Factors affecting levels of genetic diversity in natural populations

Abstract

Genetic variability is the clay of evolution, providing the base material on which adaptation and speciation depend. It is often assumed that most interspeci¢c di¡erences in variability are due primarily to population size e¡ects, with bottlenecked populations carrying less variability than those of stable size. However, we show that population bottlenecks are unlikely to be the only factor, even in classic case studies such as the northern elephant seal and the cheetah, where genetic polymorphism is virtually absent. Instead, we suggest that the low levels of variability observed in endangered populations are more likely to result from a combination of publication biases, which tend to in£ate the level of variability which is considered normal', and inbreeding e¡ects, which may hasten loss of variability due to drift. To account for species with large population sizes but low variability we advance three hypotheses. First, it is known that certain metapopulation structures can result in e¡ective population sizes far below the census size. Second, there is increasing evidence that heterozygous sites mutate more frequently than equivalent homozygous sites, plausibly because mismatch repair between homologous chromosomes during meiosis provides extra opportunities to mutate. Such a mechanism would undermine the simple relationship between heterozygosity and e¡ective population size. Third, the fact that related species that di¡er greatly in variability implies that large amounts of variability can be gained or lost rapidly. We argue that such cases are best explained by rapid loss through a genome-wide selective sweep, and suggest a mechanism by which this could come about, based on forced changes to a control gene inducing coevolution in the genes it controls. Our model, based on meiotic drive in mammals, but easily extended to other systems, would tend to facilitate population isolation by generating molecular incompatabilities. Circumstances can even be envisioned in which the process could provide intrinsic impetus to speciation.

Ancient DNA reveals Holocene loss of genetic diversity in a South American rodent

“…While low levels of genetic variation are often attributed to historical bottlenecks (Amos & Harwood 1998), few studies have tracked directly the genetic changes associated with such events, particularly over a millennial time-scale (but see Shapiro et al 2004). The reduction in genetic variation we have documented is particularly unusual given that (i) C. sociabilis is a rodent, rather than a megafaunal mammal, and (ii) no single environmental event (e.g.…”

High mutation rate and mutational bias at (TAA)n microsatellite loci in chickpea (Cicer arietinum L.)

“…The population size of a species might aect mutation rates at microsatellites (Rubinsztein et al 1995;Amos and Harwood 1998). In a larger population the frequency of heterozygote individuals is higher and this heterozygous condition might therefore increase instability of microsatellite loci (Rubinsztein et al 1995).…”

Genetic neighbourhood and effective population size in the endangered European mink Mustela lutreola

“…The decline in population results in an increase of inbreeding, which in turn may worsen the decline by depressing the fitness and restricting the evolutionary potential (Amos and Harwood 1998;Frankham 2003). The effective population size N e basically influences conservation biology of wildlife populations because the effective size determines the loss in genetic variability due to drift at a rate of 1=2N e per generation (Soulé 1976;Franklin 1980;Soulé 1987;Franklin and Frankham 1998).…”