Most species are not driven to extinction before genetic factors impact them

Abstract

There is controversy concerning the role of genetic factors in species extinctions. Many authors have asserted that species are usually driven to extinction before genetic factors have time to impact them, but few studies have seriously addressed this issue. If this assertion is true, there will be little difference in genetic diversity between threatened and taxonomically related nonthreatened species. We compared average heterozygosities in 170 threatened taxa with those in taxonomically related nonthreatened taxa in a comprehensive metaanalysis. Heterozygosity was lower in threatened taxa in 77% of comparisons, a highly significant departure from the predictions of the no genetic impact hypothesis. Heterozygosity was on average 35% lower (median 40%) in threatened taxa than in related nonthreatened ones. These differences in heterozygosity indicate lowered evolutionary potential, compromised reproductive fitness, and elevated extinction risk in the wild. Independent evidence from stochastic computer projections has demonstrated that inbreeding depression elevates extinction risk for threatened species in natural habitats when all other threatening processes are included in the models. Thus, most taxa are not driven to extinction before genetic factors affect them adversely.

An investigation of inbreeding depression and purging in captive pedigreed populations

“…inbreeding depression when brought into captivity, a possibility supported by the observation that threatened taxa have low heterozygosity compared with equivalent non-threatened taxa (Spielman et al, 2004). We tested whether island endemicity or IUCN threat rating were correlated with the detection of inbreeding depression across populations but found no relationships (data not shown).…”

Guidelines for planning genomic assessment and monitoring of locally adaptive variation to inform species conservation

“…Even in cases where the evidence for genetic adaptation is strong, management interventions should strive to conserve adaptive variation without eroding genomewide variation (Giglio, Ivy, Jones, & Latch, 2016; Haig, Ballou, & Derrickson, 1990; Spielman, Brook, & Frankham, 2004). Conversely, management actions designed to preserve genomewide variation may either involve risks of disrupting local adaptation to nonclimatic factors (e.g., biotic interactions, soils) if local adaptation is not well understood, or could result in outbreeding depression if individuals from long‐diverged populations are mixed and interbreed (see Frankham et al., 2011 for guidance on when this might occur).…”

Genetic characterization of the Balkan endemic species, Merodon desuturinus (Diptera: Syrphidae)

“…Very low levels of allozyme heterozygosity have been frequently found in threatened natural populations (for a review see: Frankham, 1995) indicating the important impact of inbreeding, which is presumed to increase the risk of extinction. Additionally, lower average heterozygosity in threatened taxa compared to closely related nonthreatened taxa is recorded (Spielman et al, 2004). M. desuturinus as a species with a low genetic variation and evolutionary potential would be expected to have a reduced ability to cope with environmental change and to survive climatic extremes, diseases and parasites.…”