Biology Reference
In-Depth Information
Group selection
Wynne-Edwards realized this and so
proposed the idea of 'group selection' to explain
the evolution of behaviour that was for the
good of the group. He suggested that groups
consisting of selfish individuals died out because
they over-exploited their food resources. Groups
that had individuals who restricted their
birth rate did not over-exploit their resources
and so survived. By a process of differential
survival of groups, behaviour evolved that
was for the good of the group (Fig. 1.4).
In theory this can work, but it would
require that groups are selected during
evolution, with some groups dying out
faster than others. In practice, however,
groups usually do not go extinct fast enough
for group selection to be an important force
in evolution. Individuals will nearly always
die at a faster rate than groups, so individual selection will be more powerful. In
addition, for group selection to work populations must be isolated, such that
individuals cannot successfully migrate between them. Otherwise there would be
nothing to stop the migration of selfish individuals into a population of individuals
all practising reproductive restraint. Once selfish individuals arrive, their genotype
would soon spread. In nature, groups are rarely isolated sufficiently to prevent such
immigration. So group selection as proposed by Wynne-Edwards is usually going to
be a weak force and probably rarely very important (Williams, 1966a; Maynard
Smith, 1976a). We revisit this topic in the final chapter.
S
A
S
A
S
S
A
A
A
S
S
S
A
S
A
A
S
A
A
S
Fig. 1.4
Wynne-Edwards' model
of group selection. Groups of
selfish individuals (S) over-exploit
their resources and so die out.
Groups of altruistic individuals (A),
who do not over-exploit resources
(e.g. by having fewer offspring
than they could potentially raise)
survive.
Individual
selection more
powerful
Empirical studies: optimal clutch size
Apart from these theoretical objections, there is good field evidence that individuals do
not restrict their birth rate for the good of the group but rather maximize their individual
reproductive success. A classic example is the long-term study of the great tit (
Parus
major
) in Wytham Woods, near Oxford, UK, started in 1947 by David Lack (Lack, 1966).
In this population the great tits nest in boxes (Fig. 1.5a) and lay a single clutch of
eggs in the spring. All the adults and young are marked individually with small
numbered metal rings round their legs. The eggs of each pair are counted, the young
are weighed and their survival after they leave the nest is measured by re-trapping
ringed birds. This intensive field study involves several people working full-time
throughout the year, and it has been going on for over 60 years! Most pairs lay 8-9
eggs (Fig. 1.5b, bars). The limit is not set by an incubation constraint because when
more eggs are added the pair can still incubate them successfully. However, the parents
cannot feed larger broods so well. Chicks in larger broods get fed less often, are given
smaller caterpillars and, consequently, weigh less when they leave the nest (Fig. 1.6a).
It is not surprising that feeding the young produces a limit for the parents because they
have to be out searching for food from dawn to dusk and may deliver over 1000 items
per day to the brood at the peak of nestling growth. In a survey of the sustainable
Clutch size in
great tits …
