Evolution__3rd_Edition

334 PART 3 / Adaptation and Natural Selection
Figure 12.10
Barn swallows with longer tails
are preferred by females, but
the character is costly. Møller
experimentally shortened some
males’ tails and elongated
others; as one control he cut
the males’ tails off and then
immediately stuck them back
on again (control 1) and as
another he left the males
untreated (control 2). (a) Males
with elongated tails obtain
mates more quickly, (b) have
higher reproductive success,
but (c) next year grow a shorter
tail while the other males grow
a longer tail. (Møller also
measured both the mating
advantage and the cost of longer
tails by other criteria too, and
those results support the
results illustrated here.)
(0.25 in ≈ 6 mm.) Redrawn,
by permission, from Møller
(1994). © 1994 Macmillan
Magazines Ltd.
Theory predicts the male character
is heritable
Premating period (days)
Change in tail length (mm)
(a)
25
20
15
10
5
0
(c)
15
10
5
0
–5
–10
Shortened Control 1 Control 2 Elongated
Tail length
Shortened Control 1 Control 2 Elongated
Tail length
Shortened Control 1 Control 2 Elongated
Tail length
fitness because her sons would have shorter tails and be rejected as mates. This is only
true if male tail length is heritable. If all the variation in tail length were environmental,
and its heritability were zero (Section 9.6, p. 235), the tail length of the mutant female’s
sons would be no shorter on average than those of the choosy females. If mate choice
imposed any cost on a female at all, the randomly mating mutant would spread. Tail
length, therefore, must be heritable or selection will favor the female who mates at
random. This condition is testable, but has never been tested in a species with a costly
and extravagant character like the peacock’s tail.
In Zahavi’s theory, the advantage of female choice does not depend on the inheritance
of the male character. Choice could be maintained even if all members of the population
had the same genes for tail length. But the theory has an analogous condition.
In species in which males transfer only sperm, female choice is for male genetic quality.
There must be variation in male genetic quality: some males must have good genes,
others bad. This is a condition called heritability of fitness. Heritability of fitness means
that individuals of higher than average fitness (that is, they produce more offspring
Number of fledglings
12
10
8
6
4
2
0
(b)
..