Phylogenetic comparative methods strengthen evidence for reduced genetic diversity among endangered tetrapods

Abstract

The fitness of species with little genetic diversity is expected to be affected by inbreeding and an inability to respond to environmental change. Conservation theory suggests that endangered species will generally demonstrate lower genetic diversity than taxa that are not threatened. This hypothesis has been challenged because the time frame of anthropogenic extinction may be too fast to expect genetic factors to significantly contribute. I conducted a meta-analysis to examine how genetic diversity in 894 tetrapods correlates with extinction threat level. Because species are not evolutionarily independent, I used a phylogenetic regression framework to address this issue. Mean genetic diversity of tetrapods, as assessed by protein heterozygosity, was 29.7-31.5% lower on average in threatened species than in their nonthreatened relatives, a highly significant reduction. Within amphibians as diversity decreased extinction risk increased in phylogenetic models, but not in nonphylogenetic regressions. The effects of threatened status on diversity also remained significant after accounting for body size in mammals. These results support the hypothesis that genetic effects on population fitness are important in the extinction process.

Figure 1

Hypothetical array of trait data on two species trees. The top tree assumes statistical independence of species, and the bottom tree shows species relations (T, threatened species; LC, species of least concern; numbers, levels of genetic diversity).

Figure 2

Observed heterozygosity transformed to the arcsine square root. Diversity levels are stratified by taxonomic class and threat category (LC, least concern and near threatened species; T, species categorized as vulnerable, endangered, critically endangered, and extinct in the wild [IUCN 2008]; NE, species not evaluated by the International Union for Conservation of Nature).