MICROEVOLUTION IN ISLAND FORMS: THE ROLES OF DRIFT AND DIRECTIONAL SELECTION IN MORPHOLOGICAL DIVERGENCE OF A PASSERINE BIRD

• Published in: Evolution Vol. 56; no. 10; pp. 2090 - 2099
• Main Authors: Clegg, Sonya M, Degnan, Sandie M, Moritz, Craig, Estoup, Arnaud, Kikkawa, Jiro, Owens, Ian P. F
• Format: Journal Article
• Language: English
• Published: United States Society for the Study of Evolution 01-10-2002; Oxford University Press
• Subjects: Animal morphology; Animals; Australia; Biological Evolution; Birds; Body size; Evolution; Evolution, Molecular; Evolutionary genetics; Genetic Drift; islands; Microsatellite Repeats; microsatellites; morphology; Multivariate Analysis; New Zealand; Phenotypic traits; Population genetics; Population size; REGULAR ARTICLES; selection; Songbirds - genetics; Songbirds - physiology; Stochastic processes; Timed tests; Zosterops; New Zealand; Australia
• ISSN: 0014-3820; 1558-5646
• DOI: 10.1554/0014-3820(2002)056[2090:MIIFTR]2.0.CO;2

Theory predicts that in small isolated populations random genetic drift can lead to phenotypic divergence; however this prediction has rarely been tested quantitatively in natural populations. Here we utilize natural repeated island colonization events by members of the avian species complex, Zosterops lateralis, to assess whether or not genetic drift alone is an adequate explanation for the observed patterns of microevolutionary divergence in morphology. Morphological and molecular genetic characteristics of island and mainland populations are compared to test three predictions of drift theory: (1) that the pattern of morphological change is idiosyncratic to each island; (2) that there is concordance between morphological and neutral genetic shifts across island populations; and (3) for populations whose time of colonization is known, that the rate of morphological change is sufficiently slow to be accounted for solely by genetic drift. Our results are not consistent with these predictions. First, the direction of size shifts was consistently towards larger size, suggesting the action of a nonrandom process. Second, patterns of morphological divergence among recently colonized populations showed little concordance with divergence in neutral genetic characters. Third, rate tests of morphological change showed that effective population sizes were not small enough for random processes alone to account for the magnitude of microevolutionary change. Altogether, these three lines of evidence suggest that drift alone is not an adequate explanation of morphological differentiation in recently colonized island Zosterops and therefore we suggest that the observed microevolutionary changes are largely a result of directional natural selection.