Showing papers in "The American Naturalist in 1984"
TL;DR: The general relationships between abundance and distribution developed here eventually should contribute to the understanding of the biogeography, population genetics, and evolution of species as well as the ecological attributes of populations and communities.
Abstract: There appears to be a general relationship between abundance and distribution that has two parts. First, within species, population density tends to be greatest in the center of the range and to decline gradually toward the boundaries. This pattern holds over a range of spatial scales from steep environmental gradients within local regions to the entire geographic range. Exceptions include: (1) abrupt changes in abundance that usually correspond to sharp, discontinuous changes in single environmental variables; and (2) multimodal patterns of abundance that are caused by environmental patchiness. The second general relationship is that among closely related, ecologically similar species spatial distribution is positively correlated with average abundance. Again this pattern holds over a variety of spatial scales from local regions to entire geographic ranges. These empirical patterns have already been reported in the literature, but their generality is demonstrated by analysis of additional data for divers...
2,630 citations
TL;DR: Natural selection at the individual level, acting on both animals and plants to produce coevolution among members of the same trophic web, can regulate such ecosystem processes as energy flow and nutrient cycling, and contribute to species coexistence and the resultant species diversity of communities.
Abstract: Many grazing animals in both terrestrial and aquatic ecosystems form dense herds that maintain the vegetation in their concentration areas at very low statures. Studies of the effects of large ungu...
984 citations
TL;DR: Though the long-lived birds and mammals are among the most promising organisms on which to test the theory that reproductive effort increases with age, measures of fecundity commonly decline with increasing maternal age, some recent evidence suggests that offspring survival may improve toward the end of the lifespan.
Abstract: Though it has been widely predicted that in animals in which reproductive value declines with age, reproductive effort should increase toward the end of the lifespan, analysis of changes in reproductive effort are impeded by fundamental difficulties in measuring the costs of reproduction. Energetic measures may not reflect the effects of breeding on subsequent survival and breeding success, especially in organisms in which body size increases with age, while attempts to estimate reproductive costs directly are complicated by positive correlations between breeding success and parental survival. Though the long-lived birds and mammals are among the most promising organisms on which to test the theory that reproductive effort increases with age, measures of fecundity commonly decline with increasing maternal age. Some recent evidence suggests, however, that offspring survival may improve toward the end of the lifespan.
834 citations
TL;DR: The truth in advertising model describes a mechanism of sexual selection to account for the evolution of the kinds of traits used by males of polygynous species to compete for and attract mates and deemphasizes the traditional dichotomies between the effects ofsexual selection and natural selection.
Abstract: Ever since sexual selection was first described by Darwin (1871), biologists have recognized the importance of the phenomenon but have had difficulty in understanding how it operates. Sexual selection is that special form of natural selection which is responsible for the evolution of traits that promote success in competition for mates. It is particularly important in polygamous breeding systems in which individuals of one sex, usually female, invest many more resources in gametes and often also in parental care, and individuals of the other sex, usually males, compete to mate with these females (Bateman 1948; Trivers 1972). Sexual selection can take two forms. On the one hand it can favor the evolution of traits, such as those related to fighting ability, which increase success in direct competition with other males for mates. On the other hand, it can lead to the evolution of characteristics, such as those involved in courtship displays, which make males more attractive to females. Although strikingly sexually dimorphic characteristics, such as the antlers of deer, the plumes of birds of paradise, and the elaborate displays of bowerbirds have been attributed to sexual selection, the precise mechanisms responsible for the evolution of these traits have remained the subject of much debate. There are two main hypotheses. Fisher's (1930) runaway selection model proposes that sexual selection will favor females that choose mates on the basis of any trait that confers an initial survival or reproductive advantage. This process will continue, resulting in simultaneous exaggeration of the trait and enhanced female choice, until the trait reduces survival of males to the point that this outweighs the advantages of increased reproductive success. This model assumes that the advantages and disadvantages conferred by the trait are confined to the sex (male) that expresses it, and that sexual selection is a special form of directional selection that ultimately tends to oppose natural selection for traits related to survival. Zahavi's (1975) handicap hypothesis also proposes that sexual selection favors the exaggeration of traits which promote male reproductive success at the expense of survival. Females choose mates with such traits because their demonstrated ability to survive despite such a handicap indicates the overall fitness of their genotypes. Under this model both male and female offspring tend to inherit
742 citations
TL;DR: The potential role of spatial heterogeneity in promoting the coexistence of prey species is explored with both analytical and graphical models and several examples from field studies suggest that habitat partitioning and the movement behavior of both predators and prey are important factors affecting coexistence in prey communities.
Abstract: Predation may generate patterns in the structure of communities similar to those produced by competition. The potential role of spatial heterogeneity in promoting the coexistence of prey species is explored with both analytical and graphical models. In these models, prey interact indirectly through their effect on predator numbers: an interaction that leads to "apparent competition" and a problem in species coexistence. A model of prey coexistence in a homogeneous habitat is explored. If prey grow logistically, prey abundance is measured by the effect prey have on predator growth, and the predator consumes prey in a finegrained manner, the criterion for species k to remain in the community takes a simple form: its sensitivity to predation (ak/rk) cannot exceed the average sensitivity to predation of the entire community, ($$\overline {a/r}$$), divided by a measure of the intensity of predation on the community (Δ). At high Δ, species must have very similar values for ai/ri to coexist. This suggests that i...
724 citations
TL;DR: A review of 24 data sets on tropical woody plants showed that most evidence indicates either density-dependence or distance-Dependence in progeny mortality, as hypothesized by Janzen and Connell, and some positive evidence also exists for the minimum critical distance effect for tropical trees.
Abstract: More than a decade ago Janzen (1970) and Connell (1971) proposed that natural enemies could be a key factor in maintaining the high tree species diversity of tropical forests. They predicted disproportionately high mortality of juvenile trees close to adults, because of host-specific seed predators, herbivores, or pathogens. Connell predicted higher progeny mortality under adult crowns because of herbivores which normally feed on adults. Janzen proposed that, in addition to such "distance-responsive" agents, density-responsive predators or pathogens will concentrate their activities near adults, where initial seed input is highest. With recruitment probability lowered near nonspecific adults, the probability of nonconspecific establishment in these sites increases. Both authors postulated that host-specific natural enemies would be particularly abundant and effective in tropical rain forests, and thus could be a significant force in maintaining the exceptionally high diversity of trees in these communities. This "Escape Hypothesis" (Howe and Smallwood 1982) was at first widely accepted, although relatively few studies had provided evidence to support it. More recently, data on spatial patterns of tropical trees have been used as a basis for rejection of the hypothesis (e.g., Hubbell 1979, 1980; Fleming and Heithaus 1981).
598 citations
TL;DR: Host plant selection is mainly a behavioral process which is governed primarily by chemoreception, and the emergence of specific insect/host plant relationships most likely results from evolutionary changes in the insects' chemosensory systems.
Abstract: The evolutionary interactions between plants and phytophagous insects are asymmetric: the biochemical and structural diversity of the angiosperms provide a profusion of niches for the evolutionary radiation (cladogenesis) of the insects, while the insects do not affect plant evolution or, at most, may cause anagenic changes in the plants. (Figwasps and figs may represent a rare case of coevolution sensu stricto.) Thus, the evolution of insects generally follows that of the plants ("sequential evolution"). Because the selection pressure exerted by insect attacks is weak or lacking, they could not have been the main cause of the appearance and maintenance of allelochemicals in plants. Nevertheless, these compounds basically determine the plants' "biochemical profile" by which the insects distinguish between host and nonhost plants. Interspecific competition is largely lacking among phytophagous insects in natural communities, so it could not have evoked stenophagy (i.e., resource partitioning) in the insect...
467 citations
TL;DR: A view of the vegetative structure of a herb as the functional fruit ("bait" for the dispersal agent) confounds attempts at understanding leaf defensive chemistry and seed coat traits solely in the context of fitness-depressing herbivory or seed dormancy.
Abstract: Many species of herbs (including grasses) have some of their seeds dispersed by the large grazing mammals that consume the seeds along with foliage. This is an interaction that has probably been oc...
443 citations
TL;DR: It is argued that group foraging, or flocking, can increase individual feeding rates as a result of the sharing of information among group members and may tend to be dissipated by "overflocking," which is in fact an evolutionarily stable strategy.
Abstract: In this paper we have investigated the effects of uncertainty and information on the foraging strategies of animals. We first argue that group foraging, or flocking, can increase individual feeding rates as a result of the sharing of information among group members. Also, members of a foraging group may experience reduced variation in their feeding rates. Both of these advantages increase with the scarcity and patchiness of the food supply. On the other hand, our analysis also suggests that the informational advantages of flocking may tend to be dissipated by "overflocking," which is in fact an evolutionarily stable strategy. (Possible defense strategies against overflocking, such as territoriality or aggression, have not been considered here. We also discuss the strategy of individual foragers. We suggest that the traditional analysis in terms of marginal values, residence times, and giving-up times, may be misleading. It turns out to be important to "probe," especially in patches where food abundance is...
427 citations
TL;DR: A general model of size-related population dynamics, based on a modified Leslie matrix, designed to include demographic processes, such as fragmentation and shrinkage, which are excluded from conventional age-related demographic analysis.
Abstract: Models of population dynamics based on age-related parameters do not provide accurate predictions if within-age-class variance in those parameters is high. Often the demographic fate of an individu...
391 citations
TL;DR: With the onset of spring in the deciduous forests of eastern North America, tree leaves do not all emerge in perfect synchrony, and species produce leaves at quite different rates during the rest of the growing season.
Abstract: With the onset of spring in the deciduous forests of eastern North America, tree leaves do not all emerge in perfect synchrony. Geographic variation in the timing of bud break within species (Lamb 1915; Altman and Dittmer 1962, table 104) can reasonably be attributed to latitudinal climatic trends (Schnelle 1955; Lieth 1974). It is less clear what causes phonological differences among tree species in a particular region (Smith 1915; Ahlgren 1957). Even within single forests, leaf emergence in spring varies over several weeks among coexisting native trees (fig. 1), and species produce leaves at quite different rates during the rest of the growing season. For example, species of Popillds (Critchfield 1960) and Betdla (Kozlowski and Clausen 1966) produce a flush of early leaves in the spring followed by a series of individual late leaves through the summer, but species of Fraxinus (Gill 1971) and Carya (Foster 1931) normally produce only a spring flush of foliage unless defoliated. Querciis has a primary flush of spring leaves, which in young trees can be followed by secondary flushes in the summer (Johannestriebe, lammas shoots) even without defoliation (Lavarenne-Allary 1965; Reich et al. 1978; Kriebel et al. 1976). The proximate control of initial leaf emergence in most temperate deciduous trees is usually the cumulative thermal sum (degree-hours, degree-days) to which buds are exposed after a prerequisite cold period (Lyr et al. 1967; Kramer and Kozlowski 1979; Valentine 1983). Occasionally, as in Fagus, which lacks a chilling requirement, bud break is under photoperiodic control (Wareing 1953; Romberger 1963). Although leaf emergence can be advanced a few days by defoliation the preceding year (Heichel and Turner 1976), in general year-to-year variation in the timing of tree phenological stages is low when related to thermal sums rather than calendar days (fig. 2; Lindsey 1963; Taylor 1974). Populations originating from different localities within a species range do differ in their times of bud break when grown in provenance trials. For example, when grown together in Ohio more northern populations of Acer saccharin leafed out earlier than populations originating more to the south (Kriebel 1957); conversely, in Juglans nigra, populations of more southern origin began growth earlier even in plantations near the northern edge of the species range (Bey 1979). While such intraspecific variations
TL;DR: Under conditions of reduced interspecific competition, there is selective pressure for age groups to diverge in resource use, and the evolution and significance of the "age structure component" of niche width is discussed.
Abstract: Many species pass through several distinct age classes (instars or year classes) as they grow. The variety of resources used by age classes effectively expands niche width. This "age-specific component" of niche width is analyzed for a desert scorpion, Paruroctonus mesaensis, whose populations exhibit both instars and discrete year classes. Another purpose of this paper is to evaluate if instars or year classes function as "ecological species" by showing the same differences in morphology and resource use as exists between biological species. The similarity of the growth ratio (Dyar's constant) to Hutchinson's Santa Rosalia ratio has promoted the speculation that the size divergence between age groups has the same ecological basis as that between competing species. Empirical differences between species for a variety of resource-related parameters were determined from a survey of the literature. These interspecific differences are compared to differences between age classes of the same population. Intraspe...
TL;DR: It is proposed that discrete character differences are more common in plants than in animals and are a direct consequence of the open, less integrative, and plastic patterns of plant morphogenesis which permit large changes in morphology on the basis of relatively few genetic changes.
Abstract: The genetic basis of differences in morphology within and between flowering plant species is reviewed in order to elucidate how many genetic changes are responsible for the evolution of new characters. Two broad morphological categories are evident. Differences in structure, shape, orientation, and presence versus absence are frequently discrete and appear to be governed by one or two genes. Differences in dimensions, weight, and number usually exhibit continuous variation and are influenced by numerous genes, though many of them probably act only indirectly via general effects at the whole organ or whole plant levels. Although it is difficult to specify the relative contributions of the two morphological categories during evolutionary divergence, it is clear that discrete character differences are more common in plants than in animals. I propose that their prevalence in plants is a direct consequence of the open, less integrative, and plastic patterns of plant morphogenesis which permit large changes in ...
TL;DR: A cost/benefit model for the evolution of carnivory is developed to analyze why carnivorous plants are restricted mainly to sunny, moist, nutrient-poor sites and seasons, and why carnivory are rare in epiphytes and other bromeliads.
Abstract: Brocchinia reducta is the first documented case of carnivory in the Bromeliaceae. Its erect leaves form a yellowish cylinder with a cuticular lubricant on its inner surface, impound fluid that emits a nectarlike fragrance, and bear trichomes capable of absorbing amino acids from this fluid in which numerous insects, mainly ants, drown. Trichome absorptivity and aspects of trichome structure appear unique in the primitive subfamily Pitcairnioideae. We present the first rigorous definition of carnivory in plants, and discuss its implications for the identification of cases of carnivory and protocarnivory in bromeliads. A cost/benefit model for the evolution of carnivory is developed to analyze why carnivorous plants are restricted mainly to sunny, moist, nutrient-poor sites and seasons, and why carnivory is rare in epiphytes and other bromeliads. The relative advantages of carnivory and ant-fed myrmecophily are discussed in terms of this model, and predictions made regarding the nature of the ant-plant mutu...
TL;DR: It is shown how the concepts of canalizing and decanalizing selection, which result, respectively, in a buffered or unbuffered developmental system, can be responsible for the maintenance of specific levels of intrapopulational variation.
Abstract: It has been suggested that the gulf between ecological and genetic theories of life history evolution can be reconciled by renewed emphasis on studies of mechanisms observed at the whole organism level. These are recognized as developmental plasticity, canalization, and constraint (Stearns 1982). Here we extend a new approach to this problem (Cooper and Kaplan 1982) by focusing on the evolution of developmental mechanisms that generate intrapopulational variation in reproductive characteristics.
TL;DR: The consequences of foraging time optimization by one species in a food web on interactions in the food web are investigated using several simple mathematical models to show that trophically mediated indirect effects theoretically can be comparable to or larger in magnitude than the direct interactions between adjacent trophic levels.
Abstract: The consequences of foraging time optimization by one species in a food web on interactions in the food web are investigated using several simple mathematical models. The situation modeled is one in which a forager must expose itself to a greater risk of predation while foraging, and it adjusts foraging time adaptively to maximize fitness. In this system the forager's behavior results in a number of indirect effects between species: it causes interactions between the forager's predator and the forager's food; it causes predator self-limitation; and it may cause interactions between different food species. Some simple models are used to show that these trophically mediated indirect effects theoretically can be comparable to or larger in magnitude than the direct interactions between adjacent trophic levels. The possibility of such indirect effects has important implications for theoretical and empirical studies of food webs.
TL;DR: On tente de preciser par la methode de regression lineaire ce qui determine les rapports de force relatifs en ce qui concerne les relations predateur-proie.
Abstract: On tente de preciser par la methode de regression lineaire ce qui determine les rapports de force relatifs en ce qui concerne les relations predateur-proie (on donne comme exemple Notonectidae-Culicidae)
TL;DR: The hypothesis that both food supply and interspecific competition have determined morphological properties in communities of Darwin's ground finches is supported, with simple models incorporating the effects of both foodsupply and competition shown to accurately predict observed morphological patterns.
Abstract: Lack's (1947) comparative study of Darwin's finches led him to two influential conclusions concerning the processes that shape morphological properties of species communities. First, he stressed the adaptive nature of much of the morphological variation among populations and species, particularly in relation to diet. Second, Lack argued that morphological differences between species were frequently a consequence of competition for food resources. Such differences arose in two ways: (1) by the extinction of species morphologically (and hence ecologically) similar to others, and (2) by natural selection causing morphologically similar species to diverge. The former process is competitive exclusion; the latter is character displacement. As a result of these claims and Lack's evidence to support them, Darwin's finches have become a classic example of the workings of natural selection, and of the general influence of interspecific competition on morphological patterns in species communities (Grant 1981b; Sulloway 1982). These views have been challenged from several sources. Bowman (1961) pointed out that the Galapagos islands are floristically different from one another,
TL;DR: The results presented here contradict Owen-Smith and Novellie's (1982) claim that linear programming is a special case and the contingency model is a representation of herbivore diet choice and the model of food energy content weighted by abundance might not be expected to work a priori, because it is not an optimal foraging model.
Abstract: Three models of herbivore foraging are tested in this paper: contingency models (Owen-Smith and Novellie 1982), nutrient content weighted by food abundance (Stenseth and Hansson 1979), and linear programming (Belovsky 1978). The models are tested using data from two microtine populations, kudu, and moose. In all four cases, the linear programming model solved for nutrient maximization best predicted the diet composition by food classes (grass, forb, etc.) and energy intake. Foraging time and digestive capacity are the two most important constraints in the linear programming model. Contingency models do not appear to predict herbivore diets for several reasons. First, either they often do not contain constraints or the constraints are incorrectly applied. Second, simultaneous search is assumed which may not hold for plant foods. The results presented here contradict Owen-Smith and Novellie's (1982) claim that linear programming is a special case and the contingency model is a representation of herbivore di...
TL;DR: TSD in Menidia supports the Charnov-Bull model for the evolution of environmental sex determination and provides a mechanism by which a sexual dimorphism can arise without sacrificing growth rates in either sex.
Abstract: In the Atlantic silverside, Menidia menidia, temperature during the larval development affects primary sex determination. Most offspring produced under lower temperature regimes of the early breeding season become females while most offspring produced at the higher temperatures prevailing during the late breeding season become males. The end result is a sexual dimorphism in size because females are provided with a longer growing season by virtue of earlier birth. The adaptive significance of TSD in M. menidia appears to be related to this sexual dimorphism and the way in which size affects the relative fitnesses of males and females. Based on a survivorship schedule that includes a winter period of severe, size-selective mortality, selection favors large individuals of either sex in survival to the breeding season. However, large size is apparently more of an advantage to female than male RS during breeding. Atlantic silversides are group spawners that openly broadcast their gametes with those of many oth...
TL;DR: It is reasonable to consider the evolution of self-pollination in terms of traditional population genetic models, despite the possibility that the fitness of progeny produced by selling might be less than that of outcrossed progeny.
Abstract: From at least the time of Darwin (1876) botanists have tried to understand why there are plants that are primarily self-fertilizing even though the progeny produced by self-fertilization (whether of normally outcrossing or normally selling plants) are often less vigorous than the progeny produced by cross-fertilization (see, e.g., Schoen 1983). The explanation commonly proposed has been that the breeding system of a species is a compromise between the conflicting demands of \"immediate f -ss\" and \"long-range fitness\" (Mather 1943). The ecological conditions in wnich a given species is found are supposed to determine whether it is predominantly self-fertilized or predominantly outcrossed (Stebbins 1950, 1957; Grant 1958). That there is variation in outcrossing rates between species is common knowledge. Studies documenting intraspecific variation in outcrossing rates are rarer, but they do exist, e.g., the studies of Harding (1970) and Harding and Horovitz (1969) on Lupinus nanus, the study of Schoen (1982) on Gilia achilleifolia, and the study of Brown et al. (1978) on Hordeum spontaneum. In some groups of plants the frequency of outcrossing has been shown to be correlated with a phenotypic state that is under simple genetic ontrol, e.g., in Phaseolus lunatus (Harding and Tucker 1964; Tucker and Harding 1965) and in Ipomoea purpurea (Ennos and Clegg 1983). Thus, it is reasonable to consider the evolution of self-pollination i terms of traditional population genetic models. Fisher (1941; with corrections by Moran [1962]) was the first o formulate an explicit genetic model to study the effect of differential selling rates in one population. He studied a one-locus two-allele model in which one homozygote was completely outcrossing, the heterozygote outcrossed with a frequency of one half, and the other homozygote was completely selling. One of the models that Wells (1979) studied extended these results to allow for intermediate s lling rates in all three genotypes. These models did not allow for the possibility that if a genotype selfs its contribution tothe pollen pool might be reduced. In the second model he considered, Wells (1979) allowed for the possibility that selling might reduce an individual's contribution to the pollen pool. Nagylaki (1976) also studied a model in which discounting of pollen (the removal of pollen from the pollen pool by selling) was possible. Though one of the genotypes was constrained always to outcross, the outcrossing frequencies and discounting rates of the other two genotypes were arbitrary. None of the models discussed so far allow for the possibility that the fitness of progeny produced by selling might be less than that of outcrossed progeny. Maynard Smith (1977, 1978) considered a model in which such selfed progeny might be less fit han the outcrossed progeny, but the model he considered id not
TL;DR: Examination of the degree and extent of multiple paternity within individual fruits set in a small, isolated population of Raphanus sativus in southern California found multiple paternity occurred frequently, involving all of the assayed parents and at least 85% of the fruits overall.
Abstract: Evidence for concurrent multiple paternity has been gathered for a large number of diverse animal species. Despite conflicting theory regarding the likelihood of multiple paternity within the fruits of angiosperms, this phenomenon has yet to be investigated quantitatively in any natural plant population. Thus, I sought to examine the degree and extent of multiple paternity within individual fruits set in a small, isolated population of Raphanus sativus in southern California. In all, 246 seeds from 59 fruits and the tissue from their nine maternal parents were examined at six allozyme loci. Comparison of the multilocus genotypes of offspring to that of their maternal parent permitted calculation of minimum estimates of multiple paternity for each fruit. Multiple paternity occurred frequently in this sample, involving all of the assayed parents and at least 85% of the fruits overall. The minimum paternal donor number ranged from 1-4 (x = 2.27). Likely sources of multiple paternity within radish fruits are...
TL;DR: A method is described for using the modified von Bertalanffy growth equation, developed in this paper, to estimate the in situ production of zooplankton.
Abstract: 1. The growth of marine, particle-feeding zooplankton is likely to be limited by the availability of food in open ocean environments, but not in coastal regions. 2. The maximum production ratio of zooplankton to phytoplankton is predicted to be greater at higher temperatures, ranging from 6.4% at 0⚬ C to 20.3% at 25⚬ C. 3. A method is described for using the modified von Bertalanffy growth equation, developed in this paper, to estimate the in situ production of zooplankton.
TL;DR: A comparison of ramet population growth rates in a flowering and a nonflowering population of Eichhornia crassipes Solms.
Abstract: A comparison of ramet population growth rates in a flowering and a nonflowering population of Eichhornia crassipes Solms., the water hyacinth, indicates that inflorescence production is inversely correlated with ramet population growth rate. The differences in patterns of resource allocation exhibited by the two populations are explained in terms of developmental constraints acting on the differentiation and growth of a limited meristem population. The meristem population is limited in size by the developmental morphology of the ramets. This interpretation contrasts with more traditional ones in which fixed carbon, measured as dry weight, is viewed as the limiting currency. The inadequacy of fixed carbon as a limiting currency for plant resource allocation studies is discussed in relation to the origins of its use in the zoological literature. While most animals must acquire all usable carbon through a single mouth and then apportion it among competing needs, herbaceous plants generate new "mouths" (= car...
TL;DR: A mathematically tractable stochastic model is presented in which animals forage systematically for prey distributed in patches, and the best rule is found and compared with the giving-up time (GUT) rule and a fixed-time rule in which the forager remains until it has exhausted each patch.
Abstract: If an animal forages for prey that are distributed in patches, it must use some rule to decide when to leave one patch and go on to another. Charnov's marginal value theorem tells when an animal should leave a patch, but it does not tell how an animal should decide when to leave a patch. This is important if patches vary in quality and the forager must use its experience in a patch to decide when best to leave a patch. In this paper a mathematically tractable stochastic model is presented in which animals forage systematically for prey distributed in patches, and three possible strategies (stopping rules) are considered. The best rule is found and compared with the giving-up time (GUT) rule and a fixed-time rule in which the forager remains until it has exhausted each patch. The GUT rule is better than the fixed-time rule for variable patches, and it is relatively insensitive to environmental changes, but it is not as good as the best rule, which I call the assessment rule. The assessment rule is not only...
TL;DR: Directional and fluctuating asymmetry were assessed for 10 paired osteometric traits in inbred and hybrid house mice primarily in order to test five separate hypotheses, and FA did exhibit statistical significance for heterosis, and there was a significant positive association of heterosis with the magnitude of heritability of each trait.
Abstract: Directional and fluctuating asymmetry were assessed for 10 paired osteometric traits in inbred and hybrid house mice primarily in order to test five separate hypotheses. Hypothesis (1) stated that directional asymmetry (DA) would be relatively low for these characters, especially those directly involved in locomotion. The DA for most characters was nonsignificant or negligible in importance, including that for the posterior limb bones most used in locomotion. Hypothesis (2), that the heritability of DA would be low, was confirmed, averaging only 2% in inbreds and 4% in hybrids. Hypothesis (3) was that fluctuating asymmetry (FA) would be moderate in magnitude. The average contribution of FA to the total variance was in fact considerable, 18% across populations and characters. Hypothesis (4) proposed that the inbreds should show more FA than hybrids, particularly for those characters with high heritabilities. FA did exhibit statistical significance for heterosis, and there was a significant positive associa...
TL;DR: Several hypotheses for the evolution of sexual dimorphism in body size are outlined in terms of the types of selection likely to be observed in contemporary studies of populations in a population of Darwin's medium ground finch, Geospiza fortis.
Abstract: Comparative methods have generally been used in attempts to understand the evolution of sexual dimorphism. Here I outline several hypotheses for the evolution of sexual dimorphism in body size in terms of the types of selection likely to be observed in contemporary studies of populations. I use this to interpret observations of selection in a population of Darwin's medium ground finch, Geospiza fortis, in which males average about 5% larger than females. Results are supplemented with observations on feeding and breeding behavior. There is estimated to be a very high between-sex genetic correlation for homologous characters. This may account for a lack of differentiation in the degree of dimorphism demonstrated among populations and species of Geospiza. There is no evidence that resource partitioning between the sexes has played any significant part in the evolution or maintenance of the dimorphism in this population. Two contemporary selection forces are identified and implicated in the past evolution of ...
TL;DR: It is found that individuals with obvious morphological deformities in three populations have increased fluctuating asymmetry, suggesting that heterozygosity is positively correlated with fitness in these fishes.
Abstract: We examined the association between heterozygosity at 42 enzyme loci and fluctuating asymmetry at five bilateral meristic characters in 14 population samples of rainbow trout, cutthroat trout, and brook trout. There is a significant negative correlation between heterozygosity and the proportion of asymmetric characters per individual in two populations of rainbow trout and two populations of cutthroat trout. This correlation is negative in 13 of the 14 population samples (sign test, P < .001). Thus, individuals that are more heterozygous at isozyme loci have reduced fluctuating asymmetry; this appears to be general among salmonid fishes. We also found that individuals with obvious morphological deformities in three populations have increased fluctuating asymmetry. Thus, fluctuating asymmetry appears to be a reliable indicator of overall developmental stability and is therefore negatively correlated with fitness. The association between heterozygosity and asymmetry and between asymmetry and morphological d...
TL;DR: It is suggested that apyrene sperm are a morphological manifestation of a more widespread phenomenon wherein accessory sperm may play an important role in sperm competition.
Abstract: Moths and butterflies (Lepidoptera) generally produce two types of spermatozoa: a typical nucleated (eupyrene) spermatozoon and a smaller anucleate (apyrene) spermatozoon. The apyrene sperm often predominate over the eupyrene sperm in an ejaculate and, in the female, they migrate actively to the sperm storage organ, the spermatheca. There they usually degenerate, apparently not playing any role in fertilization of the eggs. Several hypotheses for the function of the apyrene sperm have been proposed. These center around the notions that the apyrene sperm may assist the eupyrene sperm in their migration from the testes in the male to the spermatheca in the female or that they function as a nutritional supplement in the female. No experimental support for these notions has been adduced and observational evidence seems to argue against them. We propose that apyrene sperm may play, at least additionally, a role in competition between rival sperm deposited by different males. They may either eliminate, by displ...
TL;DR: A brief discussion of pollination and seed dispersal suggests checks to mutualistic coevolution, and indicates that reciprocal adaptation, where it occurs, is general rather than specific.
Abstract: Obligate one-to-one mutualisms between species pairs are rare in practice and anomalous in theory. A complex community holds the potential for thousands of such interactions; a negligible number exists. A brief discussion of pollination and seed dispersal suggests checks to mutualistic coevolution, and indicates that reciprocal adaptation, where it occurs, is general rather than specific. First, diversity diffuses selection from any one source, reducing the possibility of direct reciprocal selection among species pairs. Second, normal processes of succession, disturbance, and geographic flux of species ranges continually change selective regimes over evolutionary time. Third, commonplace variation in such population attributes as age and spatial distributions of plants that provide flower and fruit resources alter the intensity of interaction between plants and animals, and thereby select for general rather than narrowly specific relationships. Fourth, polygenic inheritance promotes uneven rates of evolut...