L. Nunney, THE INFLUENCE OF VARIATION IN FEMALE FECUNDITY ON EFFECTIVE POPULATION-SIZE, Biological Journal of the Linnean Society, 59(4), 1996, pp. 411-425
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