Minor Isolates and Minor Geographical Barriers in Avian Speciation in Continental Australia

Abstract

Ornithologists generally believe that speciation in birds proceeds by geographical splitting of populations into isolates and subsequent divergence. This allopatric mode has been subdivided into the dumbbell or classical mechanism and the peripheral or peripatric mechanism. Speciation theory predicts that peripatric divergence should proceed more rapidly than classical allopatric differentiation. This was examined in the Australian mainland avifauna by comparing the ratios of differentiates to isolates in major and minor geographical refuges on the assumption that populations isolated in minor areas were smaller than those in major ones. An array of distinctive avian species and subspecies was found to have evolved in peripheral refuge areas of mainland Australia. However, minor regions have produced considerably fewer new forms and, moreover, at the same rate. Therefore, the prediction of speciation theory was not confirmed. Differentiates on offshore islands around Australia were also considered and found not to have contributed to species enrichment on the mainland. Birds with patchy distributions were examined and discontinuities in habitat, especially substrate and dependency on specific resources, were the main causes in the semiarid and arid parts of the Australian continent. Another active pathway of adaptation from humid to arid habitat in eastern Bassian elements was revealed: the Grey Range Divide separates some avian populations in the Lake Eyre Basin from parental populations in more coastal regions.