Sexual Selection, Sperm Competition and Sexual Conflict - An Introduction to Behavioural Ecology - page 197

BOX 7.3 SEXUAL SELECTION FOR NOSE LENGTH: THE IMPORTANCE
OF GENETIC COVARIANCE FOR FISHER'S HYPOTHESIS
(LANDE, 1981)
(1) Imagine that at the start there was a range of nose lengths and of female
preferences in the population. Females with a preference for slightly longer
than average noses would be mated to males with longer noses and vice versa.
The crucial fact to note is that offspring of these matings would have both the
nose and preference genes: either genes for long nose plus long preference or
short nose and short preference. The preference is expressed only in females
and the nose in males, but everyone carries both kinds of gene. In short, there
will arise an association or covariance between nose and preference genes. You
could look at a female's preference and predict what kind of nose genes she
carries to give to her sons (Fig. B7.3.1).
(2) How will evolution proceed, given this covariance? If equal numbers of females
have preference above and below the mean nose length ( x ), there will be no
change. But if by chance there was a slight predominance of females on one
side of the mean (it could be long or short but let us take long), then positive
feedback will start. This is shown by the arrows in the figure. Females select
for long noses (long-nosed males have a higher chance of mating) and, thereby,
because of the covariance , select for long preference. This in turn produces a
further push to long noses, and hence an increase in preference.
(3) The final outcome of sexual selection in quantitative models of this hypothesis
depends on the exact assumptions made in the model, for example whether or
not there is a cost of female choice (Pomiankowski et al ., 1991). However, the
important general point is that covariance between the male trait and female
preference underlies Fisher's hypothesis.
(x)
Length of son's nose
Fig. B7.3.1 Genes for long nose and long preference go together in the offspring.
The slope of the line represents the degree of association or covariance.