Genetic variation, physiology, and survival in a natural population of pocket gophers

In recent years, theoretical and laboratory studies have investigated the significance of genetic variation to the viability of animal populations. Although these studies suggest that loss of genetic variation is rarely beneficial, there is little empirical evidence to indicate how genetic variation can affect fitness under natural conditions. Here the relationship between genetic variation and several components of fitness in the pocket gopher (Thomomys bottae ) is examined. This study shows that in populations with extremely low levels of genetic variation, skin grafts transferred between individuals (within-population grafts) were not rejected, whereas in a high genetic variation population, such grafts were consistently rejected. These low genetic variation individuals were capable of mounting an immune response, however, as shown by the rejection of all between-population skin grafts. This study also shows that in a cohort of juvenile pocket gophers, individual physiological performance (growth rate, oxygen consumption while burrowing, and oxygen consumption while resting) was correlated with both a DNA-based estimate of individual genetic variation (DNA minisatellite variation) and allozyme heterozygosity. Specifically, individuals with high levels of genetic variation grew faster, used less oxygen during burrowing, and had an elevated oxygen consumption when resting and presumably digesting food. In addition, when these juvenile animals were released into a large semi-natural enclosure, individuals with high levels of genetic variation and superior physiological performance showed a greater probability of long-term (6 months and 10 months) survival. The results of this study suggests that populations with low levels of genetic variation may be at an increased risk of extinction because of a uniform immune response with all individuals equally susceptible (or resistant) to a particular pathogen. It also suggests that relatively heterozygous or genetically variable animals have superior physiological fitness and greater probability of survival in the wild. This does not necessarily mean that low genetic variation is the primary cause of recent extinction events. Nevertheless, as demonstrated here, genetic variation does influence fitness in natural populations, and as such should be considered an important factor for population viability.