Inbreeding and Extinction in Island Populations: a Cautionary Note

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Extinction rates may appear generally higher on island than mainland populations, but the reason for this difference is controversial (e.g., Pimm 1991). Characteristically, small island populations are susceptible to demographic and environmental stochasticity (e.g., Pimm 1991), but genetic factors such as inbreeding and lack of genetic variation may also decrease their viability (e.g., Frankham 1995). For example, there is compelling evidence that inbreeding depresses individual reproductive success in typically out-breeding domestic plants and animals (e.g., Wright 1977; Falconer 1989; Frankham 1999) and captive populations of vertebrate wildlife (e.g., Ralls et al. 1988). But drawing general conclusions from individual species requires quantitative comparative analyses, rather than appeals to particular studies. Frankham (1998) addresses this issue by using interspecific comparative data to test several hypotheses derived from theoretical population genetics. In particular, Frankham (1998) claims that inbreeding coefficients are significantly higher in endemic than nonendemic island populations and that many island populations show levels of inbreeding associated with elevated extinction rates in domestic and laboratory species. Such interspecific comparative studies and meta-analyses of different populations are useful techniques to address apparently inconsistent empirical evidence (e.g., Harvey 1996; Frankham 1999). However, phylogenetic information should be taken into account in such analyses (Harvey & Pagel 1991), because closely related species share many traits as a result of common ancestory (Harvey 1996). Unfortunately, Frankham (1998) did not incorporate phylogenetic data into his analyses; hence, his conclusions may be misleading. We reanalyzed the data presented by Frankham (1998), correcting for taxonomic relationships. In particular, we explored the relationship between inbreeding and population viability and compared the inbreeding coeffi-