Rare alleles and selection

A subpopulation D of rare alleles is considered. The subpopulation is part of a large population that evolves according to a Moran model with selectio n and growth. Conditional on the current frequency, q, of the rare allele, an approximation to the distribution of the genealogy of D is derived. In p articular, the density of the age, T-1, of the rare allele is approximated. It is shown that time naturally is measured in units of qN(0) generations, where N(0) is the present day population size, and that the distribution o f the genealogy of D, depends on the compound parameters rho= rqN(0) and si gma = sqN(0) only. Here, s is the fitness per generation of heterozygote ca rriers of the rare allele and r is the growth rate per generation of the po pulation. Amongst more, it is shown that for constant population size (rho= 0) the distribution of D depends on sigma only through the absolute value \ sigma \, not the direction of selection. (C) 2001 Academic Press.