Differences In Selection Regimes Research Articles

Contextual Constraints: Dynamic Evolution of Snake Venom Phospholipase A2

Venom is a dynamic trait that has contributed to the success of numerous organismal lineages. Predominantly composed of proteins, these complex cocktails are deployed for predation and/or self-defence. Many non-toxic physiological proteins have been convergently and recurrently recruited by venomous animals into their toxin arsenal. Phospholipase A2 (PLA2) is one such protein and features in the venoms of many organisms across the animal kingdom, including snakes of the families Elapidae and Viperidae. Understanding the evolutionary history of this superfamily would therefore provide insight into the origin and diversification of venom toxins and the evolution of novelty more broadly. The literature is replete with studies that have identified diversifying selection as the sole influence on PLA2 evolution. However, these studies have largely neglected the structural/functional constraints on PLA2s, and the ecology and evolutionary histories of the diverse snake lineages that produce them. By considering these crucial factors and employing evolutionary analyses integrated with a schema for the classification of PLA2s, we uncovered lineage-specific differences in selection regimes. Thus, our work provides novel insights into the evolution of this major snake venom toxin superfamily and underscores the importance of considering the influence of evolutionary and ecological contexts on molecular evolution.

Contrasting post-settlement selection results in many-to-one mapping of high performance phenotypes in the Hawaiian waterfall-climbing goby Sicyopterus stimpsoni

Natural selection drives adaptive evolution, but contrasting environmental pressures may lead to trade-offs between phenotypes that confer different performances. Such trade-offs may weaken the strength of selection and/or generate complex fitness surfaces with multiple local optima that correspond to different selection regimes. We evaluated how differences in patterns of phenotypic selection might promote morphological differences between subpopulations of the amphidromous Hawaiian waterfall-climbing goby, Sicyopterus stimpsoni. We conducted laboratory experiments on fish from the islands of Kaua‘i and Hawai‘i (the “Big Island”) to compare patterns of linear and nonlinear selection, and the opportunity for selection, that result from two contrasting pressures, predator evasion and waterfall climbing, which vary in intensity between islands. We found directional and nonlinear selection were strongest when individuals were exposed to their primary selective pressures (predator evasion on Kaua‘i, waterfall climbing on the Big Island). However, the opportunity for selection was greater for the non-primary pressure: climbing on Kaua‘i, predator evasion on the Big Island. Canonical rotation of the nonlinear gamma matrix demonstrated that individuals from Kaua‘i and the Big Island occupy regions near their local fitness peaks for some traits. Therefore, selection for predator evasion on Kaua‘i and climbing on the Big Island may be less effective in promoting morphological changes in this species, because variation of functionally important traits in their respective environments may have been reduced by directional or stabilizing selection. These results demonstrate that despite constraints on the opportunities for selection, population differences in phenotypic traits can arise due to differences in selective regimes. For S. stimpsoni, sufficient variation exists in other locomotor traits, allowing for necessary levels of performance in the contrasting selective regime (i.e., climbing on Kaua‘i and predator evasion on the Big Island) through many-to-one-mapping, which may be essential for the survival of local populations in an evanescent island environment.

Toxic hydrogen sulphide shapes brain anatomy: a comparative study of sulphide‐adapted ecotypes in the Poecilia mexicana complex

AbstractThe teleost brain is an energetically costly organ, which raises the question of how brain anatomy is shaped by divergent ecological factors in contrasting (extreme/resource‐limited vs. benign) environments. A previous study compared different ecotypes of the teleost Poecilia mexicana in the Tacotalpa drainage system and found that cave fish had a smaller eye diameter, a smaller optic tectum and larger telencephalic lobes relative to ancestral surface‐dwelling fish. Smaller eyes and a smaller optic tectum but larger telencephalic lobes were also found in fish from a sulphidic surface habitat near one of the caves, which the authors hypothesized to result from limited vision in turbid sulphide waters. In this study, we tested if repeated transitions along a replicated, natural toxicity gradient result in repeated (‘convergent’) anatomical changes of the teleost brain. We compared ecotypes in the P. mexicana species complex that have independently evolved increased tolerance to hydrogen sulphide (H2S) in three river drainages in southern Mexico, including a phylogenetically old H2S‐adapted form (P. sulphuraria) and two P. mexicana ecotypes that represent earlier stages of adaptation to H2S. All H2S‐adapted ecotypes exhibited smaller eyes, a smaller optic tectum volume and a smaller brain volume, but larger corpora cerebelli and hypothalamic volume than fish from non‐sulphidic habitats. Drainage‐specific effects were found for the telencephalic lobes, the total brain and eye size, as sexes responded differently to the presence of H2S depending on the drainage of origin. Turbidity and toxicity in sulphidic habitats may explain patterns of brain size divergence similar in direction (but not degree) to those observed in cave ecotypes. Hence, variation in brain anatomy reflects major ecological differences, and repeated ecological gradients can result in convergent differences in brain anatomy. Nonetheless, some unique patterns of brain differentiation suggest as yet unidentified differences in selection regimes between different sulphidic springs.

Phenotypic correlates of melanization in two Sceloporus occidentalis (Phrynosomatidae) populations: Behavior, androgens, stress reactivity, and ectoparasites

Mechanisms underlying production of animal coloration can affect key traits besides coloration. Melanin, and molecules regulating melanin, can directly and indirectly affect other phenotypic traits including aggression, stress-reactivity, and immune function. We studied correlation of melanization with these other traits, comparing within- and between-population differences of adult male western fence lizards, Sceloporus occidentalis. We compared one high- and one low-elevation population in California where individuals are increasingly darker at higher elevations, working during comparable periods of the breeding season at each site (first egg clutch). We measured agonistic behaviors of free-ranging males in response to staged territorial intrusions (STIs). In other sets of males we measured baseline testosterone and corticosterone levels, and hormonal-reactivity to a stress handling paradigm. We counted ectoparasite loads for all males. There were no significant associations between individual variation in melanization and individual variation in any of the variables measured. However, analysis of behavior from the STIs revealed that males in the darker high-elevation population responded with more aggressive behavior compared to males in the lighter low-elevation population. Males in the low-elevation population had significantly higher mean baseline testosterone, but the two populations did not differ in adrenal function (baseline corticosterone or corticosterone after 1-h confinement stress). Males in the darker high-elevation population had higher mean mite loads compared to males in the lighter population. This array of phenotypic differences between the two populations, and the absence of trait associations when assessing individual variation, do not parallel the patterns in other vertebrates. We describe potential differences in selective regimes that could produce these different patterns across vertebrates. These data suggest that hormonal pleiotropy does not constrain phenotypic variation.

Available Climate Regimes Drive Niche Diversification during Range Expansion.

Climate is a main predictor of biodiversity on a global scale, yet how climate availability affects niche evolution remains poorly explored. Here we assess how intercontinental climate differences may affect the evolution of climate niches and suggest three possible processes: niche truncation along major environmental gradients, intercontinental differences in available climate causing differences in selective regimes, and niche shifts associated with long-distance dispersals leading to a pattern of punctuated evolution. Using the globally distributed danthonioid grasses, we show significant niche differentiation among continents and several instances of niche truncation. The comparison of inferred selective regimes with differences in available climatic space among continents demonstrates adaptation resulting from opportunistic evolution toward available climatic space. Our results suggest that niche evolution in this clade is punctuated, consistent with accelerated niche evolution after long-distance dispersal events. Finally, we discuss how intrinsic constraints (genetic, developmental, or functional) and biotic interactions could have interacted with these three processes during range expansion. Integrating these mechanisms could improve predictions for invasive taxa and long-term evolutionary responses of expanding clades to climate change.

Constraint and adaptation in newt toll-like receptor genes.

Acute die-offs of amphibian populations worldwide have been linked to the emergence of viral and fungal diseases. Inter and intraspecific immunogenetic differences may influence the outcome of infection. Toll-like receptors (TLRs) are an essential component of innate immunity and also prime acquired defenses. We report the first comprehensive assessment of TLR gene variation for urodele amphibians. The Lissotriton newt TLR repertoire includes representatives of 13 families and is compositionally most similar to that of the anuran Xenopus. Both ancient and recent gene duplications have occurred in urodeles, bringing the total number of TLR genes to at least 21. Purifying selection has predominated the evolution of newt TLRs in both long (∼70 Ma) and medium (∼18 Ma) timescales. However, we find evidence for both purifying and positive selection acting on TLRs in two recently diverged (2–5 Ma) allopatric evolutionary lineages (Lissotriton montandoni and L. vulgaris graecus). Overall, both forms of selection have been stronger in L. v. graecus, while constraint on most TLR genes in L. montandoni appears relaxed. The differences in selection regimes are unlikely to be biased by demographic effects because these were controlled by means of a historical demographic model derived from an independent data set of 62 loci. We infer that TLR genes undergo distinct trajectories of adaptive evolution in closely related amphibian lineages, highlight the potential of TLRs to capture the signatures of different assemblages of pathogenic microorganisms, and suggest differences between lineages in the relative roles of innate and acquired immunity.

Reduced lifespan and increased ageing driven by genetic drift in small populations.

Explaining the strong variation in lifespan among organisms remains a major challenge in evolutionary biology. Whereas previous work has concentrated mainly on differences in selection regimes and selection pressures, we hypothesize that differences in genetic drift may explain some of this variation. We develop a model to formalize this idea and show that the strong positive relationship between lifespan and genetic diversity predicted by this model indeed exists among populations of Daphnia magna, and that ageing is accelerated in small populations. Additional results suggest that this is due to increased drift in small populations rather than adaptation to environments favoring faster life histories. First, the correlation between genetic diversity and lifespan remains significant after statistical correction for potential environmental covariates. Second, no trade-offs are observed; rather, all investigated traits show clear signs of increased genetic load in the small populations. Third, hybrid vigor with respect to lifespan is observed in crosses between small but not between large populations. Together, these results suggest that the evolution of lifespan and ageing can be strongly affected by genetic drift, especially in small populations, and that variation in lifespan and ageing may often be nonadaptive, due to a strong contribution from mutation accumulation.

Divergence in floral scent profiles among and within cultivated species of Phlox

Variation in floral scent profiles within and among plant species may be attributed to a number of different phenomena, including selection by pollinators or enemies, direct or indirect artificial selection in horticulture, stochastic processes such as genetic drift, and pleiotropy between scent and other floral traits, including floral color. The genus Phlox represents a unique opportunity to study scent variation across multiple species using horticultural specimens derived, in part, to generate color diversity. In this study, we assessed floral scent variation among horticultural cultivars of nine different Phlox species and the related species Polemonium caeruleum, as well as differences in scent variation between wild-originated and horticultural Phlox drummondii color morphs using dynamic headspace extraction and gas chromatography–mass spectroscopy. We identified significant variation in floral scent among species of Phlox, between color morphs pooled across species, and significant scent variation between cultivars showing different floral colors within the species P. subulata, P. stolonifera, P. carolina, and P. drummondii. We also uncovered significant differences between the volatile profiles of wild-originated and horticultural specimens of P. drummondii. Color morphs of horticultural P. drummondii differed in their scent, while morphs from wild-originated plants did not. This study documents rampant variation in floral scent composition within and among Phlox species and their horticultural cultivars. Species-level differences are likely to have been shaped by differences in selection regimes (both artificial selection in cultivation and natural selection in wild progenitors) and/or stochastic processes among groups. Differences among color morphs suggest some possibility of pleiotropy between color and scent. Species with particularly high levels of between-cultivar variation further highlight the potential for pleiotropic interactions between color and scent phenotypes, although no clear patterns connecting pigment to production of particular scent compounds emerge. Our finding that horticultural P. drummondii shows greater variation in scent profiles among color morphs suggests that either selection in wild populations minimizes scent differences between color morphs or relaxation of selection/incidental artificial selection on scent in horticultural settings allows accumulation of variation between color morphs.

Discordant patterns of evolutionary differentiation in two Neotropical treefrogs

Comparative studies of codistributed taxa test the degree to which historical processes have shaped contemporary population structure. Discordant patterns of lineage divergence among taxa indicate that species differ in their response to common historical processes. The complex geologic landscape of the Isthmus of Central America provides an ideal setting to test the effects of vicariance and other biogeographic factors on population history. We compared divergence patterns between two codistributed Neotropical frogs (Dendropsophus ebraccatus and Agalychnis callidryas) that exhibit colour pattern polymorphisms among populations, and found significant differences between them in phenotypic and genetic divergence among populations. Colour pattern in D. ebraccatus did not vary with genetic or geographic distance, while colour pattern co-varied with patterns of gene flow in A. callidryas. In addition, we detected significant species differences in the phylogenetic history of populations, gene flow among them, and the extent to which historical diversification and recent gene flow have been restricted by five biogeographic barriers in Costa Rica and Panama. We inferred that alternate microevolutionary processes explain the unique patterns of diversification in each taxon. Our study underscores how differences in selective regimes and species-typical ecological and life-history traits maintain spatial patterns of diversification.

Evolutionary genetics of ageing in the wild: empirical patterns and future perspectives

Summary 1 Classical evolutionary theory states that senescence should arise as a consequence of the declining force of selection late in life. Although the quantitative genetic predictions of hypotheses derived from this theory have been extensively tested in laboratory studies of invertebrate systems, relatively little is known about the genetics of ageing in the wild. 2 Data from long-term ecological studies is increasingly allowing quantitative genetic approaches to be used in studies of senescence in free-living populations of vertebrates. We review work to date and argue that the patterns are broadly consistent with theoretical predictions, although there is also a clear need for more empirical work. 3 We argue that further advances in this field of research might be facilitated by increased use of reaction norm models, and a decreased emphasis on attempting to discriminate between mutation accumulation and antagonistic pleiotropy models of senescence. We also suggest a framework for the better integration of environmental and genetic effects on ageing. 4 Finally, we discuss some of the difficulties in applying quantitative genetic models to studies of senescence outside the laboratory. In particular we highlight the problems that viability selection can cause for an accurate estimation of parameters used in the prediction of age-trajectory evolution.

EVOLUTIONARY RESPONSES OF NATIVE PLANTS TO NOVEL COMMUNITY MEMBERS

Both ecological and evolutionary processes can influence community assembly and stability, and native community members may respond both ecologically and evolutionarily as additional species enter established communities. Biological invasions provide a unique opportunity to examine these responses of native community members to novel species additions. Here, I use reciprocal transplant experiments among naturally invaded and uninvaded environments, along with experimental removals of exotic species, to determine whether exotic plant competitors and exotic insect herbivores evoke evolutionary changes in native plants. Specifically, I address whether the common native plant species Lotus wrangelianus has responded evolutionarily to a series of biological invasions by adapting to the presence of the exotic plant Medicago polymorpha and the exotic insect herbivore Hypera brunneipennis. Despite differences in selection regimes between invaded and uninvaded environments and the presence of genetic variation for traits relevant to the novel competitive and plant-herbivore interactions, these experiments failed to reveal evidence that Lotus has responded evolutionarily to the double invasion of Medicago followed by H. brunneipennis. However, when herbivory from H. brunneipennis was experimentally reduced, Lotus plants from source populations invaded by Medicago outperformed plants from uninvaded source populations when transplanted into heavily invaded destination environments. Therefore, Lotus showed evidence of adaptation to Medicago invasion but not to the newer invasion of an exotic shared herbivore. The presence of this exotic insect herbivore alters the outcome of evolutionary responses in this system and counteracts adaptation by the native Lotus to invasion by the exotic plant Medicago. This result has broad implications for the conservation of native communities. While native species may be able to adapt to the presence of one or a few exotics, a multitude of invasions may limit the ability of natives to respond evolutionarily to the novel and frequently changing selection pressures that arise with subsequent invasions.

EVOLUTIONARY RESPONSES OF NATIVE PLANTS TO NOVEL COMMUNITY MEMBERS

Both ecological and evolutionary processes can influence community assembly and stability, and native community members may respond both ecologically and evolutionarily as additional species enter established communities. Biological invasions provide a unique opportunity to examine these responses of native community members to novel species additions. Here, I use reciprocal transplant experiments among naturally invaded and uninvaded environments, along with experimental removals of exotic species, to determine whether exotic plant competitors and exotic insect herbivores evoke evolutionary changes in native plants. Specifically, I address whether the common native plant species Lotus wrangelianus has responded evolutionarily to a series of biological invasions by adapting to the presence of the exotic plant Medicago polymorpha and the exotic insect herbivore Hypera brunneipennis. Despite differences in selection regimes between invaded and uninvaded environments and the presence of genetic variation for traits relevant to the novel competitive and plant-herbivore interactions, these experiments failed to reveal evidence that Lotus has responded evolutionarily to the double invasion of Medicago followed by H. brunneipennis. However, when herbivory from H. brunneipennis was experimentally reduced, Lotus plants from source populations invaded by Medicago outperformed plants from uninvaded source populations when transplanted into heavily invaded destination environments. Therefore, Lotus showed evidence of adaptation to Medicago invasion but not to the newer invasion of an exotic shared herbivore. The presence of this exotic insect herbivore alters the outcome of evolutionary responses in this system and counteracts adaptation by the native Lotus to invasion by the exotic plant Medicago. This result has broad implications for the conservation of native communities. While native species may be able to adapt to the presence of one or a few exotics, a multitude of invasions may limit the ability of natives to respond evolutionarily to the novel and frequently changing selection pressures that arise with subsequent invasions.

The Inconsistent and Inefficient Constricting Behavior of Colorado Western Terrestrial Garter Snakes, Thamnophis elegans

The Inconsistent and Inefficient Constricting Behavior of Colorado Western Terrestrial Garter Snakes, Thamnophis elegans

The effects of hatchery and wild ancestry and experience on the relative ability of steelhead trout fry (Oncorhynchus mykiss) to avoid a benthic predator

Differences in selection regimes between hatchery and natural environments and environmental stimuli, among other factors, have the potential to cause differences in predator avoidance ability between hatchery and wild steelhead trout fry (Oncorhynchus mykiss). In two separate laboratory experiments, fry raised from eggs of wild Quinault River steelhead trout survived predation by prickly sculpin (Cottus asper) significantly better than size-matched offspring of a locally derived hatchery population, which were reared under similar conditions. Wild fry also survived predation better than hatchery fry in natural stream enclosures over a 3-day test period. Experience, in the form of 50-min visual exposure to sculpin predation on "sacrificial" steelhead trout, improved the ability of fry from both populations to avoid predation by sculpin. Wild-experienced fry were eaten in the fewest number of trials followed by wild-naive, hatchery-experienced, and hatchery-naive fry. The results of this study are consistent with the hypothesis that innate predator avoidance ability has been negatively altered through domestication and that attempts to condition hatchery-reared steelhead to avoid predators may be limited for domesticated populations.

Population differentiation in Pacific salmons: local adaptation genetic drift, or the environment?

Morphological, behavioral, and life-history differences between Pacific salmon (Oncorhynchus spp.) populations are commonly thought to reflect local adaptation, and it is likewise common to assume that salmon populations separated by small distances are locally adapted. Two alternatives to local adaptation exist: random genetic differentiation owing to genetic drift and founder events, and genetic homogeneity among populations, in which differences reflect differential trait expression in differing environments. Population genetics theory and simulations suggest that both alternatives are possible. With selectively neutral alleles, genetic drift can result in random differentiation despite many strays per generation. Even weak selection can prevent genetic drift in stable populations; however, founder effects can result in random differentiation despite selective pressures. Overlapping generations reduce the potential for random differentiation. Genetic homogeneity can occur despite differences in selective regimes when straying rates are high. In sum, localized differences in selection should not always result in local adaptation. Local adaptation is favored when population sizes are large and stable, selection is consistent over large areas, selective differentials are large, and straying rates are neither too high nor too low. Consideration of alternatives to adaptation would improve both biological research and salmon conservation efforts.

Phenotypic integration of life history and morphology: an example from three‐spined stickleback,Gasterosteus aculeatusL.

Although typically life history and morphology are studied separately, they may evolve most often in concert. Therefore, a full understanding of the evolution of fish life‐history patterns may require exploration of the evolutionary interplay between components of life history and other aspects of phenotype. One of the most promising approaches to understanding phenotypic integration is population comparison. This approach is particularly effective when ancestralderived relationships are understood and when multiple populations can be inferred to have evolved derived character states independently. Here we provide an example of this approach using five allopatric populations of freshwater three‐spined stickleback (Gasterosteus aculcatusL.) that have diverged in response to differences in selection regimes among the lakes they inhabit. We demonstrate a relationship between one aspect of reproductive life history, clutch volume and relative body shape. The differences are consistent with those predicted on the basis of differences in trophic habit and overall body form. Finally, we discuss the value of particular groups of fish for use in comparative studies and explore the kinds of evolutionary issues that can be addressed through population comparison.

SPATIAL VARIATION IN SELECTIVE REGIMES: SEXUAL SELECTION IN THE WATER STRIDER, GERRIS ODONTOGASTER.

Studies of phenotypic selection in natural populations are often concerned with simply detecting selection. In adopting a more mechanistic approach, this study compares the sexual selection regimes in natural populations of the water strider Gerris odontogaster with a priori predictions of selection, based on a number of previous field and laboratory studies of the behavioral mechanisms of selection. In this species, a general reluctance of females to mate allows for intersexual selection for ability to subdue reluctant females in males. Female reluctance to mate has been shown to decrease with increasing population density, suggesting that sexual selection should be weaker in high density populations. Three different populations with large differences in population density were studied. A number of traits including parasite load, body mass, body size and male abdominal process length were found to experience significant sexual selection. The investigated populations differed considerably with regard to the total strength of selection on the measured traits and the form of selection on single traits. In general, males in the population with the highest density experienced the weakest selection for grasping ability. This pattern is ascribed to density-related alterations of female mating behavior. Selection for male grasping ability, as reflected by selection on male abdominal process length, is reduced in high-density populations where reluctant females are more easily subdued. Further, the studied populations differed significantly in mean phenotype and phenotypic variance for male abdominal process length. It is suggested that interpopulational differences in selective regimes may generate local adaptations with respect to male abdominal process length, and that gene flow may contribute to the maintenance of the high genetic variation in this trait. It is further suggested that more empirical effort should be made in quantifying and understanding spatial and temporal variation in selection in natural populations, since this may provide information on the prevalence of local adaptations in metric traits and on the mechanisms of selection.

Phenetic Relationships among Populations of Podarcis Sicula and P. Melisellensis (Sauria: Lacertidae) from Islands in the Adriatic Sea

Clover, R. C. (Department of Biology, California State University, Long Beach, California 90840) 1979. Phenetic relationships among populations of Podarcis sicula and P. melisellensis (Sauria: Lacertidae) from islands in the Adriatic Sea. Syst. Zool. 28:284-298.-Thirtyone island populations of Podarcis sicula and P. melisellensis from Yugoslavia were examined to determine phenetic similarities based on means of 15 characters. Both cluster analysis and multidimensional scaling (MDS) revealed the following patterns of variation: 1) the two species formed distinct groups; 2) within each species, OTUs from northern and southern islands grouped separately; 3) within P. melisellensis, OTUs from small islands tended to resemble other small island OTUs from geographically close islands. These patterns appear to contradict similarities implied by previous subspecific designations based largely on color and pattern differences. Genetic drift, gene flow, and selection are evaluated as explanations for observed phenetic patterns. Drift appears to be highly unlikely as a major force determining regional, large island, and small island similarities among OTUs. Channel depths and estimates of the rate of net increase in sea level suggest that the northern islands are younger than the southern islands, supporting the idea that gene flow has been restricted among the southern islands and between the two regions for a longer time than among the northern islands. Combined with differences in selection regimes, the recency of gene flow appears to have been important in determining north-south phenetic similarities. Populations on small islands are exposed to qualitatively different selection regimes than those on large islands. The closer similarity of northern, small island OTUs to large island OTUs suggests that the northern OTUs have not responded to small island selection regimes to the same extent that southern island OTUs have. [Phenetics; Podarcis sicula; P. melisellensis; Yugoslavia; Adriatic Sea, island populations.] The taxonomic and ecological diversity of the lacertid lizards of the Mediterranean region make them favorable subjects for a variety of evolutionary studies. A number of studies have been reported, but the majority have concentrated on the taxonomy of geographic isolates. Of the 13 species of Podarcis (formerly in the genus Lacerta; Arnold, 1973) occurring in the region, more than 172 subspecies have been described, most of these being populations restricted to single islands (Mertens and Wermuth, 1960; Brelih, 1961; Lanza et al., 1971; Lanza and Capolongo, 1972). The extensive works on subspecific nomenclature have emphasized differences among populations showing geographic variation, but unfortunately may also have obscured patterns of similarity for which general explanations may exist (Wilson and Brown, 1953). Population biologists are especially interested in attempting to discover the underlying cause and effect networks responsible for widespread patterns of similarity since such patterns should be the result of the dynamic interactions of widespread ecological and evolutionary forces (Ehrlich and Holm, 1962). The nature and extent of phenetic variation among local populations can generally be attributed to gene flow, genetic drift, and natural selection, yet it is difficult to distinguish the relative importance of these forces in determining patterns of geographic variation in natural populations (Gould and Johnston, 1972). This may be possible by examination of phenetic relationships among insular populations of lizards, however, since rates of gene flow are extremely low, environmental heterogeneity varies as a