Abstract
Founder effects during colonization of a novel environment are expected to change the genetic composition of populations, leading to differentiation between the colonizer population and its source population. Another expected outcome is differentiation among populations derived from repeated independent colonizations starting from the same source. We have previously detected significant founder effects affecting rate of laboratory adaptation among Drosophila subobscura laboratory populations derived from the wild. We also showed that during the first generations in the laboratory, considerable genetic differentiation occurs between foundations. The present study deepens that analysis, taking into account the natural sampling hierarchy of six foundations, derived from different locations, different years and from two samples in one of the years. We show that striking stochastic effects occur in the first two generations of laboratory culture, effects that produce immediate differentiation between foundations, independent of the source of origin and despite similarity among all founders. This divergence is probably due to powerful genetic sampling effects during the first few generations of culture in the novel laboratory environment, as a result of a significant drop in Ne. Changes in demography as well as high variance in reproductive success in the novel environment may contribute to the low values of Ne. This study shows that estimates of genetic differentiation between natural populations may be accurate when based on the initial samples collected in the wild, though considerable genetic differentiation may occur in the very first generations of evolution in a new, confined environment. Rapid and significant evolutionary changes can thus occur during the early generations of a founding event, both in the wild and under domestication, effects of interest for both scientific and conservation purposes.
Highlights
Population size plays a key role in determining the relative importance of natural selection and genetic drift, with small isolated populations more exposed to stochastic loss of genetic variability, potentially reducing their subsequent response to selection (Robertson 1960)
High and consistent genetic variability among initial samples collected from the wild Genetic variability was very high for all founder populations and in total not significantly different between them, indicating negligible initial sampling effects across multiple collections from the wild
The lack of differentiation between founders from Sintra and Arrábida suggests that there is extensive gene flow between the two locations, suggesting in turn that they are a single deme. This natural population has maintained remarkably stable genetic composition for neutral markers across years and seasons involved in the two sampling points of this study
Summary
Population size plays a key role in determining the relative importance of natural selection and genetic drift, with small isolated populations more exposed to stochastic loss of genetic variability, potentially reducing their subsequent response to selection (Robertson 1960). Quantitative genetic models show that a strong bottleneck may increase the evolutionary potential of a population, due to conversion of nonadditive into additive component of genetic variance (Goodnight 1988). This may lead to divergence from the ancestral population. Empirical evidence of such increases in evolutionary potential after a bottleneck comes from several species and traits (see Bryant et al 1986; Regan et al 2003).
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