Showing papers in "The American Naturalist in 1969"

A Note on Trophic Complexity and Community Stability

Abstract: That "stability" is conferred approximately in proportion to the diversity of energy pathways characterizing any particular community is a widely held axiom among ecologists. As examples of this belief, one can read that "Stability increases as the number of links increase" (MacArthur, 1955, p. 535) or "a rich fauna and flora . . . tends to be very stable because of a multiplicity of ecological checks and balances" (Watt, 1964, p. 1434). There appears to be little or no sound evidence available to accept or reject these statements, because an operational definition of stability is lacking, as are data from the more complex associations. The most workable definition involves statements about the relative variability of population numbers in space and/or time, although limits on the extent of the spatial dimension are not usually stated, and collections of acceptable data through time are tedious to gather, and hence minimal. The basic data for a community tend to be species lists with comparisons and evaluations made between years on a presence or absence basis, an analysis providing a measure of predictability, not necessarily stability. At the population level the direction and magnitude of change in numbers, that is, a measure of the tendency to oscillate, may be adequate as a measure. Even impressions based on the intuitive understanding of a competent naturalist may prove acceptable. Any or all of these would suffice for the present purpose, although they fail to make the critical distinction between describing the relative continuity of a community, that static component which is relatively easily measured, and the ability of that community to withstand perturbation, the dynamic and presumably more fundamental component which is discernible only upon alteration. Thus, the

On the Evolution of Mating Systems in Birds and Mammals

Abstract: Predictions from a theory assuming mate selection on the part of females, which maximizes reproductive success of individuals, are found to accord closely, though not completely, with known mating patterns. These predictions are that (1) polyandry should be rare, (2) polygyny should be more common among mammals than among birds, (3) polygyny should be more prevalent among precocial than among altricial birds, (4) conditions for polygyny should be met in marshes more regularly than among terrestrial environments, (5) polygyny should be more prevalent among species of early successional habitats, (6) polygyny should be more prevalent among species in which feeding areas are widespread but nesting sites are restricted, and (7) polygyny should evolve more readily among species in which clutch size is strongly influenced by factors other than the ability of the adults to provide food for the young. Most cases of polygyny in birds, a group in which monogamy is the most common mating pattern, can be explained on...

Hybridization, Unisexuality, and Polyploidy in the Teleost Poeciliopsis (Poeciliidae) and Other Vertebrates

Abstract: The discovery of a second gynogenetic, triploid "species" in the genus Poeciliopsis is reported. This all-female form is part of a five-fish hybrid complex from the Rio Fuerte of northwestern Mexico. The complex consists of two bisexual species, P. monacha and P. lucida, and three unisexual derivatives, two of which are triploid and one of which is diploid. The 2n unisexual approximates 1n of P. monacha and 1n of P. lucida. During oogenesis the paternally derived P. lucida chromosomes are lost; thus it is primarily the P. monacha set that is transmitted to the egg. Since the allfemale form mates mainly with males of P. lucida, the hybrid nature of this fish is maintained. The term "hybridogenesis" is applied to this mode of reproduction. The two triploid forms apparently stem from the occasional failure of this mechanism and the subsequent production of 2n ova. Such ova, when combined with P. lucida sperm, result in a triploid form that morphologically appears to be 2n of P. lucida and 1n of P. monacha; b...

The Concept of Reproductive Effort and Its Relation to the Evolution of Life Histories of Lizards

Abstract: Most aspects of the life histories of individual organisms are explicable in terms of natural selection, a fact often neglected in life history studies, with the result that such studies rarely contain the type of information necessary to test evolutionary hypotheses concerning life history phenomena. Williams (1966a, 1966b) has directed attention to life history phenomena that generally should be explicable from evolutionary considerations. He has made the prediction that a higher per season reproductive effort should characterize small, short-lived rather than larger, long-lived species. He has, as a corollary, cited differences in behavior that might be expected between species with differing demographic properties. Tinkle (1969) presented data on the lizard Uta stansburiana which accord with Williams's hypothesis. One purpose of the present paper is to compare reproductive effort of many species, using criteria suggested by Williams (1966a, 1966b) to determine how well lizard species accord with Williams's hypotheses. A second purpose is to formulate a general theory for the evolution of life history types in lizards.

Models of Optimal Size for Solitary Predators

Abstract: Models are presented which predict an optimal size, defined as that size which takes the least amount of time to satisfy its energy requirements, for several types of predators: I-predators which expend no time or energy in food search alone and which pursue their prey over varying distances; II-predators which expend time and energy while searching for items but none in pursuit; IIa and IIb-predators which expend time and energy both in search and pursuit. All predators are assumed to expend time and energy in handling and swallowing prey. In the simplest models considered, equations are constructed which combine to produce three major properties: (a) larger predators expend more energy in pursuing an item over a given distance than do smaller ones; (b) for most prey items, larger animals expend less time and for somewhat fewer items less energy in handling and swallowing than do smaller predators; (c) time and to a lesser extent energy lost while searching between two items are often not very different ...

Thermal Acclimation and Heat Resistance in Drosophila Species

Abstract: Physiological and developmental acclimation to dry heat as measured by survival at 38⚬ C was studied in several species of Drosophila Physiological acclimation occurs rapidly (about half-completed in 12 hr), is quickly lost, and does not depend on water uptake In broad-niched species, and especially in D melanogaster, adaptation to different climates depends mostly on individual flexibility and less on genetic differences among populations None of the narrow-niched species acclimate The D willistoni, a moderately broad-niched species, does not acclimate nor show much genetic variation It is suggested that this species avoids desiccation stress behaviorally

Bird Species Diversity and Habitat Diversity in Australia and North America

Abstract: Censuses of breeding bird populations in Australia support the conclusion reached in North American studies, that habitat diversity as measured by foliage profile is a good predictor of bird species diversity. The data also indicate that the avifaunas of forest and scrub habitats in the temperate zones of Australia and North America have reached equilibrium and are probably saturated. Equilibrium or saturation levels appear to be independent of the histories and ancestries of the avifaunas concerned.

The Buffer Effect and Productivity in Tit Populations

Abstract: Certain discussions of the effects of territorial behavior on population density and productivity (e.g., Kluyver and Tinbergen 1953; Tinbergen N., 1957; Wynne-Edwards 1962, p. 149) have concluded that under conditions which are here defined as "level 2," production of young is limited by territorial behavior at some level below the potential maximal production for the population. The present considerations of "optimum mix" and "exchange ratio" suggest an alternative interpretation, namely, that productivity in the Great Tit, at least, is not limited by territorial behavior, but actually might be maximized by it.

Enzymic Mechanisms of Temperature Compensation: Immediate and Evolutionary Effects of Temperature on Enzymes of Aquatic Poikilotherms

Abstract: Immediate and evolutionary effects of temperature on enzymes from several Arctic marine poikilotherms have been examined. The results of these studies have been compared with existing information on temperature effects on enzymes of warm- and cold-blooded animals. The following conclusions have been drawn about temperature effects on enzymes of aquatic poikilotherms: 1. Immediate compensation to temperature changes is promoted by an inverse relationship between temperature and enzyme-substrate affinity (as measured by the reciprocal of the Km of substrate). Activation energy is not an important parameter in immediate compensation. 2. The sharp decreases in enzyme-substrate affinity which frequently occur at the extremes of an organism's habitat temperature may be important in establishing thermal tolerance limits for the organism. 3. In evolutionary adaptation of enzymes to temperature, minimal Km values (maximal enzyme-substrate affinity) normally occur near the lower extreme of a species' adaptation tem...

Population Studies in Predominantly Self-Pollinating Species. XIII. Intergenotypic Competition and Population Structure in Barley and Wheat

Abstract: Interactions between various genotypes were studied in four varieties of barley, four varieties of wheat, and eight barley genotypes which had survived up to 18 generations of mutual selection in a heterogeneous population. The results showed that first-order interactions between genotypes from the heterogeneous barley population were much larger than those between the varieties of wheat and barley. Natural selection therefore appears to preserve genotypes which interact synergistically. Deterministic and stochastic simulations predicted fixation of the variety with the highest pure-stand reproductive value in populations of the wheat and barley varieties. However, they predicted the development of stable multigenotypic associations in populations formed from the eight genotypes isolated from the heterogeneous barley population. The effect of random variation in the reproductive values was to decrease the time to fixation when there was no stable nontrivial equilibrium; however, when such an equilibrium e...

The Turnover Ratio in Production Ecology of Freshwater Invertebrates

Abstract: The turnover ratio of freshwater benthic invertebrates, expressed as the ratio of a cohort's production to the mean standing crop, has been observed to be relatively constant, about 2.5 to 5, with a mode of about 3.5. Turnover ratios were computed from Allen growth-survivorship curves under various combinations of hypothetical conditions to determine the theoretical range. The effects of varying curve shape, initial individual weight relative to maximum, final population size in numbers relative to initial population, and growth pattern were tested with several series of Allen curves. With moderate variation in these factors around the most probable conditions, the theoretical turnover ratio varied from about 3 to 4 for aquatic insects, but it is probable somewhat larger for crustaceans. Turnover ratios were also considered as equal to instantaneous growth rates computed over an entire single life cycle for several invertebrate species. These were similar to those obtained with the Allen curves, although ...

Embryonic Genetic Load in Coastal Douglas-Fir, Pseudotsuga menziesii var. Menziesii

Abstract: Thirty-five coastal Douglas-fir trees were evaluated for embryonic genetic load from comparisons of sound seed set following self- and cross-pollinations, that is, from determinations of relative self-fertility. Estimates for the 35 trees ranged from about 3 to about 27 lethal equivalents per zygote active in the embryo stage, with the median tree carrying about 10 lethal equivalents per zygote. Relative self-fertility of Douglas-fir was compared with that reported for other coniferous species, and genetic load was compared with that reported for Drosophila, Tribolium, and man.

Density-dependent gene dispersal in liatris

Abstract: The effect of colony density on gene migration has been interpreted in the perennial herb, Liatris aspera. Colonies with 1, 3.2, 5, and 11 plants/square meter, respectively, were studied. The pollen component of gene migration was inferred from pollinator flight distances, 1,614 of which were analyzed. Bees were the primary pollinators. The seed component of gene migration was measured in the experimental garden, utilizing transplants as the seed source. Axial distances were determined for 2,309 wind-borne seeds. Means of pollinator flight distances displayed a close curvilinear relationship to colony density, an increase in density being accompanied by a decrease in flight distance. Flights are predominately to near neighbors, and there is strong positive correlation between the variance of flight distance and the variance of plant spacing. Thus we conclude that pollinator behavior and pollen-dispersal distance are density-dependent. Gene migration has been considered within the framework of Wright's nei...

The Relationship of Fresh-Water Protozoan Communities to the MacArthur-Wilson Equilibrium Model

Abstract: Two series of 10 artificial substrates each were placed in Douglas Lake, Michigan, and their colonization by species of fresh-water protozoans studied. Identifications were made to species whenever possible at intervals of approximately one week, and rough estimates of density were made as well. Although the aggregations of species colonizing each of the substrates were not identical, the colonization process itself was remarkably similar for the entire series. When the number of species was plotted against time in days, a simple exponential curve adequately described the relationship. Colonization rates and extinction rates were compared with the equilibrium model for island faunas proposed by MacArthur and Wilson. These results suggest that the formation and composition of protozoan communities on artificial substrates are the result of interactions comparable to those proposed by MacArthur and Wilson.

An Analysis of Gene-Enzyme Variability in Natural Populations of Drosophila melanogaster and D. simulans

Abstract: Nine populations of D melanogaster and two populations of D simulans were analyzed for polymorphism in 10 gene-enzyme systems by the technique of gel electrophoresis In the eight natural populations of D melanogaster, an average of 54% of the enzymes were polymorphic, and the average heterozygosity was 227% An experimental population of D melanogaster, which has been maintained in a laboratory cage for 20 years, showed levels of polymorphism equivalent to those of natural populations The D simulans populations had much less variability The possible factors involved in maintaining these polymorphisms are discussed

Why the prey curve has a hump

Abstract: Available evidence and a new argument support the graphical theory of predation In particular, the prey isocline can have positive slope over lower prey densities Hence, from what is already known about the predator-prey interaction, high predatory efficiency, meaning the predator's ability to reproduce successfully in the presence of few prey, can be a source of instability, perhaps even resulting in the predator's extinction

Heterosis and the Evolution of Duplications

Abstract: Single locus heterosis affords a sufficient drive for the incorporation of a newly arisen duplication into a species gene pool. Heterosis is here assumed to result from the formation of heterodimers between polypeptides specified by alleles. In consequence, the normal homologue of the duplication-bearing chromosome is eliminated rapidly unless the product of the locus is regulated in amount and recombination occurs so infrequently as to approximate mutation rates. In the latter case, the duplication comes to a frequency that is the higher, the fewer alleles initially present. This frequency would then continue until the slower processes such as mutation or recombination between adjacent cistrons permit a renewed climb to fixation. The allele included in the duplication will be lost at the original site unless the original number of alleles is low and it forms homozygotes of fitness nearly equal to the heterozygote. In general, segregation would be expected to continue for at least two alleles in nearly eq...

An Example of Gene Fixation Resulting from Selective Advantage in Suboptimal Conditions

Abstract: In maize, alcohol dehydrogenase (enzyme commission 1.1.1.1) occurs in the kernel and in the pollen. Enzyme activity falls off rapidly during seed germination (Efron and Schwartz 1968). As a result of treatment with the mutagen ethylmethane sulfonate (Schwartz, in preparation) we have induced a mutation at the Adhl locus which in homozygous condition fails to produce active enzyme. This mutant has been designated Adh10. Experiments are in progress to determine the nature of the mutation; however, whether the mutation resulted from gene inactivation or deletion is not germane to the subject of this communication. Starch gel zymograms (Schwartz 1966) of mutant embryos and pollen are devoid of alcohol dehydrogenase isozyme bands, while a prominent band is observed in sibs which carry an active Adh1 allele. In heterozygotes of Adh10 with Adhis, Adh1F, and Adh10, only the SS, FF, and CC isozyme bands appear in the zymograms. On the surface, alcohol dehydrogenase does not appear to play an essential role in the development of the plant. Kernels which lack an active Adh1 gene develop normally, and in an F2 segregating ear mutant and nonmutant kernels can be distinguished only by enzyme assay. Furthermore, no difference can be detected in the germination and subsequent growth of mutant and nonmutant sibs. The enzyme does not appear to play a role in the development and functioning of pollen. When pollen from Adh,1/Adho heterozygotes is used to fertilize Adh1 F/Adhl' plants, equal numbers of Adh1F/Adh1s and Adh1F/Adho kernels result (223:220). This is a very sensitive test for pollen function, since a number of pollen tubes grow down each style, but only one achieves fertilization. If the gene does not play an active role in plant development, why did it persist in active form in the population? It must confer some selective advantage on the plant; otherwise it should have been lost by accumulated mutations. A possible explanation is that even though the gene is not essential for normal development of the plant under the normal range of cultural conditions, it may be essential for survival under adverse conditions. In the search for such a role I was able to make an educated guess based on the biological activity of this enzyme. Alcohol dehydrogenase

Uniparental Inheritance in Chlamydomonas reinhardi

Abstract: This article reviews what is known about uniparental inheritance in the green alga Chlamydomonas reinhardi. The most important conclusion is that these genes are unlike Mendelian genes in their transmissional genetics, but are similar to Mendelian genes in that they mutate, segregate, and recombine. Experimental evidence is presented which suggests that the number of copies of uniparental genes per cell is probably small. What is known of the transmissional genetics of UP genes is compared with what is known about the transmission of chromosomal and satellite DNA species in C. reinhardi.

Brain Evolution and Dinosaur Brains

Abstract: Dinosaur brains were typically reptilian in size relative to body size. Living and fossil reptiles, amphibians, and fish are "lower" vertebrates with respect to brain evolution and may be described collectively by a single set of brain: body relationships. Their brains evolved conservatively: selection pressures apparently did not require gross changes of the brain: body function in these classes. Birds and mammals, on the other hand, may be considered to be "higher" vertebrates with respect to brain evolution. The size of their brains relative to body size increased in successive geological eras, presumably in response to selection pressures on higher vertebrates for increased neural control and information processing. Within the several vertebrate classes there are, of course, neural specializations reflected in enlargement of different brain structures in different species. These are adaptations to niches which require different kinds of information processing and neural control. Changes in total brain...

Temperature Adaptations of Amphibian Embryos

Abstract: A simple equation is used to describe the rate of development of amphibian embryos between any two developmental stages at various temperatures. This equation yields values for two constants describing the adaptive temperature and the relative time for development at this temperature for every species. The constants can be used to measure adaptability of the embryos to temperature and to study the process of temperature adaptation during speciation. Genetic adaptation to temperature is a very slow process in the amphibia. Many species develop in nature at temperatures near the limit of their adaptive range rather than at optimal temperatures. Different geographical races within one species may show significant differences in their adaptive temperatures. These are, however, considerably smaller than the differences between the actual environmental temperatures encountered by the embryos. In some species no differences between the adaptive temperatures of different geographical races can be found. Variable ...

Species Diversity in Lacustrine Phytoplankton. I. The Components of the Index of Diversity from Shannon's Formula

Abstract: Seasonal observations on the diversity of phytoplankton communities were made in three lakes in Wisconsin. Extremes in nutrient availability and morphometry in the lakes yielded a range in diversity indexes calculated from Shannon's formula. Examination of the relative importance of the two components in the index indicated that the variability of the index can in large part be attributed to the component of equitability as expressed in the 10 to 15 most abundant species. It appears that with additional species, the positive effect of the component of species richness is countered by the negative effect of the equitability component. Species of low abundance appear to have a minor effect on the index of diversity. The response of the components is discussed in terms of interactions, relationships, and problems associated with estimating the diversity of indefinitely large communities.

Avian Niche Size and Morphological Variation

Abstract: Variation in the bills of primarily passerine birds from tropical low-land and north temperate regions is compared by several statistical methods. No evidence of a trend toward greater variation in the region of lower species number could be detected.

Natural Primate Troops and a Stochastic Population Model

Abstract: A simple stochastic population model which assumes constant birth, loss (death or emigration), and immigration rates equally and independently applicable to all individuals in a theoretical population predicts at equilibrium a negative binomial distribution of population size if all three rates are strictly positive and a Poisson distribution of population size if only the birth rate is zero or if births occur at a rate independent of population size. If the theoretical population of the model is interpreted as a troop of free-ranging primates (and not necessarily as a genetic or ecological population), several species of primates appear to confirm the equilibrium predictions of the model. The observed frequency distributions of size of troops of howler monkeys approximate a truncated negative binomial except after an epidemic which removed young monkeys; the size distribution, then, is nearly truncated Poisson, as expected. In gibbons, whose troops have a birth rate independent of troop size, the observe...

Rare-Type Mating Advantage in Drosophila

Abstract: Garcia-Bellido, A., and J. R. Merriam. 1969. Cell lineage of the imaginal discs in Drosophila gyna'ndromnorphs. J. Exp. Zool. 170:61-76. Plunkett, C. R. 1926. The interaction of genetic and environmental factors in development. J. Exp. Zool. 46:181-244. Rendel, J. M. 1965. Bristle pattern in scute stocks of Drosophila melanogaster. Amer. Natur. 97:25-32. Stern, C. 1954. Two or three bristles. Amer. Sci. 42:212-247. Young, S. S. Y., and R. C. Lewontin. 1966. Differences in bristle-making abilities in scute and wild-type Drosophila melanogaster. Genet. Res. 7:295-301.

Minority Advantage in Interpopulational Matings of Drosophila persimilis

Abstract: It may be assumed that frequency dependence with a minority advantage in mating propensity of Drosophila implies a difference between rival types which must be "recognizable" as a difference among mating participants. Mating propensity between strains of D. persimilis taken from two widely separated localities (Humboldt and White Wolf) but possessing an identical chromosomal arrangement (Klamath, KL) was measured throughout a series of tests in which members of the two localities were varied in relative frequency either among males or among females. Flies were either raised under identical temperatures or under two temperatures (15⚬ and 25⚬C). The findings follow: 1. Males either raised at different temperatures or from separate localities or both have approximately equal mating propensities when they are the common type, but when rare, each type participates more frequently in mating than when common. 2. Females from either population when raised at cool temperature (15⚬C) accept males at one-tenth to on...

Evolution in Sexual and Asexual Populations: A Reply

Abstract: In a recent article in this journal, J. M\\Iaynard Smith (1968) has questioned the validity of the concept, originally due to R. A. Fisher (1930) and H. J. Muller (1932), that sexual reproduction is advantageous because it permits favorable mutants that occur in different individuals or lines to be recombined into one. In particular he questions the conclusions that we (1965) reached from a quantitative examination of Muller's model: that recombination is most advantageous when (1) the population is large, (2) the frequency of beneficial mutants is high, and (3) the individual selective advantages of the mutants are small. Maynard Smith bases his main argument on a \"counter example\". He considers a two-locus, two-allele haploid case where the fitnesses of the normal, the two single mutants, and the double mutant are in the ratio 1:1 s:I t: (1 s) (1 t). The four types are assumed to be in equilibriumn under mutation and selection. If, now, the environment changes so that the mutants become favorable, while the four fitnesses remain in geometric ratio, the course of evolution is the same whether there is recombination or not, and the double mutant is eventually fixed. We agree with the example. This result, we believe, does invalidate a very minor part of Muller's original argument, but not the main part. Muller said that a second disadvantage of asexual reproduction is that the various favored genotypes must compete with each other, each slowing the evolution of the others until the best type finally wins. With recombination, on the other hand, the different mutants can interpenetrate and cooperate. Maynard Smith's example illustrates that M\\uller's argument is wrong, since there is exactly the same rate of change with recombination as without it. Curiously, it was exactly this example that in 1964 convinced Muller that the competition between genotypes is the same in sexual and asexual systems. Muller once scribbled a note on a postcard that he was glad to have the \"competition hoax\" demolished and hoped to write an article to this effect. For this reason we did not discuss the point in our paper. Muller's death came before he had a chance to write the article. The main point of Muller's papers, and the one on which we elaborated, is the value of recombination in putting together rare advantageous genes that occur in separate individuals. The rate of gene substitution in a species with recombination exceeds that in a species without recombination by a

The effect of migration on the maintenance of a lethal polymorphism in the house mouse

Abstract: Most wild populations of house mice are polymorphic for alleles at the Brachury, T, locus. These alleles are either recessive lethals or viable male steriles and are maintained by an abnormal segregation (transmission) ratio in heterozygous males. The observed frequencies of t alleles in natural populations are considerably less than those expected from deterministic models. This discrepancy has been attributed to the operation of random genetic drift in the small, relatively isolated breeding units (demes) of which a mouse population is comprised. In this investigation, a Monte Carlo simulation of a geographical population has been employed to examine the range of breeding-unit sizes and interdemic migration rates for which the random drift would serve as a likely explanation for the low observed frequencies of the t alleles. In the runs made, lethal t alleles with a transmission ratio of 0.95 were examined. The results of these runs indicate that random drift would only have an important effect on t all...

Tit Niches in Two Worlds; or Homage to Evelyn Hutchinson

Abstract: 1. Seven species of Parus in Europe have close ecological counterparts in America, two due to affinity and the others due to convergence. 2. In the middle of Europe, six species of Parus coexist, segregated partly by habitat and partly by feeding stations and size of prey. But only two species normally coexist in North America; the others primarily replace each other geographically. 3. Probably the American species are at an earlier stage in their evolution than the European, and ecological segregation permitting coexistence may normally evolve slowly along a common boundary between two species which initially replace each other geographically.