Gene flow

摘要

Insect populations have provided many of the best studies of gene flow, a parameter which plays a central role in topics as diverse as population structure, adaptation (e.g. insecticide resistance), conservation genetics and speciation. In the literature, 'gene flow' often refers to N_em (effective population size x migration rate) estimated from frequencies of genetic markers, typically allozymes, by assuming an equilibrium between gene flow and neutral genetic drift. Ne_m and its continuous population analogue, 'neigh-bourhood population size', N_b, are actually both dimensionless parameters that measure the relative strength of gene flow and drift, rather than being simple measures of 'gene flow' or 'population size' alone. Inferring gene flow using this method therefore has many pitfalls. A more robust procedure is to exploit equilibria between gene flow and selection when suitable geographic variation in selection is present. New population genetic theory makes selection-based methods particularly useful, and selection is often a stronger force than genetic drift in many insect populations, leading to estimates with a lower margin of error. Finally, gene flow may also occur between sympatric host races or closely related species that hybridize, albeit rarely. I give examples from ongoing work with insects, especially fungus-eating beetles, Heliconius butterflies (Nymphalidae) and other Lepidoptera, Culex mosquitoes (Culicidae), and Drosophila (Drosophilidae). These newer methods provide the power to estimate actual gene flow (m, #sigma#_x~2), rather than merely dimensionless gene flow measures such as N_em and N_b.