Evolution__3rd_Edition

332 PART 3 / Adaptation and Natural Selection
The costliness of a signal makes it
reliable
Table 12.3
The handicap principle. If only males with good genes can survive the possession of a handicap,
females who mate with handicapped males will mate only with males who possess good genes.
A female who mates with males lacking a handicap will mate with males possessing good and
bad genes in their population proportions.
Males with bad genes Males with good genes
No handicap Alive Alive 5 in population
6
Handicap Dead Alive 7 proportions
The handicap acts as an indicator of genetic quality. But why does the indicator have
to be costly? The reason is that the cost guarantees that the indicator will be reliable. A
male’s genetic quality does not come written on him: it has to be inferred, and if females
inferred it from an an inexpensive signal, there would be selection on males to cheat. If
females preferentially mated with males who merely said “I have good genes” (or
rather, in a non-human species, something analogous to saying this) and rejected those
that said “I have poor genes,” mutant males who said the former independently of their
true genetic quality would be favored. Words (and their analogs) are cheap. But if the
criterion favored by females is costly, as growing a long and ostentatious tail is, then
selection will less automatically favor cheats. In particular, if the cost of growing a
handicap is less for a truly high quality male than for a low quality male, handicaps will
be grown only by high quality males and will be reliable signals for females to use. (This
condition was met in the simple example in Table 12.3: the cost of the handicap for the
males with bad genes was far higher than for males with good genes.)
So the reason for the costliness of the male character is completely different in Fisher
and Zahavi’s theories. In Fisher’s theory, the cost arose as the end product of a runaway
process. To begin with, long tails were not costly, but as an open-ended female preference
for males with longer tails was selected into the population, the tails evolved past
their optimum and ended up reducing the survival of their bearers. In Zahavi’s theory,
the male character had to be costly from the start, and to remain costly as the female
preference spreads. The function of the chosen male character is to indicate genetic
quality at other loci, and it has to be costly in order to be reliable.
12.4.5 Female choice in most models of Fisher’s and Zahavi’s theories is
open ended, and this condition can be tested
There are two crucial means by which Fisher and Zahavi’s ideas can be tested. The first
concerns the exact kind of female preference that they require. The preference is openended.
We can distinguish between absolute preferences, which are of the form “mate
preferentially with males whose tails are 12 in long,” and open-ended preferences, such
as “mate preferentially with the male who has the longest tail you can find.”
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