Are Insular Populations of the Philippine Falconet ( Microhierax erythrogenys ) Steps in a Cline?

Abstract

Founder effects, new environments, and competition often produce changesin species colonizing islands, although the resulting endemism sometimesrequires molecular identification. One method to identify fruitful areasfor more detailed genetic study is through comparative morphologicalanalyses. We measured 210 museum specimens to evaluate the potentialmorphological consequences of colonization across the Philippinearchipelago by the Philippine Falconet (Microhierax erythrogenys).Measurements of both males and females differed clearly from island toisland. Univariate and multivariate analysis of characteristics showed alatitudinal gradient, with the bill, wing, and tail of southern birdsbeing larger than those of northern birds, forming the pattern of astepped cline across a succession of islands. The stepped gradient inmorphology and extensive differences between islands we observed provideevidence for multiple perspectives on phylogeny, including concordancewith aggregate complexes expected on the basis of sea-levelfluctuations. However, calculation of diagnosability indices did notsupport subspecific designations. Sex-specific dominance and dispersalpatterns may explain this unusual south-to-north stepped cline, and theyalso provide a useful format for understanding biogeographical patternsby island. Finally, these morphological data suggest a potentiallyfruitful area for future genetic studies.