THE INFLUENCE OF VARIATION IN FEMALE FECUNDITY ON EFFECTIVE POPULATION-SIZE

Understanding the relationship between effective population size (N-e) and the number of adults in a population (N) is important for predict ing genetic change in small populations. In general, N-e is expected t o be close to N/2, i.e. in the range N/4-3N/4, provided that the power ful effect of population bottlenecks on reducing N-e is factored out ( using the harmonic mean of N). However, some very low published estima tes of N-e/N(< 0.1) raise the possibility that other factors acting to reduce N-e have been underestimated. Here one such factor, variation in female fecundity, is investigated. Its effect on N-e, depends on th e standardized variance in fecundity (per breeding season), a measure that is generally independent of mean fecundity. Empirical estimates o f this standardized variance from 16 animal studies yielded an average value of 0.44, and a maximum Value less than 1.5. To investigate the effect of such values, three kinds of fecundity variation were modelle d: random (seasonal); individual; and age-related Fixed individual dif ferences among females reduce N-e the most. However, to reduce N-e, to N/10, the resulting standardized variance must usually be 10 or more. Random differences need to be even larger to achieve the same reducti on. One possible mechanism, the random loss of whole families, require s very high family mortality (90% or more). The third model, fecundity that increases linearly with age, is ineffective at causing a marked decrease in N-e. Given the finding that very unusual conditions are re quired to reduce N-e below N/10, low estimates of N-e/N need to be exa mined critically: the lowest published ratio, for a natural population of oysters, was found to be questionable because of possible immigrat ion into the population by cultivated oysters. (C) 1996 The Linmean So ciety of London