ABSTRACT

The goal of this chapter is to review inbreeding and outbreeding depression in the context of habitat fragmentation and to show how smaller, fewer populations of any organism separated by distance may exasperate the effects of these two genetic phenomena. We review the genetic basis of each, provide examples, and discuss specific empirical issues that need to be addressed in future research. We conclude with an illustrative case study of how both genetic phenomena can act simultaneously in a single species.

INTRODUCTION

Most rare and endangered species exist as small, isolated populations (Holsinger & Gottlieb, 1989). Unfortunately this seems to be the fate of even common species as natural populations are becoming increasingly fragmented. Fragmentation reduces the number of breeding individuals within a population while reducing gene flow between populations. Consequently, mating between individuals in fragmented populations is more likely to represent selfing (if genetically feasible) and/or biparental inbreeding (matings between related individuals) resulting in inbred offspring. The deleterious consequences of inbreeding are manifold. Inbred progeny may suffer from inbreeding depression, i.e. a decline in fitness, where the relative performance of the resulting inbred progeny is lower compared to progeny produced from matings between unrelated individuals within a population (Falconer & Mackay, 1996). Continued inbreeding associated with small populations also results in the loss of within-population genetic diversity (e.g. Schoen & Brown, 1991). Genetic diversity may influence the colonising ability and persistence of a population (Barrett & Kohn, 1991; Lande, 1994). Decreased genetic diversity may also be associated with increased susceptibility to pathogens and pests (Frankham, 19951b).