Evolution in a tropical archipelago: comparisons within and among 50 species of Philippine birds

Abstract

Tropical islands are hotspots for speciation and the generation of genetic diversity within species. The islands of the Philippines have experienced dynamic biogeographic histories, making them excellent for studying the processes affecting the generation and distribution of genetic diversity. I tested hypotheses about the distribution of population genetic diversity within species and examined levels of genetic and phenotypic divergence within 50 species of Philippine birds. The first study empirically tested two models for the distribution of population genetic diversity (increasing diversity with decreasing latitude and decreasing diversity approaching range edges) and found no support for either model. This was not unexpected; in tropical island systems populations are fragmented and potentially more isolated, and their genetic diversity can be limited by island size. However, island size was not correlated with population genetic diversity. Instead, estimates of time spent in evolutionary isolation, inferred from genetic distances between populations within species, were positively correlated with population genetic diversity. This result suggests a possible link between the generation of genetic diversity within populations and the generation of new evolutionary lineages in this system. The second study measured and compared genetic and phenotypic divergence between 136 populations within 50 species and tested the prediction that avian diversity is underestimated in the Philippines. More than half of these species included at least one population that exceeded species-level divergence under conservative thresholds. These results support an urgent need for the taxonomic revision of Philippine birds and have implications for the management of biodiversity in island systems.