Evolution__3rd_Edition

330 PART 3 / Adaptation and Natural Selection
Figure 12.9
(a) The early stage in the
evolution of a bizarre character
such as the peacock’s tail.
Before females preferred to
mate with long-tailed males,
there might have been a positive
correlation between tail length
(then much shorter than in
their descendants) and male
fitness. (b) Full relation
between degree of exaggeration
of character (tail length) and
survivorship. There is an
intermediate optimum.
Modern species like the peacock
occur toward the right of the
graph.
Fisher described “runaway” sexual
selection
(a) (b)
Fitness
Tail length
before the female preference arose, things might have been different. Male tails would
have been shorter then. Suppose that, before some mutant female arose who picked longtailed
males, most females picked their mates at random; suppose also that there was at
that time a positive correlation between male tail length and survival (Figure 12.9a).
Selection would then favor a mutant female with a preference for males with longer
tails as she would produce sons with longer than average tails, with an associated higher
survival. Then, as the mutation spread, the males with longer tails would start to
acquire a second advantage. There are increasing numbers of females in the population
who prefer to mate with longer tailed males, and the males so endowed will not only
survive better but also enjoy an advantage in mating. The evolution of longer tails in
males, and a mating preference for them in females, thus come to reinforce each other,
in what Fisher called a runaway process.
Technically, they reinforce each other because the genes encoding them are in
linkage disequilibrium (Section 8.5, p. 199). The offspring of a female who mates preferentially
with longer tailed males will possess both their mother’s genes for choice and
their father’s genes for long tails. These two kinds of genes thus become non-randomly
associated, and the genes for female choice increase in frequency by hitch-hiking with
the advantageous genes for long tails in males.
If we consider a sufficiently wide range of tail lengths, the full relation between it
and male survival presumably shows some increase and decrease on either side of an
optimum (Figure 12.9b). Eventually, powered by female choice, the average tail length
in the population will reach the optimum; but evolution does not stop there. As the
population evolves towards the optimal tail length, the longer tailed males are still preferred.
By now the female preference will have spread through the population and the
majority of females will prefer longer tailed mates. Now the mating preference alone
drives the evolution of longer tails. The preference may have become strong enough to
compensate lower male survival, and evolution will proceed into the interesting zone in
which the male character, in a complete reversal of the original selective forces, evolves
to become increasingly costly to its bearers.
The evolution of the long tail therefore proceeds through three stages. Long tails initially
have only a survival advantage. Then the survival advantage is supplemented by a
Fitness
Initial evolutionary stage
Optimum
Tail length
Modern peacocks
..