Showing papers in "Evolution in 1984"

Estimating F-statistics for the analysis of population structure.

Abstract: This journal frequently contains papers that report values of F-statistics estimated from genetic data collected from several populations. These parameters, FST, FIT, and FIS, were introduced by Wright (1951), and offer a convenient means of summarizing population structure. While there is some disagreement about the interpretation of the quantities, there is considerably more disagreement on the method of evaluating them. Different authors make different assumptions about sample sizes or numbers of populations and handle the difficulties of multiple alleles and unequal sample sizes in different ways. Wright himself, for example, did not consider the effects of finite sample size. The purpose of this discussion is to offer some unity to various estimation formulae and to point out that correlations of genes in structured populations, with which F-statistics are concerned, are expressed very conveniently with a set of parameters treated by Cockerham (1 969, 1973). We start with the parameters and construct appropriate estimators for them, rather than beginning the discussion with various data functions. The extension of Cockerham's work to multiple alleles and loci will be made explicit, and the use of jackknife procedures for estimating variances will be advocated. All of this may be regarded as an extension of a recent treatment of estimating the coancestry coefficient to serve as a mea-

On the measurement of natural and sexual selection: theory.

Abstract: The aim of this paper is to illustrate an approach to the empirical measurement of selection that is directly related to formal evolutionary theory. Recent field studies have demonstrated that it is feasible to measure fitness in natural populations. The most successful studies have yielded accurate tallies of survivorship, mating success and fertility (e.g., Tinkle, 1965; Howard, 1979; Downhower and Brown, 1980; Lennington, 1980; Kluge, 1981; Clutton-Brock et al., 1982). Despite this success, no concensus has been reached on how to analyze the data and relate them to evolutionary theory. We present here a mode of data analysis that describes selection in useful, theoretical terms, so that field or experimental results will have a tangible relationship to equations for evolutionary change. Multivariate, polygenic theory (Lande, 1979, 1980, 1981; Bulmer, 1980) is particularly useful as a conceptual framework because it is concerned with the evolution of continuously distributed traits such as those commonly studied in laboratory and field situations. Multivariate equations have been used for many years by plant and animal breeders in order to impose selection and predict its impact (Smith, 1936; Hazel, 1943; Dickerson et al., 1954, 1974; Yamada, 1977), but this quantitative genetic theory has only recently been applied to evolutionary problems. Definitions and Aims. -It is critical to distinguish between selection and evolutionary response to selection (Fisher, 1930; Haldane, 1954). Selection causes observable changes within a generation in the means, variances and covariances of phenotypic distributions. Thus selection can be described in purely phenotypic terms without recourse to the inheritance of characters. In contrast, evolutionary response to selection, for example, the change in phenotypic mean from one generation to the next, certainly does depend on inheritance. In the following sections we show how knowledge of inheritance can be combined with purely phenotypic measures of selection to predict evolutionary response to selection. By distinguishing between selection and response to selection we can measure selection on characters whose mode of inheritance may be unknown and make prediction of evolutionary response a separate issue. Thus knowledge of inheritance is essential for complete

Sex chromosomes and the evolution of sexual dimorphism.

Abstract: The evolutionary significance of sex chromosomes has generally been associated with their effect on sex determination (see Mittwoch, 1967; Ohno 1967, 1979 for review). Here I will evaluate the idea that sex chromosomes also play an important role in the evolution of sexually dimorphic traits. As a premise I will assume that sexual dimorphism results from natural selection that favors different phenotypic characteristics in the two sexes. For example, in bighorn sheep (Ovis canadensis) horn size and shape are sexually dimorphic. The massive recurved horns of males are presumably an adaptive compromise between the need to blunt head-to-head collisions between fighting males and to act as weapons against predators. The smaller dagger-like horns of females are favored in this' sex since their sole function is defense against predators (Geist, 1971). Many less obvious characteristics are also likely to differ in their selective value between the sexes. In an extensive review of sexual dimorphism in mammals, Glucksmann (1981) suggested that traits such as growth rate, thermoregulation, metabolic rate, biorhythms, sensory modality, and a wide variety of other traits differ between the sexes in their optimal value. Characteristics that are selectively favored in one sex but selected against in the other will be referred to as "sexually-antagonistic" traits. Consider the evolution of a sexually dimorphic trait from a monomorphic state. For example, Geist (1971) used paleontological evidence and taxonomy to argue that the sexually dimorphic horns of bighorn sheep evolved from a sexually monomorphic ancestor. Because an exact record of this evolutionary change is unavailable, I will arbitrarily assume for the purpose of illustration that the size of female horns remained unchanged while the size of male horns increased. The evolution of sexual dimorphism in horn size could have proceeded in at least two ways: 1) The increase in frequency of genes that enhanced horn size in males but not in females, and 2) The increase in frequency of genes that enhanced horn size in both sexes followed by the evolution of modifier genes that restricted the expression of increased horn size to males. The first way will be referred to as the "pleiotropy-mechanism." It requires genetic variability that simultaneously produces the sexually-antagonistic trait (increased horn size) and is sex-limited in its expression. The second way will be referred to as the "modifermechanism." It requires genetic variability for both the sexually-antagonistic trait (increased horn size) and sex-limited expression of this trait. A similar classification was previously proposed by Turner (1978) for the evolution of sexlimited traits in butterflies. Most mutations that have been studied carefully in the laboratory have not been found to be completely sex-limited in their expression. Thus it seems reasonable to assume that most of the genetic variability available for the evolution of sexual dimorphism would be initially expressed in both sexes. This assumption may be unreasonable when considering the enhancement of an established sexually dimorphic trait since developmental canalization (Waddington, 1962) may facilitate sex-limited gene expression. However, during the initial evolution of sexual dimorphism from a monomorphic state, a feasible sequence of events would be the "modifier-mechanism" described above. In the following model I will suggest how the sex chromosomes can facilitate

On the measurement of natural and sexual selection: applications.

Abstract: In this paper, we use measures of selection developed by quantitative geneticists and some new results (Arnold and Wade, 1984) to analyze multiple episodes of selection in natural populations of amphibians, reptiles, and insects. These examples show how different methods of data collection influence the potential for relating field observations to formal evolutionary theory. We adhere to the Darwinian tradition of distinguishing between natural and sexual selection (Darwin, 1859, 1871; Ghiselin, 1974). We view sexual selection as selection arising from variance in mating success and natural selection as arising from variance in other components of fitness. The justification for this formal distinction is developed by Wade (1979), Lande (1980), Wade and Arnold (1980), Arnold and Houck (1982) and Arnold (1 983 a). (We define mating success as the number of mates that bear progeny given survival of the mating organsim to sexual maturity. We do not equate mating success with mere copulatory success.) The utility of the distinction between sexual and natural selection is that the two forms of selection may often act in opposite directions on particular characters (Darwin, 1859, 1871). While we find the distinction between these two forms of selection useful, the difference is not crucial to our analysis. The essential point is that the recognition of selection episodes permits analysis of selection that may change in magnitude and direction during the life cycle. Defining Fitness Components. -The key first step in the analysis of data is to define multiplicative components of fitness so that selection can be partitioned into parts corresponding to these components or episodes of selection. Using an animal example, if the number of offspring zygotes is taken as total fitness, we can define the following components of fitness: viability (survivorship to sexual maturity), mating success (the number of mates) and fertility per mate (the average number of zygotes produced per mate). These components of fitness are defined so that their product gives total fitness. As a second example, consider the components of fitness in a plant in which yield (seeds/plant) is taken as the measure of total fitness (Primack and Antonovics, 1981). We might define the following components of fitness: number of stems per plant, average number of inflorescences per stem, average number of seed capsules per inflorescence, and average number of seeds per capsule. Again, these four fitness components are defined so that their product gives total fitness. We will need to measure each component of fitness and each character on each individual in order to partition selection into parts corresponding to the separate episodes of selection or to the separate components of fitness. Thus in the animal example, we need to measure the viability, mating success and fertility of each individual. With this accomplished we can estimate the separate forces of viability, sexual and fertility selection on each phenotypic character. In addition we can calculate the opportunities of selection corresponding to these three episodes and covariances between the different kinds of selection. In the plant example, we might begin with the intuition that larger plants have a greater yield. Using our methodology we can reword and extend this intuition. We can not only test the proposition of

Laboratory evolution of postponed senescence in drosophila melanogaster.

Abstract: Evolutionary genetics seems to have found the fundamental cause of senescence: the decline in the sensitivity of natural selection to gene effects expressed at later ages in most populations of organisms with separate somatic and germline tissue. (Here "senescence" refers to decline in age-specific fitness-components after the onset of reproductive maturity.) This idea traces back to Haldane (1941) and Medawar (1946, 1952), with considerable elaboration and elucidation since then (Williams, 1957; Hamilton, 1966; Edney and Gill, 1968; Emlen, 1970; Charlesworth and Williamson, 1975; Charlesworth, 1980; Rose, 1983a). While there are still clear limitations to the mathematical formulation of this theory (cf. Hamilton, 1966; Charlesworth, 1980), the basic formal analysis leads to a straightforward conclusion: the first partial derivative of fitness with respect to appropriately scaled changes in age-specific life-history characters usually declines in magnitude with the age of these changes. The force of natural selection thus declines with age. This overall theory and its particular subsidiary variants lead to a number of empirically testable corollaries (Rose, 1983a, 1983b). Some of these corollaries are specific to the subsidiary variants of the theory (Rose and Charlesworth, 1980, 1981a, 1981b; Rose, 1983b), sothattests of them individually do not test the theory as a whole. Fortunately, there are two corollaries which follow from the general theory itself: the reproductive schedule of an outbred population will give rise to natural selection acting to (i) accelerate senescence in populations with a relatively earlier age of reproduction and (ii) postpone senescence in populations with a relatively later age of reproduction (Edney and Gill, 1968; Rose, 1983a). The former prediction has been corroborated by Sokal (1970) using Tribolium castaneum, while the latter has been corroborated by Wattiaux (1968a, 1968b) and by Rose and Charlesworth (1980, 198 lb), using Drosophila species. Once a theory has been well-developed mathematically and then empirically corroborated, attention turns to experiments in which the theory either is not clearly corroborated or is ostensibly refuted. It would be misleading to claim that all relevant experimental results directly corroborate the evolutionary theory of senescence. Sokal (1 970) and Mertz (1975) using Tribolium castaneum and Taylor and Condra (1980) using Drosophila pseudoobscura found heterogeneity between lines in experiments with replication, such that some lines did not exhibit the predicted response to the imposed selective regime. Taylor and Condra (1980) also found a difference in the response of the sexes which was later attributed to the pattern of female mating preference (Taylor et al., 1981). More problematic still are the studies from the Lints laboratory, one of which failed to obtain a direct response to artificial selection for longevity (Lints et al., 1979), while another gave puzzling fluctuations in life-history attributes (Lints and Hoste, 1974, 1977). Lints (1978, 1983) has made a great deal of these problems, contending that they cast doubt on all proposed evolutionary theories of senescence. While it can be argued that these puzzling results are due to technical artifacts such as inbreeding, genetic disequilibrium, and inadequate controls (cf. Rose and Charlesworth, 1981b), the only ef-

Ecological causes of sexual dimorphism.

Abstract: !:!.f= [~:Sm + h/Sf ]l2, (3b) (2) (1) ofthe trait measurable in both males and females and assume that within each sex the trait is normally distributed with means, m and f, and variances Vm and V.f Assume also that the trait is determined additively by several autosomal loci with an independent environmental component. Let Gm and Gf be the additive genetic components and Em and Ef be the environmental components of the variances in males and females. The heritabilities of the traits in males and females are given by

Selection for delayed senescence in drosophila melanogaster

Abstract: Understanding the mechanism whereby the aging process is controlled has proven to be a uniquely difficult biological problem. Many theories have been put forth offering explanations for the phenomenon of senescence on a variety of different levels ranging from cellular, biochemical, and physiological to genetic and evolutionary. Many of these explanations are nonexclusive, which adds redundancy to confusion in considering the whole body of theory. Many of the cellular and/or biochemical mechanisms proposed amount to little more than detailed discussions of various possible gene end-products, which are themselves the subject of genetic and evolutionary theories. And even among these, no single theory predominates. J. B. S. Haldane (1941) and P. B. Medawar (1952) advanced the first theory of senescence incorporating a modem genetic and evolutionary perspective on the aging process. Their theory postulates the existence of specialized age-of-onset modifier genes which repress the action of other genes that are deleterious until an advanced age has been reached. Little harm results from the expression of the mutations then, however, and senescence gradually ensues with their derepression. In this theory, selection modifies life span by simply increasing or decreasing the period over which such modifiers are effective. Williams (19 57) later expanded on this, introducing the notion that the genes influencing senescence might themselves act pleiotropically with reciprocal effects at early and late ages. In this theory, the beneficial effects of genes early in life are weighed in evolution against their late life effects; youthful vigor must be accompanied by an early senescence and short life, while a delayed senescence and long life occur at the cost of youthful vitality. Apart from further extension of these ideas by Hamilton (1966) and Emlen (1 970), no new major theories of the evolution of senescence have arisen since Williams (1957). One reason for this may be that until recently, the few experimental tests performed contributed comparatively little substantiating information toward these theories. Early attempts at modifying life span through artificial selection include that of Glass (1960), who withheld mating in Drosophila to enforce an early versus late age-specific pattern of reproduction. This produced a slight increase in the longevity of late-reproducing lines. Wattiaux (1968) also found an increase in longevity in Drosophila under selection for an agespecific pattern of reproduction. This was followed by Sokal's (1970) study showing that continuous reproduction at an early age reduced median life span in Tribolium. Mertz (1975) found similar trends in an even later study. Taylor and Condra (1980) and Barclay and Gregory (1982) report changes in the longevity of Drosophila populations under rand K-selection or when exposed to predation. Concurrently with these, Lints and Hoste (1974, 1977) published the results of a well designed and extensive experiment that also selected for increased longevity in D. melanogaster through an early or late age-specific schedule of reproduction. But life span fluctuated wildly throughout the 13 generations of selection here, declining by 70% in the first few generations and then recovering. Further experiments (Lints et al., 1979)

The quantitative genetics of polyphagy in an insect herbivore. ii. genetic correlations in larval performance within and among host plants.

Abstract: Because the overall evolution of the phenotype is a composite of direct and correlated responses to selection, the evolutionary trajectories of genetically correlated characters are fundamentally interdependent (Hazel, 1943; Falconer, 1981 Ch. 19). The effects of genetic correlations on phenotypic evolution are twofold. First, genetic covariance can influence the rate of response to selection. If characters simultaneously selected to increase are influenced by genes with positively correlated effects, the response to selection will be more rapid for the pair than for either character selected separately until the underlying genes are fixed. Conversely, negative genetic correlations among characters selected to increase can slow their rate of simultaneous evolution from that predicted by the amount of genetic variation which exists for each character separately (Dickerson, 1955; Antonovics, 1976; Lande, 1982b). Secondly, genetic correlations can affect the direction of evolution in suites of cliaracters such that traits no longer evolve directly toward their individual optima. In some cases, traits may even be drawn away from their optima for many generations due to selection on genetically correlated characters (Lande, 1980; Via and Lande, unpubl.). The manifold evolutionary effects of genetic correlations dictate a multivariate approach to the study of the quantitative genetic basis of evolution (Lande, 1979, 1980, 1982a, 1982b). Inmost non-

Distance methods for inferring phylogenies: a justification.

Abstract: Most methods for inferring phylogenies assume that the data consist of a series of variables measured across a series of taxa. This general scheme holds whether these variables are measurements of quantitative characters, discretely coded morphological states, gene frequencies, or amino acids or nucleoti'des in a sequence. Sometimes, however, the data are in the form of a table of all pairwise distances among the taxa. For some kinds of data, such as immunological distances or measurements of DNA hybridization, the data are originally collected as pairwise measures of difference between the taxa. In other cases, the investigator has computed distances from the original data table. These cases are distinguished by the possibility, in the latter case, of discarding the distances and returning to an analysis of the original data. The first papers using distance methods to infer phylogenies were of the latter type. Fitch and Margoliash ( 1967) started with amino acid sequences of cytochrome c, then reduced the data to a table of the percentages of sites differing between each pair of species. Cavalli-Sforza and Edwards (1 967) developed a distance measure based on gene frequency data, presenting a least-squares method based on the distances. The readiness with which distance approaches were adopted by these authors owed in part to the popularity of Sokal and Sneath's (1963) clustering methods for the mathematically related, but logically distinct, task of erecting classifications. However, phylogenetic distance methods have their own rationale, independent of any analogy to clustering: they are not intrinsically phenetic as opposed to phylogenetic methods. Since these original papers, a number of other distance methods have been introduced (Hartigan, 1967; Farris, 1972; Moore et al., 1973; Beyer et al., 1974; Tateno et al., 1982; Chakraborty, 1977; Sattath and Tversky, 1977; Waterman et al., 1977; Fitch, 1981). It is not my intention to review all of these methods here: I have described them briefly elsewhere (Felsenstein, 1982). What all have in common is that they try to find a rooted or unrooted tree, usually interpreted as a phylogeny, which most closely fits the observed distances. They differ in the measure of fit of the distances to the tree and the constraints they impose on the tree. All of the methods consider that there is a perfect fit of a tree to the data if all of the observed distances are sums of the lengths of the intervening branches of the tree. Such a perfect fit is shown in Figure 1, which shows a table of distances which can be perfectly fit by a particular tree, and a rooted version of that tree. Each branch of the tree has a length indicated next to it, and the distance between each pair of tips is in this example simply the sum of the lengths of the intervening branches. Thus the distance between tips A and D is 5 + 10 + 2 + 8 = 25. For any tree proposed, for each pair of tips we can compute the sum of the lengths of the branches between them. This quantity, which we call dij', should be close to the observed distance dij if the tree is a good fit. Different methods use different formulas for assessing the goodness of fit. For example, the methods of Fitch and Margoliash (1967) and Cavalli-

Molecular evolution over the mutational landscape

Abstract: A common observation in phylogenetic comparisons of the amino acid sequences of a particular protein is that the rate of evolution of the protein is nearly constant over extended periods of time. This constancy was first noticed by Zuckerkandl and Pauling (1965) and prompted them to call the amino acid substitution process a "molecular evolutionary clock." Since 1965 a great deal of additional data has supported the basic idea of the molecular clock although detailed studies have shown the clock to be a rather erratic one. The detailed studies have been of two different sorts: broadly based statistical studies of a number of proteins over relatively few species (e.g., Langley and Fitch, 1974), or very detailed looks at a particular protein over a large number of species (e.g., Baba et al., 1981). In general, these studies support the crucial observation made by Ohta and Kimura (1971) that the variance in the evolutionary rate is higher than would be expected if the substitution process were a Poisson process. A major goal of theoretical population genetics must be to account for this elevated variance. There are formidable statistical problems associated with the estimation of the variance in the rate of substitutions of amino acids. The problem is compounted by the fact that the variance is only interesting when compared to the mean as in the ratio K = Var(N,)/E(N1), where N, is the number of substitutions in a period of time, t. Obtaining accurate estimates of ratios is difficult in the best of statistical settings. For protein evolution data where the number of substitutions on each leg must themselves be inferred by a procedure such as Fitch and Margoliash's (1967) maximum parsimony procedure, the sampling variance of the final estimate must be relatively high (and itself almost impossible to estimate). Nonetheless, a number of studies have all pointed to a value of K of around 2 to 3. Ohta and Kimura (1971) were the first to estimate K and did so using the available data on hemoglobins and cytochrome c. They reported a value in the range 1.5 to 2.5. The studies by Langley and Fitch (1974) improved on this in the sense of using more data although their procedure was not expressly designed to estimate K. They concluded K was around 2.5. Later, Gillespie and Langley (1979) re-examined the statistical procedure used in the earlier studies and through a very crude argument also claimed that K was around 2.5. In studies that examine only a single protein over a large number of species the aim has been not so much to estimate K as to look more directly for periods of relatively fast or slow evolution in the protein. These studies, such as those by Goodman et al. (1982) generally present fairly convincing evidence of variations in the rates of evolution although Kimura (1981) has called into question the ability of these studies to uncover variation in the rates of evolution. These observations are critical for an understanding of the forces responsible for amino acid substitutions. One of the most appealing theories for the evolution of proteins is the neutral allele theory first proposed by Kimura (1968a, 1968b) and King and Jukes (1969). This theory predicts that the rate of substitutions will be constant although it does not imply that K will equal one as has been commonly assumed. Rather it was shown by Gillespie and Langley (1979) that K will always be greater than one under the neutral allele theory and will actually increase with

Is a Jack-of-All-Temperatures a Master of None?

Abstract: FIG. I. Hypothetical performance curves of ectotherms as function of body temperature. (a) Example predicted from the Principle of Allocation, involving a tradeoff between maximum performance and breadth of performance. The categories \"specialist\" and \"generalist\" are not discrete but are endpoints on a continuum. (b) Example contradicting the Principle, in which traits that promote performance at one temperature promote performance at all temperatures. .. u c o E

The quantitative genetics of polyphagy in an insect herbivore. i. genotype-environment interaction in larval performance on different host plant species.

Abstract: Agriculturalists have long been aware that a cultivar or variety which is superior in yield of performance in one location may not retain its relative advantage in other environments(Dickerson,1962;Comstock and Moll,1963;Allard and Bradshaw,1964).

Coevolution in ecosystems: red queen evolution or stasis?

Abstract: Tentative preliminaire de developpement d'une theorie (basee sur l'hypothese de la «Red Queen», sur la theorie de la biogeographie des iles et sur d'autres concepts) sur le comportement a long terme des ecosystemes, comprenant des changements dans le nombre d'especes, leur constitution genetique et leur abondance relative

Pattern beneath the chaos: the effect of recruitment on genetic patchiness in an intertidal limpet.

Abstract: Gene exchange among widely separated areas characterizes many marine organisms with planktonic dispersal. From an evolutionary perspective, the essential feature of such dispersal is that recruits to local populations come from somewhere else. Thus, localized adaptation is not accumulated over time, and changes in the genetic composition of adults reflect single-generation effects of selection and recruitment. The most obvious effect of planktonic dispersal is the reduction of geographic variation in genetic composition (e.g., Scheltema, 1971, 1978), and low variances of allelic frequencies have been found to be associated with planktonic dispersal (e.g., Berger, 1973; Levinton and Suchanek, 1978; Winans,

Annual variation of survival advantage of large juvenile side-blotched lizards, uta stansburiana: its causes and evolutionary significance.

Abstract: On confirme l'hypothese selon laquelle la variation de l'intensite de la predation et la competition pour l'alimentation sont responsables de l'importance de l'avantage quant a la survie des jeunes de grandes dimensions. Implications evolutives de ces resultats

Population structure and local selection in impatiens pallida (balsaminaceae), a selfing annual.

Abstract: The genetic structure of populations plays a dominant role in evolution. Levels of selection (Wade, 1978), the evolutionary consequences of interactions among conspecifics (Wilson, 1979; Wade, 1980) and the potential for differentiation (Slatkin, 1981) and speciation (Wright, 1980) are all influenced by population structure. Documentation of the factors which contribute to population structure is essential to an understanding of the process of evolution within populations. In addition, the amount of genetic variation within populations, and the extent to which local populations differ in absolute genetic variation influence the evolutionary potential of populations and species. Wright (1946, 1969) emphasized the importance of mating system on population structure, and demonstrated that self-fertilization and inbreeding promote genetic subdivision of populations. Because of the great diversity of breeding systems in plants, from almost complete selfing to outcrossing, breeding systems have a major impact on the structure of plant populations (Baker, 1953; Allard et al., 1968; Jain, 1975). Local populations of selfers may have less genetic variability than outcrossers (Solbrig, 1972; Jain, 1976; Gottlieb, 1977), but the genetic variation maintained by selfers is often far greater than expected (Imam and Allard, 1965; Kannenberg and Allard, 1967; Allard et al., 1968; Brown, 1979). Restricted recombination in selfing species results in high between-population variation (Wright, 1946; Jain, 1975) and extreme geographic differentiation (Harlan, 1945; Imam and Allard, 1965; Jain and Marshall, 1968; Hillel et al., 1 973a; Rick and Fobes, 1975). Although local population differentiation is well documented for outbreeders (Gregor, 1930; Bradshaw, 1959; Cook, 1962; Weil and Allard, 1964; Schaal, 1975), the reduced pollen flow of selfers increases their potential for between-population differentiation (Wright, 1969; Levin and Kerster, 1974). Because small effective population size is characteristic of many plant species (Levin and Kerster, 1974; Beattie and Culver, 1979), genetic drift is expected to be a major differentiating force. However, there is limited evidence for drift in natural plant populations (see Schall, 1975), and the relative importance of drift and selection continue to be debated (Bradshaw, 1972; Jain and Rai, 1974). In that mating system has a marked effect on the potential for differentiation through drift, a critical objective in plant microevolutionary studies is to determine the contributions of mating system and local selection to population structure. There are many examples of population differentiation in plants (see above), but few studies have examined population structure, local genetic variation and selection intensities in the context of plant mating system (see Antonovics, 1968). In this paper, I quantify -the extent of differentiation and local adaptation in Impatiens pallida (Balsaminaceae), a selfing annual. The goals of this research were to 1) determine the withinand between-population components of variation in quantitative characters, including several components of fitness, 2) quantify and compare the genetic variation present in local populations, and 3) experimentally determine the role of selection in population differentiation.

Experimental studies of the evolutionary significance of sexual reproduction ii. a test of the density-dependent selection hypothesis

Abstract: This study tests the hypothesis that one evolutionary advantage of sexual reproduction is that it produces genetically variable progeny with a density-dependent advantage mediated by resource partitioning or pest pressure. Our experimental approach involved planting separate plots of sexually-derived and asexually-derived tillers of the grass Anthoxanthum odoratum in density gradients at the two natural sites from which the source material was taken. The sexual progeny displayed a significant fitness advantage compared to the asexual progeny. But, in contrast to the expectations of the density-dependent selection hypothesis, the advantage of the sexually produced progeny is most marked at lower densities. Thus, the results of this experiment and our previous report (Antonovics and Ellstrand, 1984) seem to best support the frequency-dependent selection hypothesis for the advantage of sexual reproduction.

Disruptive selection on habitat preference and the evolution of reproductive isolation: a simulation study.

Abstract: There has been a wide diversity of theoretical work on the genetic mechanisms that promote speciation under sympatric (non-allopatric) conditions (see Thoday and Gibson, 1970; Bush, 1975; Endler, 1977; White, 1978; Futuyma and Mayer, 1980; Templeton, 1981 for review). The conclusion from this work is that sympatric speciation is genetically possible but it is not clear whether or not it has played a major role in the generation of species under natural conditions. Assessing the evolutionary importance of sympatric speciation awaits the identification of those environmental circumstances that are most likely to promote the process. Here I suggest that environmental conditions which produce disruptive selection on habitat preference represent a special case in which sympatric speciation is particularly likely to occur. By habitat preference I mean any tendency of an organism to become non-randomly associated with a particular spatial and/ or temporal part of an environment.

Inbreeding depression and proximity-dependent crossing success in phlox drummondii.

Abstract: Viability depression is a typical consequence of inbreeding in cross-fertilizing species. In some populations of Homo sapiens (Schull and Neel, 1965; FreireMaia and Azevedo, 1971; Schull et al., 1970) and Drosophila (Dobzhansky et al., 1963; Stone et al., 1963; MalogolowkinCohen et al., 1964; Mettler et al., 1966), the level of viability depression is linearly dependent on the inbreeding coefficient of an individual. If extrapolated to the maximum level (F = 1) from sib and cousin values, mortality due to homozygosity for detrimental genes would exceed 50% in most Homo and Drosophila populations. In most populations of Homo and Drosophila, the mean number of lethal equivalents per zygote is between 1 and 4. Populations of Tribolium also fall within this range (Levene et al., 1965). Viability depression following inbreeding in angiosperms is similar to that in the aforementioned animals. The mean numbers of lethal equivalents per zygote are as follows: Secale cereale, 2.7 (Landes, 1939); Medicago sativa, 1.2-4.5 (Cooper and Brink, 1940; Sayers and Murphy, 1966); Fagopyrum esculentum, 1.3-5.2 (Komaki, 1982); and Stylidium spathulatum, 3.4 (James, 1979). Among conifer species, the mean number of lethal equivalents per zygote varies from 1 to 10, values exceeding 8 in Pinus and Pseudotsuga (Sorensen, 1969; Franklin, 1972; Koski, 1973). Ferns are similar to conifers in their range of lethal equivalents (Klekowski, 1970; Ganders, 1972; Lloyd, 1974; Saus and Lloyd, 1976). The magnitude of viability depression from inbreeding in plants is dependent upon genetic and environmental variables. The frequency of lethal and detrimental genes in populations is a prime factor. It exceeds .15 in several species (Apirion and Zohary, 1961; Crumpacker, 1967; Kiang and Libby, 1972; Ohnishi, 1979; Komaki, 1982). The level of viability depression also is governed by the tolerance of species to higher levels of homozygosity (Mayo, 1980). Species whose genetic systems are adapted to relatively high levels of homozygosity are likely to show the least viability depression with inbreeding. Finally, the viability differential between outcross and inbred progeny is influenced by environmental quality. Under conditions of drought, disease and other stresses, the relative viability of inbreds is much poorer than under favorable conditions (Allard and Hansche, 1964; Pawsey, 1964; Koski, 1973; Libby et al., 1981). Viability depression in plants is most pronounced during seed development. For example, in Pseudotsuga menziesii, about 95% of the lethality following inbreeding occurs prior to germination (Orr-Ewing, 1957). Seed abortion usually is 2 to 10 times greater following selffertilization than cross-fertilization in conifers (Sorensen, 1969; Koski, 1971, 1973; Franklin, 1972; Birshir and Pepper, 1977) and angiosperms (Brink and Cooper, 1947; Linck, 1961; Rowlands, 1960; Sayers and Murphy, 1966; James, 1979). If the products of self-fertilization or crosses among sibs have reduced viability relative to outcrosses, we may expect progeny from neighboring plants in natural populations to have lower viability than those from distant plants when plant relatedness declines with distance. This relationship between proximity and relatedness would be a consequence of restricted pollen and seed dispersal, and is

Sexual selection on body size, territory and plumage variables in a population of darwin's finches.

Abstract: Sexual selection on a phenotypic trait arises when that trait covaries with mating success among individuals of the same sex, usually males. As such sexual selection is thought to occur most readily and strongly in polygamous species where variance in mating success is high, but it can occur also in monogamous species where there is competition among males for early breeding females (Darwin, 1871 Ch. 8; O'Donald, 1980). As Darwin originally recognized, sexual selection in monogamous systems will be intensified when the sex ratio is skewed in favor of males. This paper reports on sexual selection on male body size, plumage and territory size characteristics in a small population of Darwin's Medium Ground Finches (Geospiza fortis) on I. Daphne Major, Galapagos, Ecuador. The species is monogamous, but during this study (from 1979 to 198 1) males have outnumbered females by a factor of three to two.

Tradeoffs in performance on different hosts: evidence from within- and between-site variation in the beetle deloyala guttata.

Abstract: Host races of phytophagous insects are sympatric populations that have different host preferences and between which gene flow is restricted because of the difference in host preference. Sympatric, host-associated sibling species are sympatric populations that use different host plants and that do not interbreed because of the presence of isolating mechanisms not related to host preference (Jaenike, 1981; see Mayr, 1970, and Bush, 1969, for slightly different definitions of these terms). Although the existence of host races and host-associated sibling species of phytophagous insects has been suspected for over 50 years (e.g., Thorpe, 1930), the evolutionary mechanism of host race formation remains controversial. In particular, there is disagreement about the importance of sympatric divergence in generating host races and species. White (1978) has argued that sympatric speciation must be invoked to explain, in part, the great diversity of specialized herbivorous insects, while Bush (1974, 1975) has forcefully advocated the operation of sympatric divergence in the creation of sympatric host races of tephritid flies. By contrast, Futuyma and Mayer (1980; see also Futuyma, 1983a) have argued that there is no reliable experimental evidence to support these claims and that models of sympatric speciation are based on assumptions that are probably not met by most phytophagous insects. Several authors have developed formal models of sympatric divergence and speciation (Maynard Smith, 1966; Dickinson and Antonovics, 1973; Caisse and Antonovics, 1978; Pimm, 1979; Felsenstein, 1981). Although these models do not account for the evolution of differences in host plant preference associated with purported sympatric speciation in phytophagous insects, Bush and Diehl (1983) and Rausher (1984) have suggested how modification of these models could remedy this problem. A common assumption of all these models is that fitness on one host is negatively correlated with fitness on a second. This type of negative correlation is manifested in these models in the assumption that genetic variation exists at loci that exhibit a "crossing" genotype x host plant interaction, i.e., at loci at which some genotypes have high fitness on one host but low fitness on a second host while other genotypes have low fitness on the first but high fitness on the second host. This negative correlation seems to be necessary for sympatric speciation because it permits linkage disequilibrium to be established between loci influencing, say, viability on different hosts and loci influencing host preference. The resulting coadapted preference-viability gene complexes represent incipient host races or species. Moreover, the breakdown of these coadapted gene complexes by recombination provides the selection pressure that improves reproductive isolation, whether isolation is achieved via mating on the host (e.g., Bush, 1974, 1975) or by a separate assortative mating locus in linkage disequilibrium with loci affecting preference and fitness (e.g., Felsenstein, 1981). Without a crossing interaction, one homozygote genotype would have maximal fitness on both host species. Consequently, unless reproductive isolation were achieved instantaneously by a mutation

The evolution of cooperative breeding by delayed reciprocity and queuing for favorable social positions.

Abstract: Cooperative breeding, in which some members of a social group help others to reproduce, recurs as a normal feature of social organization in a variety of birds and mammals. In many cases, these helpers do not themselves reproduce while assisting in the care of others' young. How such apparently altruistic helping could evolve has been a central question in the recent development of sociobiology. The hypotheses advanced so far fit into three general categories (Brown, 1978): (1) immediate, direct benefits for helpers, such as immediate improvement in survival or chances for breeding; (2) indirect benefits as a result of selectively helping genealogical relatives; and (3) delayed, direct benefits as a result of eventually acquiring a favorable position for breeding. This paper focuses on the third category of explanation. The aim is to develop quantitative conditions under which delayed benefits can provide a sufficient explanation for the evolution of helping and to consider some of the evolutionary problems of delayed benefits. It is important to emphasize that the three kinds of hypotheses above are not mutually exclusive. All three effects might contribute to the evolution of cooperative breeding in a particular species or population. Indeed, it is possible that none of the three alone could provide a sufficient explanation for a particular case of cooperative breeding, whereas a combination of two or all three could. Nevertheless, a first step is to test the adequiacy of each hypothesis separately. To make these tests, we need to know the conditions under which each hypothesis can provide a sufficient explanation for the evolution of cooperative breeding. These conditions are comparatively well formulated for the first two hypotheses. For instance, immediate, direct benefits can provide a sufficient explanation for cooperative breeding provided that each individual in a group realizes an immediate gain in fitness, either through production of young or increased survival, in comparison to its fitness when breeding alone. Immediate, direct benefits to group members play an important part in the evolution of cooperative breeding in some species (Vehrencamp, 1978). In many species, however, helpers do not reproduce, or at least have very low chances for reproduction, so that immediate, direct benefits cannot provide a complete explanation for the evolution of helping in these cases. The indirect benefits from kin selection are also relatively well understood. Although these benefits are clearly important in the many species of cooperative breeders in which helpers join their natal groups, kin selection has nevertheless remained a controversial explanation for the evolution of cooperative breeding (Brown, 1978; Brown and Brown, 1981; Emlen, 1978, 1981; Gaston, 1978b; Ligon and Ligon, 1978a, 1978b, 1982; Koenig and Pitelka, 1981; Woolfenden, 1981). To determine whether or not indirect benefits can pro-

The comparison of phenotypic plasticity and genetic variation in populations of the grass danthonia spicata

Abstract: A species can persist in a heterogeneous environment either if individuals of that species are phenotypically flexible or if there is genetic variation among individuals (Thoday, 1953; Baker, 1965; Bradshaw, 1965; Jain, 1979). Phenotypic flexibility may be defined as the extent to which an organism can grow and reproduce in a range of environments either by varying its phenotype (plasticity) or by maintaining a constant phenotype (homeostasis or stability) (sensu Thoday, 1953; Hume and Cavers, 1982). Bradshaw (1965) recognized that phenotypic plasticity could itself be under genetic control and would therefore be subject to selective pressures. Bradshaw (1965) and others (Thoday, 1953; Levins, 1963; Marshall and Jain, 1968; Jain, 1979) have postulated that selection for phenotypic flexibility and genetic variation would be antagonistic, that there would be selection for a population to be either phenotypically flexible or genetically variable. Several studies comparing congeneric species (Cumming, 1959; Marshall and Jain, 1968; Jain, 1979) have found evidence that one of the species is more genetically variable and the other more phenotypically plastic. One study (Grant, 1974) has found differences in genetic variation and phenotypic plasticity among adjacent populations of a single species. The purpose of this paper is two-fold. First, we present mathematical definitions for plastic variation and plasticity. Second, we compare plastic vari-

Sexual selection and sexual differences in mormon crickets (orthoptera: tettigoniidae, anabrus simplex).

Abstract: Darwin (1871) suggested that differences in the intensity of sexual selection on the sexes was a cause of secondary sexual differences. He proposed that behavioral and morphological traits possessed by males had evolved in the context of sexual competition, because fertilizable females are typically in short supply. Emlen and Oring (1977) pointed to several factors that are expected to change the ratio of fertilizable females to sexually active males (the operational sex ratio) and thus, the intensity of sexual selection on the sexes; theoretically both the investment of the sexes in offspring (parental investment; Trivers, 1972) and certain features of the social and physical environment are important. Parental investment theory (Trivers, 1972) predicts that females in species where males invest most in individual offspring should exhibit typically masculine traits by competing for access to sexually active males and males should exhibit typically feminine traits by being selective of mates. In vertebrate species where males provide the majority of parental care there is good evidence for a role-reversal in courtship behavior (Wittenberger, 1979; Petrie, 1983). In insects, paternal care of eggs and offspring is rare (Smith, 1980) but males of several groups supply their mates with nutritious prey items or glandular products (Thornhill, 1976a). These male efforts represent mating effort, as they apparently function in acquiring copulations (Alexander and Borgia, 1979). They are, however, "nonpromiscuous" mating efforts, potentially able to limit female reproduction (Thormhill, 1976b; Gwynne, 1984a) and thus are expected to influence the evolution of sexual differences in a similar way to male parental investment (Gwynne, 1984b). Males of many katydids (Orthoptera: Tettigoniidae) feed their mates with a large spermatophore which enhances female reproduction (Gwynne, 1984a). Mormon cricket males (Anabrus simplex Haldeman) produce a large spermatophore (Gillette, 1904), investing some fifth of their body weight in a single mating. I reported a reversal in the typical courtship roles for this species; females competed for access to singing males and males discriminated among females by selecting heavier individuals as mates (Gwynne, 1981). Here I show that this courtship rolereversal is found at certain sites but not at others. I examine sexual selection and its consequences at different sites by (1) testing the prediction that sexual selection on females, as estimated by variance in mating success, should be greater at sites where sexual competition among females is observed, (2) detailing the differences in courtship behavior between sites that have apparently resulted from differences in sexual selection, (3) testing the prediction that female body size should be greater at role-reversed sites as a consequence of intersexual selection for larger females, and (4) describing possible environmental differences between sites that have caused interpopulation variation in sexual differences.

Differences in fitness between seedlings derived from cleistogamous and chasmogamous flowers in impatiens capensis.

Abstract: It often occurred to me that it would be advisable to try whether seedlings from cross-fertilized flowers were in any way superior to those from selffertilized flowers. . .. I ought to have reflected that such elaborate provisions favouring cross-fertilization, as we see in innumerable plants, would not have been acquired for the sake of gaining a distant and slight advantage, or of avoiding a distant and slight evil.

Population density, outcrossing rate, and heterozygote superiority in ponderosa pine.

Abstract: viously been shown to be negatively correlated with self-fertilization, clumped pollen transfer, pollen size, and the ratio of stigma area to the pollen-bearing area on the pollinator. Here, the four correlations are shown to be consistent with evolutionary models of optimal allocation of resources to male and female functions, providing evidence that selection on sex allocation in hermaphrodites is governed by the same principles as selection on the sex ratio.

Sperm dependence of female receptivity to remating in drosophila melanogaster.

Abstract: As a consequence of copulation, males of Drosophila melanogaster induce a variety of physiological and behavioral effects in the female. Egg production and oviposition are stimulated by products of the male's accessory glands (Kummer, 1960; Garcia-Bellido, 1964; Merle, 1968; Bumet et al., 1973), and female attractiveness and receptivity are both reduced following copulation. Changes in female attractiveness are mediated pheromonally. A change in pheromonal profile from courtship-eliciting to courtship-discouraging pheromones is induced during the first 3 min of copulation (Tompkins et al., 1980; Tompkins and Hall, 1981; Venard and Jallon, 1980). The seminal fluid enzyme esterase-6 metabolizes the seminal fluid component cis-vaccenyl acetate to produce the antiaphrodisiac cis-vaccenyl alcohol (Mane et al., 1983). Behavioral effects of this metabolite on female attractiveness have been demonstrated and have been shown to be short-lived. There is a close correspondence between the time course of these behavioral effects and those first identified by Manning (1967) as the "copulation effect." Manning (1962, 1967) also identified a "sperm effect" on female receptivity which is longer lasting and causes females to remain unreceptive to male courtship. The strength of the sperm effect seems to be proportional to the number of sperm in storage (Gromko and Pyle, 1978; Gromko et al., 1984). The sperm effect is characterized behaviorally by the use of ovipositor extrusion to reject males, a behavior used only in very low frequency by virgin females (Connolly and Cook, 1973; Tompkins and Hall, 1981). The "sperm effect" -or the sperm dependence of the return of female receptivity-is not evident when mated females are confined with second males continuously for 24 h (Newport and Gromko, 1984). In this paper we investigate the details of the sperm dependence of female receptivity within the context of an experimental design which allows females periodic interactions with second males. We also quantify the impact of sperm-dependent female receptivity on the reproductive outcome of double matings. We show that by causing females to delay remating, first males suffer very little reduction in reproductive success due to female remating.

Influence of flower color polymorphism on genetic transmission in a natural population of the common morning glory, ipomoea purpurea

Abstract: The processes of genetic transmission are fundamental to evolutionary change. Because mating forms the link between generations an essential aspect ofgenetic transmission is the mating process. In animals the choice of mates is often mediated by behavior, with such criteria as courtship displays, position in a dominance hierarchy, or specific morphological features playing an important role. In plants, mating struture is in part controlled by the vectors utilized in transferring pollen from anther to stigma. These vectors may be simply wind or gravity, or they may be insects or other animals manipulated into performing pollination duty by a plant's floral characteristics. The behavioral characteristics of insect pollinators are thought to have a profound influence on the mating process. For instance, insect pollinators may exhibit consistent preferences when presented with a choice of flower color or morphology (Waser and Price, 1981, 1983). The concept ofan insect pollinator exhibiting flower constancy is very old, having been described by investigators in botany, agriculture and insect behavior (e.g., Grant, 1950). When choices are made among flower color types or morphological types in a polymorphic population, a system ofassortative mating is often imposed on the population. In the study of pollination biology it has been generally assumed that the transmission of genes is consistent with the observed

Evolution by kin selection: a quantitative genetic model illustrated by maternal performance in mice

Abstract: Models of evolution by kin selection have been developed primarily at the single locus level (Hamilton, 1964; Wade, 1978, 1980; Michod and Abugov, 1980; D. S. Wilson, 1980), even though most characters of evolutionary interest are polygenic. Recently, several quantitative genetic models have been derived for the evolution of altruism by kin or group selection (Yokoyama and Felsenstein, 1978; Aoki, 1982a, 1982b; Crow and Aoki, 1982; Engels, 1983). All of these models, both single locus and polygenic, have essentially confirmed the reliability of Hamilton's rule, that altruism will evolve when the additive genetic regression of the recipient on the altruist is greater than the ratio of selective costs to benefits. Alternatively, the evolution of altruism through kin selection can be approached as a special case of the joint evolution of correlated characters. In such a quantitative genetic model, there are two traits, the target phenotype and kin performance for this phenotype. The target phenotype can be any trait which is affected by interaction with kin. In most previous models, the target phenotype has been fitness, but in the model presented here this is generalized so that it can be any trait of interest to the researcher, such as weight. Kin performance subsumes all aspects of an individual's phenotype which have an effect on the target phenotype in its relative. Thus kin performance is the source of kin effects on the target phenotype. These kin effects are the measured phenotypic effects of kin performance on the target phenotype. Kin performance is, by definition, only measurable in terms of its effects on the target phenotype and therefore only exists in reference to a particular aspect of a relative's phenotype. Kin performance can be considered altruistic under certain selection regimes. This view of kin selection as a special case of the joint evolution of correlated characters highlights previously unrecognized or underrated features of the models cited above. First, the evolution of altruism through kin selection is a special case of a more general model of the evolution of traits which are affected by and affect the phenotypes of relatives. This has been especially appreciated by those using "trait group" models (Wade, 1978, 1980; Yokoyama and Felsenstein, 1978; D. S. Wilson, 1980; Crow and Aoki, 1982). Second, the only genetic effects on the target phenotype which have been considered are kin effects, which are the phenotypic manifestations of kin performance. The effects of genes carried by the target on its own phenotype, usually referred to as the direct effects of genes on the target phenotype, have not been included in the models. This is because the models have concentrated on the evolution of the potentially altruistic kin performance trait to the exclusion of the evolution of the target phenotype itself. This second feature of the previous models has also led to an unstated assumption that there is no net pleiotropic effect of genes directly affecting the target phenotype and kin performance. In the model presented below, direct effects of genes on the target phenotype and pleiotropic effects on the target phenotype and kin performance will be taken into account. The influence of kin performance on polygenic phenotypes has long been a