The natural habitats of many species have become fragmented into small "islands," principally by human activities. In this paper we discuss the long-term genetic and evolutionary consequences of fragmentation as inferred from studies on populations that have undergone natural habitat fragmentation in the Ozark Mountains. The Ozarks are the highest land formation found in the midwestern United States. Because of the absence of major geographical barriers around the Ozarks, plants and animals from diverse parts of the continent have been able to invade the area during post-Pleistocene climatic periods. Many of these invasions were short-lived, but the geological and topographical complexity of the Ozarks provided numerous relictual habitats. As a

The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks. Epistasis plays a critical role in a variety of theories of evolution and speciation, including Wright's (1978) shifting-balance theory, Schmalhausen's (1949) theory of stabilizing selection , Waddington's (1957) theory of canalization and genetic homeostasis, Mayr's (1963) concept of the unity of the genotype and genetic revolutions at speciation, Carson's (1968, 1982) founder-flush theory of speciation, and Templeton's (l980a,b) theory of genetic transilience. Although these theories are distinct (Carson and Templeton 1984; Barton and Charlesworth 1984), they also share much common ground in that all of them are based on evolution of phenotypes that result from the harmonious interaction of alleles at different loci. In these theories, epistasis is considered ubiquitous because of intense interaction among genes and their products during development. Stabilizing selection acts to build a balanced system of harmoniously interacting genes through coadaptation, or the adaptation of genes to their genetic environment (the collection of alleles present in the population at other, interacting loci). This harmonious system of epi-statically interacting genes creates phenotypic uniformity at a favored, intermediate, phenotypic value despite underlying genetic diversity. The phenotypic value of an allele is not constant, but changes depending on its genetic background. Given such a balanced system of gene interaction, evolutionary change becomes difficult because alterations at any one locus will usually affect the genotype as a whole in an adverse way, hence, Mayr's (1963) unity of the genotype. The evolutionary inertia that results from a coadapted system of interacting genes is adaptive when it is formed but needs to be dismantled if further adaptation is to occur. A potentially important means for disrupting the cohesion of the gene pool is by a founding event, where a population of smal...

The genotypic basis of morphological variation is largely unknown. In this study we examine patterns of pleiotropic effects on mandibular morphology at individual gene loci to determine whether the pleiotropic effects of individual genes are restricted to functionally and developmentally related traits. Mandibular measurements were obtained from 480 mice from the F generation of an intercross between the LG/J and SM/J mouse strains. DNA was also extracted from these animals, and 76 microsatellite loci covering the autosomes were scored. Interval mapping was used to detect chromosomal locations with significant effects on various mandibular measurements. Sets of traits mapping to a common chromosomal region were considered as being affected by a single quantitative trait locus (QTL) for mandibular morphology. Thirty-seven such chromosomal regions were identified spread throughout the autosomes. Gene effects were small to moderate with the allele derived from the LG/J strain typically leading to larger size. When dominance was present, the LG/J allele was typically dominant to the SM/J allele. Most loci affected restricted functional and developmental regions of the mandible. Of the 26 chromosomal regions affecting more than two traits, 50% affect the muscular processes of the ascending ramus, 27% affect the alveolar processes carrying the teeth, and 23% affect the whole mandible. Four additional locations affecting two traits had effects significantly associated with alveolar regions. Pleiotropic effects are typically restricted to morphologically integrated complexes.

The role of epistasis as a source of trait variation is well established, but its role as a source of covariation among traits (i.e., as a source of ''epistatic pleiotropy'') is rarely considered. In this study we examine the relative importance of epistatic pleiotropy in producing covariation within early and late-developing skull trait complexes in a population of mice derived from an intercross of the Large and Small inbred strains. Significant epistasis was found for several pairwise combinations of the 21 quantitative trait loci (QTL) affecting early developing traits and among the 20 QTL affecting late-developing traits. The majority of the epistatic effects were restricted to single traits but epistatic pleiotropy still contributed significantly to covariances. Because of their proportionally larger effects on variances than on covariances, epistatic effects tended to reduce within-group correlations of traits and reduce their overall degree of integration. The expected contributions of single-locus and two-locus epistatic pleiotropic QTL effects to the genetic covariance between traits were analyzed using a two-locus population genetic model. The model demonstrates that, for single-locus or epistatic pleiotropy to contribute to trait covariances in the study population, both traits must show the same pattern of single-locus or epistatic effects. As a result, a large number of the cases where loci show pleiotropic effects do not contribute to the covariance between traits in this population because the loci show a different pattern of effect on the different traits. In general, covariance patterns produced by single-locus and epistatic pleiotropy predicted by the model agreed well with actual values calculated from the QTL analysis. Nearly all single-locus and epistatic pleiotropic effects contributed positive components to covariances between traits, suggesting that genetic integration in the skull is achieved by a complex combination of pleiotropic effects.

Abstract.-The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks.Key words.-Additive genetic variance, epistasis, mouse, population bottlenecks, quantitative trait loci.Received January 9, 1995. Accepted July 6, 1995.Epistasis plays a critical role in a variety of theories of evolution and speciation, including Wright's (1978) shifting-balance theory, Schmalhausen's (1949) theory of stabilizing selection, Waddington's (1957) theory of canalization and genetic homeostasis, Mayr's (1963) concept of the unity of the genotype and genetic revolutions at speciation, Carson's (1968Carson's ( , 1982 founder-flush theory of speciation, and Templeton's (l980a,b) theory of genetic transilience. Although these theories are distinct (Carson and Templeton 1984; Barton and Charlesworth 1984), they also share much common ground in that all of them are based on evolution of phenotypes that result from the harmonious interaction of alleles at different loci.In these theories, epistasis is considered ubiquitous because of intense interaction among genes and their products during development. Stabilizing selection acts to build a balanced system of harmoniously interacting genes through coadaptation, or the adaptation of genes to their genetic environment (the collection of alleles present in the population at other, interacting loci). This harmonious system of epistatically interacting genes creates phenotypic uniformity at a favored, intermediate, phenotypic value despite underlying genetic diversity. The phenotypic value of an allele is not constant, but changes depending on its genetic background. Given such a balanced system of gene interaction, evolutionary change becomes difficult because alterations at any one locus will usually affect the genotype as a whole in an adverse way, hence, Mayr's (1963) unity of the genotype.The evolutionary inertia that results from a coadapted system of interacting genes is adaptive when it is formed but needs to be dism...