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Evolution__3rd_Edition

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334 PART 3 / Adaptation and Natural Selection<br />

Figure 12.10<br />

Barn swallows with longer tails<br />

are preferred by females, but<br />

the character is costly. Møller<br />

experimentally shortened some<br />

males’ tails and elongated<br />

others; as one control he cut<br />

the males’ tails off and then<br />

immediately stuck them back<br />

on again (control 1) and as<br />

another he left the males<br />

untreated (control 2). (a) Males<br />

with elongated tails obtain<br />

mates more quickly, (b) have<br />

higher reproductive success,<br />

but (c) next year grow a shorter<br />

tail while the other males grow<br />

a longer tail. (Møller also<br />

measured both the mating<br />

advantage and the cost of longer<br />

tails by other criteria too, and<br />

those results support the<br />

results illustrated here.)<br />

(0.25 in ≈ 6 mm.) Redrawn,<br />

by permission, from Møller<br />

(1994). © 1994 Macmillan<br />

Magazines Ltd.<br />

Theory predicts the male character<br />

is heritable<br />

Premating period (days)<br />

Change in tail length (mm)<br />

(a)<br />

25<br />

20<br />

15<br />

10<br />

5<br />

0<br />

(c)<br />

15<br />

10<br />

5<br />

0<br />

–5<br />

–10<br />

Shortened Control 1 Control 2 Elongated<br />

Tail length<br />

Shortened Control 1 Control 2 Elongated<br />

Tail length<br />

Shortened Control 1 Control 2 Elongated<br />

Tail length<br />

fitness because her sons would have shorter tails and be rejected as mates. This is only<br />

true if male tail length is heritable. If all the variation in tail length were environmental,<br />

and its heritability were zero (Section 9.6, p. 235), the tail length of the mutant female’s<br />

sons would be no shorter on average than those of the choosy females. If mate choice<br />

imposed any cost on a female at all, the randomly mating mutant would spread. Tail<br />

length, therefore, must be heritable or selection will favor the female who mates at<br />

random. This condition is testable, but has never been tested in a species with a costly<br />

and extravagant character like the peacock’s tail.<br />

In Zahavi’s theory, the advantage of female choice does not depend on the inheritance<br />

of the male character. Choice could be maintained even if all members of the population<br />

had the same genes for tail length. But the theory has an analogous condition.<br />

In species in which males transfer only sperm, female choice is for male genetic quality.<br />

There must be variation in male genetic quality: some males must have good genes,<br />

others bad. This is a condition called heritability of fitness. Heritability of fitness means<br />

that individuals of higher than average fitness (that is, they produce more offspring<br />

Number of fledglings<br />

12<br />

10<br />

8<br />

6<br />

4<br />

2<br />

0<br />

(b)<br />

..

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