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Part I
THE FUNDAMENTALS OF
EVOLUTIONARY COOPERATION
A
lthough most of this book is about the new topics that are being
treated as part of social evolution, such as genes, microbes, and
medicine, the old fundamental subjects still matter and remain the
object of vigorous research. The first four chapters revisit some of these
standard arenas, including social insects, cooperatively breeding birds,
mutualisms, and how to model social evolution.
There are many ways to think about and model social evolution.
Inclusive fitness is one of the most venerable and most useful, and is
the framework used by many authors in this book. In Chapter 1, David
Queller revisits why inclusive fitness has been so useful and suggests ways
to expand it to make it speak more directly to interactions besides kin
selection. He delimits two other kinds of social selection that can be treated
more explicitly in Hamilton’s rule. “Kind selection,” which involves syn -
ergisms between individuals expressing the same traits, groups together
greenbeards (genes that in effect can identify the presence of copies of
themselves in other individuals) and many cases of frequency-dependent
games because these share the feature that individuals expressing the trait
have different effects on other expressers compared to nonexpressers and
they also share many differences from pure kin selection. “Kith selection”
requires neither kin nor kind, but instead involves actors affecting partners
in ways that feed back to the actor’s fitness. Mutualism and manipula-
tion are included in this category. The expanded version of Hamilton’s
rule with kin, kith, and kind could bring the advantages of Hamilton’s
methods to a broader range of social interactions.
1
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2 / Part I
Interactions between individuals of different species are a major type
of kith selection, where individuals are selected to affect their partners
in ways that ultimately benefit themselves (or their kin). Such interac -
tions need not be cooperative, but when they are, they typically involve
exchange of different services that one partner needs and the other can
easily provide, so partners can be very different. Accordingly, in Chapter
2, Joel Sachs and colleagues explore associations or symbiosis among
partners that are very different indeed, one being eukaryotic and the
other prokaryotic. Such symbioses, by leading to mitochondria and chlo -
roplasts, were responsible for the evolution of the eukaryotic cell itself.
But additional symbioses are widespread and sometimes ancient. These
authors use a combination of broad-scale phylogenetic analyses and case
histories of particular systems to explore several transitions. They find,
for example, that there is little phylogenetic signal to indicate that some
bacterial groups are preadapted for eukaryotic symbiosis. Instead, the
genes required appear to be quite widely available through horizontal
transmission. Mutualistic interactions appear to arise from both parasitic
and free-living ancestors. Once acquired, these mutualistic interactions
seem to be quite stable, with few reversions to nonmutualistic forms.
Given the tendency of vertically transmitted symbionts to degrade and
the propensity of horizontally transmitted ones to cheat, this stability is
somewhat surprising.
The social insects have long been viewed as the pinnacle of coopera -
tion. This view is most tenable if one ignores the cooperation that goes on
in transitions that are already complete, such as to multicellular animals or
the eukaryotic cell. But some social insect colonies are so cooperative and
integrated that they are viewed as superorganisms (organisms made up
of other organisms). The motive force behind the evolution of these soci-
eties, which consist of close relatives, is kin selection (Hamilton, 1964a).
In Chapter 3, Peter Nonacs points out that predictions from kin selection
theory have been both successful and also disappointing. The difference,
he suggests, is not due to chance. The successful predictions from sex-ratio
theory and worker-policing theory work because the predicted behaviors
can be achieved using simple environmental cues that correlate with kin -
ship. It is easy to treat males differently from females, or workers from
queens. The less successful kin selection predictions, such as parts of skew
theory, may fail because they require genetic kin recognition mechanisms
sufficient to detect closer from more distant relatives within colonies. This
may not explain everything, because genetic kin recognition systems do
exist, at least for distinguishing colony members from noncolony mem -
bers. The interaction between environmental and genetic recognition sys -
tems has scarcely been explored, and Nonacs runs computer simulations
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The Fundamentals of Evolutionary Cooperation / 3
showing how greenbeard loci can perturb the outcomes expected under
pedigree relatedness alone.
After the social insects, cooperative birds and mammals have attracted
the most attention. Many bird species have helpers at the nest, usually
offspring from previous broods who have remained at their natal site
(Cockburn, 2006). Kinship is important here too. Helping systems usually
evolve from monogamous ones, and discrimination evolves in systems
that show variation in relatedness (Cornwallis et al., 2010). But the story
is more complicated, for two reasons. First, although, some helpers gain
kin-selected benefits through helping close kin, others may gain direct
benefits. Compared with the social insects, more research on birds has
addressed the particular benefits of remaining at home and on the ecologi-
cal constraints that may limit independent breeding. Variance in repro-
ductive success has played a role in these discussions, but in Chapter 4,
Dustin Rubenstein moves it to a more central position. He suggests that
cooperative breeders may be bet hedgers, gaining advantage from a more
uniform reproductive output in cooperative groups. Rubenstein draws on
many years of his field data on starlings in Africa, where there is much
variation in both time and space, and he finds support for several predic -
tions of this hypothesis.
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