Biology Reference
In-Depth Information
as only a few male genotypes would be successful in breeding. In the same way, artificial
selection for traits tends to have strong effects at first but then becomes less effective as
genetic variation for the trait gets 'used up'. If female choice is costly, for example in
time searching for a mate, then, in theory at least, females should stop choosing as
genetic variation declines (Andersson, 1994).
Current thinking suggests that this is not so much of a problem in practice, for four
reasons. Firstly, populations suffer from a continuous input of deleterious mutations
and so it may pay females to always be choosy to avoid these in potential mates
(Kondrashov, 1988; Agrawal, 2001). Secondly, if only males in the best condition can
afford to grow elaborate ornaments, or have vigorous displays, then female choice is
unlikely to deplete the genetic variation available simply because so many genes are
involved in influencing male condition. In fact, just about every physiological process
going on in a male's body will have some influence on condition, so the genetic variation
will be enormous (Rowe & Houle, 1996). Thirdly, females may choose different male
traits in different years, which could maintain genetic variation underlying multiple
sexual ornaments. In lark buntings,
Calamospiza melanocorys
, for example, in some
years females prefer the blackest males, in others those with the largest wing patches
and in others those with the largest beaks, and so on (Chaine & Lyon, 2008). These
rapidly changing patterns of sexual selection parallel the oscillating patterns of natural
selection described for Darwin's finches, where a rapidly changing food supply selects
for small beaks in some years and large beaks in others (Grant & Grant, 2002).
The fourth factor that might maintain genetic variation in fitness is host-parasite
arms races. Bill Hamilton, who lived from 1936 to 2000, was one of the most influential
evolutionary biologists since Darwin. He once commented that two of the most dramatic
changes in the English countryside during his lifetime were caused by disease: the
myxoma virus that periodically killed the rabbit population and led to vegetation
change, and Dutch elm disease, a fungus which killed most of the elm trees that once
graced lowland England. This convinced him that disease must be a powerful selection
pressure on organisms in nature. Together with Marlene Zuk (Hamilton & Zuk, 1982),
he suggested that sexual displays were reliable indicators of genetic resistance to disease.
According to this view, by choosing for elaborate displays females are acting as diagnostic
veterinarians, selecting males which are genetically equipped to resist current infection.
Because the parasites and hosts are engaged in a never-ending arms race of adaptation
and counter-adaptation, involving genetic changes in both sides, the 'good genes' are
changing all the time and so it always pays females to be choosy.
How is genetic
variation for
quality
maintained over
the generations?
Host-parasite
arms races:
female choice for
disease resistance
Testing the hypotheses for genetic
benefits
To demonstrate that a trait could have evolved by the Fisher process, it would be
necessary to show that there is genetic variation in both female preference and the male
trait, and that preference and trait genes covary (Box 7.3). Several studies have now
shown this predicted genetic correlation between trait and preference.
Wilkinson and Reillo (1994) studied stalk-eyed flies,
Cyrtodiopsis dalmanni
. These
small flies have their eyes held out on stalks which are particularly long in the males,
Covariance of
male trait and
female preference
in stalk-eyed flies
…
