Evolution__3rd_Edition

302 PART 3 / Adaptation and Natural Selection
The evidence for group selection
has been criticised ...
. . . as has the theory
Migration between groups
undermines group selection
selection is a weak and unimportant process. There are both theoretical and empirical
reasons. Empirically, there are no definite examples of adaptations that need to be
interpreted in terms of group advantage: Williams (1966) argued that the characters
that Wynne-Edwards had suggested evolved to regulate population size can all be
explained as adaptations that benefit individuals. Individuals generally reproduce at
the maximum rate they can. The only obvious exceptions concern genetically related
individuals, and can be explained by kin selection. Moreover, living things have characteristics
that contradict the theory of group selection. The 50 : 50 sex ratio, which we
discuss in Section 12.5 (p. 337), is a case in point. In polygynous species, it is inefficient
for the population to produce 50% males, most of whom are not needed. The widespread
existence of the 50 : 50 sex ratio suggests that group selection has been ineffective
on this trait.
Group selection is also implausible in theory. Consider a population containing two
genotypes. One codes for an altruistic, or group adaptive, trait like reproductive
restraint. The other codes for a selfish trait, like reproducing as fast as possible. The
population is made up of a number of groups, which can contain any proportion of
altrusitic and selfish individuals; groups with mainly altruistic members we call altruistic
groups and those with mainly selfish members, selfish groups. The altruistic groups
will go extinct at a lower rate as group selection favors altruism. Individual selection
favors the selfish individuals within all groups so within each group the selfish individuals
increase in frequency. An altruistic group may temporarily contain no selfish
members, but as soon as it is “infected” with one the selfish trait will proliferate and
become fixed in the group. What result should we expect to find? It depends on the balance
between the two processes. In theory, we can imagine a rate of group extinction so
high that altruists will predominate: just imagine, for sake of argument, what would
happen if all groups with more than 10% selfish types instantly went extinct. All the
groups we should see would clearly have at least 90% altruists. But that is only a thought
experiment. The interesting question is what we should expect to happen naturally.
The reason most biologists suppose that group selection is a weak force in opposition
to individual selection stems from the slow life cycles of groups as compared to individuals.
Individuals die and reproduce at the rate of once per generation, and many
individuals can move between groups within a generation. Groups go extinct at a much
slower rate. The amount of time it will take for selfish individuals to infect and proliferate
in a group is a small part of the group’s lifespan; at any one time, therefore, individual
adaptations will predominate.
Many models of group and individual selection exist, but they can mainly be reduced
to a common form (Figure 11.1). The groups are supposed to occupy “patches” in
nature. As before, some patches are occupied by altruistic and others by selfish groups.
There are also empty patches. A selfish group in the model drives itself extinct by
overeating its patch’s resources. The result of the model depends on whether a selfish
group can infect an empty or altristic patch before going extinct. Maynard Smith
(1976) defines the number m as the number of successful migrants produced by one
selfish group on average between its origin and extinction. (Successful means that the
migrant establishes itself in another group and breeds.) If m = 1 the system will be stable;
if m < 1 the selfish groups decrease in number, and if m > 1 they increase. In other
words, a selfish group only needs to produce more than one successful emigrant during
..