Showing papers in "Evolution in 1997"

Stabilizing selection and the comparative analysis of adaptation.

Abstract: Comparative studies tend to differ from optimality and functionality studies in how they treat adaptation. While the comparative approach focuses on the origin and change of traits, optimality studies assume that adaptations are maintained at an optimum by stabilizing selection. This paper presents a model of adaptive evolution on a macroevolutionary time scale that includes the maintenance of traits at adaptive optima by stabilizing selection as the dominant evolutionary force. Interspecific variation is treated as variation in the position of adaptive optima. The model illustrates how phylogenetic constraints not only lead to correlations between phylogenetically related species, but also to imperfect adaptations. From this model, a statistical comparative method is derived that can be used to estimate the effect of a selective factor on adaptive optima in a way that would be consistent with an optimality study of adaptation to this factor. The method is illustrated with an analysis of dental evolution in fossil horses. The use of comparative methods to study evolutionary trends is also discussed.

Likelihood of ancestor states in adaptive radiation.

Abstract: Theories of ecological diversification make predictions about the timing and ordering of character state changes through history. These theories are testable by "reconstructing" ancestor states using phylogenetic trees and measurements of contemporary species. Here we use maximum likelihood to estimate and evaluate the accuracy of ancestor reconstructions. We present likelihoods of discrete ancestor states and derive probability distributions for continuous ancestral traits. The methods are applied to several examples: diets of ancestral Darwin's finches; origin of inquilinism in gall wasps; microhabitat partitioning and body size evolution in scrubwrens; digestive enzyme evolution in artiodactyl mammals; origin of a sexually selected male trait, the sword, in platies and swordtails; and evolution of specialization in Anolis lizards. When changes between discrete character states are rare, the maximum-likelihood results are similar to parsimony estimates. In this case the accuracy of estimates is often high, with the exception of some nodes deep in the tree. If change is frequent then reconstructions are highly uncertain, especially of distant ancestors. Ancestor states for continuous traits are typically highly uncertain. We conclude that measures of uncertainty are useful and should always be provided, despite simplistic assumptions about the probabilistic models that underlie them. If uncertainty is too high, reconstruction should be abandoned in favor of approaches that fit different models of trait evolution to species data and phylogenetic trees, taking into account the range of ancestor states permitted by the data.

Patterns of speciation in drosophila revisited

Abstract: In a paper published seven years ago in this journal (Coyne and Orr 1989a), we analyzed the time course of speciation in Drosophila by correlating electrophoretic genetic distance between pairs of species (a number roughly proportional to their divergence time) with the strength of reproductive isolation between them. That analysis yielded five conclusions. First, both prezygotic and postzygotic reproductive isolation increase with divergence time between taxa. Second, prezygotic (sexual) isolation evolves more rapidly than postzygotic isolation (sterility and inviability of hybrids). This difference is, however, due entirely to much stronger prezygotic isolation between sympatric than between allopatric pairs of species. We suggested that this difference was due to "reinforcement," or direct selection for sexual isolation that occurs among sympatric taxa that produce unfit hybrids (Dobzhansky 1937). Third, hybrid sterility and inviability evolve at similar rates. This conclusion now appears to be incorrect because average divergence time between taxa is not a sensitive way to measure evolutionary rates of reproductive isolation, and more sensitive analyses show that hybrid sterility may in fact evolve more rapidly than inviability (Wu 1992). Fourth, the usual pathway for the production of postzygotic isolation is the initial appearance of sterility or inviability in hybrid males, followed by its appearance in females. This explains the frequent observation of Haldane's rule: the pattern that if only one gender of hybrids is sterile or inviable in species crosses, it is nearly always the heterogametic (XY or XO) sex (Haldane 1922; Coyne and Orr 1989b). Finally, there is a large increase in genetic distance between those species pairs producing sterile or inviable males only and those producing sterile or inviable hybrids of both sexes. This implies that there is a long time lag between the evolution of postzygotic isolation in males and in females. While a similar (but much smaller) analysis has since been conducted in salamanders (Tilley et al. 1990), the data from Drosophila are unique-and are likely to remain so-because of the large number of crossable species and the ease of estimating sexual and postzygotic isolation in the laboratory. These Drosophila data have hence attracted some interest. Because of this, we have continued to accumulate new data as they have appeared. We have also found a few errors in our original data set, and have revised some estimates of reproductive isolation and phylogenetic relatedness when better data became available. We now have data for 171 interspecific hybridizations in Drosophila, an increase of 43% over the 119 hybridizations described in our previous paper. Because DNA sequencing has largely supplanted gel electrophoresis as a way of measuring divergence between species, it is unlikely that this data set will grow much larger; and it will be many years before we possess DNA-based estimates of divergence between many pairs of Drosophila species. We therefore thought it timely to check our earlier conclusions using the new and larger data set.

Interacting phenotypes and the evolutionary process: i. direct and indirect genetic effects of social interactions.

Abstract: Interacting phenotypes are traits whose expression is affected by interactions with conspecifics. Commonly- studied interacting phenotypes include aggression, courtship, and communication. More extreme examples of inter- acting phenotypes-traits that exist exclusively as a product of interactions-include social dominance, intraspecific competitive ability, and mating systems. We adopt a quantitative genetic approach to assess genetic influences on interacting phenotypes. We partition genetic and environmental effects so that traits in conspecifics that influence the expression of interacting phenotypes are a component of the environment. When the trait having the effect is heritable, the environmental influence arising from the interaction has a genetic basis and can be incorporated as an indirect genetic effect. However, because it has a genetic basis, this environmental component can evolve. Therefore, to consider the evolution of interacting phenotypes we simultaneously consider changes in the direct genetic contributions to a trait (as a standard quantitative genetic approach would evaluate) as well as changes in the environmental (indirect genetic) contribution to the phenotype. We then explore the ramifications of this model of inheritance on the evolution of interacting phenotypes. The relative rate of evolution in interacting phenotypes can be quite different from that predicted by a standard quantitative genetic analysis. Phenotypic evolution is greatly enhanced or inhibited depending on the nature of the direct and indirect genetic effects. Further, unlike most models of phenotypic evolution, a lack of variation in direct genetic effects does not preclude evolution if there is genetic variance in the indirect genetic contributions. The available empirical evidence regarding the evolution of behavior expressed in interactions, although limited, supports the predictions of our model.

Increased probability of extinction due to decreased genetic effective population size: experimental populations of clarkia pulchella.

Abstract: We established replicated experimental populations of the annual plant Clarkia pulchella to evaluate the existence of a causal relationship between loss of genetic variation and population survival probability. Two treatments differing in the relatedness of the founders, and thus in the genetic effective population size (Ne ), were maintained as isolated populations in a natural environment. After three generations, the low Ne treatment had significantly lower germination and survival rates than did the high Ne treatment. These lower germination and survival rates led to decreased mean fitness in the low Ne populations: estimated mean fitness in the low Ne populations was only 21% of the estimated mean fitness in the high Ne populations. This inbreeding depression led to a reduction in population survival: at the conclusion of the experiment, 75% of the high Ne populations were still extant, whereas only 31% of the low Ne populations had survived. Decreased genetic effective population size, which leads to both inbreeding and the loss of alleles by genetic drift, increased the probability of population extinction over that expected from demographic and environmental stochasticity alone. This demonstrates that the genetic effective population size can strongly affect the probability of population persistence.

Genetic models of adaptation and gene flow in peripheral populations.

Abstract: We investigate the interplay between gene flow and adaptation in peripheral populations of a widespread species. Models are developed for the evolution of a quantitative trait under clinally varying selection in a species whose density decreases from the center of the range to its periphery. Two major results emerge. First, gene flow from populations at the range center can be a strong force that inhibits peripheral populations from evolving to their local ecological optima. As a result, peripheral populations experience persistent directional selection. Second, response to local selection pressures can cause rapid and substantial evolution when a peripheral population is isolated from gene flow. The amount of evolutionary change depends on gene flow, selection, the ecological gradient, and the trait's heritability. Rapid divergence can also occur between the two halves of a formerly continuous population that is divided by a vicariant event. A general conclusion is that disruption of gene flow can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random genetic drift.

A population genetic theory of canalization.

Abstract: Canalization is the suppression of phenotypic variation. Depending on the causes of phenotypic variation, one speaks either of genetic or environmental canalization. Genetic canalization describes insensitivity of a character to mutations, and the insensitivity to environmental factors is called environmental canalization. Genetic canalization is of interest because it influences the availability of heritable phenotypic variation to natural selection, and is thus potentially important in determining the pattern of phenotypic evolution. In this paper a number of population genetic models are considered of a quantitative character under stabilizing selection. The main purpose of this study is to define the population genetic conditions and constraints for the evolution of canalization. Environmental canalization is modeled as genotype specific environmental variance. It is shown that stabilizing selection favors genes that decrease environmental variance of quantitative characters. However, the theoretical limit of zero environmental variance has never been observed. Of the many ways to explain this fact, two are addressed by our model. It is shown that a "canalization limit" is reached if canalizing effects of mutations are correlated with direct effects on the same character. This canalization limit is predicted to be independent of the strength of stabilizing selection, which is inconsistent with recent experimental data (Sterns et al. 1995). The second model assumes that the canalizing genes have deleterious pleiotropic effects. If these deleterious effects are of the same magnitude as all the other mutations affecting fitness very strong stabilizing selection is required to allow the evolution of environmental canalization. Genetic canalization is modeled as an influence on the average effect of mutations at a locus of other genes. It is found that the selection for genetic canalization critically depends on the amount of genetic variation present in the population. The more genetic variation, the stronger the selection for canalizing effects. All factors that increase genetic variation favor the evolution of genetic canalization (large population size, high mutation rate, large number of genes). If genetic variation is maintained by mutation-selection balance, strong stabilizing selection can inhibit the evolution of genetic canalization. Strong stabilizing selection eliminates genetic variation to a level where selection for canalization does not work anymore. It is predicted that the most important characters (in terms of fitness) are not necessarily the most canalized ones, if they are under very strong stabilizing selection (k > 0.2Ve ). The rate of decrease of mutational variance Vm is found to be less than 10% of the initial Vm . From this result it is concluded that characters with typical mutational variances of about 10-3 Ve are in a metastable state where further evolution of genetic canalization is too slow to be of importance at a microevolutionary time scale. The implications for the explanation of macroevolutionary patterns are discussed.

Experimental manipulation of putative selective agents provides evidence for the role of natural enemies in the evolution of plant defense.

Abstract: Although biologists have long assumed that plant resistance characters evolved under selection exerted by such natural enemies as herbivores and pathogens, experimental evidence for this assumption is sparse. We present evidence that natural enemies exert selection on particular plant resistance characters. Specifically, we demonstrate that elimination of natural enemies from an experimental field population of Arabidopsis thaliana alters the pattern of selection on genetic variation in two characters that have been shown to reduce herbivore damage in the field: total glucosinolate concentration and trichome density. The change in pattern of selection reveals that natural enemies imposed selection favoring increased glucosinolate concentration and increased trichome density, and thus, supports one of the major assumptions of the coevolution hypothesis. We also demonstrate that a pattern of stabilizing selection on glucosinolate concentration results from a balance between the costs and benefits associated with increasing levels of this resistance character. This result provides direct confirmation of the appropriateness of cost-benefit models for characterizing the evolution of plant defenses.

A performance constraint on the evolution of trilled vocalizations in a songbird family (passeriformes: emberizidae)

Abstract: Behavioral evolution can be influenced by constraints, for example, of phylogeny and performance. In this paper I describe a pattern in the evolution of birdsongs that may reflect a constraint on vocal performance. Trilled vocalizations from 34 species of songbirds (Passeriformes: Emberizidae) were analyzed. Two acoustic variables, trill rate and frequency bandwidth, were measured for different trill types. In most species, maximal values of frequency bandwidth were found to decrease with increasing trill rates. Further, trills with low trill rates exhibited wide variance in frequency bandwidth, and trills with high trill rates exhibited only narrow frequency bandwidths. The bounded nature of this pattern suggests that performance constraints have limited the evolutionary diversification of trills. In particular, I explore the role of constraints associated with vocal tract modulations during song production and evolution. Identification of this constraint may enhance our ability to explain particular patterns of trill evolution.

Perspective: a critique of sewall wright's shifting balance theory of evolution.

Abstract: We evaluate Sewall Wright's three-phase "shifting balance" theory of evolution, examining both the theoretical issues and the relevant data from nature and the laboratory. We conclude that while phases I and II of Wright's theory (the movement of populations from one "adaptive peak" to another via drift and selection) can occur under some conditions, genetic drift is often unnecessary for movement between peaks. Phase III of the shifting balance, in which adaptations spread from particular populations to the entire species, faces two major theoretical obstacles: (1) unlike adaptations favored by simple directional selection, adaptations whose fixation requires some genetic drift are often prevented from spreading by barriers to gene flow; and (2) it is difficult to assemble complex adaptations whose constituent parts arise via peak shifts in different demes. Our review of the data from nature shows that although there is some evidence for individual phases of the shifting balance process, there are few empirical observations explained better by Wright's three-phase mechanism than by simple mass selection. Similarly, artificial selection experiments fail to show that selection in subdivided populations produces greater response than does mass selection in large populations. The complexity of the shifting balance process and the difficulty of establishing that adaptive valleys have been crossed by genetic drift make it impossible to test Wright's claim that adaptations commonly originate by this process. In view of these problems, it seems unreasonable to consider the shifting balance process as an important explanation for the evolution of adaptations.

Ectotherms follow the converse to bergmann's rule.

Abstract: In a recent paper, Van Voorhies (1996) suggested that Bergmann size clines in ectotherms might result from developmental processes that cause cells to grow larger at lower temperatures. Van Voorhies (1996) found that when Caenorhabditis elegans was grown at cool temperatures cell sizes were larger than control groups reared in a warmer environment, and that this observation may be generally true for other ectotherms (e.g., nematode egg size and fish red blood cells). This is an interesting observation that may play an important role for intrapopulation phenotypic plasticity in body size. However, I would suggest that temperature induced variation in cell size is not a likely mechanism underlying observed patterns of body size variation along latitudinal and altitudinal clines that have been reported for many invertebrates. There are three lines of evidence to support my proposition. First, in many studies of geographic variation in body size in insects that I have reviewed, body size generally decreases with decreasing mean temperature. For example, in an early review of North American field crickets (genus Gryllus), Lutz (1908) found that populations from high latitudes had much smaller tegmina and femora than their southern counterparts. In studies of several Japanese cricket species (e.g., Teleogryllus emma and T. yezoemma), Masaki and others (see Masaki 1978 for a review) found that body size also decreased at higher latitudes. Crickets from northern sites were generally smaller that their southern conspecifics. Similarly, in the North American striped ground cricket, Allonemobius socius, body size shows a "saw-tooth" pattern, with size generally decreasing with decreasing summer season length within a life-cycle type (i.e., univoltine or bivoltine; Mousseau and Roff 1989). Saw-tooth body size clines are expected for any organism capable of adjusting the number of generations per growing season (Roff 1980), with a dip in body size corresponding to the transition from bivoltine to univoltine life cycles. Curiously, one could erroneously conclude that Bergmann's Rule applied if one only examined body size variation without regard to shifts in voltinism (i.e., in the transition zone, body size shifts from small, bivoltine individuals to large, univoltine individuals; see Fig. 1). Similar patterns have been observed for other North American crickets by Alexander and Bigelow (1960). In the lesser migratory grasshopper, Melanoplus sanguinipes, body size also decreases with decreasing mean annual temperatures (Dean 1982; Scott and Dingle 1990; Orr 1996). Scott and Dingle (1990) and Orr (1996) found that grasshoppers from high altitudes in the Sierra Nevada's of California were considerably smaller (and had much shorter development times) than populations from valley sites (Fig. 2), while Dean (1982) found a similar pattern of decreasing size with increasing latitude. Thus, there is strong empirical evidence that refutes the Bergmann's Rule paradigm, and in fact points to a converse of Bergmann's Rule for ectotherms, as has been suggested by Masaki and others (Masaki 1978; Roff 1980, 1986; Scott and Dingle 1990). There is a second line of evidence that refutes the applicability of Bergmann's Rule to ectotherms, that also points to the developmental mechanism underlying geographic variation in body size. When reared in a common garden in the lab, body-size variation among populations tends to follow that observed in the field, suggesting that much of the variation observed in the wild reflects genetic differentiation rather than environmentally induced phenotypic plasticity. In a study of 10 populations of Allonemobius socius, lab-reared (after two generations of common garden conditions to remove maternal effects) and field-collected crickets showed essentially the same pattern of body size variation (Mousseau and Roff 1989). Similar findings have been reported for grasshoppers (Dingle et al. 1990; Orr 1996) and several Japanese cricket species (Masaki 1967, 1983). These observations add further support to the notion that ectotherms tend to follow the converse of Bergmann's Rule, and indicate a high degree of genetic determinism underlying this pattern. Such patterns of body-size variation are an expected and predictable consequence of the interaction between season length at a given latitude (or altitude) and the physiological time available for development in an ectothermic organism (Masaki 1978; and see Roff 1980, 1986 for detailed models). At high latitudes (or altitudes), the growing season is short, and for a univoltine organism development is constrained to a single season or less. At lower latitudes (or altitudes), the growing season is relatively longer and an individual's development time can be extended. Given a positive relationship between development time and body size that is frequently observed in ectotherms (e.g., Peters 1983), body size tends to directly parallel development time, and likely leads to the converse of Bergmann's Rule that has been reported for insects. Although in crickets and grasshoppers, body size appears to have a large genetic basis (with northern and high altitude populations being genetically smaller), this does not need to be so: the converse to Bergmann's Rule would follow for any organism whose life cycle is linked to season length, given a relationship between development time and body size, even if variation in development time reflects environmentally modulated plasticity rather than genetic determinism. A third line of evidence refuting the importance of temperature induced cell size effects comes from studies of rear-

INFERRING PHYLOGENIES FROM mtDNA VARIATION: MITOCHONDRIAL-GENE TREES VERSUS NUCLEAR-GENE TREES REVISITED.

Abstract: The paper by Moore (1995) provided a theoretical basis for preferring mitochondrial-DNA(mtDNA) gene trees over nuclear-gene trees when estimating species phylogenies. This commentary examines conditions that contradict Moore's analysis, suggesting that mtDNA-gene trees for clades exhibiting female philopatry and male dispersal, or a polygynous mating system, may be less reliable indicators of a species phylogeny than nuclear-gene trees. Moore (1995) provides a clear discussion of the differences between gene trees and species trees. The fundamental reason for this distinction is that new alleles, or haplotypes, can originate at any point in time and are not constrained to begin evolutionary divergence at the time of speciation. Indeed, the fact that mutation generates polymorphism within a species is the foundation of population genetics. This point alone allows true branch lengths on a gene tree to differ from true branch lengths on the species tree for the same set of taxa. Of greater concern is that the true branching topology of a gene tree can also differ from that of the species tree when polymorphisms are maintained for long periods relative to internodal distances on the species tree (Moore 1995, fig. 1). If polymorphisms are resolved relatively quickly by the extinction and fixation of alternative alleles (i.e., lineage sorting sensu Avise et al. 1987), then the topology of the species tree will be imposed on the gene tree, because homologous gene sequences will coalesce in the most recent common ancestor of any two species. If, however, ancient polymorphisms coexisted for periods longer than the time between speciation events then the order of speciation may not be reflected in the gene phylogeny. This is expected because loss of particular lineages from descendant taxa should be random with respect to the age of the lineage. The loss of genetic lineages over a time period that includes a speciation event can occur by subsampling of ancestral polymorphisms in the original descendant population or by the subsequent extinction of alleles in descendant taxa. For the purpose of data analysis, undersampling extant intrataxonomic variation also mimics the effects of lineage sorting. Two strategies have been suggested to deal with the potential discordance between gene trees and species trees. The first recommends the simultaneous analysis of multiple unlinked genes (Pamilo and Nei 1988; Wu 1991). This strategy, however, is based on the assumption that the pattern of loss of alleles in descendant taxa is not correlated across loci. Although this may often be the case, there are also circumstances that would invalidate this assumption. For example, the subsampling of ancestral alleles during speciation may not be independent across unlinked loci if genetic variation was geographically structured in the ancestor (Hoelzer and Melnick 1994). Even when this critical assumption is true, the number of unlinked genes that must be examined to give one confidence in the topology can be prohibitive (Moore 1995). The second strategy, which is promoted by Moore (1995), is to use a gene that exhibits short coalescence times. This will maximize the chance that the historical durations of polymorphism retention will not exceed internodal distances on the species tree. Moore (1995) goes on to show that mtDNA is expected to exhibit a coalescence time that is 25% as long as for nuclear genes. This assertion is based on the expectation that effective population size of mtDNA (Nemit) is one-fourth that of nuclear genes (Nenuc) because the mitochondrial genome is both haploid and transmitted only through females. In a random mating population with an equal number of males and females, there will be four times more nuclear-gene copies passed on to the next generation because the nuclear genome is diploid, and alleles are contributed by both sexes. This is an excellent point with important implications for our interpretations of results in molecular systematics and our choices of approach to future studies; however, there are also other factors that can increase Nemit relative to Nenuc. Consideration of these factors shows that Nem it can exceed Nenuc and render a mtDNA tree a less reliable estimate of the species phylogeny than a nuclear-gene tree.

Long-tongued fly pollination and evolution of floral spur length in the Disa draconis complex (Orchidaceae)

Abstract: Field studies in South Africa showed that floral spur length in the Disa draconis complex (Orchidaceae) varies enormously between populations in the southern mountains (means = 32-38 mm), lowland sandplain (mean = 48 mm), and northern mountains (means = 57-72 mm) We tested the hypothesis that divergence in spur length has resulted from selection exerted through pollinator proboscis length Short-spurred plants in several southern mountain populations, as well as long-spurred plants in one northern mountain population, were pollinated by a horsefly, Philoliche rostrata (Tabanidae), with a proboscis length that varied from 22 to 35 mm among sites Long-spurred plants on the sandplain were pollinated by the tanglewing fly, Moegistorynchus longirostris (Nemestrinidae), which has a very long proboscis (mean = 57 mm) Selection apparently favors long spurs in sandplain plants, as artificial shortening of spurs resulted in a significant decline in pollen receipt and fruit set, although pollinaria removal was not significantly affected Fruit set in the study populations was limited by pollen availability, which further suggests that selection on spur length occurs mainly through the female component of reproductive success

The price equation, fisher's fundamental theorem, kin selection, and causal analysis.

Abstract: A general framework is presented to unify diverse models of natural selection. This framework is based on the Price Equation, with two additional steps. First, characters are described by their multiple regression on a set of predictor variables. The most common predictors in genetics are alleles and their interactions, but any predictor may be used. The second step is to describe fitness by multiple regression on characters. Once again, characters may be chosen arbitrarily. This expanded Price Equation provides an exact description of total evolutionary change under all conditions, and for all systems of inheritance and selection. The model is first used for a new proof of Fisher's fundamental theorem of natural selection. The relations are then made clear among Fisher's theorem, Robertson's covariance theorem for quantitative genetics, the Lande-Arnold model for the causal analysis of natural selection, and Hamilton's rule for kin selection. Each of these models is a partial analysis of total evolutionary change. The Price Equation extends each model to an exact, total analysis of evolutionary change for any system of inheritance and selection. This exact analysis is used to develop an expanded Hamilton's rule for total change. The expanded rule clarifies the distinction between two types of kin selection coefficients. The first measures components of selection caused by correlated phenotypes of social partners. The second measures components of heritability via transmission by direct and indirect components of fitness.

Speciation and population genetic structure in tropical pacific sea urchins.

Abstract: Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.

Natural selection for environmentally induced phenotypes in tadpoles.

Abstract: Models suggest that phenotypic plasticity is maintained in situations where the optimal phenotype differs through time or space, so that selection acts in different directions in different environments. Some empirical work supports the general premise of this prediction because phenotypes induced by a particular environment sometimes perform better than other phenotypes when tested in that environment. We have extended these results by estimating the targets of selection in Pseudacris triseriata tadpoles in environments without predators and with larval Anax dragonflies. Tadpoles displayed significant behavioral and morphological plasticity when reared in the presence and absence of nonlethal dragonflies for 32 days in cattle tanks. We measured selection in the absence of free predators by regressing growth and survival in the tanks against activity and several measures of tail and body shape. We measured selection in the presence of predators by exposing groups of 10 tadpoles to Anax in overnight predation trials and regressing the average phenotype of survivors against the number of tadpoles killed. Selection in the two environments acted in opposite directions on both tail and body shape, although the affected fitness components were different. In the presence of Anax, tadpoles with shallow and narrow body, deep tail fin, and wide tail muscle survived best. In the absence of free predators, tadpoles with narrow tail muscle grew significantly faster, and those with shallow tail fin and deep body grew somewhat faster. Activity was unrelated to survival or growth in either environment. Developmental plasticity in tail shape closely paralleled selection, because tail fin depth increased after long-term exposure to Anax and tail muscle width tended to increase. In contrast, there was no plasticity in body shape in spite of strong selection for decreasing body depth. Thus, when confronted with a dragonfly predator, P. triseriata tadpoles adjusted their tail shape (but not body shape) almost exactly in the direction of selection imposed by Anax. These results suggest that phenotypic plasticity in some morphological traits, such as tail depth and tail muscle width, has evolved under intermittent selection by dragonflies. Other traits that undergo selection by dragonflies, such as body morphology, appear developmentally rigid, perhaps because of historically strong opposing selection in nature or other constraints.

Prey adaptation as a cause of predator-prey cycles

Abstract: We analyze simple models of predator-prey systems in which there is adaptive change in a trait of the prey that determines the rate at which it is captured by searching predators. Two models of adaptive change are explored: (1) change within a single reproducing prey population that has genetic variation for vulnerability to capture by the predator; and (2) direct competition between two independently reproducing prey populations that differ in their vulnerability. When an individual predator's consumption increases at a decreasing rate with prey availability, prey adaptation via either of these mechanisms may produce sustained cycles in both species' population densities and in the prey's mean trait value. Sufficiently rapid adaptive change (e.g., behavioral adaptation or evolution of traits with a large additive genetic variance), or sufficiently low predator birth and death rates will produce sustained cycles or chaos, even when the predator-prey dynamics with fixed prey capture rates would have been stable. Adaptive dynamics can also stabilize a system that would exhibit limit cycles if traits were fixed at their equilibrium values. When evolution fails to stabilize inherently unstable population interactions, selection decreases the prey's escape ability, which further destabilizes population dynamics. When the predator has a linear functional response, evolution of prey vulnerability always promotes stability. The relevance of these results to observed predator-prey cycles is discussed.

The evolution of sperm size in birds.

Abstract: Sperm size varies enormously among species, but the reasons for this variation remain obscure. Since it has been suggested that swimming velocity increases with sperm length, earlier studies proposed longer (and therefore faster) sperm are advantageous under conditions of intense sperm competition. Nonetheless, previous work has been equivocal, perhaps because the intensity of sperm competition was measured indirectly. DNA profiling now provides a more direct measure of the number of offspring sired by extrapair males, and thus a more direct method of assessing the potential for sperm competition. Using a sample of 21 species of passerine birds for which DNA profiling data were available, we found a positive relation between sperm length and the degree of extrapair paternity. A path analysis, however, revealed that this relationship arises only indirectly through the positive relationship between the rate of extrapair paternity and length of sperm storage tubules (SSTs) in the female. As sperm length is correlated positively with SST length, an increase in the intensity of sperm competition leads to an increase in sperm length only through its effect on SST length. Why females vary SST length with the intensity of sperm competition is not clear, but one possibility is that it increases female control over how sperm are used in fertilization. Males, in turn, may respond on an evolutionary time scale to changes in SST size by increasing sperm length to prevent displacement from rival sperm. Previous theoretical analyses predicting that sperm size should decrease as sperm competition becomes more intense were not supported by our findings. We suggest that future models of sperm-size evolution consider not only the role of sperm competition, but also how female control and manipulation of ejaculates after

Molecular phylogenetic analysis of life-history evolution in asterinid starfish.

Abstract: We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.

Comparative patterns of craniofacial development in eutherian and metatherian mammals

Abstract: The sequence of differentiation of major elements of the skeletal, muscular and nervous systems of the head is examined in developmental series of five eutherian (placental) and four metatherian (marsupial) mammals. The analysis identifies the elements that are conserved across the Theria, those that are unique to the Metatheria and to the Eutheria, and those that are variable. It is shown that although there are slight shifts in the sequence of development within the somatic tissues of the head, the primary difference between marsupial and placental mammals involves the timing and rate of differentiation of structures of the central nervous system (CNS) relative to a specific subset of structures of the cranial skeleton and musculature. In eutherians, CNS morphogenesis is well underway before the somatic tissues of the head begin differentiation. In metatherians, CNS development is delayed considerably and certain elements of the skeletal and muscular systems are advanced. It is concluded that the developmental differences between marsupial and placental mammals are best explained by the interaction of several processes including neurogenesis as a potential rate-limiting step, the developmental requirements of somatic elements, and the extremely short period of organogenesis of marsupial mammals. Several other issues, including the way that these data may be applied to determine the primitive therian developmental condition, and the use of comparative developmental data to address basic questions on morphogenetic processes, are discussed.

Examining two standard assumptions of ancestral reconstructions: repeated loss of dichromatism in dabbling ducks (anatini)

Abstract: Although phylogenetic reconstruction of ancestral character states is becoming an increasingly common technique for studying evolution, few researchers have assessed the reliability of these reconstructions. Here I test for congruence between a phylogenetic reconstruction and a widely accepted scenario based on independent lines of evidence. I used Livezey's (1991) phylogeny to reconstruct ancestral states of plumage dichromatism in dabbling ducks (Anatini). Character state mapping reconstructs monochromatic ancestors for the genus Anas as well as most of its main clades. This reconstruction differs strongly from the widely accepted scenario of speciation and plumage evolution in the group (e.g., Delacour and Mayr 1945; Sibley 1957). This incongruence may occur because two standard assumptions of character state reconstruction are probably not met in this case. Violating either of these two assumptions would be a source of error sufficient to create misleading reconstructions. The first assumption that probably does not apply to ducks is that terminal taxa, in this case species, are monophyletic. Many of the widespread dichromatic species of ducks may be paraphyletic and ancestral to isolated monochromatic species. Three lines of evidence support this scenario: population-level phylogenies, biogeography, and vestigial plumage patterns. The second assumption that probably does not apply to duck plumage color is that gains and losses of character states are equally likely. Four lines of evidence suggest that dichromatic plumage might be lost more easily than gained: weak female preferences for bright male plumage, biases toward the loss of sexually dichromatic characters, biases toward the loss of complex characters, and repeated loss of dichromatism in other groups of birds. These seven lines of evidence support the accepted scenario that widespread dichromatic species repeatedly budded off isolated monochromatic species. Drift and genetic biases probably caused the easy loss of dichromatism in ducks and other birds during peripatric speciation. In order to recover the accepted scenario using Livezey's tree, losses of dichromatism must be five times more likely than gains. The results of this study caution against the uncritical use of unordered parsimony as the sole criterion for inferring ancestral states. Detailed population-level sampling is needed and altered transfor- mation weighting may be warranted in ducks and in many other groups and character types with similar attributes.

The effects of gene flow on reinforcement.

Abstract: We explore the possibility that differences in the pattern of gene flow between populations may affect the evolution of reinforcement by comparing pairs of populations undergoing one-way migration versus symmetric mi- gration. The case of symmetric migration is modeled by a two-island model, where the two populations exchange equal proportions of migrants each generation. One-way migration is modeled by a continent-island model, where migration is in one direction from a large continental population with a fixed genotype to an island population whose genotype frequencies can vary. Hybrid inviability is assumed to be caused by epistatic interactions between background loci. We examine the spread of an introduced preference allele for a previously unpreferred male trait that characterizes one of the populations. Computer simulations indicate that with a weak introduced preference, reinforcement is possible under a wide range of parameter values in a symmetric migration model but cannot occur in a one-way migration model. Reinforcement with one-way migration can occur only with a very strong introduced preference and very strong selection against hybrids. Our results suggest that the speciation of a peripheral isolate, which undergoes essentially one-way migration, may be difficult to complete if secondary contact occurs before reproductive isolation is fully developed.

Bergmann's rule in ectotherms: is it adaptive?

Abstract: . 1986. Predicting body size with life history models. Bioscience 36:316-323. SCOTT, S. M., AND H. DINGLE. 1990. Developmental programmes and adaptive syndromes in insect life-cycles. Pp. 69-85 in F Gilbert, ed. Insect life cycles: Genetics, evolution and co-ordination. Springer-Verlag, London. VAN VOORHIES, W. A. 1996. Bergmann size clines: A simple explanation for their occurrence in ectotherms. Evolution 50:12591264.

Natural variation in the expression of the heat-shock protein hsp70 in a population of drosophila melanogaster and its correlation with tolerance of ecologically relevant thermal stress

Abstract: Although Hsp70, the principal inducible heat-shock protein of Drosophila melanogaster, has received intense scrutiny in laboratory strains, its variation within natural populations and the consequences of such variation for thermotolerance are unknown. We have characterized variation in first-instar larvae of 20 isofemale lines isolated from a single natural population of D. melanogaster, in which larvae are prone to thermal stress in nature. Hsp70 expression varied more than twofold among lines after induction by exposure to 36°C for one hour, with an estimated proportion of the variation due to genetic differences of 0.24 ± 0.08. Thermotolerance with and without a Hsp70-inducing pretreatment, survival at 25°C, and developmental time also varied significantly. As expected, expression of Hsp70 correlated positively with larval thermotolerance. By contrast, lines in which larval survival was high in the absence of heat stress showed lower than average Hsp70 expression and lower than average inducible thermotolerance. This conditional performance suggests an evolutionary trade-off between thermotolerance and the ability to produce higher concentrations of Hsp70, and survival in a benign environment.

Evolution of dispersal rates in metapopulation models: branching and cyclic dynamics in phenotype space

Abstract: We study the evolution of dispersal rates in a two patch metapopulation model. The local dynamics in each patch are given by difference equations, which, together with the rate of dispersal between the patches, determine the ecological dynamics of the metapopulation. We assume that phenotypes are given by their dispersal rate. The evolutionary dynamics in phenotype space are determined by invasion exponents, which describe whether a mutant can invade a given resident population. If the resident metapopulation is at a stable equilibrium, then selection on dispersal rates is neutral if the population sizes in the two patches are the same, while selection drives dispersal rates to zero if the local abundances are different. With non-equilibrium metapopulation dynamics, non-zero dispersal rates can be maintained by selection. In this case, and if the patches are ecologically identical, dispersal rates always evolve to values which induce synchronized metapopulation dynamics. If the patches are ecologically different, evolutionary branching into two coexisting dispersal phenotypes can be observed. Such branching can happen repeatedly, leading to polymorphisms with more than two phenotypes. If there is a cost to dispersal, evolutionary cycling in phenotype space can occur due to the dependence of selection pressures on the ecological attractor of the resident population, or because phenotypic branching alternates with the extinction of one of the branches. Our results extend those of Holt and McPeek (1996), and suggest that phenotypic branching is an important evolutionary process. This process may be relevant for sympatric speciation.

Multiple modes of speciation involved in the parallel evolution of sympatric morphotypes of lake whitefish (coregonus clupeaformis, salmonidae).

Abstract: We performed a phylogenetic analysis of mtDNA variation among seven sympatric pairs of dwarf and normal morphotypes of whitefish from northern Quebec and the St. John River drainage to address three questions relevant to understanding their radiation. Are all sympatric pairs reproductively isolated? Do phylogenetic analyses confirm that sympatric whitefish morphotypes found in eastern North America represent the outcome of polyphyletic evolutionary events? If so, did all sympatric pairs from the St. John River drainage originate from the same scenario of allopatric divergence and secondary contact? The hypothesis of genetic differentiation was supported for all sympatric pairs from the St. John River drainage, whereas lack of mtDNA diversity precluded any test of reproductive isolation for northern Quebec populations. Patterns of mtDNA variation confirmed that dwarf and normal morphotypes evolved in parallel among independent, yet closely related, lineages, thus providing indirect evidence for the role of natural selection in promoting phenotypic radiation in whitefish. Patterns of mtDNA diversity among sympatric pairs of the St. John River indicated a complex picture of whitefish evolution that implied sympatric divergence and multiple allopatric divergence/secondary contact events on a small geographic scale. These results suggests that ecological opportunities, namely trophic niche availability, may promote population divergence in whitefish.

Fluctuating population size and the ratio of effective to census population size.

Abstract: The effective size of a population (Ne) quantifies the rate at which genetic diversity is eroded by genetic drift (i.e., 112Ne per generation), a fundamental process of evolutionary change. Genetic diversity and its rate of decay have been linked with key components of population fitness (Allendorf and Leary 1987; Ralls et al. 1988; Briscoe et al. 1992; Newman and Pilson 1997; but see Britten 1996). Ne is thus a central parameter both in studies aimed at understanding evolution (Falconer and Mackay 1996) and in the field of conservation genetics (Lande and Barrowclough 1987; Nunney and Campbell 1993; Nunney and Elam 1994). Unfortunately, accounting for all factors that influence Ne is notoriously difficult (reviewed by Caballero 1994). This difficulty is apparently responsible for significant disagreement between theoretical (Nunney 1993) and observed values of the ratio, Ne/N (Frankham 1995). Here we investigate whether this disagreement can be reconciled by incorporating the effect of a factor long known to reduce Ne, namely temporal fluctuations in population size (FPS; Wright 1938). More specifically, we consider the extent to which Ne/N is depressed by FPS over the range of fluctuations observed in wild animal populations. In addition, we present a method for predicting Ne/N from a standard measure of population variability, and we discuss the implications of this theoretical relationship. Several factors affect the effective size of a population: fluctuations in size, variance in fecundity, sex ratio, and the degree to which generations overlap (Crow and Kimura 1970). One difficulty in estimating Ne is that no single formula simultaneously accounts for all these factors. This difficulty would be largely inconsequential if the ratio Ne/N were known to fall consistently within a narrow range. Estimating Ne would be trivial because N is often relatively easily estimated. Theoretical and empirical studies have searched for such a range of Ne/N. Theoretical studies have explored the plausible range of Ne/N through analysis of a reparameterized version of Hill's (1972) expression for Ne (Nunney 1991, 1993, 1996). This reparameterization provides several advantages. Ne is expressed in parameters that are biologically interpretable, and for which typical ranges are known. In addition, the parameters can be estimated from data commonly available from single-season studies of real populations (Nunney and Elam 1994). Through thorough numerical exploration of the parameter space, these studies led to the conclusion that Ne/N is usually close to 0.5 and only rarely outside the range 0.250.75 (Nunney 1991, 1993, 1996; hereafter, referred to as the theoretical expectation.) In contrast with this theoretical expectation, a review of 192 empirical estimates (based on a variety of demographic and genetic methods) revealed that Ne/N was usually less than 0.5 (Frankham 1995; hereafter, referred to as empirical estimates.) In fact, approximately one-third of the Ne/N estimates were less than 0.25, and a subset of these estimates (37 from animal taxa) accounting for all factors that influence Ne had an average Ne/N of 0.15 (median = 0.08). By contrast, a subset of estimates (27 from animal taxa) accounting for all factors except FPS had an average Ne/N of 0.38 (median = 0.38). The discrepancy between theoretical expectation and empirical estimates may thus be largely attributable to the fact that the theoretical expectation is based on the assumption of constant N. The theoretical expectation may provide a reasonable estimate of the short-term Ne/N, but the longerterm ratio may often be less than 0.25, owing to the effect of FPS. A long-term estimate of Ne that accounts for FPS is obtained by transforming a series of short-term effective sizes (Wright 1938; see also Crow and Kimura 1970; Lande and Barrowclough 1987):

HISTORICAL DEMOGRAPHY AND PRESENT DAY POPULATION STRUCTURE OF THE GREENFINCH, CARDUEUS CHLORIS-AN ANALYSIS OF mtDNA CONTROL-REGION SEQUENCES.

Abstract: Genetic variability within and among 10 geographically distinct populations of Greenfinches (Carduelis chloris) was assayed by directly sequencing a 637 BP part of the mtDNA control region from 194 individuals. Thirteen variable positions defined 18 haplotypes with a maximum sequence divergence of 0.8%. Haplotype (h = 0.28-0.77) and nucleotide (r = 0.058-0.17%) diversities within populations were low, and decreased with increasing latitude (h:r, = -0.81; r: r, = -0.89). The distribution of pairwise nucleotide differences fit better with expectations of a "sudden expansion" than of an "equilibrium" model, and the estimates of long term effective population sizes were considerably lower than current census estimates, especially in northern European samples. Selection is an unlikely cause of observed patterns because the distribution of variability conformed to expectations of neutral infinite alleles model and haplotype diversity across populations was positively correlated with heterozygosity (HE) in nuclear genes (r, = 0.74, P mtDNA > allozymes).

Genetic differentiation of fitness-associated traits among rapidly evolving populations of the soapberry bug.

Abstract: In this study we used reciprocal rearing experiments to test the hypothesis that there is a genetic basis for the adaptive differences in host-use traits among host-associated soapberry bug populations (described in Carroll and Boyd 1992). These experiments were conducted on two host races from Florida, in which differences in beak length and development were found between natural populations on a native host plant species and those on a recently introduced plant species (colonized mainly post-1950). Performance was generally superior on the host species from which each lab population originated (i.e., on the "Home" host species): in analysis of variance, there was significant population-by-host interaction for size, development time, and growth rate. These results indicate that the population differences in nature are evolved rather than host induced. Increased performance on the introduced host was accompanied by reduced performance on the native host, a pattern that could theoretically promote further differentiation between the host races.

Narrow hybrid zone between two subspecies of big sagebrush (artemisia tridentata: asteraceae). iv. reciprocal transplant experiments

Abstract: Does endogenous or exogenous selection stabilize the big sagebrush (Artemisia tridentata) hybrid zone? After two years of study, our reciprocal transplant experiments showed significant genotype by environment interactions for a number of fitness components, including germination, growth, and reproduction. Hybrids were the most fit within the hybrid garden. In the parental gardens, the native parental taxon was more fit than either the alien parental or hybrids. These results are consistent with the bounded hybrid superiority model, which assumes exogenous selection, but are clearly at odds with the dynamic equilibrium model, which assumes endogenous selection and universal hybrid unfitness.