Showing papers in "Evolution in 1983"

The measurement of selection on correlated characters

Abstract: Natural selection acts on phenotypes, regardless of their genetic basis, and produces immediate phenotypic effects within a generation that can be measured without recourse to principles of heredity or evolution. In contrast, evolutionary response to selection, the genetic change that occurs from one generation to the next, does depend on genetic variation. Animal and plant breeders routinely distinguish phenotypic selection from evolutionary response to selection (Mayo, 1980; Falconer, 1981). Upon making this critical distinction, emphasized by Haldane (1954), precise methods can be formulated for the measurement of phenotypic natural selection. Correlations between characters seriously complicate the measurement of phenotypic selection, because selection on a particular trait produces not only a direct effect on the distribution of that trait in a population, but also produces indirect effects on the distribution of correlated characters. The problem of character correlations has been largely ignored in current methods for measuring natural selection on quantitative traits. Selection has usually been treated as if it acted only on single characters (e.g., Haldane, 1954; Van Valen, 1965a; O'Donald, 1968, 1970; reviewed by Johnson, 1976 Ch. 7). This is obviously a tremendous oversimplification, since natural selection acts on many characters simultaneously and phenotypic correlations between traits are ubiquitous. In an important but neglected paper, Pearson (1903) showed that multivariate statistics could be used to disentangle the direct and indirect effects of selection to determine which traits in a correlated ensemble are the focus of direct selection. Here we extend and generalize Pearson's major results. The purpose of this paper is to derive measures of directional and stabilizing (or disruptive) selection on each of a set of phenotypically correlated characters. The analysis is retrospective, based on observed changes in the multivariate distribution of characters within a generation, not on the evolutionary response to selection. Nevertheless, the measures we propose have a close connection with equations for evolutionary change. Many other commonly used measures of the intensity of selection (such as selective mortality, change in mean fitness, variance in fitness, or estimates of particular forms of fitness functions) have little predictive value in relation to evolutionary change in quantitative traits. To demonstrate the utility of our approach, we analyze selection on four morphological characters in a population of pentatomid bugs during a brief period of high mortality. We also summarize a multivariate selection analysis on nine morphological characters of house sparrows caught in a severe winter storm, using the classic data of Bumpus (1899). Direct observations and measurements of natural selection serve to clarify one of the major factors of evolution. Critiques of the "adaptationist program" (Lewontin, 1978; Gould and Lewontin, 1979) stress that adaptation and selection are often invoked without strong supporting evidence. We suggest quantitative measurements of selection as the best alternative to the fabrication of adaptive scenarios. Our optimism that measurement can replace rhetorical claims for adaptation and selection is founded in the growing success of field workers in their efforts to measure major components of fitness in natural populations (e.g., Thornhill, 1976; Howard, 1979; Downhower and Brown, 1980; Boag and Grant, 1981; Clutton-Brock et

Phylogenetic inference from restriction endonuclease cleavage site maps with particular reference to the evolution of humans and the apes.

Abstract: Recombinant DNA technology provides evolutionary biologists with another tool for making phylogenetic inference through contrasts of restriction endounuclease cleavage site maps or DNA sequences between homologous DNA segments found in different groups. This paper is limited to the problem of making phylogenetic inference from restriction site maps. Several methods for making such inference have already been used or proposed (Avise et al., 1979a, 1979b;NeiandLi, 1979; Ferris et al., 1981), but all these methods depend upon the assumption that shared restriction sites reflect common evolutionary origins and are not the result of convergent evolution. Unfortunately, convergent evolution occurs with high probability for this type of data (Templeton, 1983). In addition, data from several different restriction enzymes are generally pooled in these analyses. Recently, Adams and Rothman (1982) have examined the distributions of cleavage sites and related sequences for 54 restriction endonucleases. They concluded 1) that cleavage sites and related sequences are distributed non-randomly in most DNA sequences, 2) that there is considerable heterogeneity between different restriction enzymes (even those with recognition sequences of the same length) with respect to the number and distribution of their respective cleavage sites and related sequences, and 3) that inference of phylogenetic relationship based on distances will be biased. In addition, Brown et al. (1982) sequenced 896 base pairs of the mitochondrial DNA from humans and apes and concluded that about 90% of the substitutions were transitions. The predominance of transitions over transversions increases the probability of convergence over that expected when all base substitutions are assumed to be equally likely (Templeton, 1983). Therefore, a need exists for an algorithm of phylogenetic inference that deals more directly with the problem of convergent evolution and statistical inhomogeneity between different restriction enzymes. In this paper, I propose such an algorithm. After discussing the problem of estimation of a phylogenetic tree, the task of statistical testing is then addressed. First, I present a non-parametric statistical framework for testing the fit of one hypothesized phylogeny versus an alternative phylogeny. Second, non-parametric statistical procedures are presented for testing hypotheses about relative rates of evolution among the various lineages.

Coumarins and caterpillars: a case for coevolution.

Abstract: Ehrlich and Raven (1964) were among the first to focus on coevolution as a distinct evolutionary process. In their formulation, insect-plant coevolution is a five-step sequence: 1. by mutation and recombination, angiosperms produce novel secondary substances; 2. by chance, these new secondary substances alter the suitability of the plant as food for insects; 3. the plants, released from the restraints imposed by herbivory, undergo evolutionary radiation in a new adaptive zone; 4. by mutation or recombination, insects evolve mechanisms of resistance to the secondary substances; 5. able to exploit a plant resource hitherto excluded from herbivores, the adapted insects enter a new adaptive zone and undergo their own evolutionary radiation. This scenario was inspired by broad patterns of hostplant utilization among families of butterflies (Rhopalocera). Although the schema gained widespread acceptance, to date no specific example demonstrates most or all of the steps in the sequence. This lack of empirical evidence has been the subject of considerable criticism (e.g., Jermy, 1976; Janzen, 1980). Recent experimental work on associations between various insects and plants containing furanocoumarins and related compounds (Berenbaum, 1978, 1980, 1981a, 1981b, 1981c), however, provides a case study with either direct or circumstantial evidence for each part of the coevolutionary process.

Testing the constant-rate neutral allele model with protein sequence data.

Abstract: Langley and Fitch (1973, 1974) and Fitch and Langley (1976) statistically analyzed the pattern of nucleotide substitutions in seven proteins and 17 taxa and rejected the null hypothesis of a constantrate Poisson model of protein evolution. Their rejection of the constant-rate model was based on the extremely large observed value of a statistic (henceforth referred to as X2LF), which has a standard Chi-squared distribution under their null hypothesis. Gillespie and Langley (1979) showed that under a constant-rate neutral allele model, X2LF is not Chi-squared distributed. Though they did not obtain the exact distribution Of X2LF, they were able to show that the expectation Of X2LF is an increasing function of 0 = 4Nu, where N is the population size and u is the neutral mutation rate. Thus, the Langley and Fitch analysis does not constitute a test of the constant-rate neutral model. Evidently no test of the neutral model is possible using only the statistic X2LF, since no matter how large the observed value Of X2LF, a sufficiently large value of 0 could account for the observation. If one knew the distribution Of X2LF as a function of 0, one could use the observed value Of X2LF to estimate 0. I report here the results of Monte Carlo simulations that were used to characterize the distribution Of X2LF for several values of 0. The simulations were used to determine 0, the value of 0 for which the mean value Of X2LF over many simulation trials equals the observed value Of X2LF. The simulations were also used to determine Omil, the value of 6 for which approximately 5% of the simulation trials produced values Of X2LF greater than or equal to the observed val-

Quantitative genetics of development: genetic correlations among age-specific trait values and the evolution of ontogeny.

Abstract: It has recently been re-emphasized, by Gould (1977) and others, that evolution of adult forms takes place through the evolution of ontogenies. Direct selection at any age results in a correlated response at all other life history stages. This selection alters growth curves, or causes the evolution of ontogeny. Lande (1982) has presented a quantitative genetic model for the evolution of ontogenetically varying traits which clearly defines the parameters required for an analysis of ontogeny and evolution: (1) the genetic variance/covariance matrix; (2) the phentoypic variance/ covariance matrix; and (3) the vector of selection differentials. Genetic covariances or correlations among age-specific trait values quantitatively describe the genetic link between expressions of the same trait at different points in ontogeny. These genetic links between age-specific trait values are the result of pleiotropy, in this case the effects of one gene on the phenotype as expressed at more than one age, and linkage disequilibrium. The general effects of genetic variation on growth curves may be summarized using two parameters, curve height and curve shape (see Fig. 1). The average height of the curve can be defined as the average value of a trait over the ontogenetic period under consideration or the area under the growth curve divided by the length of the growth period. When there are only height differences between two growthi curves, the two curves will be parallel. Differences in curve shape, or the rate of growth at all ages, can be quantified independent of height, by standardizing the curves so that they have the same average height, or enclose the same area, and then calculating the absolute value of the area remaining between curves and dividing by the length of the growth period (see Fig. 1). This curve shape measure does not specify particular shapes but rather gives a general measure of differences in growth rates to compare with the independent measure of curve height differences. Specific curve shapes are presented without summarization. When heritabilities and phenotypic variances are the same at all ages, one can discern the independent effects of genetic correlations among age-specific trait values on ontogenetic variation and evolution. For example, in Figure 2 the ancestral ontogeny is displayed as a horizontal line (A) for purposes of illustration. Selection on the adult, or any other age, when the genetic correlations between ages are all 1.000, results in the descendant ontogeny, curve B. Note that the average height of the curve has changed but not its shape. When all the genetic correlations are one, the spectral decomposition of the age-specific traits correlation matrix includes only one non-zero eigenvalue equal to 'n,' the number of traits represented in the matrix, and an associated isometric eigenvector with loadings of 1/\Vn for each trait. This vector measures variation in growth curve height independent of variation in curve shape. Any deviation of the correlations

Competition between high and low mutating strains of escherichia coli.

Abstract: The dynamics of bacterial populations are often characterized by several distinctive features: under optimal growth conditions they double every few hours; they usually contain in excess of 106 individuals; higher fitness mutants have a good chance of arising in a population since average mutation rates are approximately 10-6 to 10-7 per gene replication; new favorable mutations, in the absence of genetic recombination, always increase to fixation in linkage with the genome of the parent clone in which they originally occurred; and higher fitness mutants often exhibit 10% to 20% higher growth rates than their parental clones. Consequently, when populations of such organisms are exposed to a new environment, a series of replacement cycles rapidly ensues, each cycle corresponding to the fixation of a higher fitness mutation in linkage with the genome of its parent clone. The linkage between the new mutation and the genome of the parent clone, and the rapidity of these clonal replacements, are two features that distinguish such asexual populations from ones that reproduce sexually. The existence of such cycles in asexual populations was first studied systematically by Atwood et al. (1951) in a study with laboratory populations of Escherichia coli in long-term cultures. By comparing the relative fitness of a series of bacterial clones isolated from these cultures, Atwood et al. were able to show that populations underwent a succession of clonal changes, each clonal replacement

Homage to santa anita: thermal sensitivity of sprint speed in agamid lizards

Abstract: Etude en Israel de l'evolution de la sensibilite thermique de 2 especes de lezards Agama savignyi et Stellio stellio en Israel

Sex ratio evolution under local mate competition in a parasitic wasp.

Abstract: A fundamental distinction in sex ratio theory concerns how selection acts upon autosomal genes in panmictic populations versus spatially structured populations. Fisher (1930) originally predicted that natural selection favors "equal investment" of resources in male and female offspring in panmictic populations. This results in a 50:50 primary sex ratio for species in which the cost of producing a son or a daughter is equal. Fisher's conclusion has been repeatedly confirmed mathematically (e.g., Shaw and Mohler, 1953; Bodmer and Edwards, 1960; Leigh, 1970). Hamilton (1967) observed that altering the assumption of panmixia led to quite different results. He considered a large population divided into many mating groups, each composed of the progeny of one or more female foundresses. After mating, the daughters disperse throughout the larger population before dividing into groups which produce the next generation. Such a population structure selects for a female-biased sex ratio because daughters of a parent compete for reproductive success within the large population whereas sons tend to compete with each other in the local mating group (Local Mate Competition). As the number of foundresses contributing progeny to a local mating group increases, the optimal sex ratio becomes less female biased (Hamilton, 1967; Taylor and Bulmer, 1980). With many foundresses contributing to a mating population, the optimum is one-half sons, as predicted for panmictic populations by Fisher (1930). Sex ratio evolution in spatially structured populations is of special interest be-

Breeding system and habitat effects on fitness components in three neotropical costus (zingiberaceae).

Abstract: Understanding the evolutionary significance of the diverse reproductive systems of plants and animals remains one of the most difficult problems in biology. The primary issue concerns the relative costs and benefits of varying degrees of genetic recombination (Muller, 1932; Maynard Smith, 1971, 1977; Ghiselin, 1974; Williams, 1975; Treisman, 1976; Charnov, 1979; Maynard Smith, 1979). Although this question is of general theoretical interest, the diverse reproductive systems and relative ease of experimental manipulation in plants renders them particularly suitable for a quantitative study of breeding-system evolution. The observation that many plant species combine selfing and outcrossing (see Jain, 1976; Clegg, 1980) suggests complex evolutionary responses to the selective forces influencing plant fitness. The advantage of outcrossing is generally thought to result from 1)an increase in the genetic variability among progeny (Williams, 1975; Maynard Smith, 1979), and 2) the greater fitness of individual progeny due to heterotic effects following outcrossing. Selfpollination may increase the probability and efficiency of seed production (Stebbins, 1957; Schemske, 1978a, 1978b), and provides local adaptation through reduced pollen flow (Antonovics, 1968; Jain, 1976) at the potential cost of a 50% decrease in heterozygosity every generation. As emphasized by Lloyd (1979), the genetic consequences of selfing have received considerable attention (Hayman and Mather, 1953; Allard et al., 1968; Jain and Marshall, 1968; Solbrig, 1972; Hillel et al., 1973; Gottleib, 1977; Jain, 1978; Brown, 1979), but the factors which influence the extent of selfand cross-fertilization have Revised August 3, 1982

Determinants of multiple host use by a phytophagous insect population

Abstract: The number of host species used by an insect population is an important component of niche breadth. The use of several species may arise from preference differences among the insects. Some may prefer one species and some another, and they may all use the plants which they prefer. However, there are at least two ways in which oligophagy could be generated without differences among insects in preference. First, the insects may have "neutral" preference, and use the first plant they encounter within the range of acceptable host species. Second, all insects may prefer A over B and C, but some may fail to encounter A and may then use B or C. It follows that some insects may not use their most highly preferred hosts, and that the relationship between host preference and host use may be indirect. These terms are employed in the following way: two insects differ in host use if they actually feed on different plant categories, usually different species. On the other hand, insects which, in the same situation, have different likelihoods of using particular plant categories are described as differing in their host preference. This distinction is important, since natural selection acts directly on host use, not preference. One can make no assumptions about the nature of selection pressure on preference without knowing how variation in use is related to variation in preference. In consequence, a study of this relationship, as undertaken in this paper, is an essential step in any investigation of the evolution of either preference or use. The work reported here is part of a more general study concerning the behaviors by which butterflies select their host plants and the ways in which natural selection may act upon these behaviors. How and why do conspecific populations differ in the identities of the host plants they use or in the diversities of the hosts they attack? Such questions can be answered by examining the ways in which the behavior of individuals generates patterns of host exploitation by populations, and by investigating the consequences of host selection decisions that the insects make. The principal question posed in this paper is: how does the use of four host species result from the behavior of individual insects in a population of the nymphaline butterfly Euphydryas editha? I also ask in what ways the behavior of butterflies in this oligophagous population differs from that of the insects in monophagous populations of the same species. The answers to these questions lie principally in the oviposition behavior of E. editha rather than in larval host selection behavior. This is because newly-hatched larvae must find food within two or three inches of the oviposition site (Singer, 1971). E. editha was chosen as the study organism for three reasons. 1) It shows extensive ecotypic variation in host use that depends at least in part on heritable variation in oviposition preference (Singer, 1971; White and Singer, 1974, and unpubl.). 2) There is equally striking variation in the degree of host specialization. Some populations are strictly monophagous. Others are usually monophagous, but occasionally include a second host species in their diet. In a few populations, such as the one described in detail in this paper, eggs are regularly laid on as many as four host genera. This variation among populations could have a simple explanation: if there were five acceptable host species, the insects could be monophagous where only one of these plants grows and oligopha-

Sexual selection and communication in a neotropical frog, physalaemus pustulosus.

Abstract: Many sexually dimorphic characters have evolved under the influence of sexual selection, either because they increase an individual's ability to compete for access to members of the opposite sex, or because they increase an individual's probability of being chosen as a mate (Darwin, 1871). Some of the most elaborate displays in animal communication are those used by males to attract and court females. Male sexual displays may provide information which allows females to identify males as conspecifics (Mayr, 1963) or possibly to assess mate quality (Fisher, 1958; Trivers, 1972). Other suggested functions of sexual displays, such as synchronizing male and female receptivity (Lehrman, 1965) or persuading a female to approach an aggressive male (West Eberhard, 1979) probably are proximate effects of a signal which evolved to enhance mate discrimination (also see Dawkins and Krebs, 1978). Most sexual displays seem to contain information far in excess of that needed for species recognition (but see Rand and Williams, 1970). Darwin (1871) maintained that male sexual displays allowed females to pick more vigorous males and that female choice thus caused the further elaboration of male sexual displays. This idea was extended by Fisher's theory of runaway sexual selection (Fisher, 1958; also see Trivers, 1972; West Eberhard, 1979; Lande, 1980, 1981). Thus there is an important relation between sexual selection and communication. However, few studies have demon-

Ecological genetics of daphnia pulex.

Abstract: Planktonic cladocerans provide a valuable resource for examining the interface between population ecology and genetics. Selective factors such as predators, food availability and permanency of the environment are known to vary dramatically between populations as well as seasons (Lynch, 1980). As cladocerans reproduce by ameiotic parthenogensis during most of the year, selection operates on specific genotypes for repeated generations. However, the long-term response of a population to selection is related to the operation of the mating system which serves the dual function of producing resting eggs and in most cases generating genetic variability (Banta, 1939). The intensity and timing of sexual reproduction is in turn a function of the biological and physical environment; most cladocerans only engage in sexual reproduction when they become physiologically stressed by high densities or some other factor (Berg, 1934; Banta, 1939; D'Abramo, 1980). The importance of the physical environment was revealed by Hebert (1974a, 1974b). Analysis of a number of Daphnia magna populations inhabiting temporary ponds showed that genotypic frequencies at polymorphic loci were generally in close agreement with Hardy-Weinberg expectations and temporally stable within years, suggesting that mating was random and that selection among genotypes was negligible. Populations of the same species in permanent habitats tended not to be in Hardy-Weinberg equilibrium and frequently exhibited violent fluctuations in genotype frequency over short periods of time. Hebert suggested that prolonged parthenogensis in permanent populations leads to the selection of a small number of highly adapted clones, but that the recurrent bouts of sexual reproduction that must Revised April 17, 1982

Gene diversity and genetic structure in a narrow endemic, torrey pine (pinus torreyana parry ex carr.).

Abstract: Recent reviews have suggested that tree species are the most variable of organisms, as measured by proportion of polymorphic loci or average heterozygosity (Hamrick, 1979; Hamrick et al., 1979). Average heterozygosity is greater than 0.30 for several conifers. By contrast, annual herbaceous species have a mean heterozygosity of 0.13 (Hamrick et al., 1979). Conifers have several mechanisms that promote outcrossing, and are expected to maintain high levels of genetic variation. Despite these mechanisms, it is uncertain whether the breeding system is capable of maintaining variability in small populations and in the absence of migration. While the genetic consequences of reduced population size have long been understood in theory, empirical evidence is scarce in plants, particularly in tree species. The potential to maintain high levels of genetic variation in reduced and scattered populations is important because of its implications for genetic resource conservation, and in fact, for the ability of species to respond to environmental change and avoid extinction. Because most conifers are commercially valued and exploited for lumber and paper products, much of the original forest in North America has been destroyed, and the tendency under management will be to reduce native populations to scattered relicts. One way of forecasting the fate of species reduced in numbers is to make use of natural experiments, by examining the genetic structure of species that occur in disjunct populations (Shaffer, 1981). We chose to investigate how much variation might be preserved in conifers by observing a narrow endemic, Torrey pine (Pinus torreyana Parry ex Carr.). The investigation also contributes to an understanding of the biogeography of the California Channel Islands. Torrey pine has the smallest population of any known pine. The 1973 count in the 445 ha (1,100 acre) Torrey Pines State Reserve on the Pacific Coast at San Diego, California was 3,401 mature trees (Calif. Dept. Parks Rec., 1975). Including seedlings, the Reserve's naturalist estimated ca. 7,000 individuals in 1979 (H. Nicol, pers. comm.). The population includes only two other small stands, contiguous with the Reserve. Another population occurs on Santa Rosa Island, one of the Northern Channel Islands off the California coast near Santa Barbara. There may be 2,000 individuals on the northeast coast of the island. Climatic, edaphic, and floristic characteristics of the two sites were summarized by Haller (1967). The San Diego and Santa Rosa Island populations are separated by 280 km and the island is 40 km from the mainland. It is highly unlikely that there has been any significant opportunity for gene exchange within recent centuries. Nor is Torrey pine likely to exchange genes with its closest relatives, digger pine (Pinus sabiniana Dougl.) and Coulter pine (Pinus coulteri D. Don), both of which are allopatric. Controlled crosses with Digger pine succeed only with difficulty and there have been no hybrids with Coulter pine despite several attempts (Critchfield, 1966).

The heritability of external morphology in darwin's ground finches (geospiza) on isla daphne major, galápagos.

Abstract: Ecologists use avian morphological measurements to develop and test evolutionary theories. The theories are usually based on genetic models, although little is known about the inheritance of such characters. In field studies it is commonly assumed that phenotypic variation closely reflects underlying genetic variation (Grant et al., 1976), while theoreticians sometimes assume that heritabilities are equal to one (Long, 1974). P. R. Grant and his colleagues (Grant et al., 1976; Abbott et al., 1977; Boag and Grant, 1981; Grant, 198 lb) have been studying Darwin's ground finches (Geospiza) in the Galapagos, focusing on relationships between finch morphology and food supplies. Unlike Mendelian characters such as plumage polymorphisms (Mineau and Cooke, 1979), finch morphology involves metric characters and is studied using quantitative genetics (Falconer, 1981). Quantitative genetics describes the phenotypic value of an individual (e.g., its

Structural relationships between spines and lateral plates in threespine stickleback (gasterosteus aculeatus).

Abstract: Of the five genera of stickleback (Gasterosteidae), Casterosteus has the longest dorsal and pelvic spines and the most extensive expression of lateral bony plates (Nelson, 1971). Spines, which can be locked into an erect position (Hoogland, 1951), enlarge the effective body diameter of the stickleback and may pierce the mouthparts of a predator during manipulation, increasing the opportunity for escape (Hoogland et al., 1957). As might be expected, in localities where predatory fish are common, the stickleback spines are relatively longer than in populations where such predators are rare or absent (Hagen and Gilbertson, 1972; Gross, 1978a). Numbers of lateral plates are also geographically variable (for review see Bell, 1976; Wootton, 1976). Most marine populations have a row of 35 lateral plates on each side of the body extending from the head to the caudal peduncle (Fig. 1a). These plates are greatly reduced in number in many freshwater populations in western North America and southwest Europe. While populations are known in which fish lack all lateral plates (Miller and Hubbs, 1969; Moodie and Reimchen, 1973), the majority have from three to eight plates per side in the anterior region of the body (Fig. 1b). Early investigators (Bertin, 1925) considered this variation to be environmentally induced, but subsequent genetic analysis showed that a large component of the variance is heritable (Munzing, 1959; Lindsey, 1962; Hagen, 1973; Hagen and Gilbertson, 1973a; Avise, 1976). However, the adaptive significance

Extensive genetic variation in mitochondrial dna's among geographic populations of the deer mouse, peromyscus maniculatus.

Abstract: Restriction endonucleases have recently permitted the detection of extensive intraspecific mitochondrial DNA (mtDNA) sequence heterogeneity in a number of mammalian species (e.g., Potter et al., 1975; Upholt and Dawid, 1977; Avise et al., 1979b; Brown, 1980; Brown and Simpson, 1981; Ferris et al., 1981). Because mtDNA appears to evolve very rapidly (Brown et al., 1979), and because mtDNA seems to be strictly maternally inherited (Hutchison et al., 1974; Lansman et al., 1983), the molecule has been advocated as a potentially useful marker for estimating matriarchal phylogenies within and among conspecific populations and closely related species (Lansman et al., 1981). To date, however, only a small handful of studies have begun to exploit this potential for natural population analysis (Avise et al., 1979b; Brown and Wright, 1979; Brown, 1980; Brown and Simpson, 1981; Ferris et al., 1981a, 1981b). In the first paper of this series, we included a description of mtDNA restriction fragment divergence among three geographically distant samples of the deer mouse, Peromyscus maniculatus (Avise et al., 1979a). Of particular interest was the observation that samples from Colorado and southern Michigan were very similar in mtDNA sequence (estimated nucleotide divergence, P .005), while a sample from North Carolina was very distinct from these (P .040). Here we greatly extend our survey of the deer mouse by analyzing mtDNA sequence variation in 135 animals collected across much of the vast range of the species in North America. Our procedure has been to map restriction sites recognized by eight endonucleases. The restriction maps permit comparisons of the pattern and character of changes in the mtDNA molecule that have occurred during its evolutionary history in P. maniculatus. Thus the data presented in this paper are of considerable significance to two different but closely interrelated aspects of mtDNA evolution: (1) the ' Number IV in the series, "The use of restriction endonucleases to measure mitochondrial DNA sequence relatedness in natural populations." 2 Current address: Laboratory of Genetics, National Institute of Environmental Health Sciences, Research Triangle Park; North Carolina 27709.

Genetic variability within and among populations of the black-tailed prairie dog.

Abstract: Genetic heterogeneity over short geographic distances has been observed for many populations of small mammals (house mice: Selander, 1970; deer mice: Wright, 1978; pocket gophers: Patton and Yang, 1977; Smith et al., 1978; Wright, 1978; Patton and Feder, 1981) as well as for large, highly mobile species such as the elephant (Osterhoff et al., 1974), moose (Ryman et al., 1977, 1980), red deer (Gyllensten et al., 1980), and white-tailed deer (Manlove et al., 1976; Chesser et al., 1982). For most studies of the genetic structure of populations the specific mechanisms of genetic differentiation have not been identified. To understand the causes of population subdivision more fully, comparison of genetic variability should be made among the breeding and/or social units, rather than arbitrarily selected samples. Allele frequency differences among observed social groups within populations have been documented for house mice (Selander, 1970), dark-eyed juncos (Baker and Fox, 1978), marmots (Schwartz and Armitage, 1980), and humans (Neel and Ward, 1972). The organization of populations into independent breeding units may have important effects on the shortterm evolution of populations (Wright, 1980) as well as on the maintenance of genetic polymorphisms (Chesser et al., 1980; Karlin and Campbell, 1980). The black-tailed prairie dog (Cynomys ludovicianus) is perhaps the most socially complex of any rodent species (King, 1955; Koford, 1958). Prairie dog populations are comprised of several small coteries (har-

A natural experiment in life-history evolution: field data on the introduction of mosquitofish (gambusia affinis) to hawaii.

Abstract: Patterns in life-history variation were first explained by differences in mode of population regulation (rK Mertz, 1975; Taylor and Condra, 1980; Barclay and Gregory, 1981) and by comparisons of wild populations (see articles reviewed in Stearns 1976, 1977, 1980) are these: (1) High, variable, or unpredictable adult mortality rates select for increased reproduction early in life. (2) High, variable, or unpredictable juvenile mortality rates select for decreased reproduction and longer adult life., This paper begins a test of age-specific models. In designing this study, I kept in mind certain criteria for assessing life-history studies (Stearns, 1977): they should contain measurements of the environmental factors invoked as evolutionary causes, of the variation of life-history traits among populations in the field, of the genetic component of that variation measured in the laboratory under constant conditions, of the time-series of age-specific mortality rates, and of reproductive efforts. Here I report evidence bearing on the first two criteria. In Stearns (1983) I report evidence bearing on the third. Without field data on ageand size-specific mortality rates, I cannot directly test current predictions. This paper tests assumptions about the differences between stable and fluctuating environments and evaluates alternative explanations of what causes variation among populations.

The relation of growth to heterozygosity in pitch pine.

Abstract: The connection between fitness and heterozygosity has eluded geneticists for decades. The classic form of the Neo-Darwinian argument hypothesizes that heterozygosity confers genetic homeostasis (Lerner, 1954); i.e., multiple, molecular forms of the same enzyme endow the organism with a broader range of tolerance to environmental variation because different forms may differ in their optima for temperature, pH, and other factors (Johnson, 1976). In maize (Zea mays L.), inbred lines and their hybrids are similar in growth rate when raised under constant conditions at optimal temperatures, but hybrid superiority becomes progressively more obvious as conditions deviate from the optimum (McWilliam and Griffing, 1965). Apparently, homozygotes in maize can only deal successfully with a narrow range of conditions as compared to heterozygotes. In a variety of organisms, including butterflies, fish, and oysters, variance for morphological traits decreased with increasing heterozygosity at enzyme loci (Eanes, 1978; Mitton, 1978; Zouros et al., 1980), and bilateral symmetry, a measure of developmental homeostasis, increased with heterozygosity in lizards and bivalves (Soule, 1979; Kat, 1982). Growth rate increased with heterozygosity in oysters and salamanders (Singh and Zouros, 1978; Zouros et al., 1980; Pierce and Mitton, 1982). The perennial herb, cylindric blazing star (Liatris cylindracea Michx.), can be roughly aged by counting rings in the corm, the subterranean perennating organ. In natural populations, heterozygosity was higher among older than among younger individuals, suggesting poor survival for homozygotes (Schaal and Levin, 1976). Grown in a uniform environment, biomass production and reproductive potential of the herb were weakly, but positively, related to the level of enzyme heterozygosity. However, Mitton and colleagues (Mitton and Grant, 1980; Knowles and Grant, 1981; Mitton et al., 1981) noted conflicting results in forest trees, which are among the most polymorphic of organisms (Hamrick, 1979; Hamrick et al., 1979). In only one of three tree species was there a positive association between mean width of the annual ring, taken as a measure of growth and fitness, and heterozygosity. In two of the species, variance in ring width increased with increasing heterozygosity and in the third, it decreased. The relationship between growth and heterozygosity in trees is confusing, and more data are needed to draw firm conclusions. Our objective was to relate growth to heterozygosity in pitch pine (Pinus rigida Mill.), for which we had information on 21 enzyme loci in several populations. We were also interested in whether heterozygotes had greater longevity and whether annual growth was more or less well-buffered in heterozygotes than in homozygotes. A positive association be-

Density-dependent pollinator foraging, flowering phenology, and temporal pollen dispersal patterns in linanthus bicolor.

Abstract: Gene flow in flowering plant populations occurs through the dispersal of pollen and seeds. In animal-pollinated plants, the foraging behavior of pollinators can affect the distance and frequency of pollen transfer between and within plants, and thus can have an important impact on gene dispersal and outcrossing potential (Levin, 1979a, 1979b). The distribution of gene dispersal distances, the rate of outcrossing and the density of reproductive individuals in a population determine the size of a genetic neighborhood, in the sense of Wright's (1943, 1946) isolation by distance model. In general, as neighborhood size and area increase, a population's evolutionary potential for microgeographic genetic differentiation will diminish (Wright, 1943, 1946). Several authors have estimated neighborhood size and area by using pollen dispersal distances inferred from pollinator flight patterns (Kerster and Levin, 1968; Levin and Kerster, 1968, 1969; Schaal and Levin, 1978; Beattie, 1979; Beattie and Culver, 1979; Schmitt, 1980a, 1980b; Waser, 1982; Zimmerman, 1982). Recent studies of artificial populations indicate, however, that actual gene flow may often be more extensive than that estimated from pollinator movements (Schaal, 1980; Levin, 1981; Ennos and Clegg, 1982; Handel, 1982). These findings probably result from the carryover of pollen on a pollinator's body from a given plant successively to many other plants (Thomson and Plowright, 1980; Morse, 1982; Price and Waser, 1979, 1982; Waser

Relative fitnesses of selfed and outcrossed progeny in gilia achilleifolia (polemoniaceae).

Abstract: The evolution of self-pollination is a recurrent theme in the history of the angiosperms, and has long been of interest to evolutionists (Darwin, 1876; Darlington, 1939; Mather, 1943; Baker, 1955; Stebbins, 1957; Allard et al., 1968; Jain, 1976; Maynard Smith, 1977; Lloyd, 1979). Yet despite the widespread attention that the topic has received, the factors important to the evolution of self-pollination (autogamy) are poorly understood. As noted by Jain (1976), there are many hypothesized advantages associated with autogamy, but few data bearing on these hypotheses. One of the commonly cited advantages of selfing, and the most general, has been referred to as the automatic selection advantage (Jain, 1976). This advantage arises from the fact that while an outcrossing individual contributes on the average, one haploid genome through its ovules and one through its pollen, a selfing variant contributes up to three haploid genomes on the average, two through its selfed offspring, and one by outcrossing as a pollen parent. Unless the inbred progeny have reduced fitness, mutations promoting selfing are expected to increase in frequency by selection (Fisher, 1941; Maynard Smith, 1977; Lloyd, 1979, 1980). Inbreeding depression in the progeny of selfed parents may impart a reduction in viability, fecundity or both, sufficient to impede the selection of genes for self-pollination. According to Lloyd (1979), in situations where a selfing mutant arises in an outcrossing population, the critical magnitude of inbreeding depression above which mutant genes promoting selfing are

Host range evolution: the shift from native legume hosts to alfalfa by the butterfly, colias philodice eriphyle

Abstract: Many insects have expanded their host range to include crop plants, and thereby have become major pests. Host plant shifts to crops are not only economically important, but also present an excellent opportunity to study the evolutionary relationships between herbivorous insects and their food plants. "Host plant shift" is defined here as the process by which one or more formerly used host plant species are abandoned in favor of one or more new host plant species. If some populations of an herbivorous insect shift hosts while other populations do not, the net result is "host range expansion," the addition of one or more host plant species to the total number used by the herbivore species. Genetic changes in host utilization may be required for some host shifts (Ehrlich and Raven, 1964). For example, mutations affecting host choice might be necessary for oviposition on a novel host, or mutations which improve detoxification capability might enable an herbivore to exploit a previously toxic host species. In some cases, changes in extrinsic factors, such as the availability of host plant species, may lead to host shifts without requiring genetic changes in the herbivore. Some shifts to crop plants may fit this pattern. Even when genetic changes are not required for the initial shift to a new host, differential host use may lead to evolutionary divergence between populations. Evolutionary theory predicts that popu-

The genetic basis of differences in life-history traits among six populations of mosquitofish (gambusia affinis) that shared ancestors in 1905.

Abstract: Students of life-history evolution have relied for some time on inferences drawn from differences observed among populations and species. Such differences usually have both environmental and genetic components, but the studies in which the two components have been distinguished (e.g., Birch, 1953; Clatworthy and Harper, 1962; Birch et al., 1963; Keith et al., 1966; Gadgil and Solbrig, 1972; Hickman, 1975; Oka, 1976; Dawson, 1975; McCauley, 1978; Stearns and Sage, 1980) have remained a small, but important, part of the literature. This paper documents genetically-based differences in life-history traits among six stocks of mosquitofish, Gambusia affinis, whose approximately 150 common ancestors were introduced to Hawaii in 1905 for mosquito control. Previous work (Stearns, 1983) has shown that 24 stocks of mosquitofish in Hawaiian reservoirs differ significantly in life-history traits in the field, that more of the variation in life-history traits is explained by differences among individual stocks than by classifying the reservoirs as stable or fluctuating in water level, and that three hypotheses could account for this pattern. The first, that the stable-fluctuating dichotomy misleads, concealing considerable diversity of types of fluctuations, is well-supported (Stearns, 1983). The second, that the differences contain a genetic component and have risen through either founder effects, drift, or local adaptation, is examined here. The third, that the differences are due to developmental plasticity, is evaluated in Stearns (1983). This paper demonstrates that there is a genetic component to many of the differences, and that some of the genetic differences are large and have arisen rapidly, but it does not isolate the evolutionary origin of the genetic differences. The method used here to detect genetic differences between stocks is a compromise between power and difficulty. I held wild-caught fish in the laboratory for three months under similar conditions to reduce maternal effects, then raised their offspring on controlled rations and at constant temperatures in individual containers. This straight-forward method is best suited for surveys of a number of stocks; crosses between stocks and backcrosses to parental types would be required to rule out maternal effects definitively. The fish discussed in this paper are the F 1 generation born to wild-caught parents. I later reared an F2 generation from full-sib/half-sib crosses within each of two stocks, and report on patterns of heritabilities elsewhere (Stearns, in press). In the F 1 generation I worked with six stocks, two from reservoirs with stable water levels and four from reservoirs with fluctuating water levels.

Pollen dispersal by hummingbirds and butterflies: a comparative study of two lowland tropical plants.

Abstract: Because of the difficulties in measuring the distances to which pollen or seed is dispersed, information on gene flow in plant populations is quite limited (Levin and Kerster, 1974). This is particularly true of plants in complex species-rich tropical communities for which such information is of much value in models of speciation for tropical forest trees (Fedorov, 1966; Ashton, 1969; Bawa, 1974, 1979). To the best of our knowledge there are only four reports of pollen dispersal in tropical plants (Linhart, 1973; Frankie et al., 1976; Linhart and Mendenhall, 1977; see also Linhart and Feinsinger, 1980). Here we report the results of a comparative study of pollen flow for two early successional species in Costa Rica: Malvaviscus arboreus is a hummingbird-pollinated shrub or small tree, with hermaphrodite flowers and Cnidoscolus urens is a monoecious, butterfly-pollinated annual herb. The subject populations of the two species occupied adjacent habitats; M. arboreus was confined to margins and light gaps in the river forest and C. urens was found in a fallow rice field adjoining the river forest. Apart from differences in sexual and pollination systems, habit, and habitat, the two species also differed in population density: C. urens occurred in relatively dense patches, 3.0 x 10-2/m2, whereas the population of M. arboreus was more scattered, 3.11 x 10-3/m2 in our sample. Although our principal objective was to determine the effect of differences in pollinator foraging behavior and plant density on pollen flow, our studies also differ

Dna/dna hybridization studies of muroid rodents: symmetry and rates of molecular evolution

Abstract: Data from several sources suggest that molecular and organismal rates of evolution often differ (Sarich and Wilson, 1967a, 1967b; Wilson et al., 1974a, 1974b, 1977; Prager and Wilson, 1975; Bruce and Ayala, 1979; Ferris et al., 1981). Protein molecules, for example, are useful systematic tools because they seem to evolve in a stochastically regular manner (Fitch, 1976). In contrast, others note that rates of morphological evolution seldom exhibit such time-dependence (Eldredge and Gould, 1972). The source of comparative data thus influences perceptions of species relatedness, taxonomic status, and divergence time (c.f. Byrd, 1981). For example, on the basis of electrophoretic and immunological analyses, human and chimpanzee might be considered as sibling species that diverged 4-6 million years ago (MYA) (Sarich and Wilson, 1967a, 1967b; Wilson and Sarich, 1969; King and Wilson, 1975; Sarich and Cronin, 1976; Wilson et al., 1977). However, on the basis of morphological characters, human and chimpanzee are placed in different families (Hominidae and Pongidae), and their divergence from a common ancestor is placed earlier (Pilbeam, 1970; Simons, 1976; see also Bruce and Ayala, 1979). To reconcile the apparent disagreement between morphologicallyand biochemically-based primate classifications and divergence estimates, Uzzell and Pilbeam (1971) and Goodman et al. (1971) proposed that biochemical similarities might be a result of an evolutionary slowdown within the hominoid lineage. Sarich and Wilson (1973) evaluated this hypoth-

Clonal diversity and population structure in a reef-building coral, acropora cervicornis: self-recognition analysis and demographic interpretation.

Abstract: Description of genetic variation in a population, recognized by Hubby and Lewontin (1966) as providing "the fundamental datum of evolutionary studies," is now routinely accomplished by electrophoretic characterization of allozyme variation and by other molecular techniques. These studies have revealed considerable variation at the molecular level, as well as the structure of this variation within and among populations. Although the value of the molecular approach in population genetics is beyond dispute, there remain interesting aspects of population variation not directly assayable by these methods. Biological self-recognition phenomena, exemplified by self-sterility in flowering plants and immune systems in animals, have been utilized in a population genetic context for only a few species. An analysis of genetic population structure based upon a self-recognition phenomenon requires a relatively straightforward assay incorporating the phenomenon, and an established relationship between the detectable polymorphisms and a more general aspect of population structure. Harberd used a combination of morphological characters and self-sterility relationships to deduce clonal structure in populations of the grasses, Festuca rubra and F. ovina (Harberd, 1961, 1962a) and the clover, Trifolium repens (Harberd, 1962b). In most vertebrate populations nearly every individual possesses a unique histocompatibility type, but some vertebrate population structures have been characterized by a departure from this condition of complete diversification. Kallman (1964) interpreted the occurrence of duplicated histocompatibility types within isolated populations of platyfish as an indication of inbreeding. For populations in which asexual reproduction occurs, some authors (Maslin, 1967; Cuellar, 1976, 1977; Angus and Schultz, 1979; Angus, 1980) have assumed that each clone is distinguished by a unique histocompatibility type, thus equating the diversity of incompatible strains with clonal diversity. In one such study (Angus and Schultz, 1979), a tissue graft analysis applied to populations of the unisexual fish, Poeciliopsis monacha-lucida, resolved more clones than had been detected in an earlier electrophoretic survey (Vrijenhoek et al.,

An electrophoretic study of evolution in capsicum (solanaceae).

Abstract: Most recent treatments of the peppers, genus Capsicum (Solanaceae), recognize four or five domesticated taxa and 20 or more wild species (Eshbaugh, 1980a). The distribution of the species is fairly well documented with the exception of the Brazilian taxa. The genus includes a number of wide ranging species as well as several narrow endemics, e.g., C. galapagoensis, C. cardenasii, etc. All taxa are at least facultative self-fertilizers with the exception of C. cardenasii. A survey of chromosome numbers indicates n = 12 or 13 (Pickersgill, 1977), suggesting two lines of evolution which may eventually require some realignment at the generic level. Changing concepts on evolution of the genus have led from the hypothesis of C. frutescens as the single wild progenitor of all the domesticates (Davenport, 1970; Ramalingam, 1972; Jett, 1973), to four or five distinct progenitors each giving rise to a domesticate (Heiser et al., 1971) and alternatively, to three independent lines of evolution leading to domesticated C. baccatum, C. pubescens, and a complex of C. annuum, C. chinense, and C. frutescens (Eshbaugh, 1980b). Various approaches to investigating and understanding evolution in the genus have included comparative morphological investigation, karyotype analysis (Ohta, 1962; Shopova, 1966; Pickersgill, 1977) and the study of breeding behavior (Pickersgill, unpubl.; Eshbaugh, 1975). We use the technique of starch gel electrophoresis to provide data for a different perspective of evolution in Capsicum. Re-

Morphological variation in the neotropical salamander genus bolitoglossa.

Abstract: There are two aspects to any process of evolutionary change. One is the probability of survival of any given trait within a given context (i.e., other traits present in the individual and in the population, physical environment, population structure, etc. . . .). That is, the probability of fixation of a trait under the action of natural selection and stochastic processes. The other aspect is the nature, and origin, of the variation upon which the agents previously mentioned can operate. Mutation and recombination have been the mechanisms traditionally invoked in the generation of variation at the genetic level. However, selection operates at the level of the phenotype and, therefore, it is justified to ask if the properties of the variation at the genetic level can be extrapolated to the phenotypic level, particularly to the level of complex morphologies. Recently, Ho and Saunders (1979), Alberch (1980, 1982a, 1982b), Gould (1980), Rachootin and Thomson (198 1), Oster and Alberch (1982) and Wagner (1982) among others have contended that there are qualitative differences between modes of evolution at the genetic and epigenetic levels. A simple gene-phenotype extrapolation would only be correct if there were a oneto-one correspondence between a given morphological trait and a given region of DNA in the genome. This is obviously not the case. Morphology is the product of complex, and very often regulatory, genetic and epigenetic interactions. By epigenetic I mean interactions among gene products and their environment, not, as often assumed, nongenetic. These developmental interactions constrain the expression of morphological variation upon which natural selection operates. At the morphological level, one can abstract these processes and view phyletic phenotypic change as the result of genetically mediated regulation of a resilient developmental program. The present study addresses the above issue from an empirical perspective. I ask the question: What kind of phenotypic variation do we find in natural populations? However, I purposely do not focus on quantitative variations in metric traits as is often done (e.g., Lande, 1976; Cheverud, 1982). Rather, I analyze the variation encountered in osteological characters used in taxonomy and systematics. Taxonomists have chosen these traits, not because of their adaptive properties, but due to the diagnostic properties of these discrete and highly invariant traits. Furthermore, when paleontologists discuss evolution above the species level in the fossil record, they are basically studying the patterns of change in these species-diagnostic traits. Since the paleontological definition of species is based on purely morphological criteria, analyses of mechanisms of morphological change of such features among extant species can be readily applied to actual macroevolutionary patterns in the fossil record and related to contemporary controversies (e.g., Stanley, 1979). I describe the intra-specific (mostly intra-populational) variation found in the dermal bones of the nasal region, and in the number of tarsal elements and phalanges in various species of the neotropical salamander genus Bolitoglossa. This is a highly diverse genus, characterized by its adaptive radiation into the New World tropics (see Wake and Lynch, 1976, for an introduction to the genus). The main conclusion of this paper is that the variation encountered is characterized by a recurrence of the same variants in