Epistasis and the Effect of Founder Events on the Additive Genetic Variance

Abstract

Models of founder events have focused on the reduction in the genetic variation following a founder event. However, recent work (Bryant et al., 1986; Goodnight, 1987) suggests that when there is epistatic genetic variance in a population, the total genetic variance within demes may actually increase following a founder event. Since the additive genetic variance is a statistical property of a population and can change with the level of inbreeding, some of the epistatic genetic variance may be converted to additive genetic variance during a founder event. The model presented here demonstrates that some of the additive-by-additive epistatic genetic variance is converted to additive genetic variance following a founder event. Furthermore, the amount of epistasis converted to additive genetic variance is a function of the recombination rate and the propagule size. For a single founder event of two individuals, as much as 75% of the epistatic variance in the ancestral population may become additive genetic variance following the founder event. For founder events involving two individuals with free recombination, the relative contribution of epistasis to the additive genetic variance following a founder event is equal to its proportion of the total genetic variance prior to the founder event. Traits closely related to fitness are expected to have relatively little additive genetic variance but may have substantial nonadditive genetic variance. Founder events may be important in the evolution of fitness traits, not because they lead to a reduction in the genetic variance, but rather because they lead to an increase in the additive genetic variance.

Offspring fitness in relation to population size and genetic variation in the rare perennial plant species Gentiana pneumonanthe (Gentianaceae)

“…These data correspond with the general finding that reduced heterozygosity does not only lower individual fitness, but may also increase the amount of variation among progeny since 'developmental homeostasis' is disturbed in homozygotes (Mitton and Grant 1984;Mitton 1989). Other studies have found that genetic variance increased rather than decreased after founder events or population bottlenecks (Goodnight 1988;Bryant et al 1986;Carson 1990). Small populations can therefore also be expected to show increased instead of reduced variation in fitness components among their progeny, which may be an explanation for the negative correlation coefficients between most of the coefficients of variation and population size that were found in this study.…”

Hallauer’s Tusón: a decade of selection for tropical-to-temperate phenological adaptation in maize

“…The sustained, although subtler, reduction in mean flowering time across generations, coupled with the inflection in generational variance (Figure 2) as the population advanced beyond physiologicalmodel expectations that would implicate photoperiodism (Table 2), suggested an additional adaptive mechanism was at play. Genetically, different allele frequency-dependent models could be invoked to hypothesize on the dynamics of change in this latter phase of selection (for example, with respect to increasing variance: Goodnight, 1988), and ongoing analysis using genotype data will hopefully permit such inference. However, at the level of the whole genotype (individual), variation in temperature-dependent flowering time has been noted for maize across the range of temperatures that occurred in the environments of the present study (Ellis et al, 1992), and analysis of environmental variables associated with gEI in this study revealed adaptation to low temperature as a relevant factor (Figure 6).…”

GENETIC VARIATION IN INBREEDING DEPRESSION IN THE RED FLOUR BEETLE TRIBOLIUM CASTANEUM

“…For example, in conservation, the loss of genetic variability that results from a period of reduced population size and the consequent loss of evolutionary po tential to respond to new selective pressures may be as im portant as the immediate manifestation of inbreeding de pression. However, theoretical (Goodnight 1987(Goodnight , 1988 and empirical (Bryant et al 1986a,b;Bryant and Meffert 1988;Carson and Wisotzkey 1989;Schwartz and Goodnight unpubl. data) studies demonstrate that nonadditive genetic vari ance in the ancestral population can lead to novel, unpre-dieted evolutionary change following a period of reduced population size.…”