ASSOCIATIONS BETWEEN PHYSICAL ISOLATION AND GEOGRAPHICAL VARIATION WITHIN 3 SPECIES OF NEOTROPICAL BIRDS

We studied effects of physical isolation on geographical variation in mtDNA RFLP polymorphisms and a suite of morphological characters withi n three species of neotropical forest birds; the crimson-backed tanage r Ramphocelus dimidiatus, the blue-gray tanager Thraupis episcopus, an d the streaked saltator Saltator albicollis. Variation among populatio ns within continuous habitat on the Isthmus of Panama was compared wit h that among island populations isolated for about 10 000 years. Putat ive barriers to dispersal were influential, but apparent isolation eff ects varied by species, geographical scale, and whether molecular or m orphological traits were being assessed. We found no geographical stru cturing among the contiguous, mainland sampling sites. Migration rates among the islands appeared sufficient to maintain homogeneity in mtDN A haplotype frequencies. In contrast, variation in external morphology among islands was significant within two of three species. For all sp ecies, we found significant variation in genetic and morphological tra its between the island (collectively) and mainland populations. Inters pecific variation in the effects of isolation was likely related to di fferential vagility. These data generally corroborate other studies re porting relatively great geographical structuring within tropical bird s over short distances. Behaviourally based traits - low vagility and high 'sensitivity' to geographical barriers - may underlie extensive d iversification within neotropical forest birds, but more extensive eco logical and phylogeographic information are needed on a diverse sample of species.