-

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TABLE 1. Portion of Kerr and Godfrey-Smith’s Table 1 showing fitness structures of
the individualist and multi-level perspectives and their relations.

Individualist Multi-Level

Definition of fitness
structure

�i � number of copies for
an A type in group with i
A types

pi � total number of copies
for a group with i A
types

bi � number of copies for
a B type in group with i
A types

�i � number of A copies
in a group with i A types
/ total number of copies
in a group with i A types

Relation to other
perspective

�i � pi�i / i pi � i �i � (n � i) bi

bi � (1 � �i) pi / n � i �i � i �i / i �i � (n � i) bi
Reproduced with permission of the authors and Kluwer Academic Publishers.

1. Their paper appears with commentaries by Dugatkin, Maynard Smith, and Sober
and Wilson, as well as their reply to these (Godfrey-Smith and Kerr 2002) and a com-
panion paper that continues their argument by focusing on models that use Price’s
equation and average fitness (Kerr and Godfrey-Smith 2002b).

stances of the same kind of thing; this itself may constitute a major inte-
grative advance. Moreover, the selective nature of mathematical
representations is no barrier to their capturing what is central about a
phenomenon. One might point to the “frameshifting” between genic, in-
dividual, and group selection at the heart of Sober and Wilson’s multilevel
view of selection as illustrative of both points.

Second, and more importantly, model pluralists about selection have
putatively identified an extensive, fine-grained isomorphism between vari-
ous models of selection. Model pluralists do not simply claim that the
models are result equivalent but that they are causally equivalent in their
structures. For example, Dugatkin and Reeve (1994, 120, Table 1) provide
a mapping between individualist and trait-group explanations, and Kerr
and Godfrey-Smith claim that their “two parameterizations of the fitness
structure in the system contain exactly the same information, parceled up
differently” (2002a, 484). Minimally, the onus is on someone who rejects
model pluralism to identify differences between the models that cannot
reasonably be called “pragmatic” or “heuristic” in nature. I focus below
on the parameterizations that Kerr and Godfrey-Smith provide. Their
more precise characterization of many of the ideas expressed by Dugatkin,
Reeve, and Sterelny reveals the depth of the formal equivalence between
“individualist” and “multi-level” perspectives on selection and, in my view,
makes the strongest case for model realism yet provided.1

Consider Kerr and Godfrey-Smith’s Table 1 (2002a, 487), part of which
is redrawn above. The fitness structure posited in the individualist per-

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 . 540

spective is parameterized in terms of the number of copies of the relevant
genotypes or phenotypes, �i and bi, where the index signifies a relativiza-
tion to a group with i A types. This reflects a standard individualist view,
adapted so as to explicitly recognize a role for population structure as the
context in which types occur. The fitness structure in the multi-level per-
spective, by contrast, is parameterized in such a way as to reflect the group
as a collective that itself can serve as the bearer of fitness values, and so
captures the idea that natural selection can operate on groups themselves.
Thus, pi and �i aim to express, respectively, group productivity and the
contribution of A types to that productivity. So parameterized, the math-
ematical equivalences between the two perspectives falls out with some
simple algebra.

Suppose that we grant the parameterizations for now. What follows?
To derive a conclusion about equivalence in the two perspectives, a con-
clusion about the translatability between them, we need at least the as-
sumption that these formalizations express the core of those perspectives.
One reason to question this, even granting the formalizations, is that the
two perspectives differ in their quantificational range. In particular, the
individualist perspective quantifies over just individuals while the multi-
level perspective quantifies over groups, not only as the bearers of fitness
values or interactors but also as the manifestors of adaptations and the
beneficiaries of natural selection, to use Elisabeth Lloyd’s (2001) useful
terminology. A group may play one or more of these roles in any given
case in the multi-level perspective, but none of these roles in the individual
perspective. Groups, in short, are agents in the process of natural selection
in the multi-level perspective, but are never such in the individualist per-
spective. This is an ontological difference between the two perspectives,
and insofar as it is not reflected in the formalizations, they leave something
important out. Moreover, it is difficult to see how adding in parameters
to the multi-level model to capture such agentive roles would allow one
to preserve the isomorphism between the two formal models, since that
would in turn require corresponding parameters in the individualist model.
But since individualists do not think that groups play such roles—cer-
tainly, not in all cases (see Maynard Smith 2002)—this would seemingly
require a more radical departure from their perspective than simply rela-
tivizing types to population structure.

There is a dilemma here for the model pluralist that we might express
in several ways. Consider one of these. If the models are exhaustively
constituted by the formal definitions provided by Table 1 (and its exten-
sions), then these models leave out something crucial about each of the
perspectives they represent. So formal equivalences in the models do not
support the claim that the two perspectives are simply different ways of
expressing a shared view of the biological world (and so do not compete

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, ,      541

for the truth about that world). If, on the other hand, we need to extend
the formalizations of each perspective so as to reflect the deeper ontolog-
ical differences between them, then the onus is on the model pluralist to
show how to (a) perform the formalization, and (b) preserve the isomor-
phism between the two formal models. I have no definitive argument to
show that this is impossible, though I have said something briefly about
why I suspect that it cannot be done.

Let us return to the parameterization itself, for one of its interesting
features is that it provides a way to translate formally between the indi-
vidualist and multi-level views of the bearer of fitness values, interactors.
If this is successful, then it captures one of the “ontological” differences
between the perspectives—whether groups play this role—itself a signifi-
cant achievement. But the characterization of both parameters, p and �,
should be questioned by proponents of the multi-level perspective, pre-
cisely because each ties a group’s productivity too closely to the expected
numbers of copies of the genotypes or phenotypes in that group. In some
circumstances a group’s productivity may be measured just by the total
number of copies of individuals it leaves, as the parameterization suggests,
but such a measure will be misleading in others where the group structure
itself plays a significant role in directing evolutionary change.

For example, consider a group, T, with A and B types that reproduces
three groups, U, V, and W, each of its original size, as depicted in Figure
1a. At generations 1 and 2, this lineage has precisely the same group pro-
ductivity (fitness) as defined by p as the lineage in Figure 1b. Yet since
group size and structure may be ecologically significant, there may be a
critical fitness difference between the two lineages, as revealed in genera-
tion 3. For example, the T lineage contains Z, which will allow the repli-
cation pattern in generation 2 to be reproduced again. And in the Q line-
age, R may exceed a threshold, leading to a lower fitness level. Moreover,
differences between the groups in Figure 1a after the first generation may
themselves be significant for evolutionary change. For example, a “pure
group” like W breaks the dynamic of decreasing within-group fitness of
As relative to Bs. Note that this dynamic is broken in the transition from
T to W, and from V to Z, through the “defection” of just one B to A,
something that itself may be a function of group structure. p is insensitive
to these sorts of causal differences precisely because it conceptualizes
group productivity solely in terms of the composition of groups rather
than the reproduction and structure of groups themselves.

One might grant that this is true but insignificant because it is the other
parameter, � that is relevant here, for it is here that the underlying causal
story is to be told within the multi-level perspective. Again, however, this
should be questioned by a proponent of the multi-level perspective, for
this parameterization is also cast exclusively in terms of individual off-

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 . 542

Figure 1. (a) The T lineage, containing A and B types. The portion of generation 3 depicted
shows how the group structure iterates through group Z. (b) The Q lineage, whose group R
in generation 2 exceeds some threshold, leading to a fitness drop. In general, B increases in
relative fitness across generations, with the exceptions (from T to W, and from V to Z)
mediated by the group structure in the T lineage.

spring or copies. But just as group productivity should sometimes be un-
derstood within the multi-level perspective in terms of the production of
groups (and not just individuals in those groups), we also need to look
beyond an individual’s within-group frequency in order to understand the
causal interplay between individuals and their population-structured en-
vironments. As Figure 1 illustrates, having a certain proportion of A types
within a group may be necessary for that group to flourish or even survive,
and this role may vary across different ecological circumstances. In such
a case, the parameterization will offer at best a partial characterization or
approximation of the contribution of A types to group productivity.

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, ,      543

The basic problem with both parameters is precisely what allows them
to serve as the bridge between the individualist and multi-level perspec-
tives: they are themselves too individualistic. Group productivity is pa-
rameterized solely in terms of the individual composition of groups and
then used to define the particular contributions of individuals to overall
group productivity. While this is sometimes an appropriate way to un-
derstand groups themselves as bearers of fitness values—since group fit-
ness sometimes just is the sum of the fitness of the individuals it produces—
it does not allow one to represent circumstances in which groups play a
more agent-like role in natural selection.

One final point about model pluralism. I noted at the outset that model
pluralism has proven attractive across the spectrum of views over the levels
of selection. But this attraction has a very different basis for proponents
of group selection and individualists about selection. David Sloan Wilson,
for example, has sought to restore the theory of group selection to re-
spectability over an extended period in which individualistic views have
predominated. Thus, he has both distanced his views from “naive group
selection theory” of the past and sought to integrate his “new group se-
lection theory” with predominant individualistic views. In this context,
model pluralism serves a legitimating function. Individualists, by contrast,
are attracted by model pluralism because it allows them to treat new group
selection theory as old wine in new bottles. Cases that seem to call out for
viewing groups themselves as occupying centre-stage in the process of nat-
ural selection are redescribed from the individualist perspective with
groups present but very much in the wings. Phenomena that feature
groups themselves as units of selection exist but are rare (cf. Sterelny 1996,
Maynard Smith 2002), very much the view that Williams (1966) originally
defended. In this context, model pluralism functions to maintain the status
quo.

Disagreements that lie beneath the surface of the pluralist consensus
reflect this contextual difference. One such disagreement concerns unit
pluralism and its realist and anti-fundamentalist standpoint (see also So-
ber and Wilson 2002). We have seen that Sober and Wilson endorse both
model pluralism and unit pluralism, while individualists remain tempted
by fundamentalism and are, at best, neutral or guarded about unit plu-
ralism. One way to read my criticism of the translatability argument for
model pluralism is as suggesting that unless model pluralism allows us to
resolve the ontological issues that unit pluralism raises, it stops short of
dissolving the disagreement between individualists and multi-selectionists.

5. Entwined Levels of Selection. Model pluralism is an enticing position
for making sense of debate over the units of selection, a temptation harder
to resist in light of the appeals to the “mathematical equivalences” between

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 . 544

various models of selection. But I have argued that these equivalences do
not have the significance that model pluralists have ascribed to them. And
I view model pluralism itself as a mistaken way to spell out the intuition
that there may not be a determinate answer to the question of just which
level is “the” level at which selection occurs in any particular case. Unit
pluralism shares this intuition but adopts a realist view of whether selec-
tion is operating at the genic, individual, or group level (or all three) in
any particular case. The unit pluralist’s supposition is that the levels at
which selection operates are always sufficiently separable for it to make
sense to invoke criteria for preferring one or more levels to others, at least
in particular cases.

It is this supposition that I challenge in this section. Various levels of
selection are often entwined in that while they are reliably coinstantiated,
they do not make isolatable, distinct contributions to the ultimate evolu-
tionary currency, fitness. Especially when two or more levels of selection
work in the same direction, and act in ways to mutually reinforce one
another, as I think is often the case, isolating the contribution of each level
involves an artificial partitioning of the causes of the resulting evolution-
ary change (or stasis) that does not track biological reality. In what follows
I hope to show how to articulate this idea to arrive at a position that
departs from both unit and model pluralism while accommodating the
central intuition that motivates these views.

I present the argument in two stages: with respect to properties at a
level, and then with respect to processes or mechanisms at different levels.
The first stage aims to show that properties at the group level can be
entwined, as can those at the individual level. In the second stage I argue
that the very processes that generate these entwined properties across the
two levels are themselves entwined. It is this entwinement “across levels”
that challenges the very idea that the biological world separates neatly into
layers or levels. I develop both stages of the argument through a detailed,
idealized example.

Consider a metapopulation consisting of groups of nesting organisms
that vary with respect to their invadability by outsiders. Suppose that the
least invadable groups have a certain proportion of distinctly larger in-
dividuals, and this is because those individuals are particularly adept at
blocking holes in the nest with their larger-than-average bodies. These
larger individuals come to specialize in nest defense, and there is some
division of labor within the nest of which such a specialization forms a
part. Since larger individuals require more food resources, too many of
these lead to resource-based problems for the group, and so there is an
equilibrium number, m, of large individuals for any nest of size n to pos-
sess. Here it would be natural to say that certain groups are less invadable
because they have a certain proportion, m/n, of larger individuals. Suppose

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, ,      545

2. Sober’s (1984) distinction between the selection for and the selection of properties is
irrelevant to both cases of entwinement I have sketched here. Some will find this easier
to recognize if the descriptions are cast in terms of there being laws that relate the
entwined properties. Since I think that an appeal to biological laws, especially in this
context, is misplaced, I talk instead of the properties being reliably coinstantiated.

also that individuals are able to grow distinctly larger only because they
are members of relatively uninvadable groups, since the corresponding
division of labor creates time for more individuals to forage. Suppose also,
as a result, that uninvadable groups have a higher level of fitness than
invadable groups.

In a population with very few distinctly large individuals there will be
individual selection for individuals of greater body size: their proportion
will increase within the group over time until it reaches the equilibrium
ratio of m/n. This is a form of frequency-dependent selection that selects
for an intrinsic property of individuals, body size. But since all and only
those individuals with greater body size are also those who function as
hole-blockers, there is also selection for hole-blockers in this population.
In the environment in which selection actually occurs the realization of
one of them is also a realization of the other, and this is no accident.
Natural selection is sensitive to the ways in which properties are reliably
instantiated, and so it is not fine-grained enough to distinguish between
entwined properties.

Consider now group selection. Because the groups vary with respect to
their invadability, this variation correlates with fitness variation, and that
variation is heritable (i.e., survives intergenerationally), lower levels of
invadability are adaptations and there is a (group) selective pressure for
their evolution. Either groups with low levels of invadability will differ-
entially grow in members, or such groups will be ancestors to more groups
than those with higher levels of invadability. But it is also surely true that
there is group selection for groups with, or that approximate having, m/n
large individuals. Being uninvadable and having m/n large individuals are
not identical—there are possible, even nomologically possible, situations
in which they come apart—yet in the example as I have described it they
are entwined. And as before, natural selection does not—indeed, cannot—
distinguish between entwined properties.2

Furthermore, not only are properties at both the group-level and the
individual-level entwined, but the two corresponding processes or mech-
anisms, group and individual selection, are also entwined. Individuals with
large body size and that play the hole blocker function are selected within
groups, and there is the selection between groups of lower levels of invad-
ability and having m/n large individuals. But the selective processes at
different levels here are not independent; they constitute mutually rein-

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 . 546

forcing forces whose effects, the properties that are selected, are closely
and deeply related, with neither individual- nor group-level properties be-
ing causally prior to the other. These reliably coinstantiated selective
forces do not make isolatable, distinct contributions to fitness. Moreover,
given the case as described, these selective processes can be teased apart
only in thought or through experimental interventions that alter the very
relationships between them.

Perhaps what I am calling entwinement would be more aptly named
fusion, since its occurrence implies the oneness of what might, in other
cases, be separate and separable forces, in much the way that two winds
can fuse to create a gale whose effects are not attributable separably to
each of the initially (and in other circumstances, continuingly) distinct
winds. Entwinement or fusion of properties or processes implies that it
makes no sense to apportion determinate, partial causal responsibility for
the resulting evolutionary change. This is true whether we take the prop-
erties of having a certain body size or type and being a hole-blocker at the
individual level, of being uninvadable and having m/n large individuals at
the group level, or the levels themselves.

When either properties or processes merely co-occur, they can be teased
apart through experimental means, using various control groups and ex-
perimental conditions in order to isolate the distinctive contribution that
a property or process makes to the direction and size of the force of natural
selection. This is precisely what one might think lies at the heart of ex-
perimental approaches to determine the level or levels at which selection
operates. But such experimental interventions reach a limit when prop-
erties or processes are entwined, since in that case the two do not pre-exist
as independent, separable parts of the world. When there is entwinement,
further empirical investigation reveals further complexities and entangle-
ments, and the discovery of the workings of isolated properties or pro-
cesses is an artifact of the experimental investigation, an artifice of our
imposing on the world an ontology with distinct levels in an idealized
biological hierarchy.

An examination of Michael Wade’s (1977, 1978, 1980, 1982) classic
experimental approach to group selection on flour beetles, two species of
Tribolium, and the work that it has inspired, will illustrate this point. In
a series of elegant and often-discussed experiments, Wade and his col-
leagues selected for groups of beetles with high rates of population in-
crease, pitting these against both randomized control groups and those in
which there was only individual selection. Rather than using a method of
panmixis across generation times, as had previous models of group selec-
tion, Wade used propagules as the basis for new generations in his artificial
(group) selection model. The basic finding was that there are conditions

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, ,      547

under which the effects of group selection are significant, and significantly
stronger than those of individual selection. Wade himself selected for the
group-level property of rate of population increase, but related studies
have selected for other group-level traits in Tribolium, such as emigration
rate (e.g., Craig 1982), and multi-level traits in other species, such as leaf
area in the cress Arabidopsis thaliana (Goodnight 1985).

Although each of these studies shows that a certain property can be
most effectively selected through group rather than individual selection,
even collectively they do not show that there are no other properties being
selected through either group or individual selection. Goodnight and Ste-
vens have noted that “[a]ll of the traits examined in group selection ex-
periments are influenced by interactions among individuals” (1997, S67):
population size and emigration rates (in Tribolium) by cannibalism rates
and population density, respectively; and leaf area (in Arabidopsis) by
planting density. Each of these pairs of properties is entwined in the ex-
perimental conditions, and thus both are selected through group selection
(high population size with low rates of cannibalism; high emigration rates
with high population density). But there is also a range of individual-level,
dispositional properties, such as fecundity, aggression, and cannibalism,
that are not independent of these group-only and multi-level traits, and that
are selected along with them. For example, when the rate of cannibalism is
high, there are more individuals manifesting that trait, and so we could see
individual selection as operating on it. This sort of entwinement underpins
Sterelny’s suggestion that we “reanalyse the evolution of (say) large flour
beetle groups as the evolution of a gene for (say) synchronizing breeding, a
gene advantageous only in a particular population-structured context”
(1996, 571). What I am suggesting is that we give up the fundamentalism
that I think drives Sterelny’s own talk of “reanalyzing”, but recognize the
reality of the metaphysical entwinement that gives rise to it.

Wade’s more recent work on “indirect genetic effects” (IGEs) is inter-
estingly positioned vis-à-vis traditional talk of levels of selection and the
notion of entwinement that I have introduced. IGEs occur when there are
phenotypic or fitness changes that result from changes to the genes of an
individual’s social partners, rather than simply from that individual’s own
genes. While phenotypic expression and fitness always depend on the val-
ues of environmental variables, IGEs constitute a malleable environment
whose effects on selection can be extreme. For example, the effect of plant
height on a given plant’s fitness will depend, in part, on the heights of its
surrounding neighbors, and changes in an individual’s genetic environ-
ment can very quickly change the fitness values of its phenotypes. Agra-
wal, Brodie III, and Wade (2001) extended the basic models of IGEs to
apply to cases in which population structure directs selection, finding that

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 . 548

IGEs could be significantly higher once assumptions of panmixis and lin-
ear interactions were relaxed. They remind us (2001, 318) that the non-
additive effects of individual and group selection had long been noted in
the literature on experimental approaches to group selection, and view
their own models as providing an account of the interactions between these
levels of selection. This is because the models themselves add in terms that
explicitly refer to the effects of interactions between the levels of selection.
However, one might also see this work as posing a challenge to the frame-
work that multi-selection theory operates within, and as providing con-
ceptual and mathematical tools more readily viewed as applying not to a
layered biological reality but to one whose “levels” are inherently entwined
with one another. Wolf, Wade, and Brodie III (2003), which proposes a
general way to model a variety of gene-context interactions, might be
viewed as a further step in this direction.

The entwinement of properties and processes in the biological world is
clearly a matter of degree, a point that we can use to situate the view I am
advocating with respect to our two forms of pluralism. When entwinement
is extremely strong or tight, as it was in my idealized example, then model
pluralism takes on some appeal, since the modeling “tools of the trade”,
like natural selection itself, will be insensitive to underlying ontological dif-
ferences. And when entwinement is extremely weak or loose, unit pluralism
will allow us to approximate the contributions of individual-level and
group-level properties and agents to the process of evolution by natural
selection. Since I think that we have significantly under-estimated the extent
to which properties in the biological world are entwined, the limitations of
unit pluralism are likely to be more severe than its proponents have allowed.

The idea that biological reality is entwined provides a different starting
point, however, from both model and unit pluralism in thinking about the
nature of selection. For any given organism the evolutionary interest of
various units—genes, individuals, and groups—are inextricably woven to-
gether. None of these putative units floats free of the others in evolutionary
space. To take the standard Darwinian unit, individuals typically mediate
their reproductive success through their membership in a group or groups,
and do so by means of the replication of their genetic material; genes exist
and thus reproduce only in individuals, and groups do so only through
the reproductive efforts of the individuals that comprise them (cf. Heisler
and Damuth 1987, Damuth and Heisler 1988). There is thus a clustering
of replicative, reproductive, and social interests—not always in harmony
but inextricably knitted together—that lies somewhere between the merely
fortuitous and the lawful. By taking this entwinement to be ontologically
basic, we can see the hierarchically-layered view shared by pluralists, and
so the issue of how these levels are related, as an artifact of just one way
of thinking about the metaphysics of science.

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, ,      549

6. Levels, Entwinement, and the Metaphysics of Science. In the previous
section I introduced the idea of entwinement as a tool for complicating
the conception of natural selection as operating at distinct levels that un-
derlies the forms of pluralism discussed in Sections 2–4. Given the ubiquity
of talk of levels amongst those engaged in various debates over the meta-
physics of science, if the notion of entwinement proves useful in charac-
terizing the levels of selection, it is likely to have broader application. I
want to conclude the substantive part of the paper by raising some general
issues about levels and entwinement with this in mind.

The first of these concerns the metaphorical nature of talk of levels in
many scientific contexts. Such talk might be thought to be literal, to cor-
respond to a distinctly layered reality, in physics, where the concern is to
mark scale differences between objects of different size (e.g., Dresden
1998). Whatever one thinks of this, it seems to me too optimistic as a view
of levels-speak in other sciences. For example, the common idea that we
should understand talk of the “levels of selection” literally in terms of
entities nested in a compositional hierarchy (e.g., genes, individuals, fam-
ilies, groups) is confused. First, there is no univocal relational expression
that applies to the full range of entities that have been postulated as pu-
tative objects of selection. “Compose”, “are a part of ”, “physically con-
stitute”, and “are realized in”—to take four of the more obvious predicates
relating smaller to larger biological entities that are candidate units of
selection—do not truly, literally, and univocally describe the relationship
between adjacent pairs in the following series: alleles, genotypes, cells,
organs, bodily systems, organisms, kin groups, families, nests, colonies,
species. Second, it is much less clear how properties, processes, events, and
states of such entities can stand in these sorts of mereological relationships
to one another. For example, supposing that groups are physically com-
posed of individuals, in what sense is the process of group selection phys-
ically composed of individual selection? At best, the relationship between
these processes would seem to be derivative, in some way, from that which
holds between the physical objects to which they are “attached”. This
problem is more severe in the philosophy of mind and the cognitive sci-
ences, where the levels are often simply designated “mental” and “physi-
cal” or their variants “psychological” and “neurological”.

The second of these issues concerns the relationship between a layered
and an entwined metaphysical view of scientific ontology. There seem two
distinct ways to think of the relationship. On the one hand, one could view
entwinement as an alternative to the layered view of the world. If one
thought that talk of levels was misleading, or was itself responsible for
creating some of the puzzles that keep philosophers busy (e.g., the mind-
body problem), then this is likely to be an attractive view of the entwi-
nement metaphor. If the entwinement metaphor is developed with this in

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 . 550

mind, then talk of fusion may prove more apt and useful than that of
entwinement. On the other hand, one could view entwinement as a sup-
plement to the view of the world as layered, one that either is applicable
to special cases and circumstances or that allows one to fine-tune or correct
the predominant layered view of scientific ontology. Both of these views
may be useful in thinking further about the nature of selection.

The third issue is how we can make the notion of entwinement both
more precise and more general, for at root it too gains much of whatever
appeal it has to its metaphorical nature. Despite the pioneering work of
Mary Hesse (1966) and Richard Boyd (1993), both building on Max
Black’s (1962) views of metaphor, that established a place for metaphor
in science, this remains an uncomfortable place for many offering an an-
alytic understanding of scientific processes and debates over them. I have
suggested that phenotypic properties are entwined at a level just when they
are reliably coinstantiated but do not make distinct contributions to the
fitness of their bearer, and that processes across levels are entwined just
when they are reliably coinstantiated but do not make distinct or inde-
pendent contributions to the fitness of their respective bearers. There are
natural ways to generalize such notions of entwinement to properties and
processes more generally, but how plausible these are will turn, in part,
on how we conceptualize the relationship between levels and entwinement.

7. Conclusion. Model pluralists are right that there is, in at least some
cases, no fact of the matter as to whether selection operates at the (broad-
sense) individual level or at the group level. This is not, however, because
these two models are inter-translatable, or because one is reducible to the
other, but rather because the reality that these models describe includes
entwined or fused properties and processes. Thus the resulting indeter-
minacy as to just which property is selected, or just which process governs
its selection, is not a legacy of our epistemic limitations. Instead, it is a
reflection of a biological reality that does not always fall neatly into dis-
tinct levels, as proponents of unit pluralism imply. If this introduction of
entwinement stimulates discussion in this and other areas in which phi-
losophers of science and others have complacently appealed to the meta-
phor of levels, so much the better.

REFERENCES

Agrawal, Aneil F., Edmund D. Brodie III, and Michael J. Wade (2001), “On Indirect Genetic
Effects in Structured Populations”, American Naturalist 158: 308–323.

Bernardes, A.T., and R.M. Zorzenon Dos Santos (1997), “Immune Network at the Edge of
Chaos”, Journal of Theoretical Biology 186: 173–187.

Black, Max (1962), Models and Metaphors. Ithaca, NY: Cornell University Press.
Bourke, Andrew F.G., and Nigel R. Franks (1995), Social Evolution in Ants. Princeton, NJ:

Princeton University Press.

This content downloaded from 129.128.046.162 on June 14, 2017 11:06:48 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).

http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F321324
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1006%2Fjtbi.1996.0316


, ,      551

Boyd, Richard (1993), “Metaphor and Theory Change: What is ‘Metaphor’ a Metaphor
For?”, in Andrew Ortony (ed.), Metaphor and Thought. 2d ed. Cambridge: Cambridge
University Press, 481–532.

Craig, D. M. (1982), “Group Selection Versus Individual Selection: An Experimental Anal-
ysis”, Evolution 36: 271–282.

Crozier, Ross H., and Pekka Pamilo (1996), Evolution of Social Insect Colonies. New York:
Oxford University Press.

Damuth, John, and I. Lorraine Heisler (1988), “Alternative Formulations of Multilevel Se-
lection”, Biology and Philosophy 3: 407–430.

Dawkins, Richard (1982), The Extended Phenotype. Oxford: Oxford University Press.
——— (1989), The Selfish Gene. 2d ed. Oxford: Oxford University Press.
Dresden, Max (1998), “The Klopsteg Memorial Lecture: Fundamentality and Numerical

Scales—Diversity and the Structure of Physics”, American Journal of Physics 66:
468–482.

Dugatkin, Lee Alan (2002), “Will Peace Follow?”, Biology and Philosophy 17: 519–522.
Dugatkin, Lee Alan, and Hudson K. Reeve (1994), “Behavioral Ecology and Levels of Se-

lection: Dissolving the Group Selection Controversy”, in Peter J.B. Slater et al. (eds.),
Advances in the Study of Behavior, vol. 23. New York: Academic Press, 101–133.

Frank, Steve A. (1998), Foundations of Social Evolution. Princeton, NJ: Princeton University
Press.

Godfrey-Smith, Peter, and Benjamin Kerr (2002), “Group Fitness and Multi-Level Selection:
Replies to Commentaries”, Biology and Philosophy 17: 539–549.

Goodnight, Charles (1985), “The Influence of Environmental Variation on Group and In-
dividual Selection in a Cress”, Evolution 39: 545–558.

Goodnight, Charles, and Lori Stevens (1997), “Experimental Studies of Group Selection:
What Do They Tell Us About Group Selection in Nature?”, American Naturalist 150:
S59-S79.

Grafen, Alan (1984), “Natural Selection, Kin Selection, and Group Selection”, in J. Krebs
and N. Davies (eds.), Behavioural Ecology: An Evolutionary Approach. London: Black-
well, 63–84.

Griffiths, Paul (2002), Review of Sober and Wilson 1998, Mind 111: 178–182.
Heisler, I. Lorraine, and John Damuth (1987), “A Method for Analyzing Selection in Hi-

erarchically Structured Populations”, American Naturalist 130: 582–602.
Hesse, Mary (1966), Models and Analogies in Science. South Bend, IN: University of Notre

Dame Press.
Kerr, Benjamin, and Peter Godfrey-Smith (2002a), “Individualist and Multi-Level Perspec-

tives on Selection in Structured Populations”, Biology and Philosophy 17: 477–517.
——— (2002b), “On Price’s Equation and Average Fitness”, Biology and Philosophy 17:

551–565.
Kitcher, Philip (2002), “Evolution Theory and the Social Uses of Biology”, paper read at

eighteenth biennial meeting of the Philosophy of Science Association, November 2002,
Milwaukee, WI.

Lloyd, Elisabeth A. (2001), “Units and Levels of Selection: An Anatomy of the Units of
Selection Debates”, in Rama S. Singh, Costas B. Krimbas, Diane B. Paul, and John
Beatty (eds.), Thinking About Evolution: Historical, Philosophical, and Political Per-
spectives. New York: Cambridge University Press, 267–291.

Maynard Smith, John (1987), “How to Model Evolution”, in John Dupré (ed.), The Latest
on the Best. Cambridge, MA: MIT Press, 119–131.

——— (2002), “Commentary on Kerr and Godfrey-Smith”, Biology and Philosophy 17:
523–527.

Michod, Richard (1999), Darwinian Dynamics. Princeton, NJ: Princeton University Press.
Reeve, Hudson K. (2000), “Multi-Level Selection and Human Cooperation”, Evolution and

Human Behavior 21: 65–72.
Sober, Elliott (1984), The Nature of Selection. Cambridge, MA: MIT Press.
Sober, Elliott and David Sloan Wilson (1998), Unto Others: The Evolution and Psychology

of Unselfish Behavior. Cambridge, MA: Harvard University Press.
——— (2002), “Perspectives and Parameterizations: Commentary on Benjamin Kerr and

This content downloaded from 129.128.046.162 on June 14, 2017 11:06:48 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).

http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F284732
http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2408045
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020580721145
http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F286050
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020504900646
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1007%2FBF00647962
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020545311555
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1016%2FS1090-5138%2899%2900020-3
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1016%2FS1090-5138%2899%2900020-3
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1093%2Fmind%2F111.441.178
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1017%2FCBO9781139173865.023
http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2408652
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1119%2F1.18827
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020572704223
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020583723772


 . 552

Peter Godfrey-Smith’s ‘Individualist and Multi-Level Perspectives on Selection in
Structured Populations’”, Biology and Philosophy 17: 529–537.

Sterelny, Kim (1996), “The Return of the Group”, Philosophy of Science 63: 562–84.
Sterelny, Kim and Philip Kitcher (1988), “The Return of the Gene”, Journal of Philosophy

85: 339–361.
Wade, Michael J. (1977), “An Experimental Study of Group Selection”, Evolution 31:

134–153.
——— (1978), “A Critical Review of the Models of Group Selection”, Quarterly Review of

Biology 53: 101–114.
——— (1980), “An Experimental Study of Kin Selection”, Evolution 34: 844–855.
——— (1982), “Group Selection: Migration and the Differentiation of Small Populations”,

Evolution 36: 944–961.
Williams, George C. (1966), Adaptation and Natural Selection. Princeton, NJ: Princeton

University Press.
Wilson, David Sloan, and Elliott Sober (1994), “Reintroducing Group Selection to the Hu-

man Behavioral Sciences”, Behavioral and Brain Sciences 17: 585–654.
Wilson, Robert A. (1996), “Promiscuous Realism”, British Journal for the Philosophy of

Science 47: 303–316.
——— (1999), “Realism, Essence, and Kind: Resuscitating Species Essentialism?”, in Robert

A. Wilson (ed.), Species: New Interdisciplinary Essays, MIT Press, Cambridge, MA,
187–207.

Wolf, Jason B., Michael J. Wade, and Edmund D. Brodie III (2003), “The Genotype-
Environment Interaction and Evolution when the Environment Contains Genes”, in
Thomas J. DeWitt and Samuel.M. Scheiner (eds.), Phenotypic Plasticity: Functional
and Conceptual Approaches. New York: Oxford University Press.

This content downloaded from 129.128.046.162 on June 14, 2017 11:06:48 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).

http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2407991
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1023%2FA%3A1020575531347
http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2026953
http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F410450
http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F410450
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1093%2Fbjps%2F47.2.303
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1093%2Fbjps%2F47.2.303
http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2408074
http://www.journals.uchicago.edu/action/showLinks?system=10.1086%2F289977
http://www.journals.uchicago.edu/action/showLinks?crossref=10.2307%2F2407552
http://www.journals.uchicago.edu/action/showLinks?crossref=10.1017%2FS0140525X00036104