## Gene flow and genetic drift

In the absence of gene flow, conspecific populations will generally diverge from one another as a result of genetic drift. However, very little gene flow is necessary to reduce the rate of genetic drift and thereby prevent substantial population subdivision. Consider once more the relationship between gene flow and population differentiation (FST). Earlier in this chapter we learned that when populations are in equilibrium, FSt = l/(4Nem + 1). 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). With only one migrant every fourth generation (Nem = 0.25), Fst will be reduced to 0.5. If one migrant moves between a pair of populations every generation (Nem = 1), then FST = 0.20. Since FST is a measure of the degree

Figure 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). The FST values are reduced rapidly even with low levels of gene flow

Figure 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). The FST values are reduced rapidly even with low levels of gene flow 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. 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).

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). 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). The importance of gene flow to the genetic diversity -- and hence the survival -- of threatened or endangered populations was illustrated by a study of Morelet's

Table 4.9 Some of the ways in which gene flow (and lack thereof) can influence population genetics. Gene flow tends to be positively correlated with genetic diversity and Ne, and negatively with the rate of genetic drift within populations and the extent of genetic differentiation among populations

Connected populations Isolated populations (gene flow) (no gene flow)

Genetic drift # "

Genetic diversity " #

Population differentiation # "

crocodile (Crocodylus moreletii) in Belize, Central America. In the 1960s, Morelet's crocodile declined to critically low levels because of intense hunting pressure, although populations now seem to be recovering. Despite this recent bottleneck, microsatellite analysis of crocodiles from seven populations in north-central Belize revealed surprisingly high levels of within-population genetic diversity (He = 0.49), plus low levels of among-population genetic differentiation. Both of these findings are at least partially attributable to high levels of migration (approximately five migrants per generation among localities). One of the populations included in this study, New River/New River Lagoon, is located in a relatively large and undisturbed area, and it appears that this population is acting as a genetic reservoir for the region. Regular dispersal from the New River Lagoon to other sites is increasing genetic diversity and reducing genetic drift within smaller regional populations (Dever et al., 2002).

Northern pocket gopher Montane vole

Northern pocket gopher Montane vole

2500 2000 1500 1000 500 0 Years before present

Figure 4.12 Changes in haplotype diversity over time for montane vole and northern pocket gopher populations. Adapted from Hadly et al. (2004)

2500 2000 1500 1000 500 0 Years before present

Figure 4.12 Changes in haplotype diversity over time for montane vole and northern pocket gopher populations. Adapted from Hadly et al. (2004)

Box 4.4 Climate change, gene flow and genetic diversity

Despite its growing relevance, the effects that climate change may have on the genetic diversity of populations have seldom been empirically evaluated. Part of the difficulty lies with obtaining data from a sufficiently long time period. In one study, a group of researchers managed to obtain samples dating from between 2525 and 166 years before present (BP) for the montane vole (Microtus montanus) and the northern pocket gopher (Thomomys talpoides) (Hadly et al., 2004). The authors of this study were interested in how the abundance and genetic diversity within populations of these two species had changed in Yellowstone National Park over the past 2500 years. They addressed this question by comparing abundance estimates from the fossil record with mtDNA sequences, an approach that effectively compared long-term population dynamics with genetic variation (measured as haplotype diversity). Both species prefer wet habitats, and this is reflected in their relative abundance in the fossil record during wet periods and their decline during dry periods. The decline of both species was particularly pronounced between 1438 and 470 years BP, a time that spanned the Medieval Warm Period. In the pocket gopher population this led to a decrease in haplotype diversity, whereas the opposite was true in the montane vole population (Figure 4.12).

These differences in genetic diversity have been attributed to contrasting levels of gene flow, because the two species differ in their dispersal patterns: montane voles move regularly between populations, whereas pocket gophers are highly territorial and seldom disperse. In the dispersive montane vole, gene flow increased during the Medieval Warm Period and as a result, a number of new haplotypes were introduced into the population. On the other hand, the pocket gopher population remained largely isolated, and a drop in abundance led to a decrease in haplotype diversity. Since genetic diversity is important to the long-term survival of populations, dispersal behaviour is likely to be one factor that determines which populations will survive climate change and other environmental disturbances in the future.