|Male mating success||1.00||1.39||1.45||2.99||2.55|
|Note: Data from Zapata,
Beardmore 1990. * and ** indicate values significantly different from 1.00 at the|
5% and 1% level, respectively.
lated that by choosing males with large horns, females gain mates of superior genetic quality, for both resistance to disease and superior foraging ability are needed to grow large horns.
Fitzsimmons measured the horns of 113 bighorn rams taken by hunters in Wyoming, in 1989 and 1990 and estimated the annual increase in the horns' volume. Samples of tissue from each animal were analyzed with electrophoresis to identify their genotypes at four polymorphic loci. The sample was broken into two groups, those heterozygous for zero to one and those heterozygous for two to four loci. Multilocus heterozygosity accounted for 25% of the variability of growth in years 6 through 8, a time when growth in the highly heterozygous group was significantly higher than that in the less heterozygous group. Growth rates did not differ significantly between the two groups in the first five years of growth, or after year 8, although increases in horn volume in these years favored highly heterozygous rams. Rams typically begin to breed in year 7 or 8, after two or three years of differential growth between the heterozygosity groups. Growth was related to heterozygosity at individual loci as well. The growth of heterozygotes significantly exceeded the growth of homozygotes for transferrin, gly- oxyalase, and lactate dehydrogenase. Symmetry of the horns was not related to heterozygosity ( S. Buskirk, pers. comm.).
Sexual selection favoring heterozygous mates was detected in the marine snail, Littorina mariae ( Rolán-Alvarez, Zapata, and Alvarez 1995). Snails were collected from the marine alga Fucus vesiculosus in the intertidal zone in Muros-Noya Ria, Galicia, Spain. Individuals were characterized as being solitary or copulating, and each one's genotype was identified for nine polymorphic allozyme loci. Reproductive success was estimated as the proportion of copulating individuals for each genotype or heterozygosity class. Although larger females had greater mating success, allozyme heterozygosity had no connection to it. However, mating success did increase with heterozygosity in males, a trend that was strong in young males but not statistically significant ill older males. The male mating success of heterozygosity classes 0, 1, 2, and 3 through 5 was 1.00, 1.43, 1.81, and 2.30, respectively.
A model of female choice sexual selection of "good genes" was presented. This model extended earlier models by incorporating the threshold selection of overdominant loci