Genetic quality of individuals impacts population dynamics

Abstract

Ample evidence exists that an increase in the inbreeding level of a population reduces the value of fitness components such as fecundity and survival. It does not follow, however, that these decreases in the components of fitness impact population dynamics in a way that increases extinction risk, because virtually all species produce far more offspring than can actually survive. We analyzed the effects of the genetic quality (mean fitness) of individuals on the population growth rate of seven natural populations in each of two species of wolf spider in the genus Rabidosa, statistically controlling for environmental factors. We show that populations of different sizes, and different inbreeding levels, differ in population dynamics for both species. Differences in population growth rates are especially pronounced during stressful environmental conditions (low food availability) and the stressful environment affects smaller populations (o500 individuals) disproportionately. Thus, even in an invertebrate with an extremely high potential growth rate and strong densitydependent mortality rates, genetic factors contribute directly to population dynamics and, therefore, to extinction risk. This is only the second study to demonstrate an impact of the genetic quality of individual genotypes on population dynamics in a wild population and the first to document strong inbreeding-environment interactions for fitness among populations. Endangered species typically exist at sizes of a few hundred individuals and human activities degrade habitats making them innately more stressful (e.g. global climate change). Therefore, the interaction between genetic factors and environmental stress has important implications for efforts aimed at conserving the Earth's biodiversity.

Analysis of the effects of inbreeding on lifespan and starvation resistance in Drosophila melanogaster

“…Ultimately, in small and isolated populations inbreeding and genetic drift tend to decrease genetic variability Buskirk and Willi 2006). The extent to which genetic diversity is lost and characters displaced away from the selective optima are important for individual fitness as well as for population dynamics and viability (Buskirk and Willi 2006;Willi et al 2006;Hughes et al 2008;Vandewoestijne et al 2008;Reed et al 2007a). Despite the evolved behavioral and genetic mechanisms to avoid inbreeding, mating among relatives occurs regularly in many populations of wild species and it is therefore fundamental to understand the causes and patterns of inbreeding depression, the decline in trait values as a consequence of mating among relatives (Pusey and Wolf 1996;Crnokrak and Roff 1999;Hedrick and Kalinowski 2000;Frankham et al 2002;Keller and Waller 2002;O'Grady et al 2006).…”

The need for a better understanding of inbreeding effects on population growth

“…That inbreeding can potentially reduce population growth rates and increase extinction risks was demonstrated by influential studies of experimental plant populations (Newman & Pilson, 1997) and a butterfly metapopulation (Saccheri et al, 1998) and, more recently, by studies that demonstrate how experimentally restored immigration rapidly reverses negative population growth rates of inbred populations (Hogg et al, 2006). By demonstrating a link between population size (as a proxy of genetic variation) and population growth rates Reed, Nicholas & Stratton's (2007) study adds to the growing evidence that inbred populations may experience reduced population growth rates.…”

“…The study of Saccheri et al (1998) fits this expectation, since the Glanville fritillary butterflies on Å land form part of a metapopulation that is regulated at the metapopulation level. The two spider species studied by Reed et al (2007), on the other hand, do not seem to fit the expectations: there was evidence for density dependence in survival. However, the populations of Rabidosa rabida exchange up to 1.5 migrants per generation, suggesting that they might form part of a metapopulation.…”

Conservation genetics of American crocodile, Crocodylus acutus, populations in Pacific Costa Rica

“…Maintenance of genetic diversity is of increasing importance in the preservation of threatened and endangered species (Lacy 1997;Haig 1998;Reed and Frankham 2003;Reed et al 2007). Lack of genetic diversity can lead to inbreeding depression (Frankham 1995), decreased immunity (O'Brien et al 1985), decreased reproductive performance (O'Brien et al 1985;Parker et al 1991) and eventual extinction (Frankham 2005).…”