Evolutionary dynamics of bird populations in Southeast Asia

Abstract

A confluence of factors determines the biological diversity we observe today. Here, I conducted three investigations of the historic, geographic and ecological factors that shaped the morphological and genetic diversity of rainforest birds in Southeast Asia. (1) In the Oriental Dwarf Kingfisher (Ceyx erithaca) complex, the birds are highly polymorphic in plumage. Quantitative analysis of this variation indicated that a large proportion of birds in Sumatra, the Malay Peninsula and Borneo have plumages intermediate between the northerly black form from mainland Asia and the southerly rufous form from Java. Phylogenetic analyses indicated that birds from continental Southeast Asia (north of the Malay Peninsula) were well differentiated from those from insular Southeast Asia. This genetic distinction correlates well with a fixed plumage difference (mantle coloration). Coalescent analyses showed that the plumage polymorphism was caused by past genetic introgression between the two parental forms. (2) I sampled 16 lowland rainforest bird species primarily from the Malay Peninsula and Borneo to test the long-standing hypothesis that animals on different Sundaic landmasses intermixed extensively when low sea-levels during the Last Glacial Maximum (LGM) exposed land-bridges. This hypothesis was rejected in all but five species through coalescent simulations. Environmental niche modeling showed that the presence of unsuitable habitats between western and eastern Sundaland during the LGM coincided with deeper inter-population genetic divergences. The distinctiveness of the northeastern Borneo populations of some species may be underlain by a combination of factors that included riverine barriers, LGM expansion of montane forests and regional physiography. (3) I further investigated the population divergence and demographic histories of three bird species that possessed disparate ecological characteristics. Multilocus analyses revealed changes in effective population sizes that were driven by long-term changes in the environment, instead of high-frequency glacial cycles. Populations from Borneo exhibited stronger demographic growth than those from mainland Southeast Asia, suggesting regional differences in environmental changes or directional colonization. The species with the widest habitat breadth also showed the greatest amount of inter-landmass gene flow. This adds to the growing body of empirical work indicating an association between a species’ ecological characteristics and its population connectivity over evolutionary time-scales.