Genetic Diversity and Ecological Heterogeneity in Amphibian Evolution

The evolutionary forces causing genetic divergence in amphibians were analyzed by reviewing allozyme diversity from a global literature database involving 189 amphibian species (two orders and 13 families), 123 urodeles (five families), and 66 anurans (eight families). The analysis involved genetic indices of heterozygosity (H) and polymorphism (P), averaging 23 gene loci (range: 10-50 loci) per species. Likewise, we analyzed DNA content (genome size) in 24 species, mostly urodeles. The genetic indices were correlated with 18 biotic variables, involving seven ecological, five demographic, and six life history variables. The following results were indicated: (1) Heterozygosity is significantly correlated with life zone and habitat type. Tropical species are more heterozygous than temperate species (H = 0.096 vs 0.065, respectively; P < 0.001). Heterozygosity decreases from terrestrial and arboreal to aquatic and subterranean families as follows (H in parentheses): Discoglossidae (0.140) > Rhacophoridae (0.123) > Sooglossidae (0.106) > Bufonidae (0.105) > Plethodontidae (0.077) > Ranidae (0.075) > Salamandridae (0.058) > Hylidae (0.051) > Ascaphidae (0.048) > Pelobatidae (0.044) > Proteidae (0.017) > Ambystomidae (0.009) > Cryptobranchidae (0.006). Thus, genetic diversity decreases with decreasing ecological heterogeneity from terrestrial and arboreal to aquatic and subterranean habitats, i.e., with decreasing niche breadth, or with increasing ecological stability and predictability. (2) Genetic diversity is partly correlated with, and explained by, a combination of three ecological variables. We concluded that ecological rather than demographic variables predominate in explaining and predicting genetic diversity and divergence in amphibians. Likewise, fecundity, body size, and longevity explain a considerable amount of the variance in DNA content, R2=0.72. The patterns and correlates of genetic diversity in amphibians strongly implicate environmental selection as an important evolutionary force causing divergence of heterozygosity, and are inconsistent with predictions of the neutral theory of molecular evolution.