The importance of founder events in promoting evolutionary changes on islands has been a subject of long-running controversy. Resolution of this debate has been hindered by a lack of empirical evidence from naturally founded island populations. Here we undertake a genetic analysis of a series of historically documented, natural colonization events by the silvereye species-complex (Zosterops lateralis), a group used to illustrate the process of island colonization in the original founder effect model. Our results indicate that single founder events do not affect levels of heterozygosity or allelic diversity, nor do they result in immediate genetic differentiation between populations. Instead, four to five successive founder events are required before indices of diversity and divergence approach that seen in evolutionarily old forms. A Bayesian analysis based on computer simulation allows inferences to be made on the number of effective founders and indicates that founder effects are weak because island populations are established from relatively large flocks. Indeed, statistical support for a founder event model was not significantly higher than for a gradual-drift model for all recently colonized islands. Taken together, these results suggest that single colonization events in this species complex are rarely accompanied by severe founder effects, and multiple founder events and͞or long-term genetic drift have been of greater consequence for neutral genetic diversity.islands ͉ silvereyes ͉ colonization ͉ microsatellites T he idea that establishing a population from a small number of founders can result in a cascade of genetic changes leading to evolutionary differentiation was first developed by Mayr (1) in his seminal ''genetic revolution'' model. In this model, the key role of the founder event is to reduce levels of heterozygosity that subsequently affect the nature of coadapted gene complexes (1). Other founder-effect models have since been developed (2, 3), leading to much controversy surrounding the role of founder events in population differentiation (4-10).One way to estimate the likelihood that founder events play an important role in natural island systems is to determine whether neutral genetic changes occur abruptly (by means of initial founder events) or in a more gradual manner (by long-term drift and new mutations). The rate of genetic change can be estimated by comparing the level of neutral genetic variation in island populations that have been established over a range of time periods. Recently founded populations can be used to gauge the relative impact of drift associated with the founding event itself, whereas older populations will bear additional genetic consequences of persisting as relatively small populations over evolutionary time.
.— 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.
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