Even low levels of immigration can introduce new genotypes and increase the effective size, and hence genetic diversity, of local populations (see Box 4.4). ![]() Since gene flow reduces the rate of genetic drift, it stands to reason that, all else being equal, isolated populations will have a lower Ne, a higher rate of drift, and lower genetic variation compared with populations that receive immigrants (Table 4.9). In fact, theory predicts that Nem values as low as one per generation may be sufficient to prevent the differentiation of populations by genetic drift (Wright, 1931). Of inbreeding within a local population relative to the collective population, and reflects the probability that two alleles drawn at random from within a population are identical by descent, an FST value of 0.2 means that the local population is only 20 per cent more inbred than the collective population, even though gene flow is still very low. The FST values are reduced rapidly even with low levels of gene flow Since FST is a measure of the degreeįigure 4.11 Comparison of the genetic differentiation between populations (FST) and the accompanying indirect estimates of gene flow (Nem), based on the relationship FST = 1/(4Nem+1). If one migrant moves between a pair of populations every generation (Nem = 1), then FST = 0.20. With only one migrant every fourth generation (Nem = 0.25), Fst will be reduced to 0.5. From this equation we can see that although populations are maximally divergent when Nem = 0 and FST =1, even small increases in gene flow (Nem) will markedly reduce population differentiation (Fst) (Figure 4.11). ![]() Earlier in this chapter we learned that when populations are in equilibrium, FSt = l/(4Nem + 1). Consider once more the relationship between gene flow and population differentiation (FST). ![]() However, very little gene flow is necessary to reduce the rate of genetic drift and thereby prevent substantial population subdivision. In the absence of gene flow, conspecific populations will generally diverge from one another as a result of genetic drift.
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