Showing papers in "The American Naturalist in 1970"

Herbivores and the Number of Tree Species in Tropical Forests

Abstract: A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in ...

Life Historical Consequences of Natural Selection

Abstract: The tremendous variation in the life-history patterns of organisms is best explained as adaptive.any organism has a limited amount of resources at its disposal, and these have to be partitioned between reproductive and nonreproductive activities. A larger share of resources to reproductive activities, that is, a higher reproductive effort at any age, leads to a better reproductive performance at that age; this may be considered a as profit function. This reproductive effort also leads to a reduction in survival and growth and consequent diminution of the reproductive contribution of the succeeding stages in the life history; this may be considered as a cost function. Natural selection would tend to an adjustment of the reproductive effort at every age such that the overall fitness of the life history would be maximized. A model of life history processes has been developed on the basis of these considerations. It leads to the following predictions: 1. If the form of the profit function is convex, or that o...

Community equilibria and stability, and an extension of the competitive exclusion principle

Abstract: It is shown in this paper that no stable equilibrium can be attained in an ecological community in which some r of the components are limited by less than r limiting factors. The limiting factors are thus put forward as those aspects of the niche crucial in the determination of whether species can coexist. For example, consider the following simple food web: Despite the similar positions occupied by the two prey species in this web, it is possible for them to coexist if each is limited by an independent combination of predation and resource limitation, since then two independent factors are serving to limit two species. On the other hand, if two species feed on distinct but superabundant food sources, but are limited by the same single predator, they cannot continue to coexist indefinitely. Thus these two species, although apparently filling distinct ecological niches, cannot survive together. In general, each species will increase if the predator becomes scarce, will decrease where it is abundant, and wi...

The Biological Species Concept: A Critical Evaluation

Abstract: The term "species" has been a central tenet of biological belief since the early days of biology. But the concepts attached to the term have varied and often were not defined rigorously. The purpose of this paper is to investigate the biological species concept (BSC) : to consider its theoretical aspects, how one would actually delimit a biological species in nature, whether such species exist in nature, and whether the concept is of any unique value to the study of evolution. The classical definition of the BSC is partitioned into its essential components, and some of their aspects and problems are discussed. Three fundamental operations necessary for the delimitation of biological species in nature are described in detail. These are operations based on criteria of: (1) geographic contiguity, (2) phenetic similarity, and (3) interbreeding. Two properties of interbreeding, connectedness and success of reproduction, are defined and discussed. A flow chart for recognizing biological species is constructed f...

Competition for Pollinators between Simultaneously Flowering Species

Abstract: Species with similar floral structures and with similar flowering periods and time of pollen and nectar presentation may compete for the service of the same pollinators. The outcome of interspecific competition was analyzed with a simple two-species model. The reproductive success of a species depends on its relative frequency. The minority species will be at a reproductive handicap because it suffers a larger percentage of heterospecific pollinations. The reproductive handicap of the minority species leads to a smaller standing crop in the next generation, which in turn enhances the minority disadvantage. This should lead to rapid exclusion of the minority species from the immediate flora. An increase in pollinator constancy to a given plant species retards the elimination of the rarer species. Pollinator preference for a given plant species may enhance or retard this progression depending on whether the minority species is preferred, since preference alters the effective frequencies of the two species. ...

Relationships between Fruiting Seasons and Seed Dispersal Methods in a Neotropical Forest

Abstract: The weight of fruits and seeds that fell into 75 polyethylene sheets, each 152 x 152 cm, totalling 175 m2 in area, was measured over a period of 17 months in the humid forest of Barro Colorado Island, in the Panama Canal Zone. A total of 72 species of fruits and seeds fell into the traps, and the number falling into any individual trap varied from three to 14. The number of fruiting species per month varied from 11 to 26, while the combined weight of fruits and seeds varied from a monthly average of 0.61-1.930 gm/square meter per day. Small-seeded fruits ripened rather evenly throughout the year, whereas large-seeded fruits tended to be seasonal. Although the primary determinants of fruiting times may be physical, small fruits the seeds of which pass unharmed through the guts of animals tend to be nonseasonal and thus avoid competition for dispersal. Fruits with large seeds that are destroyed by the animals eating them fruit synchronously. The agouti (Dasyprocta punctata), being unable to devour the entir...

Natural Selection and Mutability

Abstract: In sexual organisms, alleles increasing the mutation rate of the entire genome are invariably selected against, whereas in asexual organisms such "general mutators" are favored if they adjust the mutation rate closer to an optimum balancing the advantages of responsiveness to environmental change against those of a small mutational load. Unless very small selective pressures are effective, it seems that no mechanisms exist to establish optimum mutation rates in sexual organisms.

Photosynthetic Systems of Mediterranean-Climate Shrubs and Trees of California and Chile

Abstract: The growth forms of the dominant plants found in Chile and California within mediterranean-type climates are similar. As the length of the drought increases, communities dominated by evergreen trees are replaced by evergreen shrubs, then drought-deciduous shrubs, and finally drought-deciduous shrubs and succulents. The shrubby evergreen elements generally have extensive root systems, a long photosynthetic period, but low photosynthetic capacities. The drought-deciduous shrubby elements have high carbon fixation rates but short periods of activity and generally shallow roots. The evergreen systems expend energy for a certain degree of drought tolerance for the gain of potential year-round production. In regions of long drought periods, the potential seasonal carbon gain of evergreens decreases and they are no longer at an advantage over drought-deciduous elements. The highly drought-resistant but slow-growing succulents can persist in communities where water, and not light, is the primary limiting factor.

Insular Biogeography in Continental Regions. I. The Northern Andes of South America

Abstract: In order to test whether the theory of insular biogeography also applies to a continental situation, species diversity and endemism were studied among the birds living in islands of paramo vegetation above timberline in the northern Andes of South America Stepwise regression analyses were performed, assuming that measures of environmental diversity (especially area) and of isolation (interisland distances) permit prediction of species numbers and numbers of endemics in a continental situation, as they do in archipelagos With seven independent variables included in the equations, prediction of species diversity and endemism was extremely good From 92% to 97% of the variance in species numbers can be accounted for by regression, the best fit (97%) being obtained with a linear model From 87% to 93% of the variance in number of endemics can be predicted by regression, a semilog model providing the best fit (93 % ) The Andean islands conform to archipelagos in two ways: first, because the species-area cur

Size Patterns in West Indian Anolis Lizards. II. Correlations with the Sizes of Particular Sympatric Species-Displacement and Convergence

Abstract: Differences in head length and snout-vent length were computed for all combinations of Anolis species taken two at a time on the Greater Antilles. There is a tendency to converge associated with structural-habitat similarity: Males of species whose range projections on a map do not overlap are significantly closer in size if structural habitats are similar than if different. There is a tendency to diverge associated with spatial overlap: Species with substantial structural habitat similarity are more different in head and snout-vent length if their ranges overlap than if allopatric. When a given species overlaps in part of its range with a second smaller species of similar structural habitat, convergence in head and snout-vent length is about as frequent as divergence; that is, the given species is as likely to increase its size as to decrease that size. Relatively large species converge significantly more often in this situation than do smaller species. However, where a given species overlaps in part of ...

The effect of experience and novelty on avian feeding behavior with reference to the evolution of warning coloration in butterflies. ii. reactions of naive birds to novel insects

Abstract: This paper confirms the findings of the previous study (Coppinger 1969) that novel stimuli often fail to elicit an attack response from avian predators. In this experiment, hand-raised birds avoided novel insects in a manner which showed that the rejection was not learned or innate. The rejection suggests a relationship between the amount of stimulus change and the previous experience of the animal. The experiments further suggest that there need not be an association with noxiousness ia order for conspicuous coloration to be a selective advantage. The implications of these findings are discussed as they apply to the evolution of warning coloration.

Variations in Diversity within Benthic Marine Communities

Abstract: From theoretical considerations, it is suggested that the continual occurrence of small-scale disturbances can account for part of the spatial and temporal variations of diversity within benthic marine communities. This small-scale stability hypothesis is evaluated in terms of how effectively it can serve as an explanation of variations in an intertidal community. Appreciable changes occurred at 10 of 20 sites studied in Tomales Bay from 1963 to 1967. The recurrent pattern of change at nine of these 10 sites can be readily explained by the stability hypothesis. The only difference between this explanation of diversity within communities and that between communities is the frequency and scale of the perturbations of the environment. A combination of the concepts of succession and the relationship between species diversity and environmental stability provides a theoretical context for predicting short-term changes in modern environments and for the interpretation of the stratigraphic record of ancient commu...

Sex ratio and differential mortality between the sexes

Abstract: Differential mortality between the sexes after the period of parental care apparently does not alter the ratio of male to female births. A continuous model which keeps track of the numbers of each sex shows that an allele for equal division of effort between the sexes is favored over any allele causing a slight deviation from equality, no matter what the difference in mortality between the sexes.

Genetic polymorphism in a varied environment.

Abstract: With the discovery that genetic polymorphism for protein structure is more extensive than had been expected, the criteria for stability of polymorphism assume greater importance. Four authors, Levene (1953), Deakin (1966), Maynard Smith (1962, 1966), and Prout (1968) have given criteria for stability in a varied environment. When compared, their criteria turn out to be different. This note attempts to clear up discrepailcies, and to give a more general solution when there is habitat selection. Levene considered a population in an environment divided into a number of ecological niches. Selection operates within each niche, and the proportion of individuals produced in a niche is constant, regardless of the genotypes within it; thus Levene tacitly assumed that density-dependent regulation acts in each niche, and not on the whole population. After selection, individuals from different niches form a single random-mating population, and their offspring settle at random in the various niches. In his notation:

A theoretical model for the optimal timing of diapause

Abstract: A theoretical model is developed to predict the optimal timing of diapause in polycyclic insects as a function of seasonal changes in survival and reproduction of diapausing and nondiapausing individuals, respectively. For a uniform population in a uniform environment, in which maturation is spread over a long time, a relatively sharp transition between 0%-100% diapause is optimal at the time when the expectation of reproduction becomes less than the expectation of survival. On the other hand, when maturation is restricted to a short interval, the optimal diapausing fraction will take an intermediate value during an extended period. Several cases of correlations between conditions on successive days are considered, and the results are discussed with reference to observed diapausing behavior.

The "Niche-Variation" Hypothesis: A Test and Alternatives

Abstract: 1. It is highly anthropocentric to speak of the width of ecological niches. This and other reasons militate against the utility of the idea. 2. There seems to be no correlation in birds between the variety of foods taken and the variation of bill characters. Hence, the "niche width-variation model" is rejected for complex characters. 3. One or more of the following is likely to cause unusually high variability of a canalized character in a morph of a stable population: (a) immigration from genetically different populations, (b) directional selection affecting the exposure of theretofore hidden genetic variation, (c) mixing individuals from different localities (artifactual variation). 4. It is suggested that a permanent high level of variation in a complex character, even if "adaptive," is improbable because other characters overlap its genetic and epigenetic base and stabilizing selection on these related characters will set limits on its flexibility. 5. It follows from (4) that the fewer the genes influ...

The Role of Sexual Behavior in Density-Related Dispersal of Pierid Butterflies

Abstract: Sexual interactions in dense populations of a butterfly, Pieris protodice, lead to emigration of gravid females which colonize new habitats. Some other species of Pierids respond to high population density and skewed sex ratios early in each generation by aggregation of males outside the breeding area.

An Estimation of Redundancy and Information Content of Anole Dewlaps

Abstract: Redundancy is a common feature of message systems and is known to occur in animal communications on a qualitative basis. The following is an attempt to estimate quantitatively the amount of information and redundancy in the dewlap displays of certain sympatric lizards. By redundancy we mean those elements in the display beyond the minimum necessary for a particular message. This analysis of eight species of Anolis known or expected to occur sympatrically at a mountain locality, La Palma, in Hispaniola, grew from observations made during a survey of the ecological relations among these small arboreal iguanid lizards (Rand and Williams 1969). As is usual with sympatric anoles, each species at La Palma has a distinctive dewlap. This dewlap is a large, usually brightly colored and variously patterned sheet of skin which is folded against the throat when at rest and extended by a flexible cartilage during display. The dewlap, which may occur in both sexes or, more frequently, in males only, is highly specialized for signal functions. Its use has been described in advertisement, in courtship, and in aggressive displays (Rand 1967; Ruibal 1967; Carpenter 1965; Kastle 1963; Greenberg and Noble 1944; Evans 1938). Though experimental evidence is lacking, it seems likely that the dewlap is a major display, perhaps the major display, involved in species recognition. Differences between species in dewlaps may serve as premating isolating mechanisms. We have assumed that anoles could distinguish any simple colors, patterns, or shapes which we ourselves could distinguish easily. This assumption is unproven at present, but color vTision has been demonstrated in iguanid lizards (Forbes et al. 1964), and the work on the reactions of Anolis carolinensis to aposematic insects (Sexton 1964) shows that anoles can make fairly sophisticated visual discriminations. We have also assumed that species recognition among anoles may be more complex than simple releasers. Table 1 compares the dewlaps of six of the seven species known from La Palma and two additional species that may occur there. One species from La Palma, A. aliniger which lacks a dewlap in both sexes, is omitted in the following discussion. (A. aliniger possesses a striking orange and black mark in the axilla which may be used in display in situations where other

Developmental Instability and Evolution in Peripheral Isolates

Abstract: It is proposed that inbreeding, directional selection, and drift in small peripheral isolates could alter the genetic milieu to the point where the developmental feedback and regulatory devices would be disrupted. The consequence of such may be sporadic penetrance of expressions hidden by developmental canalization. These variants add to the phenotypic repertoire of an isolate, and may be incorporated and then fixed. Should an isolate succeed in establishing a new adaptive mode and barriers to gene exchange, it may give rise to a discrete population system with expressions which depart radically from those of the parental species.

Accumulation of Nonfunctional Genes on Sheltered Chromosomes

Abstract: The probability of accumulation of nonfunctional mutations on sheltered chromosomes such as the Y-chromosome in higher organisms is studied by using diffusion approximations in probability theory. It has been shown that this probability is highly dependent upon the effective population size, the heterozygous effect of nonfunctional mutations, and the recombination value between the sex-determining and nonfunctional gene loci. The present knowledge of these three factors, which has been obtained mostly from Drosophila studies, suggests that the nonfunctional genes could be accumulated on sheltered chromosomes within a reasonable period of evolutionary time.

Coefficients of Relationship and the Identity of Genes by Descent in the Hymenoptera

Abstract: Hamilton (1964a) has pointed out that social behavior can evolve through kinship selection, wherein the probability of an altruistic act is proportional to the probable genetic relationship between altruist and recipient. Therefore, to calculate likelihoods of altruistic behavior being evolved in certain situations, the probable proportions of genes identical by descent in the individuals involved must be known. Haplodiploidy, the basic genetic system in Hymenoptera, is particularly favorable for the evolution of altruistic behavior, since full sisters have three-fourths of their genes identical by descent (Hamilton 1964b). This is because they each receive the same haploid set of chromosomes from their father, making up half their genotypes, while genes from their mother, being the products of a normal meiosis, are identical by descent half of the time. Full sisters under normal autosomal inheritance, where both sexes are diploid, have only half their genes identical by descent. When I compared a number of other coefficients of relationship calculated by Hamilton (1964b) with those given by Li (1955), however, I observed several discrepancies. In some cases it was also clear that Hamilton's values must be in error. For example, since brothers, being haploid and impaternate, are the equivalents of gametes produced by the meiosis of one individual, they clearly have half their genes in common; yet Hamilton (1964b) gives a coefficient of relationship of r 1/4 for this relationship. This erroneous result is due to Hamilton 's treatment of males in calculating coefficients of relationship: each male is artificially \"made diploid\" by the addition of a \"cipher \" gene that is unique to it. Although Hamilton 's calculations are in error for some relationships involving males, the coefficients of relationship given by Wright (1922; see Li 1955) are also inadequate for determining the optimum strategy of an individual based on the probable degree of relatedness of another to it. This is because the coefficient of relationship as usually employed is actually a measure of phenotypic correlation assuming simple additive gene effects, rather than an estimate of the proportion of genes held in common through identical descent, although in normal autosomal inheritance the values obtained are correct for either purpose. Another failing of the coefficient of relationship is that it makes no distinction between the relationship of a given male to a given female and the converse female-male relationship. Thus the value r = 1/2 is obtained for the father-daughter relationship (Li 1955), whereas in fact all the father's genes are identical by descent with genes present in the daughter, while only half the daughter's genes are identical by descent with genes present in the father. The situation is, however, clearly indicated by Lji's diagrams (1955, p. 182). Letting GA(B) be the proportion of individual A's

The Community Matrix and the Number of Species in a Community.

Abstract: In this paper I am concerned with the number of species that will be held in stable equilibrium in a community of competing organisms, using the general form of the Lotka-Volterra competition equations for m species. Defining Ki as the saturation density for the ith species and αij as the competition coefficient between species i and j, and Ni as the equilibrium density of species i, the number of species will be determined by N, K, $$\overline{\alpha}$$, var (K), the covariances among the α's, and the covariance between α and N. In particular, the number of species increases as K increases but as N, $$\overline{\alpha}$$, cov (α), cov (α,N) and variance of K decrease.

Meiotic Mechanisms of Naturally Occurring Unisexual Vertebrates

Abstract: Genetic consequences of several proposed or observed oogenic pathways for unisexual vertebrates are tabulated, and compared with known modes of oogenesis or with inheritance data. Poeciliopsis Cx has a selective maturation; inheritance is haploid and matroclinous. Triploid Carassius auratus gibelio reportedly suppress Meiosis I. The triploids Ambystoma tremblayi, A. platineum, and Cnemidophorus uniparens suppress cytokinesis at a premeiotic mitosis. Ova of diploid Lacerta armeniaca undergo two meiotic divisions; the somatic ploidy level is reportedly restored by re-entry of the second polar body. On the other hand, a premeiotic mitosis without cytokinesis is more probably the oogenic pathway used by unisexual Lacerta, since it is consistent with the observations (two meiotic divisions), but avoids the difficulties associated with homozygosity, which would rapidly result from suppressing the second meiotic division. In fact, nothing that is known of inheritance patterns of naturally-occurring unisexual ver...

An Analysis of Dispersal in Mites

Abstract: Dispersal from an old to a new resource involves two components: (1) the number of dispersants, and (2) behavioral adaptations determining the probability of discovering a new resource. Dispersant numbers may be increased by dispersing at a younger stage, decreasing the biomass of individuals developed prior to dispersal and/or reducing the biomass of males. The probability of founding a new colony can be maximized if females mate before dispersal and/or through phoresy on a reliable host. The utility of the analysis is illustrated in comparisons of dispersal by three mites. Tetranychid mites maximize the number of dispersants but have a very low probability of dispersal. Poecilochirus is reliably dispersed by its host and has no adaptations to increase dispersant numbers. Iponemus feeds on scolytid eggs and is also phoretic on scolytids, thus, its probability of dispersal is maximal, but there is a direct selective disadvantage if resources are wasted so the size and frequency of males and females is mod...

A markov contingency-table model for replicated lotka-volterra systems near equilibrium*

Abstract: This paper proposes a translation of the deterministic Lotka-Volterra equations for the interaction of populations of two species in a single environment into a formally equivalent Markovian model for the interaction of populations of two species in an ensemble of similar environments. Unlike several discussions of the Lotka-Volterra equations, this paper explores the evolution of such an ensemble of interacting populations when some of the parameters of interaction are considered as subject to natural selection. The purpose of this theoretical development is eventually to illuminate interactions of infections in parasitized, particularly human, hosts, and the following exposition will speak of individuals (hosts) with various kinds of infections. But interactions of any species distributed in discrete, or patchy, and approximately similar environments could also be studied in the same way (Section 6).

A consideration of the power of the χ2 test to detect inbreeding effects in natural populations.

Abstract: The data above indicate that the sample sizes required to reject the null hypothesis, (F = 0), are much larger than hitherto employed, given the range of expected in a natural population (e.g., from .0004 to .009 in nonprimitive human populations). The use of the χ2 statistic without considering the power of the test to estimate F from small samples would seem to be unjustified since the magnitude of inbreeding expected in a natural population would not cause significant χ2 values. On the other hand, should significant χ2 values be obtained, the magnitude of F required is so great that it would be illogical to attribute the deviations from H-W proportions to inbreeding. Thus it would seem that the problem of estimating inbreeding in a natural population is analogous to the problem of estimating selection (Lewontin and Cockerham 1959; Neel and Schull 1968) because sample sizes far in excess of those normally used are required. The material presented here indicates the size of sample required to detect a sp...

From Hopeful Monsters to Bolyerine Snakes

Abstract: Two monotypic snake genera, Bolyeria and Casarea, comprise the Bolyerinae, a subfamily of the Boidae (pythons and boas). Bolyerines are known to have occurred on two or more of the most remote islands of the Mascarene assemblage and seem to be the only snakes indigenous to this region in Recent times. Today they either have reached, or are approaching, extinction on Round Island which, barely more than a mile across, is the last spot on earth from which living specimens have been collected. Bolyerines are distinct from all known amniotes, both living and fossil, in that the maxillary bone is divided into anterior and posterior sections by a movable joint. Such a modification has mechanical characteristics adaptive for feeding. It is difficult to devise a scheme whereby this adaptation evolved in smooth, uninterrupted fashion from an ancestral condition. Several possibilities are presented, all of which lead to the conclusion that the maxillary modification arose suddenly. The sudden appearance of a bizarr...