When does conservation genetics matter?

Abstract

Is this short review we explore the genetic threats facing declining populations, focusing in particular on empirical studies and the emerging questions they raise. At face value, the two primary threats are slow erosion of genetic variability by drift and short-term lowering of ®tness owing to inbreeding depression, of which the latter appears the more potent force. However, the picture is not this simple. Populations that have passed through a severe bottleneck can show a markedly reduced ability to respond to change, particularly in the face of novel challenges. At the same time, several recent studies reveal subtle ways in which species are able to retain more useful genetic variability than they`should', for example by enhanced reproductive success among the most outbred individuals in a population. Such ®ndings call into question the validity of simple models based on random mating, and emphasize the need for more empirical data aimed at elucidating precisely what happens in natural populations.Keywords: endangered species, evolutionary potential, genetic diversity, heterozygosity, inbreeding depression, population managementThe perceived importance of genetic problems in the conservation of endangered species has¯uctuated considerably over the last two decades, and remains the subject of debate. An early high-pro®le case study reported that cheetahs have low levels of genetic variability, poor sperm quality and poor reproductive success in captivity (O'Brien et al., 1983;O'Brien et al., 1985;O'Brien et al., 1986). It was concluded that the species had suered a genetic bottleneck, stripping it of variability and leaving it prone to problems associated with inbreeding depression. Although later studies (Caro & Laurenson, 1994; Caughley, 1994; Merola, 1994; Caro, 2000) have revealed many inconsistencies and the story has now been largely rewritten, the cheetah project helped stimulate growing interest in the role of genetics in conservation.Today, many conservation studies include a genetic element, and the list of possible problems being considered has expanded to embrace loss of evolutionary potential, susceptibility to disease, mutational meltdown, and more. In this review we re-examine the main genetic threats faced by small and declining populations and discuss the empirical basis for deciding which of these are most likely to pose serious problems over the sorts of time-scales that concern conservation practitioners. In doing so, we have made a conscious eort to look to the future by speculating about areas and concepts which are only now coming to light.Of the various possible genetic problems which face a declining population, loss of genetic variability and inbreeding depression have historically received most attention (O'Brien, 1994). Although often treated as one and the same phenomenon, these two processes are in reality very dierent, and operate over radically dierent timescales. In general, variability is lost very slowly, usually over hundreds or thousands of generations. Rate of loss is i...

Historical connectivity, contemporary isolation and local adaptation in a widespread but discontinuously distributed species endemic to Taiwan, Rhododendron oldhamii (Ericaceae)

“…Maintenance of genetic variation in a population is important for long-term persistence, especially in the face of novel selective pressures (Amos and Balmford, 2001;Hedrick, 2004). In plants, genomic SSRs revealed much-higher genetic diversity than that revealed by EST-SSRs (for example, Chabane et al, 2005;Peleg et al, 2008).…”

Marker-based investigation of inbreeding depression in the endangered species Brassica insularis

“…The deleterious consequences of inbreeding in small populations have been much debated with regard to conservation issues (Hedrick and Kalinowski, 2000;Amos and Balmford, 2001;Keller and Waller, 2002). A high genetic load per se does not threaten a population as long as it is not expressed, if naturally occurring inbreeding levels are low.…”

Contrasting population structures in two sympatric anurans: implications for species conservation

“…The importance of genetic diversity to the long-term viability of wild plant and animal populations is increasingly well recognised (eg Amos and Balmford, 2001;Hedrick, 2001). Neutral theory predicts that a positive relationship should exist between effective population size (N e ) and the extent of genetic variation (allelic diversity and heterozygosity) at loci not subject to strong selection (Kimura, 1983).…”