Biology Reference
In-Depth Information
Selfish individuals or group advantage?
We now return to our theme of studying the adaptive significance of behaviour, how it
contributes to an individual's chances of survival and its reproductive success. We
interpreted the behaviour of the lions in relation to individual advantage, reflecting
Darwin's emphasis on evolution as a struggle between individuals to out-compete
others in the population. Many traits evolve because of their advantage to the individual
even though they are disadvantageous to others in the population. For example, it is not
to the species' advantage to have a cub killed when a new male takes over a lion pride. It
is not to the lionesses' advantage either! However, she is smaller than the male and often
there is probably not much that she can do about it. Infanticide has evolved simply
because the advantage to the male that practises it outweighs the cost to the female in
resisting.
Not so long ago, however, many people thought that animals behaved for the good of
the group, or of the species. It was common to read (and sometimes still is) explanations
like, 'lions rarely fight to the death because, if they did so, this would endanger survival
of the species' or, 'salmon migrate thousands of miles from the open ocean into a small
stream where they spawn and die, killing themselves with exhaustion to ensure
survival of the species'. Because 'group thinking' is so easy to adopt, it is worth going
into a little detail to examine why it is the wrong way to think about the evolution of
behaviour.
The most famous proponent of the idea that animals behave for the good of the group
was V.C. Wynne-Edwards (1962, 1986). He suggested that if a population over-exploited
its food resources it would go extinct, and so adaptations have evolved to ensure that
each group or species controls its rate of consumption. Wynne-Edwards proposed that
individuals restrict their birth rate to prevent over-population, by producing fewer
young, not breeding every year, delaying the onset of breeding and so on. This is an
attractive idea because it is what humans ought to do to control their own populations.
However, there are two reasons for thinking that it is unlikely to work for animal
populations.
Behaviour of
advantage to
individuals
may be
disadvantageous
to the group
Theoretical considerations
Imagine a species of bird in which a female lays two eggs and there is no over-exploitation
of the food resources. Suppose the tendency to lay two eggs is inherited. Now consider a
mutant that lays three eggs. Since the population is not over-exploiting its food supplies,
there will be plenty of food for the young and because the three-egg genotype produces
50% more offspring it will rapidly increase at the expense of the two-egg genotype.
Will the three-egg type be replaced by birds that lay four eggs? The answer is yes, as
long as individuals laying more eggs produce more surviving young. Eventually a point
will be reached where the brood is so large that the parents cannot look after it as
efficiently as a smaller one. The clutch size we would expect to see in nature will be the
one that results in the most surviving young because natural selection will favour
individuals that do the best. A system of voluntary birth control for the good of the
group will not evolve because it is unstable; there is nothing to stop individuals behaving
in their own selfish interests.
