IMPORTANCE OF GENETIC-VARIATION TO THE VIABILITY OF MAMMALIAN POPULATIONS

Small populations lose genetic variability because of genetic drift, a nd inbreeding within populations can further decrease individual varia bility. Lower variation depresses individual fitness, resistance to di sease and parasites, and flexibility in coping with environmental chal lenges. Lower variation decreases mean fitness of populations (populat ion growth rates), resilience. and long-term adaptability. Genetic dri ft can threaten viability of populations not just by depleting variati on, but also by replacing natural selection as the predominant force d riving evolutionary change. Although most genetic studies use laborato ry or domesticated populations, evidence is accumulating that the effe cts of inbreeding are at least as severe on wild animals in natural ha bitats. Natural selection is expected to reduce the frequency of delet erious alleles in populations that persist through bottlenecks, but as yet there is little evidence for such purging of the genetic load in mammalian populations. No species of mammal has been shown to be unaff ected by inbreeding. Genetic problems are contributing to the decline and vulnerability of at least several mammalian ta,va. Genetic threats to population viability will be expressed through their effects on an d interactions with demographic and ecological processes. Theoretical analyses. experimental tests, field studies, and conservation actions should recognize the fundamental interdependency of generic and non-ge netic processes affecting viability of populations.