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Showing papers in "Evolution in 1980"


Journal ArticleDOI
TL;DR: Sexual dimorphism may result from natural and/or sexual selection, and systems of mating are often thought to evolve in response to ecological pressures, although mating preferences may be self-reinforcing.
Abstract: Conspicuous sexual dimorphism is a feature of many species of higher animals. The genetic basis of variation in metrical characters, including that in sexual dimorphism between families or lines, is usually polygenic (Falconer, 1960; Frankham, 1968b; Wright, 1968, 1977; Bird and Schaffer, 1972; Ehrman and Parsons, 1976). Genetic experiments on mice, birds and Drosophila flies indicate that artificial selection practiced on a character of one sex causes not only a direct response of the character in the selected sex, but also a correlated response of the homologous character, if any, in the opposite sex (Shaklee et al., 1952; Harrison, 1953; Korkman, 1957; Becker et al., 1964; Eisen and Legates, 1966; Frankham, 1968a, 1968b; Eisen and Hanrahan, 1972). Such correlated selective responses are attributable to pleiotropy (and linkage) of genes affecting the characters of both sexes, that is, correlations between the additive effects of genes as expressed in males and females. The genetic correlation between homologous characters of the sexes is often quite high (op. cit.). As will be shown, this greatly restricts the rate of evolution of sexual dimorphism relative to that for the average phenotype of the two sexes. Sexual dimorphism may result from natural and/or sexual selection. Darwin (1874, Part 2) elucidated how natural selection operating differently on males and females arises from their distinctive roles in reproduction, or from competition between the sexes for resources such as food, leading to adaptive sexual dimorphism. He also reasoned that intrasexual contests for mates and intersexual mating preferences exert sexual selection, usually on the males, producing sexual dimorphism which is maladaptive with respect to natural selection. Comparisons within and between closely related species led Darwin to conclude that adult males typically are more modified than adult females or juveniles of either sex, but that females have often acquired male characters by "transference." It was difficult for Darwin to believe that sex-limitation of characters could evolve by selection, but Fisher (1958, Ch. 6) outlined how divergent selection on the two sexes could accumulate genes with different effects in males and females, causing a character at first expressed equally in both sexes to become sexually dimorphic and finally sex-limited. The strength of sexual selection is enhanced by a polygamous mating system, but the possibility of sexual selection in monogamous systems of mating exists due to male competition for early-breeding females, and mate choice exercised by these females (Darwin, 1874; Fisher, 1958; O'Donald, 1977). Systems of mating are often thought to evolve in response to ecological pressures (reviewed by Selander, 1972; Brown, 1975; Emlen and Oring, 1977), although mating preferences may be self-reinforcing (Fisher, 1958; O'Donald, 1967, 1977; Lande, unpubl.). Darwin and Fisher described qualitative methods by which an observed sexual dimorphism could be attributed mainly to either natural or sexual selection. To assign natural selection as the primary cause requires ecological observations that males and females follow different ways of life and employ the dimorphic character(s) adaptively in their distinct modes of survival or reproduction. Darwin presented several such examples, mostly among the lower classes of animals. Selander (1972) 292

1,692 citations


Journal ArticleDOI
TL;DR: Until the authors know more about how and why natural selection occurs, attempts to measure it are quixotic, and discussions of its importance are theandric.
Abstract: All too often in evolutionary biology we are led to speculate or infer the mode of action of natural selection; we usually do not know why some individuals are more adaptive than others. Very often attempts to measure natural selection are unsuccessful, leading to heated arguments about' the relative importance of selection, genetic drift, and epistasis in evolution (Lewontin, 1974). Until we know more about how and why natural selection occurs, attempts to measure it are quixotic, and discussions of its importance are theandric. It is no coincidence that most of the successful studies of natural selection have dealt with animal color patterns; it should be obvious which color patterns are more adaptive in the presence of visually hunting predators. The adaptive significance of warning coloration and mimicry of distasteful species has been worked out (Cott, 1940; Wickler, 1968; Edmunds, 1974; Rothschild, 1975; Turner, 1977). But most species are neither distasteful nor mimetic; most have inconspicuous or cryptic color patterns in their natural habitats (Poulton, 1890; Thayer, 1909; Cott, 1940; Endler, 1978). Most field and experimental studies have shown that the overall color or tone of inconspicuous species matches or approximates the background (DiCesnola, 1904; Sumner, 1934, 1935; Isley, 1938; Popham, 1942; Dice, 1947; Kettlewell, 1956, 1973; Turner, 1961; Kaufman, 1974; Wicklund, 1975; Curio, 1976), but they treated species with solid colors or

1,290 citations


Journal ArticleDOI
TL;DR: The retention of the usefulness of 'coevolution' is pleaded for by removing it from synonymy of usage with 'interaction', '"symbiosis, '"mutualism,' and 'animal-plant interaction.'
Abstract: 'Coevolution' may be usefully defined as an evolutionary change in a trait of the individuals in one population in response to a trait of the individuals of a second population, followed by an evolutionary response by the second population to the change in the first. 'Diffuse coevolution' occurs when either or both populations in the above definition are represented by an array of populations that generate a selective pressure as a group. Ehrlich and Raven's (1964) classic paper on the interactions of butterflies and plants was the first essay explicitly focused on coevolution. However, they did not define coevolution, and butterflies were neither stated nor implied to have been the single populations or array of herbivores that have generated the plant traits that they discuss as causing butterfly distributions on host plants. I believe that the lack of an original definition of 'coevolution,' the inapplicability of the example chosen by the original advocates of the use of the term, and the obvious commonplace nature of coevolutionary events in the history of plant-animal interactions have led to misleading uses of the term in contemporary evolutionary thought and studies. Here, I wish to call for more careful attention to the use of 'coevolution' as a word and concept. There are three conspicuous misuses at present: 1) It is commonly assumed that a pair of species whose traits are mutualistically congruent have coevolved. For example, it is quite possible that the fruit traits of a mammal-dispersed seed coevolved with the mammal's dietary needs. However, it is also quite possible that the mammal entered the plant's habitat with its dietary preferences already established and simply began feeding on the fruits of the species that fulfilled them. When this occurs, it is those species that are most exactly congruent which will appear most coevolved yet are likely to be the least coevolved. Are the hard seeds of those aridland trees dispersed by passage through a contemporary mammal gut coevolved with the mammal? Not necessarily. 2) In similar manner, a herbivore parasitic on a plant is often thought of as coevolved with the defense timing, chemistry, morphology, etc. However, when a parasite arrives in a new habitat, it will feed on those species whose defense traits it can circumvent because of the abilities it carries at the time. Such a parasite cannot be distinguished from one that evolved the ability to circumvent a defense while in trophic contact with its host. 3) When other evidence makes it clear that a parasite has evolved traits to circumvent the defenses of its host, it is frequently automatically assumed that coevolution has occurred. However, it is not necessary to conclude that the defense trait circumvented was evolutionarily produced in response to the parasite in question. In fact, it is likely that many defense traits of plants were produced through coevolution with animals no longer present in their habitat or no longer parasitizing them if present. Strongly coevolved parasite-host systems probably as often proceed to ecological independence of the participants as to relatively benign parasitism. In summary, I plead for the retention of the usefulness of 'coevolution' by removing it from synonymy of usage with 'interaction, '"symbiosis, '"mutualism,' and 'animal-plant interaction.' A bee is not necessarily coevolved with the flower it pollinates, a caterpillar is not necessarily

893 citations


Journal ArticleDOI
TL;DR: A survey of population phenomena can be found in this paper, where the authors present an overview of the evolution of population genetics, including the Hardy-Weinberg Law.
Abstract: 0. Introductory Survey of Population Phenomena. I. THE BASICS OF POPULATION GENETICS. 1. An Overview of Population Genetics. 2. The Hardy-Weinberg Law. 3. Natural Selection and Mutation at One Locus with Two Alleles. 4. The Fundamental Theorem of Natural Selection. 5. Genetic Drift. 6. The Neutrality Controversy. II. COMPLEX GENETIC SYSTEMS. 7. Natural Selection with Multiple Alleles at One Locus. 8. Population Genetics with Multiple Loci. 9. Natural Selection and Quantitative Inheritance. 10. Nonrandom Mating. III. SPECIAL TOPICS IN EVOLUTION. 11. Evolution of the Genetic System. 12. Evolution in Spatially Varying Environments. 13. Natural Selection in Temporally Varying Environments. 14. The Evolution of Altruism: Kin Selection and Group Selection. IV. EVOLUTIONARY ECOLOGY OF SINGLE POPULATIONS. 15. An Overview of Evolutionary Ecology. 16. Exponential and Logistic Population Growth. 17. Density-Dependent Natural Selection. 18. Population Growth with Age Structure. 19. Age-Specific Selection and Life History Strategies. 20. Stochastic Environments: Extinction, Resource Tracking, and Patchiness. V. EVOLUTIONARY ECOLOGY OF INTERACTING POPULATIONS. 21. Competition. 22. Predation. 23. Coevolution in Ecological Systems. 24. Niche Theory and Island Biogeography. VI. APPENDICES. A1. The Mean and Variance. A2. How to Write a Computer Program in BASIC. A3. Matrix Algebra and Stability Theory. Index. Bibliography.

637 citations


Journal ArticleDOI

308 citations


Journal ArticleDOI
TL;DR: This paper elaborates the hypothesis proposed by Kaneshiro (1976) and proposes a uni-directional phylogeny for the six populations of the grimshawi complex of species.
Abstract: Kaneshiro (1976), in his study of behavioral isolation among four species of Hawaiian Drosophila, made the observation that isolation was asymmetrical between allopatric species. That is, between any allopatric pair of species, sexual isolation was strong in one direction but showed weak or no isolation in the other reciprocal. Based on the assumption that "elements of behavior" are "lost" during founder events due to severe drift conditions and the genetic revolution which accompanies such events, Kaneshiro proposed a hypothesis to infer the direction of evolution for the four species studied. Ohta (1978) analyzed sexual isolation among six populations of two other Hawaiian Drosophila species and observed striking asymmetry in pair-wise combinations of four of the six. By using Kaneshiro's (1976) hypothesis, Ohta was able to propose a uni-directional phylogeny for the six populations of the grimshawi complex of species. A preliminary search of the literature on ethological isolation between populations of Drosophila indicates that one-sided mating preference or asymmetrical isolation is not an uncommon phenomenon. Re-evaluation of these data in terms of the hypothesis proposed by Kaneshiro (1976) complements phylogenetic interpretations inferred from various other techniques. This paper elaborates the hypothesis proposed by Kaneshiro (1976). Additional evidence which appear to support his ideas will be presented. A discussion of the origin of premating isolation mechanisms is presented first to emphasize the role of sexual isolation in the speciation process.

262 citations


Journal ArticleDOI
TL;DR: Almost all cone-bearing species, with winddispersed seeds, are monoecious and almost all species with fleshy or showy fruits, dispersed by animals, are dioecious; the striking pattern that emerges is that, with mode of pollination controlled, breeding system correlates almost perfectly with dispersal syndrome.
Abstract: Studies on the evolution of breeding systems have recently focused on the adaptive significance of dioecy vs. monoecy and hermaphroditism in plants (Ross, 1970; Lloyd, 1972, 1975; Bawa, 1974; Arroyo and Raven, 1975; Bawa and Opler, 1975, 1978; Ross and Weir, 1975; Ashton, 1976; Charnov et al., 1976; Freeman et al., 1976; Melampy and Howe, 1977; Charlesworth and Charlesworth, 1978; Zapata and Arroyo, 1978; Charnov, 1979; Opler, 1979; Willson, 1979). Dioecy is marked by the presence of male and female flowers on separate plants; monoecy, by the presence of male and female flowers on the same plants; and hermaphroditism, by the presence of perfect, or bisexual, flowers on all plants. Whereas dioecy, or separation of the sexes, is frequent in animals and especially vertebrates, it is relatively uncommon in flowering plants (Maynard Smith, 1978). Although 22-40% of woody species are reported dioecious in certain tropical floras, the incidence of dioecy is less in temperate trees, and much less in temperate shrubs and herbs (Bawa and Opler, 1975). Several conflicting views have been advanced as to what ecological and evolutionary factors may favor dioecy (Darwin, 1877; Mather, 1940; Lewis, 1942; Stebbins, 1951; Baker, 1958; Ornduff, 1966; Opler and Bawa, 1978; and references cited above). Gymnosperms have been largely ignored in this controversy, but their combination of breeding, pollination, and dispersal systems casts important light on the evolution of dioecy. Most gymnosperms are either monoecious or apparently dioecious, although a few species have populations with both monoecious and dioecious members (Pilger, 1926; Pearson, 1929; Florin, 1933, 1958; Chamberlain, 1935; Cutler, 1939; Martinez, 1946; Hunziker, 1949; Buchholz and Gray, 1958; Johnson, 1959; Marsh, 1966; Dallimore et al., 1967; Mirov, 1967; Keng, 1969, 1978; deLaubenfels, 1969, 1972, 1978a, 1978b; Veillon, 1978). All gymnosperms are wind-pollinated (see Giddy, 1974, regarding alleged beetle pollination in cycads), but two major dispersal syndromes are known. One involves wooden or leathery cones enclosing (usually) winged seeds that are windor gravity-dispersed. The other involves fleshy, often showy "fruits" or arils surrounding wingless seeds that are animal-dispersed. The term "fruit" is used in an ecological and functional sense in this paper; gymnosperm "fruits" actually represent a great diversity of morphologically distinct, but ecologically similar, structures that attract seed dispersers. The distribution of dispersal syndromes among the 804 extant species in 14 gymnosperm families and 74 genera is shown in Table 1. The striking pattern that emerges is that, with mode of pollination controlled, breeding system correlates almost perfectly with dispersal syndrome (Table 2). Almost all cone-bearing species, with winddispersed seeds, are monoecious and almost all species with fleshy or showy fruits, dispersed by animals, are dioecious. To a certain extent, this pattern is set by familiar specialization: most species in the Araucariaceae, Pinaceae, and Taxodiaceae are monoecious and wind-dispersed, while most in the Cycadaceae, Stangeriaceae, Zamiaceae, Ginkgoaceae, Taxaceae, Cephalotaxaceae, Podocarpaceae, Ephedraceae, and Gnetaceae are

262 citations


Journal ArticleDOI
TL;DR: This study concentrates on how mass-flowering of the individual plant influences pollinator movement and consequent gene flow and suggests that these contrasting flowering patterns attract different types of pollinators.
Abstract: There are conflicting views about the regulation of gene flow in tropical plants (see Corner, 1954; Baker, 1959; Federov, 1966; Ashton, 1969; Bawa, 1974). Characteristics proposed to promote outbreeding include a long flowering period, asynchronous flowering, dioecism, protandry, protogyny, and self-incompatibility. One feature which should yield reduced gene flow is mass-flowering of the individual plant. This study concentrates on this feature and how it influences pollinator movement and consequent gene flow. Among species, the flowering phenology of the individual plant varies markedly. Among tropical plants, Janzen (1971) distinguished between two extreme patterns of daily flower abundance. In the first pattern, later designated "steady state" (Gentry, 1974), the individual plant produces small numbers of new flowers almost daily over an extended period (usually greater than 2 mo, often for 6-12 mo [Gilbert, 1975; Stiles, 1975]). In the second pattern, designated "big bang" (Gentry, 1974) or "mass-flowering" (Heinrich and Raven, 1972), an individual produces large numbers of new flowers each day over a short period (often less than one week). Earlier observations suggest that these contrasting flowering patterns attract different types of pollinators. Pollinators visiting steady-state individuals are thought to be low in number and in species richness, to shift to new plants after a shorter foraging period, and to move longer distances between conspecific individuals, relative to pollinators visiting mass-flow-

260 citations


Journal ArticleDOI
TL;DR: The main findings from a study of a cohort of oysters were observed: that the number of heterozygous individuals in the population was much lower than predicted from Hardy-Weinberg equilibrium and there was a correlation between an individual's weight and thenumber of loci at which the individual was heterozygotes.
Abstract: It is a well established phenomenon that homozygosity induced either by inbreeding or by especially designed experiments (such as, e.g., the technique of Sved and Ayala [1970] for comparing overall performance of chromosomal heterozygotes) generally results in low fitness. Because in all such cases a large part of the genome is made homozygous, the reduction in fitness can be equally well attributed to either reduced heterozygosity at heterotic loci, or to homozygosity for detrimental recessive genes. The problem of distinguishing between the two explanations is one of major importance in population genetics and is thoroughly discussed by Lewontin (1974). Another approach to studying the effect of heterozygosity is to correlate heterozygosity with a quantifiable character in individuals drawn randomly from a population. This approach does not rely on inbreeding, and although some inbreeding may occur in the population, if reduced homozygote performance is observed, it may well be attributed to other factors. Chromosomal inversions and electrophoretic variants provide suitable means for such an approach, the latter having the additional advantage of allowing one to study the effect of homozygosity or heterozygosity at a rather small segment of the genome. It appears, however, that most researchers have preferred to search for balancing selection in natural populations by comparing the observed levels of heterozygosity in the populations with that expected under the assumption of no selection (e.g., Marshall and Allard, 1970; Richmond and Powell, 1970; Allard et al., 1972). Only rarely has the individual's state in regard to homozygosity or heterozygosity been compared with its score for a quantitative character. Most of these studies have shown that heterozygotes perform differently than homozygotes (Koehn et al., 1973; Chaisson et al., 1976; Schaal and Levin, 1976; Stalker, 1976; Mitton, 1978). In a previous paper (Singh and Zouros, 1978) we reported the main findings from a study of a cohort of oysters. The cohort consisted of individuals of same age, drawn as spat (settling larvae) from a natural population, and grown under uniform conditions. We observed that the number of heterozygous individuals in the population was much lower than predicted from Hardy-Weinberg equilibrium. We also observed that there was a correlation between an individual's weight and the number of loci at which the individual was heterozygous. The first observation is common for populations of marine molluscs (for references see Singh and Zouros, 1978, and Skibinski et al., 1978). The second observation is less common in the literature and has an obvious bearing on the question of adaptive significance of enzyme heterozygosity. Here we report the findings of a second study. Our main objective was to see whether the patterns observed in the first study are reproducible. We have employed more loci and analyzed a much larger sample. This allowed us to firmly establish the correlation between heterozygosity and growth rate. It also provided us with a sizeable body of informati6n on which to evaluate certain hypotheses about the nature of the correlation, and ' Present address: Department of Zoology, University of Western Ontario, London, Ontario, Canada N6A 5B7.

252 citations


Journal ArticleDOI
TL;DR: Success in pollinia removal and fruit production as a function of inflorescence size in the orchid Brassavola nodosa is examined to examine the potential for counterselection, that is, conflicting selection pressures, on life-history traits.
Abstract: The evolutionary consequences of varying the density and dispersion of flowers on a plant include the effect on pollinator movement (Heinrich and Raven, 1972; Frankie et al., 1976; Stiles, 1975; Schemske, in press), predator pressure (Beattie et al., 1973), and reproductive success (Janzen, 1977; Schemske, 1978; Schemske, in press). In hermaphrodites, the evolution of floral display may be a consequence of sexual selection (Willson, 1979). As in Asclepias syriaca, large displays may increase an individual plant's probability of siring seeds, the "male" component of fitness, but not fruit production, the "female" part (Willson and Price, 1977). Although large floral displays may result in more offspring than smaller displays in a single season, their greater expense may reduce plant growth, survivorship, and ultimately, future reproduction. Thus, production of a particular floral display is probably a tradeoff between "conflicting" selective pressures. In this context, Williams (1966) emphasized that the evolution of life-history traits is molded by reproductive value. In this paper I examine success in pollinia removal and fruit production as a function of inflorescence size in the orchid Brassavola nodosa. I compare the success rates of different floral displays to the frequency distribution of inflorescence size in the population to examine the potential for counterselection, that is, conflicting selection pressures, on life-history traits.

252 citations


Journal ArticleDOI
TL;DR: The breeding systems and stationary spatial distributions of animal-pollinated flowering plants are consistent with Wright's models and such systems have proved excellent for the study of neighborhood size, because gene dispersal can be estimated directly by measuring pollinator movements and seed dispersal distances.
Abstract: Wright (1943, 1946) showed that in a continuous population of organisms, the pattern of genetic differentiation is largely determined by the number of individuals in a local random breeding unit, or neighborhood. When neighborhoods are small, populations are subject to greater differentiation, both randomly and in response to natural selection. Neighborhood size is a function of population density and gene dispersal. The breeding systems and stationary spatial distributions of animal-pollinated flowering plants are consistent with the assumptions of Wright's models; such systems have proved excellent for the study of neighborhood size, because gene dispersal can be estimated directly by measuring pollinator movements and seed dispersal distances (Kerster and Levin, 1968; Levin and Kerster, 1968, 1969a, 1969b, 1974; Schaal and Levin, 1978; Beattie, 1979). The foraging behavior of pollinators has major importance for patterns of gene dispersal in plant populations (Levin, 1979a, 1979b). Foraging behavior in turn may be affected by the quality and distribution of the nectar sugar rewards offered by flowers (Heinrich and Raven, 1972; Heinrich, 1975). Pollinators can utilize only flower resources which provide sufficient caloric reward to make foraging energetically profitable (Heinrich and Raven, 1972; Heinrich, 1975). Types of pollinators may differ by several orders of magnitude in the metabolic energy costs they incur during foraging and thermoregulation (Heinrich and Raven, 1972; Heinrich, 1975). For example, butterflies, which thermoregulate by basking (Watt, 1968) and have relatively low foraging costs, can profitably utilize flowers with relatively small nectar rewards (Watt et al., 1974), while bumblebees and hawkmoths, which thermoregulate metabolically and expend more energy in foraging (Heinrich, 1975), would operate at a loss on the same resource. Many flower species have specialized features of nectar presentation adapting them to a particular pollinator type (Heinrich and Raven, 1972; Faegri and van der Pijl, 1979). For example, plant species specialized for bumblebee or hawkmoth pollinators often provide rich nectar rewards in deep-spurred nectaries that are inaccessible to low-energy pollinators. On the other hand, many plants provide minute quantities of nectar that are profitable only to animals with low metabolic costs. If pollinator types with different energy requirements differ in their foraging behavior, then the neighborhood structures of plants specialized for those pollinators can be expected to differ also. Linhart (1973) has shown that pollen dispersal patterns in tropical Heliconia differ dramatically depending on whether territorial or traplining hummingbirds are the pollinators. Moreover, for plants which are generalists, pollinated by several types of animals, neighborhood structure may be significantly affected by the proportion of pollen transferred by different pollinator types. Two aspects of pollinator foraging behavior have particular importance for patterns of plant gene dispersal. First, flight distances between plants will determine the distance over which pollen is transferred. Second, in self-compatible plants the number of flowers visited per plant will determine the proportion of seeds set that are selfed or outcrossed, and thus will affect levels of inbreeding. Moreover, if pollen from a given flower is carried over to more than one of the flowers subse-


Journal ArticleDOI
TL;DR: Experimental approaches to testing the hypothesis that birds adaptively limit clutch size for the sake of enhanced survival are likely to provide the strongest inferences.
Abstract: Lack (1947) hypothesized that clutch size in nidicolous birds has evolved by natural selection to correspond with the maximum number of young that, on average, the parents can feed. Although the hypothesis gained wide acceptance in subsequent years, the evidence is equivocal and inconsistencies remain (Klomp, 1970; Cody, 1971; von Haartman, 1971; Hussell, 1972). Those cases in which the most productive brood size is larger than the most common do not support the implied concept of direct limitation of clutch size by food supply. Furthermore, the interpretation of brood manipulation experiments that support Lack's hypothesis is open to question, since the results do not distinguish between food supply limits in the environment and possible adaptive limits upon parental feeding behavior (Cody, 1971; Hussell, 1972). Mountford (1968) suggested that incorrect formulation of predictions was responsible for some apparent contradictions. Lack recognized that selection should favor the clutch size that maximizes fitness, but overemphasized the direct influence of environmental factors upon clutch size as a single trait. A more comprehensive perspective incorporates interactions between clutch size and other life history features as well (Fisher, 1958; see review in Stearns, 1976). Pertinent models predict a most common clutch that is smaller than the most productive, under the assumption that rearing larger broods places greater stress upon parents and reduces their chances of surviving to breed again (Williams, 1966; Charnov and Krebs, 1974). The assumption of a trade-off between clutch size and adult survivorship within populations is largely untested. Observations from naturally occurring brood sizes have not yielded any consistent relationship between clutch size and parental survival (e.g., Kluyver, 1963; Perrins, 1965; Lack, 1966, p. 109). However, since variation in parental ability (e.g., efficiency in gathering food for egg formation or for feeding nestlings) may contribute to adaptive modification of clutch size (cf. Klomp, 1970), the search for such a relationship is confounded. If parents that normally initiate larger clutches are more capable of rearing them, the young in.those broods are not necessarily disadvantaged (cf. Perrins and Moss, 1975), nor are those parents necessarily less likely to survive than parents raising smaller broods. Greater weight losses among parents with larger broods (Hussell, 1972; Winkel and Winkel, 1976; Bryant, 1979) and lower probabilities of initiating a second brood after a large first brood (Kluyver, 1963; Pinkowski, 1977) provide indirect evidence that rearing large broods is stressful physiologically, but have yet to be linked to differential survival. Recently, Bryant (1979) found differences in survival between singleand double-brooded female House Martins (Delichon urbica), but these differences were not related to the brood sizes reared. Experimental approaches to testing the hypothesis that birds adaptively limit clutch size for the sake of enhanced survival are likely to provide the strongest inferences (cf. Ricklefs, 1973, p. 426; Stearns, 1976, p. 42). Artificial manipulation can extend brood sizes beyond limits currently observed within a population. However, if parental ability is reflecterd in individulm] clutch sizes. nqirpnts 278

Journal ArticleDOI
TL;DR: The present study will establish the amount of intrapopulational variability in ovum size (pre-fertilization parental care) in several salamander populations to allow for an evaluation of how parameters that may be related to offspring fitness are affected by variation in pre-ovipositional parental investment.
Abstract: A prevalent attitude concerning parentoffspring relationships maintains that either preor post-fertilization care by parents toward offspring results in increased offspring survival at the expense of additional present or future reproductive success. This has led to models that have as an assumption monotonically increasing functions which relate parental investment per offspring to offspring fitness (Smith and Fretwell, 1974; Brockelman, 1975; Wilbur, 1977). A second assumption of such models, that offspring number is sacrificed for increased offspring care, has recently been found valid in some real situations in salamanders (Kaplan and Salthe, 1979). A third assumption is that there is intrapopulational variability in the energy invested in offspring which allows for the possibility of change in the system. Reproductive patterns of populations can evolve (or be maintained) only if there is sufficient intrapopulational variability in at least some of their components. The present study will establish the amount of intrapopulational variability in ovum size (pre-fertilization parental care) in several salamander populations. Focusing on the first assumption, the physiological ramifications of intrapopulational ovum size variability that may be related to fitness will be assessed. Whether such ramifications are monotonically associated with ovum size variability over the range of variability found will be determined. This approach will allow for an evaluation of how parameters that may be related to offspring fitness are affected by variation in pre-ovipositional parental investment. For example, if the timing of a developmental event (e.g., hatching) is of critical importance in some environments and changes in ovum size affect such timing, then in those environments variability in ovum size will potentially affect offspring fitness. However, in an environment where the timing of such an event is unimportant, variance in investment per offspring will not affect the offspring's fitness through this route. This study also relates to the problem of the determination of clutch size, that is, how and why parental investment during any breeding period is partitioned differentially among offspring within a population. Factors that determine the degree of total parental investment during a single breeding period have been dealt with elsewhere (Kaplan and Salthe, 1979), and for the purposes intended here can be assumed to be constant among individuals.

Journal ArticleDOI
TL;DR: Investigating the validity of Mayr's founder effect-"genetic revolution" model from a population genetic perspective indicates that a founder effect can indeed induce rapid speciation, complete with preand/or post-mating isolating barriers, but the details and implications are far different from those portrayed by Mayr (1954).
Abstract: Recent advances in molecular biology have allowed population geneticists to make genetic comparisons across species as well as within species. Such molecular information has proven to be an important tool in systematics and in reconstructing phylogenies. Besides just describing past cladogenetic events, it has also been hoped that molecular information could be used to make inferences about the modes of speciation. For example, if "genetic revolutions" (Mayr, 1954) occur, it has been argued that a large genetic distance based on electrophoretic analyses should rapidly arise between the ancestral and descendant species (Avise and Ayala, 1975, 1976; Avise, 1977, 1978). However, this expectation is made without considering the underlying population genetic mechanisms responsible for speciation and "genetic revolutions." Recently, I have investigated both empirically (Templeton, 1979) and theoretically (Templeton, in press) the validity of Mayr's founder effect-"genetic revolution" model from a population genetic perspective. These studies indicate that a founder effect can indeed induce rapid speciation, complete with preand/or post-mating isolating barriers. However, the details and implications of this rapid speciation are far different from those portrayed by Mayr (1954). Hence, I refer to this mode of speciation as the "genetic transilience" rather than the "genetic revolution" to avoid the many connotations the latter phrase has acquired throughout the years. Specifically, Mayr (1954) argued that the founder effect and its associated inbreeding would ". . . affect all loci at once." Indeed, it was this very genic extensiveness of the founder effect that provides the driving force for the "genetic revolution" in Mayr's view. Moreover, Mayr (1954, 1955) emphasized that loci were part of a polygenic interaction system in which the marginal effects of a single locus or handful of loci were unimportant. Because of these arguments, the term "genetic revolution" connotes extensive changes throughout the genome that affect virtually all loci. However, my work indicates this assumption is not true. A genetic transilience does not shake-up the whole genome; rather, it is confined principally to a polygenic system strongly affecting fitness that is characterized by having a handful of major genes with strong epistatic interactions with several minor genes. Indeed, straightforward quantitative genetic considerations imply a genetic transilience would be virtually impossible in a polygenic system lacking a few genes with major marginal effects (Templeton, in press; Lande, in press)just the opposite of what Mayr argues. Hence, all but a small number of genes will be neutral with respect to the transilience, and any expectations concerning genetic distance should be made with this inference in mind. This criticism is particularly relevant since most genetic distance measures are based on enzyme-coding loci, a type of locus that appears to be neutral with respect to the laboratory genetic transiliences I have been studying (Templeton, 1979). (This does not necessarily imply that such loci are neutral in the usual microevolutionary sense.) Moreover, there is much circumstantial evidence that enzyme-coding loci are relatively insensitive markers of speciation and macro-evolutionary events in general (Wilson, 1975; Tauber and Tauber, 1977; Larson and Highton, 1978; Nevo and Cleve, 1978; Ferris et al., 1979; Kirkpat-

Journal ArticleDOI
TL;DR: A correlative approach is useful for suggesting relevant selective forces that might favor one breeding system over another, although incapable of proving which selective forces are the most important, it may serve to discredit hypotheses whose predictions are violated.
Abstract: Many plant species produce both chasmogamous flowers, adapted to attracting pollinators, and tiny cleistogamous flowers, adapted to self-fertilization. Darwin (1877) interpreted these cases of floral dimorphism as adaptations to the two, frequently antagonistic, functions served by flowers: successful cross-fertilization with other plants, and the efficient setting of many seeds. He noted how sparing of pollen and nectar the cleistogamous flowers were, and realized how this economy would permit expanded seed set. He then asked why the more efficient mode of reproduction would not replace the other, and concluded that adaptations for crossfertilization implied the existence of some advantage for outcrossed progeny. This advantage is now taken to be heterosis, or the avoidance of inbreeding depression. From the time of Linnaeus, the environment has been known to influence the production of cleistogamous and chasmogamous flowers in many species (Uphof, 1938). Many researchers have searched for and found relationships between a species' life history or a population's environment and a particular breeding system (Darwin, 1877; Stebbins, 1950; Baker, 1955; Darlington, 1958; Grant, 1971; Levin, 1972; Solbrig and Rollins, 1976). This correlative approach is useful for suggesting relevant selective forces that might favor one breeding system over another. Although incapable of proving which selective forces are the most important, it may serve to discredit hypotheses whose predictions are violated. Jain (1976) and Sol-

Journal ArticleDOI
TL;DR: The techniques for describing the gender of plants which are proposed elsewhere are applied here to plants of four natural (self-maintained) populations, one from each of four angiosperm species, to illustrate the use of the techniques in providing accurate descriptions of plant sex and in documenting evolutionary changes in sex conditions.
Abstract: In angiosperms, gender is a quantitative phenomenon which can be measured on a continuous scale between the strictly male and strictly female extremes. Measurements of gender are of two kinds. Estimates of the "phenotypic gender" of a plant are based on the reproductive characteristics of that plant alone-counts of the percentage of seminiferous or polliniferous flowers for example. The "functional gender" of a plant takes into account the production of gametes and seeds by other individuals in the population and estimates the proportions of a plant's genes that are transmitted through its ovules, its femaleness (Gi), or through pollen, its maleness (Ai = 1 Gi) (Lloyd, 1980). Both phenotypic gender and functional gender can be estimated either from counts of pollen and ovule production (gamete estimates) or from counts of pollen and seed production (seed estimates). Such estimates of functional gender measure the prospective success of plants as pollen and seed parents based on the patterns of parental investment; hence they record parental strategies rather than the eventual success of a plant in leaving adult descendants. The techniques for describing the gender of plants which are proposed elsewhere (Lloyd, 1980) are applied here to plants of four natural (self-maintained) populations, one from each of four angiosperm species. The four populations illustrate the use of the techniques in providing accurate descriptions of plant sex and in documenting evolutionary changes in sex conditions. The four populations show the origin (populations that are monomorphic in gender) and intermediate stages (dimorphic subdioecious populations) in two important pathways leading to dioecy in angiosperms. Two species of Umbelliferae illustrate the "gynodioecy pathway" to dioecy from species with hermaphrodite flowers. Two Cotula (Compositae) populations illustrate the "paradioecy pathway" from monoecy to dioecy. The plants of all four species are perennial, and the gender estimates reported here represent the sexual reproduction of one season only. The estimates are based on counts of the numbers of flowers bearing pollen and of the number of flowers with either ovules or seeds. In the absence of information on variation in the number of pollen grains in polliniferous flowers and in the number and average fitness of seeds in fruit (one or two in the Umbelliferae but always one in Cotula), it is assumed that all polliniferous flowers of a population, and separately all ovuleor seed-bearing flowers, are equally likely to contribute to the next generation. Details of the reproductive behavior of the species and of the methods used are described elsewhere (Lloyd, 1972a, b, unpubl.; C. J. Webb, 1979). The species are considered in order of increasing complexity of the patterns of gender distribution.

Journal ArticleDOI
Nancy Knowlton1
TL;DR: Evidence for sexual selection in the snapping shrimp Alpheus armatus (Caridea, Alpheidae), an obligate symbiont of the Caribbean anemone Bartholomea annulata (Clarke, 1955) is provided.
Abstract: Highly polygamous and promiscuous species have been the subjects of most of the published studies on sexual selection (Bateman, 1948 [Drosophila]; Wiley, 1973 [sage grouse]; Le Boeuf, 1974 [elephant seals]; Parker, 1974 [dung flies]; Trivers, 1976 [Anolis]). Among monogamous species only the Arctic skua (O'Donald, 1972) and the feral pigeon (Burley, 1977) have been intensively studied with respect to the operation of sexual selection. Neglect of more monogamous species stems from the less spectacular differences between the sexes (Selander, 1972; Ralls, 1977) and from uncertainty as to whether sexual selection can operate in the absence of the potential for multiple matings (Mayr, 1972). This neglect is unfortunate as monogamous species offer valuable opportunities for examining sexual selection when the costs and benefits of mating adaptations are more closely balanced. This paper provides evidence for sexual selection in the snapping shrimp Alpheus armatus (Caridea, Alpheidae), an obligate symbiont of the Caribbean anemone Bartholomea annulata (Clarke, 1955). Adult shrimp are typically found in male-female pairs, a fact which suggests a primarily monogamous mating system. Data on sexual dimorphism in behavior and morphology, on differences between two areas in sexual dimorphism, and on the natural history needed to interpret these patterns will be presented. The overall purpose is to show how differences between the sexes in the importance of mate choice and in the potential for multiple matings interact with environmental constraints to produce sexual dimorphism.

Journal ArticleDOI
TL;DR: In this article, the authors quantify visitation by both pollinating animals and non-pollinating animals to flowers of two neotropical hummingbird-pollinated plant species, showing that visits by both pollen and nectar parasites to flowers are relatively frequent and very effectively remove resources.
Abstract: Many angiosperm flowers produce substances (e.g., nectar, pollen, lipids) which serve as rewards for animals which are morphologically and behaviorally adapted to effect pollination by transferring pollen in sequential visits to flowers of the same species (Faegri and van der Pijl, 1966; Baker and Baker, 1975). It has long been realized that these floral rewards are frequently also attractive to other animal species which, due to morphology or behavior at particular flowers, do not effect pollination (e.g., Kerner, 1896; Free and Butler, 1959; Proctor and Yeo, 1973; Inouye, in press). The relationships among plant, pollinators, and floral parasites have been documented to some extent for several insect-pollinated plant species, usually involving pollen transfer by longtongued bees and nectar parasitism by shorter-tongued species (Hawkins, 1961; Macior, 1966; Koeman-Kwak, 1973; Rust, 1977). For the closed-flowered Calathea species, Kennedy (1978) has found that the identities are reversed, with pollination by short-tongued bees and nectar parasitism by longer-tongued species. Recognition of the remarkable suite of corresponding characteristics shared by hummingbirds and the flowers they pollinate (Faegri and van der Pijl, 1966; Grant and Grant, 1968), has fostered the assumption that once the plant-pollinator(s) "unit" has been identified, the floral biology of the plant is understood. Observation of many tropical hummingbirdpollinated plants suggests that a more complicated relationship frequently exists involving floral parasitism by a variety of animals (Skutch, 1954; Lyon and Chadek, 1971; Colwell et al., 1974; Gentry, 1974; Janzen, 1975). While earlier work (e.g., Skutch, 1954) generally reported parasitism as occuring only rarely, our observations indicate that visits by both pollen and nectar parasites to flowers of many long-tubed, nectar rich, hummingbirdpollinated species are relatively frequent and very effectively remove resources. The interactions between these parasites and pollinating visitors may be both direct and, through their impact on the availability of floral rewards, plant-mediated. The ultimate effects of these interactions can be viewed in terms of changes in the relationship between a plant species and its pollinators, and, concomitantly, in the reproductive success of the plants. In this paper we quantify visitation by both pollinating animals and non-pollinating animals to flowers of two neotropical hummingbird-pollinated plant species. Several aspects of the more complex problem of determining the effect of floral parasitism on the reproductive biology of the plants are also explored. The impact of the utilization of floral resources by nonpollinators is presented in terms of the proportion and diurnal pattern of flowers visited and damaged by parasites. Since the effect of this parasitism on visitation by pollinators will depend upon its impact on the availability of floral rewards, we compare patterns of nectar availability in flowers which have been parasitized (pierced) and in those which have not been visited or have been visited only by pollinators. A very important aspect of the plant-mediated interactions among flower visitors is the relative ability of different species using different visitation modes to extract the nectar available. We investigated this problem indirectly, by determining the amount of nectar remaining in

Journal ArticleDOI
TL;DR: Using electrophoretic data, the following questions were examined for the milkfish Chanos chanos: what is the level of genic variation in this species and how does it change over large geographic distances, and how genetically isolated is the Hawaiian population of milkfish relative to other milkfish populations?
Abstract: Since the first use of protein gel electrophoresis for the assessment of genic variation in natural populations (Lewontin and Hubby, 1966), a large number of plants and animals across many phylogenetic lines has been surveyed for electrophoretic variation. Results of these surveys and synopses of the related evolutionary hypotheses have been summarized in Lewontin (1974), Ayala (1976), and Nevo (1978). In reviewing these reports, it is apparent that one group of organisms has been generally overlooked: the marine fishes. For instance, many workers have examined the population structure of freshwater fishes (Avise and Selander, 1972; Avise and Smith, 1974; Echelle et al., 1975; Allendorf et al., 1976; Vrijenhoek et al., 1977; and others), yet there are virtually no data available for widely distributed marine fishes. In comparable studies of marine fishes, the work has dealt with diadromous fishes (Koehn and Williams, 1978; Utter et al., 1980), estuarine fishes (Johnson, 1975), or fishes from restricted geographic areas (Gorman et al., 1976; Gorman and Kim, 1977; Ward and Beardmore, 1977). In only one report were genetic differences assessed for a marine fish found in separate localities across the tropics, but this study remains unpublished (Soule, reference in Ehrlich, 1975). It is generally assumed that because of a planktonic larval stage, marine fish have a high potential for gene flow and hence may show little interlocality differentiation, but this topic remains unexplored. It would be interesting, therefore, to assess patterns of geographic differentiation of fish populations within and among archipelagos across an ocean basin. It has long been realized that small and/ or isolated populations may undergo considerable genetic changes. Semi-isolated populations of cavefish Astyanax mexicanus had reduced levels of electrophoretic variation relative to conspecific surface populations (Avise and Selander, 1972), and stream populations of Etheostoma radiosum, separated in different drainages of the same tributary, exhibited high levels of population differentiation (Echelle et al., 1975). In a marine habitat the epitome of isolation of an ichthyofauna is the Hawaiian inshore fish community. Randall (1976) estimates the level of endemism for the inshore fishes is 29%-the highest value for any oceanic island. Yet, there have been no electrophoretic estimates of the degree of isolation of the fishes in Hawaii. Thus, using electrophoretic data, the following questions were examined for the milkfish Chanos chanos: 1) What is the level of genic variation in this species and how does it change over large geographic distances? 2) What is the genetic population structure for a marine fish, such as Chanos, that has a planktonic larval stage? 3) What factors affect the observed pattern of geographic variation and how does the level of geographic differentiation compare to that of other species-terrestrial and marine? 4) How genetically isolated is the Hawaiian population of milkfish relative to other milkfish populations? Life history of milkfish. -Milkfish, Chanos chanos (Forskal), are tropical marine fish distributed throughout the Pacific 1 Hawaii Institute of Marine Biology Contribution No. 586.

Journal ArticleDOI
TL;DR: The objective of this paper is to estimate gene flow, effective population sizes and genetic variance components for several avian species using available dispersal data.
Abstract: The genetic structure of natural populations is of general interest because many current models in ecology and evolution involve assumptions about the viscosity of breeding populations. For instance, theories of group and kinship selection, local adaptation, and speciation all depend on the magnitude of an among-deme component of genetic variance. In spite of this, information on genetic population structure is not available for most taxa. Such information can be obtained through two general classes of analysis. One approach is to analyze genotypic frequencies in populations to obtain direct estimates of genetic population structure using F statistics (Cockerham, 1969, 1973). This has been attempted for a few taxa with electrophoretic data (Hedgecock, 1978; Smith et al., 1978). A second approach is to infer the theoretically expected among-deme variance component using estimates of dispersal parameters in a fashion analogous to that used by Dobzhansky and Wright (1943, 1947). This approach has not been widely used, but this is certainly due in pdrt to the difficulty of obtaining quantitative estimates of dispersal distributions for natural populations. Some effort has been devoted to this method, however, in Drosophila (Crumpacker and Williams, 1973; Powell et al., 1976), a lizard (Kerster, 1964), and a snail (Greenwood, 1974). It is somewhat surprising, though, that this type of analysis has not been extended to avian species; banding, population, and behavioral studies have generated dispersal information that could be used in such a way. The objective of this paper is to estimate gene flow, effective population sizes and genetic variance components for several avian species using available dispersal data. To do this it

Journal ArticleDOI
TL;DR: Wilson et al. as discussed by the authors used phylogenetic trees from molecular data and compared the amount of change occurring along diverging branches of the trees (relative rate test) to assess the degree of regularity of molecular change between taxa.
Abstract: The concept of a molecular clock is certainly controversial among evolutionary biologists. Simply stated, the hypothesis predicts that the rate of amino acid substitutions between any pair of taxa is relatively regular and reflects the divergence time between the taxa. The substitutions can be measured directly, by sequencing proteins, or indirectly, by such techniques as immunological or electrophoretic comparisons. The molecular clock is a prediction of the neutrality hypothesis of molecular evolution. Under this hypothesis, the long-term rate of protein sequence evolution equals the rate of mutation which produces selectively equivalent amino acid substitutions in proteins. Since mutation rate is a stochastic process, the clock is viewed as stochastic (Kimura and Ohta, 1971). Much of the evidence in favor of the molecular clock has been reviewed by Wilson et al. (1977). By constructing phylogenetic trees from molecular data and comparing the amount of change occurring along diverging branches of the trees (relative rate test), Wilson et al. (1977) point out that one can assess the degree of regularity of molecular change between taxa. A useful clock has to be not only regular, but also must be calibrated. Calibration is not always easy to achieve, for it is dependent upon a knowledge of geological events, often the fossil record, and interpretations based upon such records are often open to considerable question (Carl-

Journal ArticleDOI
TL;DR: The Mate Recognition System is the essential component of a new concept of species, the Recognition Concept, which is conceptually quite distinct from the current paradigm, the Isolation Concept and is subject to modification under natural selection to improve its effectiveness should a small population become isolated in a new and distinct environment.
Abstract: In a recent response to a note by Ringo (1977) on "Why 300 species of Hawaiian Drosophila?," Templeton (1979) introduced a new aspect to the discussion. This was the idea of the Mate Recognition System, or, as I now prefer to call it, the Specific-Mate Recognition System (SMRS), which I introduced in 1976. The SMRS is the essential component of a new concept of species, the Recognition Concept, which is conceptually quite distinct from the current paradigm, the Isolation Concept. According to Dobzhansky (1951, 1976), species under the latter concept are to be regarded as "adaptive devices" whose integrity is maintained through the possession of isolating mechanisms (Dobzhansky, 1935). Here I wish to comment on Templeton's use of the term "mate recognition system" because it differs fundamentally from my own. If it is not corrected at once, his usage may lead to confusion and misunderstanding when my concept is published in full. According to the recognition concept, species are incidental consequences of adaptive evolution and cannot be regarded as "adaptive devices." The line of reasoning which leads to this conclusion runs as follows: In sexual species the achievement of syngamy is of fundamental significance. It is therefore not surprising that all sexual organisms possess adaptations which have evidently evolved to facilitate the achieving of fertilization (fertilization mechanisms). In each of these it is always possible to discern a subset, the specific-mate recognition system (SMRS). The SMRS comprises a coadapted signalresponse chain of the sort illustrated by Stitch (1963), or, in more general terms, by Desmond Morris (1956). Although under strong stabilizing selection as I have explained previously (Paterson, 1976, 1978), the SMRS is subject to modification under natural selection to improve its effectiveness should a small population become isolated in a new and distinct environment. Due to the stabilizing selection acting on the system, such adaptive change can only occur in small steps with coadaptation between male and female being re-established at each step. The evidence for these statements comes from observations on the SMRS's of species in their preferred habitats (e.g., Morton, 1975). Speciation is said to have occurred when the SMRS of the members of the daughter population has been so extensively modified that it no longer functions effectively with members of the parental or any other population. It should be said that modification of the SMRS by natural selection may be indirect, being mediated by pleiotropy. Thus, speciation occurs as an incidental consequence of the adaptation of a small isolated population to a new habitat. The SMRS, therefore, falls into the same category as other adaptive characters which are changed when a population is adjusted by natural selection to new conditions. This accounts for the common observation that modifications to the niche of members of a population generally accompanies speciation (Mayr, 1963). It is evident that a new SMRS, derived in this way, determines a new gene pool and, hence, a new species. According to the recognition concept, species are populations of individual organisms which share a common specific-mate recognition system (Paterson, 1978). Species are, thus, incidental effects of adaptive evolution. If these arguments are fully appreciated it will be obvious how greatly my ideas have been misrepresented by Templeton when he wrote (1979, p. 516): "The raison d'etre of a mate recognition system is to

Journal ArticleDOI
TL;DR: There is no evidence in their analyses or arguments to change the previous conclusion that interspecific competition has played a role in the adaptive radiation of Darwin's Finches, and some unsolved problems in biogeography are drawn attention.
Abstract: We conducted a field study of some of Darwin's Finches (Geospiza species) in order to assess the relative importance of interspecific competition and habitat features in determining the observed biogeographic, ecological and morphological characteristics of these species (Abbott et al., 1977). Strong et al. (1979) have criticized one of our methods and have reanalyzed a small portion of our data. They employed stochastic models to generate expected beak size differences between sympatric species, and then compared expected with observed differences. Finding a generally close correspondence between expected and observed differences, they concluded that random processes are sufficient to account for the observations, and that therefore there is no need to invoke deterministic processes such as competition as we had done. Strong et al. (1979) obtained the same results and drew the same conclusion from analyses of beak size differences among birds on the Tres Marias islands of Mexico and the Channel islands of California. Simberloff and his associates have also drawn the same conclusion from a series of other analyses performed in like manner (Connor and Simberloff, 1978; Simberloff, 1978). We take issue with the procedures Strong et al. (1979) have used in their analyses and with the way in which our statements and interpretations have been represented. We identify five problems in their analyses and five sources of confusion in the interpretation of results. We find no evidence in their analyses or arguments to change our previous conclusion that interspecific competition has played a role in the adaptive radiation of Darwin's Finches. Finally, we draw attention to some unsolved problems in biogeography, concerning principally the separation of potentially conflicting effects of different processes such as dispersal and competition.

Journal ArticleDOI
TL;DR: A hypothesis to explain the evolution of distyly into dioecy is proposed, which states that individuals bearing female flowers are evolutionarily derived from long-style individuals, while male plants are derived from the shortstyle form.
Abstract: Distylous flowering plant species are characterized by having two types of individuals that bear different forms of flowers: "pin" flowers with long styles and short stamens and "thrum" flowers with short styles and long stamens (Darwin, 1877; Frankel and Galun, 1977). In most cases this flower dimorphism is associated with a physiological self-incompatibility mechanism that prevents fertilization after self-pollination or pollen transfer between individuals of the same flower type, with the result that only pollination between forms results in fertilization (Frankel and Galun, 1977; de Nettancourt, 1977). In several angiosperm genera, distyly has evolved into dioecy (Baker, 1958, 1959; Bir Bahadur, 1968; Ornduff, 1966; Viulleumier, 1967; Opler et al., 1975), and in every case individuals bearing female flowers are evolutionarily derived from long-style individuals, while male plants are derived from the shortstyle form. The selective forces that may have brought about this transformation are not well understood (Lloyd, 1979). Here we propose a hypothesis to explain the evolution of distyly into dioecy.

Journal ArticleDOI
TL;DR: Information is provided on the magnitude of heritability of patterns of morphometric size and shape variation, the patterns of genetic covariation in functional character sets and the dynamics of phenotypic variance and covariance components during ontogeny for the laboratory rat.
Abstract: Size and shape of an organism can be regarded as polygenically controlled metric characters whose phenotypic expression often changes considerably during ontogeny and evolution. Multivariate statistical analyses of morphometric size and shape variation have been very popular in recent years. However, in spite of this popularity, little critical data relating to the genetic and ontogenetic aspects of size and shape variation exists. Several important problems are poorly understood including the magnitude of heritability of patterns of morphometric size and shape variation, the patterns of genetic covariation in functional character sets and the dynamics of phenotypic variance and covariance components during ontogeny. In this series of papers we provide information on these topics for the laboratory rat. Growth in mammals is accompanied by concomitant changes in the phenotypic variances and covariances and their components. Rather extensive studies on mice (El Oksh et al., 1967; Rutledge et al., 1972; Herbert et al., 1979) suggest that phenotypic variance and heritability of size and shape measures increase during ontogeny while variance due to maternal effects decreases. Dynamics of the variance components appear to differ between species, although only a few species and traits have been examined. For example, the postnatal maternal influences on body weight appear to be greater in mice (Le-

Journal ArticleDOI
TL;DR: It is shown that as a result of the interactions among individuals, it is possible, at least in principle, for small genetic differences between groups of individuals to become relatively large differences in populations.
Abstract: The biological character or phenotype of a population is determined both by the genotypes of its component individuals and by their interaction with biotic and abiotic factors in the environment. To the extent that there are genetic differences among individuals, differences among populations may be generated by both drift and selection. In most theoretical studies, the populational phenotype is completely characterized by the genotypic or phenotypic mean and variance of its component individuals. In these cases, the population characteristic in question is simply a statistical summarization of its component parts. When this description of the relationship between individual and population phenotypes is accurate, the phenotypic differentiation of an array of populations owing to drift or selection can readily be represented by the frequency distribution of within-population means. This distribution is a complete characterization because the variance within the population in most genetic models is a function of the mean gene frequency and in most quantitative models this variance is assumed to be constant. However, laboratory studies of genotype-genotype and genotype-density interactions (Lewontin and Matsuo, 1963; Sokal and Karten, 1964; Weisbrot, 1966; McCauley and Sokal, 1977) have illustrated many instances in which the biological character or phenotype of a population is not a simple linear function of its component parts. As a result of the interactions among individuals, it is possible, at least in principle, for small genetic differences between groups of individuals to become relatively large differences in

Journal ArticleDOI
TL;DR: Deep differences between males and females with respect to floral morphology, floral rewards, average number of flowers per plant and flowering periodicity in Jacaratia dolichaula (D. Smith) Woodson are reported, which provides an ecological basis for similar features of floral biology observed in other plants.
Abstract: Sexual dimorphism in animals has recently received much attention from evolutionary biologists who have sought to review differences in the reproductive strategies of males and females in the light of modern concepts in ecology and evolution (Campbell, 1972; Ghiselin, 1974; Williams, 1975; Emlen and Oring, 1977; Maynard Smith, 1978). Although dioecism is widespread among flowering plants, especially in tropical lowland forest trees (Ashton, 1969; Bawa, 1974, 1980; Tomlinson, 1974; Bawa and Opler, 1975; Croat, 1979), no serious attempt has been made to examine the differences in the reproductive behavior of the two sexes. Even differences in floral rewards of male and female flowers and the ways pollinators perceive and respond to such differences to bring about the transfer of pollen have not been generally investigated (Bawa, 1977). A possible exception is the dioecious Carica papaya L. (Caricaceae) for which Baker (1976) has demonstrated significant differences in floral rewards and concomitant differences in the behavior of pollinators on the two types of flowers. Here, I report profound differences between males and females with respect to floral morphology, floral rewards, average number of flowers per plant and flowering periodicity in Jacaratia dolichaula (D. Smith) Woodson, also of the Caricaceae, in which female flowers seem to mimic male flowers. I explain these differences and the mimicry in relation to pollination ecology and sexual selection, particularly intrasexual competition for pollinators. My explanation provides an ecological basis for similar features of floral biology observed in other plants. Revised November 2, 1979

Journal ArticleDOI
TL;DR: This study is, in part, an empirical test of sex ratio theory using a solitary bee, Osmia lignaria propinqua Cresson, to test the accuracy of theoretical formulations and document seasonal variability in the sex ratio.
Abstract: Although sex ratio and investment theory have been recently refined and made more mathematically rigorous (Hamilton, 1967; Leigh, 1970; Trivers, 1972; Trivers and Hare, 1976; Charnov, 1978; MacNair, 1978), Fisher's (1930) original arguments remain essentially unchanged In a panmictic population we expect equal amounts of time and energy to be allocated to the production of each sex A disproportionate investment in the offspring of one sex will be corrected in subsequent generations by selection for parents that produce the sex in short supply Because investment must be divided equally between the sexes, the optimal sex ratio is expected to be unity only when the parental effort expended on an average individual of each sex is also equal If production of a female requires twice the effort as production of a male, the sex ratio should be 2 d :1 9 rather than unity This study is, in part, an empirical test of sex ratio theory using a solitary bee, Osmia lignaria propinqua Cresson It was prompted by the paucity of field data currently available to test the accuracy of theoretical formulations Additionally, observations by Torchio (unpubl) suggest that the sex ratio of offspring of 0 lignaria varies seasonally Indeed, seasonal variation in the sex ratio is not uncommon in bees (Michener, 1974), although the reasons for such variability require clarification Here we document seasonal variability and offer an explanation for it Until recently (Trivers and Hare, 1976), the interrelationship between the ratio of investment in an average male and female and the sex ratio was not clearly perceived It is now apparent that sex ratio is related to investment ratio and any study of sex ratio naturally extends to questions of optimal allocation of investment within each sex Investment expended upon an average individual of each sex in many species is unequal, although total investment in each sex must be equal Such differences are most clearly manifest in those species that exhibit sexual dimorphism in size In Osmia lignaria females are distinctly larger than males Of the explanations proposed for sexual dimorphism, two are frequently cited Trivers (1972), in elaborating on Darwin (1871), hypothesized that members of the sex that invest most in offspring are subjects of intrasexual competition between members of the other sex As a result of intrasexual selection (probably sexual selection also), members of the competing sex are expected to be larger and fewer because of the usual positive relation between size and mating success This explanation is not applicable to 0 lignaria because females are responsible for all but sperm investment in offspring The second hypothesis (Amadon, 1959; Selander, 1972) argues that sexual dimorphism is frequently due to intersexual competition for resources; a divergence in size reduces overlap in resource use, thereby increasing the overall efficiency of each sex This hypothesis is not appropriate to 0 lignaria because the sexes do not differ in distribution across the limited number of flower species used for nectar (P F T, pers observ) We will demonstrate that the asymmetry in investment between individual males and females is probably due to a complicated process involving seasonal changes in resource availability

Journal ArticleDOI
TL;DR: A case in which gene flow may have prevented small scale local adaptation in one population of mosquito fish, Gambusia affinis, carries two messages: field-workers should check the assumption that their study organisms are adapted to the local environment because that assumption does not always hold, and there are limiting cases involving high dispersal rates over short distances inWhich gene flow can overwhelm local selection pressures.
Abstract: Field biologists commonly assume that the organisms with which they deal are well adapted-even optimally adaptedto local circumstances. Both Ehrlich and Raven (1969) and Endler (1977) have de-emphasized the role of gene flow in preventing large scale geographic differentiation and local adaptation. This paper documents a case in which gene flow may have prevented small scale local adaptation in one population of mosquito fish, Gambusia affinis. It carries two messages: field-workers should check the assumption that their study organisms are adapted to the local environment because that assumption does not always hold, and there are limiting cases involving high dispersal rates over short distances in which gene flow can overwhelm local selection pressures. Evidence that local populations are being maintained away from the locally optimum phenotype is rare. Camin and Ehrlich (1958) analyzed the banding patterns of water snakes (Natrix sipedon) on islands in Lake Erie. They showed that on the islands, where snakes were probably confined to a band of sandy substrate near shore by lack of water inland, patternless, light-colored snakes were most common. In contrast, on the mainland where water snakes inhabit wooded streams with darker, more complex substrates, richly patterned, dark-colored snakes predominated. However, a small proportion of the island snakes were patterned, and therefore maladapted to the local environment, an observation that