Evolution__3rd_Edition

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Selfish
The theories of group selection and
of species selection are distinct
Group selection can be
experimentally simulated
Empty
Altruist
CHAPTER 11 / The Units of Selection 303
Figure 11.1
Maynard Smith’s formulation of group selection models. A patch
may change state in the direction of the arrows. Redrawn, by
permission of the publisher, from Maynard Smith (1976). © 1976
University of Chicago Press.
its existence for the selfish trait to take over. This is a small number. So small that we
can expect selfish individual adaptations to prevail in nature. Group selection, we
conclude, is a weak force. It only works if migration rates are implausibly low and
group extinction rates implausibly high. It is also not needed to explain the facts.
The case against group selection is presented here in stark terms, but only to make
the arguments clear. The matter has not been settled finally, and group selection probably
operates sometimes. Moreover, group selection can have evolutionary consequences
even if it never overrides individual selection. In Section 23.6 (p. 658) we look
at a process called “species selection.” Species selection operates when different species
(or even higher taxa) possess different individual-level adaptations, and their different
adaptations have different consequences for the rate of extinction or speciation. Taxa
with lower extinction, or higher speciation, rates tend to proliferate. Much the same
could be true of groups within a species.
In species selection, there is no conflict between selection at lower (individual) and
higher (species, or even group) levels. In all the species (or groups), individuals act in
their own selfish interest. Species selection is theoretically uncontroversial, though its
empirical importance is open to doubt. The controversy about group selection that we
looked at above was theoretical as well as empirical. Critics of group selection doubt
whether group selection could be strong enough to cause individuals to sacrifice their
own reproductive interests to those of its group.
In nature, group selection is rarely likely to override individual selection, and to
establish individually disadvantageous behavior. In the laboratory, however, conditions
can be made extreme enough for it to do so. Let us finish by looking at such a
case: Wade’s experiment on flour beetles, Tribolium castaneum. The life cycle of the
flour beetle takes place, from egg to adult, in stored flour. They are pests, but they have
also become a population biologist’s standard experimental animal, particularly in
Chicago. Wade set up an experiment to illustrate Wynne-Edwards’ hypothesis of
reproductive restraint.
Figure 11.2a illustrates the experimental design. In each of three experimental treatments,
there were 48 different colonies of Tribolium. Each colony was allowed to breed
for 37 days. Then 48 new colonies, of 16 beetles each, were set up from the progeny of
the old ones. Wade (1976) artificially selected for groups that had showed a low (or
high) fecundity (the third treatment was a control). He selected for low fecundity by
forming a new generation of colonies from Tribolium colonies that had a low population
density at the end of the 37 days; and for high fecundity by forming each new
generation from colonies that had high population densities. He repeated a number of
rounds of the process.