Biology Reference
In-Depth Information
easier to detect or that females had a pre-existing sensory bias to respond to certain
stimuli (Ryan
et al
., 1990; Chapter 14). If there is some genetic basis for differences
between males in tail length the advantage will be passed on to the female's sons. At the
same time, a gene which causes females to prefer longer than average tails will also be
favoured, since these females will have sons better able to fly or more readily detected by
potential mates.
Now, once the female preference for longer tails starts to spread, longer-tailed males
will gain a double advantage: they will be better at flying and be more likely to get a
mate. The female similarly gets a double advantage from choosing: she will have sons
that are both good fliers and attractive to females. As the positive feedback between
female preference and longer tails develops, gradually the benefit of attractive sons will
become the more important reason for female choice, and the favoured trait might
eventually decrease the survival ability of males. When the decrease in survival
counterbalances sexual attractiveness, selection for increasing tail length will grind to
a halt (Lande, 1981; Kirkpatrick, 1982). Box 7.3 describes some aspects of Fisher's
hypothesis in more detail.
Selection for
attractiveness
alone
Good genes for sons and daughters
Amotz Zahavi (1975, 1977) suggested an alternative view of elaborate male sexual
displays. He pointed out that the peacock's long tail is a handicap in day-to-day survival.
He then went on to suggest that females prefer long tails (or other equivalent traits)
precisely
because
they are handicaps and, therefore, act as a reliable signal of a male's
genetic quality. The tail demonstrates a male's ability to survive in spite of the handicap,
which means that he must be extra good in other respects. If any of this ability is
heritable, then the tendency to be 'good' at surviving will be passed on to offspring.
Thus, females select for good genes by selecting to mate only with males whose displays
honestly indicate their genetic quality. Note that in this hypothesis the 'good genes' are
genes for the utilitarian aspects of survival and reproduction, rather than genes purely
for attracting females, as assumed in Fisher's hypothesis.
When it was first published, Zahavi's idea was not accepted, but subsequent theoretical
papers have shown that the handicap hypothesis is a plausible explanation for the
evolution of elaborate sexual displays, and perhaps of animal signals in general
(Chapter 14). The most important feature of theoretical models of the handicap
principle that 'work' (i.e. show that females could benefit from choosing males because
of their handicaps) is that males only express the handicap, in other words develop the
full sexual display, when they are in good condition (Grafen, 1990a, 1990b). This gets
around the difficulty some critics saw in Zahavi's original idea, that males were forced
to carry the handicap whether or not they could afford it, because it was viewed as a
fixed trait. There are different variants of the flexible handicap idea (some authors refer
to 'revealing handicaps' that reveal a male's current vigour, others to 'condition
dependent handicaps' expressed in proportion to the male's condition), but the essential
feature of all these models is that the degree of expression of the male sexual display
tells the female about his genetic quality.
Another problem that initially bothered theoreticians is this; if there is strong selection
by females for males with the 'best genes' then genetic variation might rapidly decline,
Amotz Zahavi's
handicap
hypothesis: only
good quality
males can afford
elaborate
ornaments and
display
Selection for
increased genetic
quality of
offspring
