Evolution__3rd_Edition

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Theory requires open-ended choice
Experiments show choice to be
open-ended
CHAPTER 12 / Adaptations in Sexual Reproduction 333
In Fisher’s theory, at the initial stage when the male character was positively correlated
with survival, either an absolute or an open-ended preference could be favored.
The average tail length might then have been 2 in (5 cm), and the longest tails in the
population might have been 12 in (30 cm). If the mutant that was selected happened to
be one encoding an absolute preference for males with 12 in tails, then evolution would
proceed until the average tail length was 12 in and then come to a stop. Only if the
preference is open-ended can there be an equilibrium with a costly male character. At
equilibrium, the lower survival of males with longer than average tails has to be compensated
by a higher frequency of mating. It is not enough for the females to prefer
average males, or to mate at random. The females must actively prefer males with
longer than average tails.
Likewise, in the handicap theory, females must prefer males with the most costly
handicaps. If the greater cost paid by the higher quality males is not compensated by
higher mating success, a less costly handicap will evolve. Therefore, in both theories,
the female choice must be open-ended in a species with a costly male ornament. If we
find evidence for such preferences, it suggests the male character is indeed maintained
by female choice. However, it does not tell us whether Fisher’s runaway theory or
Zahavi’s handicap theory are at work.
The prediction has been tested in more than one species. One example to illustrate
the procedure is Møller’s (1994) study of barn swallows (Hirundo rustica). The two
sexes are similar in the swallow, except for the outermost tail feather which is about
16% longer in the male than in the female. Møller tested whether females open-endedly
prefer males with longer tails by experimentally shortening the tails of some males, by
cutting them off with a pair of scissors, and elongating the tails of others, by sticking
those severed tail feathers on to other intact males with superglue that hardened in less
than 1 second. He then measured how long it took the different males to find a mate.
Males with elongated tails mated faster (Figure 12.10a), resulting in higher reproductive
success (Figure 12.10b).
Møller also confirmed that the male character is costly. Swallows molt in the fall and
grow a new tail for the following breeding season. A male’s new tail is on average about
0.2 in (5 mm) longer than in the previous year, but the males whose tails were elongated
grew a tail in the following year that was shorter than before the experimental treatment
(Figure 12.10c). (Møller did not tamper with their tails in the year after the
experiment.) Those males had enjoyed a good year during the experiment, but the
extra effort of flying with an elongated tail exacted a physiological cost. Next year the
cost was paid: the males took longer to find a mate and their reproductive success
decreased. In summary, Møller has shown that the sexually dimorphic tail feathers of
swallows are maintained by female choice, that the choice is open-ended, and that the
character chosen is costly.
12.4.6 Fisher’s theory requires heritable variation in the male character,
and Zahavi’s theory requires heritable variation in fitness
In Fisher’s runaway theory, the reason why females choose males with long tails at the
final equilibrium point is that a mutant female who mated at random would have lower