Genetical Research

Inbreeding depression due to mildly deleterious mutations in finite populations: size does matter

a1 INRA-SGAP, Domaine de Melgueil, 34130 Mauguio, France
a2 Génétique & Environnement – CC065, Institut des Sciences de l'Evolution, Université Montpellier II, 34095 Montpellier, France


We studied the effects of population size on the inbreeding depression and genetic load caused by deleterious mutations at a single locus. Analysis shows how the inbreeding depression decreases as population size becomes smaller and/or the rate of inbreeding increases. This pattern contrasts with that for the load, which increases as population size becomes smaller but decreases as inbreeding rate goes up. The depression and load both approach asymptotic limits when the population size becomes very large or very small. Numerical results show that the transition between the small and the large population regimes is quite rapid, and occurs largely over a range of population sizes that vary by a factor of 10. The effects of drift on inbreeding depression may bias some estimates of the genomic rate of deleterious mutation. These effects could also be important in the evolution of breeding systems in hermaphroditic organisms and in the conservation of endangered populations.

(Received February 25 1999)
(Revised May 19 1999)

c1 Corresponding author. e-mail: Tel: (+33) 4-67-29-06-20. Fax: (+33) 4-67-29-39-90.
p1 Present address: Department of Zoology, University of Texas, Austin, Texas 78712, USA.