All the very large mammals have fecundities of approximately 25. Small mammals, such as house mice and deer mice, have fecundities that are approximately twice as great as those in large mammals, providing a greater potential for the occurrence of balancing selection. In mammals, as in all other groups of species, the opportunity for balancing selection increases with fecundity, but in mammals, size and fecundity are negatively correlated. Together, these patterns may explain the weak negative correlation between heterozygosity and size in mammals ( Nevo et al. 1984; Wooten and Smith 1985). If someone will point to an immense homeotherm with high fecundity, I will predict that it will have lots of genetic variation.
Whereas the largest animals have little genetic variability, the largest plants have the highest level of heterozygosity of any group of species. Conifers have more genetic variation than do Drososphila, marine invertebrates, or amphibians ( Hamrick, Mitton, and Linhart 1979; Mitton 1983, 1995b; Nevo, Beiles, and Ben-Shlomo 1984). Again, size is probably irrelevant. Multivariate analyses of life-history variables revealed that many aspects of ecology and life history covaried among the 113 taxa of plants examined ( Hamrick, Linhart, and Mitton 1979). Principal-components analysis was used to examine the patterns of variation in the data, which included 15 variables for each species. The first principal axis explained 30% of the variation in the data, and this axis was most strongly associated with three measures of genetic variation. Other variables associated with this axis were generation length, mating system, pollination mechanism, fecundity, seed dispersal, and successional status. Species that were long lived and wind pollinated and had high fecundities tended to have the highest levels of genetic variation. This suite of characteristics is common to conifers.
Fecundity, geographic range, and the population size ( Soulé 1976) covary among species; species with large population sizes (or populations with high densities) also tend to have large geographic ranges, and it is these species that most commonly have high levels of genetic variation. This coordinated set of axes can be used to predict rates of molecular evolution and rates of speciation. These predictions are presented in chapter 10).
Genetic variation varies dramatically among species of both plants and animals; fruit flies, marine mussels, and conifers have lots of genetic variation, whereas large vertebrates and weeds have much less. Widespread species tend to have more genetic variation than do species with small geographic ranges. Genetic variation increases with niche width in some studies but not in others. Neutral theory predicts that genetic variation will increase with population size, and empirical data are usually consistent with this prediction. Both the opportunity for selection and genetic variation increase with fecundity.