Search

In this paper, we use a model by Slatkin (1977) to investigate the genetic effects of extinction and recolonization for a species whose population structure consists of an array of local demes with some migration among them. In particular, we consider the conditions under which extinction and recolonization might enhance or diminish gene flow and increase or decrease the rate of genetic differentiation relative to the static case with no extinctions. We explicitly take into account the age‐structure that is established within the array of populations by the extinction and colonization process. We also consider two different models of the colonization process, the so‐called “migrant pool” and “propagule pool” models. Our theoretical studies indicate that the genetic effects of extinction and colonization depend upon the relative magnitudes of K,the number of individuals founding new colonies, and 2Nm,twice the number of migrants moving into extant populations. We find that these genetic effects are surprisingly insensitive to the extinction rate. We conclude that, in order to assess the genetic effects of the population dynamics, we must first answer an important empirical question that is essentially ecological: is colonization a behavior distinct from migration?

The genetic and ecological effects of population subdividsion were investigated for two wild strains of Tribolium castaneumand two wild strains of T. confusumand compared with the effects of population subdivision on the synthetic laboratory strain of T. castaneum(c‐SM), used extensively in earlier experiments. For the c‐SM strain, it has been shown repeatedly, for a variety of different population structures (different combinations of effective numbers, Ne, and migration rates, m), that large heritable differences in population growth rate arise among demes during 10 to 15 generations of population subdivision. Because this laboratory strain was synthesized by mass mating several “inbred” strains in 1973 (80 to 100 generations ago), it is possible that it has genetic variation for fitness (measured as the heritable variance among demes in the rate of population increase) unusually large compared to natural populations of flour beetles. In this paper, I report that natural populations of flour beetle exhibit as much or morephenotypic and genetic variation in the effects of population structure on fitness than the laboratory strain, c‐SM. The observation of substantial heritable variation for fitness in natural populations is unexpected under additive theory and may be indicative of nonadditive genetic variance.

The norm of reaction, the set of average phenotypes produced by a genotype in different environments, can be affected by spatial variation in natural selection especially when there exists genotype‐environment interaction. In subdivided populations, the greater the genotype‐environment interaction variance and the lower the migration rate, the more independent are the possible evolutionary trajectories for local adaptation. I examined genotype‐environment interaction in the rate of population increase for lineages randomly derived from a wild population of Tribolium castaneumacross a series of ecologically important environments. The lineages were derived from an outbred, wild‐caught population by 14 generations of random genetic drift, during which the effective size of each lineage was approximately 22 breeding adults. The environments studied were the classic temperate‐wet and cold‐dry climates of Park (1954) in factorial combination with two genetic strains of a congeneric competitor, T. confusum. Much among‐lineage genetic variation for rate of population increase was found for each of these ecologically important environments of climate and competition. Genotype‐environment interaction accounted for 40.5% of the total among‐lineage variance in rate of population increase signifying that the performance of a lineage in one environment is not necessarily a good predictor of its performance in another. Changing the genetic identity of the competitor changed the rate of increase of some lineages as much or more than changing the climatic conditions of temperature and humidity. This is the first empirical study to characterize the genotype‐environment interaction variance associated with genetic variation in a competing congeneric species. This competitor‐specific genetic variation in competitive ability may play an important role in coevolution in subdivided populations.

We discuss the necessary and sufficient conditions for identifying the cause of natural selection on a phenotypic trait. We reexamine the observational methods recently proposed for measuring selection in natural populations and illustrate why the multivariate analysis of selection is insufficient for identifying the causal agents of selection. We discuss how the observational approach of multivariate selection analysis can be complemented by experimental manipulations of the phenotypic distribution and the environment to identify not only howselection is operating on the phenotypic distribution but also whyit operates in the observed manner. A significant point of departure of our work from recent discussions is in regard to the role of the environment in the study of natural selection. Instead of viewing the environment as a source of unwanted variation that obscures the relationship between phenotype and fitness, we view fitness as arising from the interaction of the phenotype with the environment. The biotic and abiotic environment is the context that gives rise to the relationship between phenotype and fitness (selection). The analysis of the causes of selection is in essence a problem in ecology. The experimental study of the association between selection gradients and environmental characteristics is necessary to identify the agents of natural selection. We recommend research methods for identifying the agency of selection that depend upon a reciprocity between the observational approach of multivariate selection analysis and the manipulative approach of field experiments in evolutionary ecology.

The distribution of genetic variance within and among socially interacting groups, often quantified as the average relatedness or r̄, is an important determinant of the evolution of social behaviors. Models of social evolution often treat this average as a constant characteristic of a species. In this paper, we present data documenting the degree of temporal and spatial variation in the average relatedness of larval groups of imported willow leaf beetles, a species whose immatures display several primitive social behaviors. Collections of groups were made over three generations at three localities in Virginia and three localities in Illinois. Average relatedness was estimated from the distribution of electrophoretically determined genotypic frequencies within and among groups for each collection. Average relatedness ranged from 0.20 at one locality in Illinois to 0.65 at one locality in Virginia. Individual cases of pairwise differences between samples indicated that there were both temporal and locality effects. Further, statistical analysis showed the set of relatedness values to be heterogeneous with significant locality effects. Geographic genetic variance was also partitioned among trees sampled within localities in Illinois, among localities within each state, and between states. Notably, significant gene‐frequency variation among groups of beetles occupying closely spaced trees was detected at several localities.