Gene flow can promote as well as limit local adaptation
Although many populations are strikingly well matched to their environments, others are not. Gene flow is one of the factors that can both promote and limit the extent to which a population is adapted to its local environment.
For example, some plant species have tolerant genotypes that can grow on soils at former mine sites containing high concentrations of heavy metals; such soils are toxic to intolerant genotypes. On “normal” soils off of the mine wastes the tolerant genotypes grow poorly compared with the intolerant genotypes. Thus, we would expect the frequencies of tolerant genotypes to approach 100% on mine soils (where they are advantageous) and 0% on normal soils (where they are disadvantageous). Researchers found that a population of the bentgrass Agrostis tenuis growing on mine soils was dominated by tolerant genotypes, as expected. However, a population growing on normal soils downwind from the mine site contained more tolerant genotypes than expected (McNeilly 1968). Bentgrass is wind-pollinated, and each year, pollen from the plants growing on mine soils carried alleles for heavy metal tolerance into the population growing on normal soils, preventing that population from becoming fully dominated by intolerant genotype. The population growing on mine soils also received pollen from plants growing on normal soils. In this population, however, gene flow had relatively little effect on allele frequencies, because selection against intolerant genotypes was so strong (they survived poorly on mine soils). In general, whenever alleles are transferred between populations that live in different environments, the extent to which adaptive evolution occurs in each population depends on whether natural selection is strong enough to overcome the effects of ongoing gene flow.
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