Derrick W. Sugg

Population genetics meets behavioral ecology.

Populations are often composed of more than just randomly mating subpopulations - many organisms from social groups with distinct patterns of mating and dispersal. Such patterns have recieved much attention in behavioral ecology, yet theories of population genetics rarely take social structures into account. Consequently, population geneticists often report high levels of apparent in breeding and concomitantly low efective sizes, even for species that avoid mating between close kin. Recently, a view of gene dynamics has been introduced that takes dispersal and social structure into account. Accounting for social structure in population genetics leads to a different perspective on how genetic variation is partitoned and the rate at which genic diversity is lost in natural populations - a view that is more consistent with observed behaviors for the minimization of inbreeding.

DO BLACK-TAILED PRAIRIE DOGS MINIMIZE INBREEDING ?

Considerable controversy surrounds the importance of inbreeding in natural populations. The rate of natural inbreeding and the influences of behavioral mechanisms that serve to promote or minimize inbreeding (e.g., philopatry vs. dispersal) are poorly understood. We studied inbreeding and social structuring of a population of black‐tailed prairie dogs (Cynomys ludovicianus) to assess the influence of dispersal and mating behavior on patterns of genetic variation. We examined 15 years of data on prairie dogs, including survival and reproduction, social behavior, pedigrees, and allozyme alleles. Pedigrees revealed mean inbreeding coefficients (F) of 1–2%. A breeding‐group model that incorporated details of prairie dog behavior and demography was used to estimate values of fixation indices (F‐statistics). Model predictions were consistent with the minimization of inbreeding within breeding groups (“coteries,” asymptotic FIL = –0.18) and random mating within the subpopulation (“colony,” asymptotic FIS = 0.00). Estimates from pedigrees (mean FIL = –0.23, mean FIS = 0.00) and allozyme data (mean FIL = –0.21, mean FIS = –0.01) were consistent with predictions of the model. The breeding‐group model, pedigrees, and allozyme data showed remarkably congruent results, and indicated strong genetic structuring within the colony (FLS = 0.16, 0.19, and 0.17, respectively). We concluded that although inbreeding occurred in the colony, the rate of inbreeding was strongly minimized at the level of breeding groups, but not at the subpopulation level. The behavioral mechanisms most important to the minimization of inbreeding appeared to be patterns of male‐biased dispersal of both subadults and adults, associated with strong philopatry of females. Incest avoidance also occurred, associated with recognition of close kin via direct social learning within the breeding groups.

Small Mammals from the Most Radioactive Sites Near the Chornobyl Nuclear Power Plant

This study was designed to estimate the impact of pollution resulting from the meltdown of Reactor 4, Chornobyl, Ukraine, on the taxonomic diversity and abundance of small mammals in the surrounding area. Trap sites included the most radioactive areas within the 10-km exclusion zone, a site within the 30-km exclusion zone that received minimal radioactive pollution, and five sites outside of the 30-km exclusion zone. Within the exclusion zones, 355 specimens representing 11 species of small mammals were obtained, whereas 224 specimens representing 12 species were obtained from outside the exclusion zone. It is concluded that the diversity and abundance of the small-mammal fauna is not presently reduced at the most radioactive sites. Specimens from the most radioactive areas do not demonstrate aberrant gross morphological features other than enlargement of the spleen. Examination of karyotypes does not document gross chromosomal rearrangements.

Breeding Groups and Gene Dynamics in a Socially Structured Population of Prairie Dogs

Genetic substructuring of a colony of black-tailed prairie dogs ( Cynomys ludovicianus ) was examined using three different sources of information: allozyme alleles, pedigrees, and demography (a “breeding-group” model based on mating and dispersal patterns). Prairie dogs and their social breeding groups (called “coteries”) were studied under natural conditions during a 15-year period. Prairie-dog coteries exhibited substantial genetic differentiation, with 15–20% of the genetic variation occurring among coteries. Mating patterns within the colony approximated random mating, and, thus, mates tended to originate from different coteries. Social groups of black-tailed prairie dogs resulted in genetic substructuring of the colony, a conclusion that was supported by estimates from allozyme alleles and colony pedigrees. Predictions of the breeding-group model also were consistent with and supported by estimates from allozyme and pedigree data. Some methodological problems were revealed during analyses. Although individuals of all ages usually are pooled for biochemical estimates of among-group genetic differentiation, our estimates of among-coterie variation from allozyme data were somewhat higher for young than for older prairie dogs, perhaps due to sampling effects caused by mating patterns and infanticide of offspring. Pedigree estimates of among-coterie genetic differentiation were significantly positive for young prairie dogs, adult females, and adult males. Those estimates were always more accurate for the offspring generation, however, because pedigree data were always more complete for young and genetic differences among coteries were diluted by virtually complete dispersal of males away from their natal coteries.

THE INFLUENCE OF SOCIAL BREEDING GROUPS ON EFFECTIVE POPULATION SIZE IN BLACK-TAILED PRAIRIE DOGS

Abstract Effective population sizes reported in the literature typically range from a small fraction of the adult population to about half the number of breeding adults. Theoretically, however, social structuring of genetic diversity could produce effective sizes as great as or even greater than population size. A colony of the highly social black-tailed prairie dog (Cynomys ludovicianus) was studied in the field for 16 years, and data were gathered for estimation of effective population sizes from pedigrees, demography, and allozyme alleles. Social breeding groups (“coteries”) within the colony exhibited high correlations of genes among individuals, and different coteries exhibited substantial genetic differentiation. Genetic diversity thus occurred within individuals, within coteries, and among coteries, and shifted among these levels of organization over time. “Instantaneous” estimates of effective size from short-term (annual) changes in genetic correlations were calculated from pedigree information but were not useful because they produced a wide diversity of estimates, due in part to the lack of demographic and genetic equilibrium in the colony. “Asymptotic” measures of effective population size that assumed eventual genetic equilibrium yielded relatively consistent estimates of effective sizes. For 10 years of empirical results from prairie dogs, effective population sizes from pedigrees (harmonic mean = 79.4), demographic model based on breeding groups (asymptote = 88.5), and allozyme data (harmonic mean = 88.9) were similar, and all were somewhat higher than the number of adults in the population (harmonic mean = 74.1). The colony of prairie dogs, therefore, exhibited a lower rate of loss of genetic diversity than expected, due to the genetic substructure created by the presence of social breeding groups.