Showing papers in "The American Naturalist in 1967"
TL;DR: The total number of species is proportional to the total range of the environment divided by the niche breadth of the species, which is reduced by unequal abundance of resources but increased by adding to the dimensionality of the niche.
Abstract: 1. There is a limit to the similarity (and hence to the number) of competing species which can coexist. The total number of species is proportional to the total range of the environment divided by the niche breadth of the species. The number is reduced by unequal abundance of resources but increased by adding to the dimensionality of the niche. Niche breadth is increased with increased environmental uncertainty and with decreased productivity. 2. There is a different evolutionary limit, L, to the similarity of two coexisting species such that a) If two species are more similar than L, a third intermediate species will converge toward the nearer of the pair. b) If two species are more different than L, a third intermediate species will diverge from either toward a phenotype intermediate between the two.
3,946 citations
TL;DR: In this article, it was suggested that the temperature gradient across a mountain range determines its effectiveness as a barrier, rather than the absolute height, and that animals and plants are evolutionarily adapted to, and/or have the ability to acclimate to, the temperatures normally encountered in their temporal and geographic habitat (or microhabitat).
Abstract: patry of related populations would therefore tend to increase density of species in a region as a whole. It will be suggested below that this is a factor in the increase of species densities in regions of high topographic relief. I do not, however, know of any evidence that it is more general or more effective in the tropics." I believe that the climatic regimes discussed below, and the reactions of organisms to them, indicate that topographic barriers may be more effective in the tropics. Mountain barriers and their temperature gradients in Central America, as contrasted to those in North America, are used as examples; but it is believed that the central idea equally applies to other tropical areas, types of barriers, and physical parameters. There are three thoughts central to the argument to be developed: (1) in respect to temperature, it is the temperature gradient across a mountain range which determines its effectiveness as a barrier, rather than the absolute height; (2) in Central America, terrestrial temperature regimes are generally more uniform than North American ones, and differ in their patterns of overlap across geographic barriers; and (3) it can be assumed that animals and plants are evolutionarily adapted to, and/or have the ability to acclimate to, the temperatures normally encountered in their temporal and geographic habitat (or microhabitat). MOUNTAIN TEMPERATURE GRADIENTS
1,629 citations
TL;DR: This rebuttal will demonstrate the internal inconsistency of the criticism of Ehrlich and Birch, clarify some of the considerations raised by Murdoch, and demonstrate in part the empirical validity of the authors' earlier conclusions.
Abstract: regret the brevity of HSS. We must also apologize for some confusion of terminology which it contains. Specifically in this rebuttal we will: demonstrate the internal inconsistency of the criticism of Ehrlich and Birch, clarify some of the considerations raised by Murdoch, and demonstrate in part the empirical validity of our earlier conclusions. It must be made clear that the dispute is not simply verbiage, but rather the interpretation of commonly accepted observations and the use of these interpretations in prediction. Neither Murdoch nor Ehrlich and Birch deny the major empirical observations of HSS. They merely cite exceptions. But, as we point out below, the exceptions themselves are as predicted in the original paper. A thoughtful reading of HSS shows that certain statements concern trophic levels as wholes, and may not necessarily apply to every subset of populations within trophic levels. The point is, we think, sufficiently clear in the body of the paper, but not in the summary. Murdoch is incorrect, therefore, when he substitutes "in general" for "as a whole." His thesis that our "hypotheses" cannot be tested derives from this misinterpretation of what we have said. We were not making statements about most herbivores, or most carnivores, but about these trophic levels as wholes. Our statements, then, apply to the quantitatively dominant species but not necessarily to the numerical majority of species in any ecosystem. We were, in fact, claiming a good deal less than many of our critics have thought. We claimed, and still claim, that, with qualifications to be included below, certain biological methods of regulation are characteristic of certain trophic levels, while Ehrlich and Birch deny this. Ehrlich and Birch (but not Murdoch) affirm that biological regulation is not universal. Note that if
240 citations
TL;DR: The thesis of Hairston, Smith, and Slobodkin that populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion is based on a series of assumptions about these trophic levels which are, in all probability, false.
Abstract: 1. The notion that nature is in some sort of "balance" with respect to population size, or that populations in general show relatively little fluctuation in size, is demonstrably false. 2. The thesis of Hairston, Smith, and Slobodkin that "populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion" is based on a series of assumptions about these trophic levels which are, in all probability, false. Even if the assumptions are true, this conclusion does not follow from them. 3. A realistic basis for building models dealing with the changes of numbers in populations would include the following propositions: a. All populations are constantly changing in size. b. The environments of all organisms are constantly changing. c. Local populations must be recognized and investigated if changes in population size are to be understood. d. The influence on population size of various components of environment varies with population density, ...
227 citations
TL;DR: It was discovered that, in general, if a population is relatively asymmetrical for one character, it has a high probability of being asymmetry for other characters as well, and a model is proposed to account for this phenomenon.
Abstract: 1) The minor, "random" differences between bilateral structures in 20 natural populations (18 insular populations in the Gulf of California, Mexico and two continental populations) of lizards were examined It was discovered that, in general, if a population is relatively asymmetrical for one character, it has a high probability of being asymmetrical for other characters as well This phenomenon is called the population asymmetry parameter (PAP) and differs from the apparently nonexistent organism-wide asymmetry property and is not dependent on it 2) Many of the populations on islands less than three square kilometers in area are very asymmetrical (ie, the average of the individual's asymmetry values are high) compared to continental populations This is especially true of the most phenetically distinct of the small island populations 3) Populations on both recently formed continental islands and large islands tend to be relatively symmetrical The lowest asymmetry values were found in what is probabl
180 citations
TL;DR: The noted increase in diversity from successional to climax communities is related in part to the progressive addition of the climax pioneers and climax exclusives while maintaining a few successionals in the population.
Abstract: Tree species diversity in forests from north central Florida is partially a function of the successional status of the community, the portion of the environment occupied, and the ecological amplitude of the species. Diversity is higher in climax communities where greater uniformity exists between tree, sapling, and seedling size classes. Mesic fertile sites support communities with higher diversities than wet or dry sterile sites. Successional species tend to have wide ecological amplitudes. They generally occupy extreme environments or areas under disturbance. They are not competitive in mixed populations, and as a consequence they are more successful in assembled populations with low diversities. As successional species ameliorate extreme environments followed by the arrest of controlling variables such as fire, climax pioneer species may invade successional communities thereby increasing diversity. With continued arrest of controlling variables, successional features are lost. The climax pioneer group ...
137 citations
TL;DR: The mechanism of eye and pigment reduction probably involves the initial reduction in genetic variability and reconstruction of the epigenotype which accompanies the speciation process, which may lead indirectly toregression of adaptively neutral characters through pleiotropy and polygeny.
Abstract: 1. The principal energy sources of cave ecosystems are (a) organic matter swept underground by sinking streams, and (b) the feces, eggs, and dead bodies of animals which remain in the cave for shelter but feed outside (trogloxenes). In temperate zone caves flooding and the entrance of cold air during winter and early spring disturb the relatively constant physical conditions of the cave environment. 2. Species density, dispersal potential, and other aspects of the ecology of troglobites (obligate cavernicoles) are profoundly influenced by continuity of limestone outcrops in which the caves are developed. In the Appalachian valley, where limestone is exposed in many narrow, anticlinal strike belts, species density per unit area is high, and dispersal of troglobites through subterranean channels is severely restricted by geologic structure. In the Mississippian plateaus, where thick, caverniferous limestone is widely and continuously exposed, there are fewer species per unit area; and subterranean dispersal...
119 citations
TL;DR: The conclusion that in the arctic, even during the summer, the environmental light cycle is the principal entraining agent of the endogenous rhythm is supported.
Abstract: In both the arctic and subarctic regions of Alaska during the spring and summer pronounced daily periodicity was observed in the field and running-wheel activity of the following diurnal and "nocturnal" rodents: Spermophilus undulatus, Clethrionomys rutilus, Lemmus trimucronatus, Microtus miurus, and M. oeconomus. Animals whose phases were shifted to abnormal times of the day by artificial light cycles regained their normal phases when exposed to the local field conditions. However, difficulty in retaining proper phase was observed in several of these nocturnal species at the time of the summer solstice, i.e., at the time when the amplitude of the daily cycle of illumination is lowest. The usually nocturnal L. trimucronatus centered their activity about noon at this time of year. Temperate zone rodents (Ammospermophilus leucurus and Peromyscus leucopus) taken to Point Barrow, Alaska, also showed pronounced daily periodicity under arctic conditions, although the latter species did not show locomotor activi...
114 citations
TL;DR: The absence of a longitudinal gradient of species number and diversity in the uniform Type II habitats over 160⚬ of longitude, despite reports of larger total numbers of species near the "faunistic center" of the Indo- West Pacific region, supports the conclusion that habitat complexity is an important determinant of species diversity.
Abstract: Species diversity of assemblages of the gastropod genus Conus occurring on tropical Indo-West Pacific reef platforms increases in the following order of habitat types: (1) extensive regions of sand substrate (mean number of species N = 3.0; mean species diversity H = 0.5); (2) intertidal smooth limestone plantforms (mean N = 7.6; mean H = 1.3); (3) topographically complex subtidal coral reefs (mean N = 13.7; mean H = 2.1). The census data show a close correlation between N and H (Spearman r = 0.93). The absence of a longitudinal gradient of species number and diversity in the uniform Type II habitats over 160⚬ of longitude, despite reports of larger total numbers of species near the "faunistic center" of the Indo- West Pacific region, also supports the conclusion that habitat complexity is an important determinant of species diversity. Type III habitats near the "faunistic center" have more diverse assemblages than those of outlying areas east and west, but it has not been possible to distinguish the role...
101 citations
TL;DR: Models for optimal migration behavior have been worked out for the case where only the density functions of the distributions of survival for the two alternatives are given, and for the cases where conditions at the time the choice has to be made are correlated with subsequent survival.
Abstract: Optimal migration and dispersal behavior is considered from a theoretical point of view. It is considered as a problem of an optimal choice between alternatives, the outcomes of which are random variables. Models for optimal migration behavior have been worked out for the case where only the density functions of the distributions of survival for the two alternatives are given, and for the case where conditions at the time the choice has to be made are correlated with subsequent survival. The conditions for dominance between the two alternatives have been derived analytically. The predictions of the two models are discussed and compared to natural patterns of migration and dispersal.
99 citations
TL;DR: Estimates of between and within-family variability suggested that while both species have a substantial genetic component of total variability, there is a relatively larger nongenetic component in A. barbata accounting for its greater overall phenotypic variation in nature.
Abstract: Quantitative estimates were made of the degree of polymorphism at three marker loci and the amount of phenotypic and genetic variability for three quantitative characters in several natural populations of two wild oat species, Avena fatua and A. barbata. The results showed that, for all the simply inherited characters studied, A. barbata was largely monomorphic whereas A. fatua was highly polymorphic for at least five different marker loci. Evidence was presented for heterozygote advantage as a factor maintaining polymorphism in certain populations of A. fatua. Estimates of between and within-family variability suggested that while both species have a substantial genetic component of total variability, there is a relatively larger nongenetic component in A. barbata accounting for its greater overall phenotypic variation in nature. The differences in the modes of variation and population structure between the two species are briefly discussed in relation to their adaptive strategies. It was postulated that...
TL;DR: Evidence of neoteny among brood parasites includes prolonged retention of juvenal plumage, juvenal-adult-like species pairs, and the retention ofjuvenal-like incomplete skull ossification throughout adult life.
Abstract: Dull plumages prevail among parasitic birds. Parasitic cuckoos are significantly less brightly marked than are nesting cuckoos. Dull parasitic birds are less readily seen and recognized by hosts, and dull plumages also appease the hosts by absence of releasers of aggressive behavior. Inconspicuousness and appeasement increase the vulnerability of hosts to the brood parasites. The dull juvenal plumages in some parasitic birds may have been incorporated into the adult communication signals by neoteny. Evidence of neoteny among brood parasites includes prolonged retention of juvenal plumage, juvenal-adult-like species pairs, and the retention of juvenal-like incomplete skull ossification throughout adult life. Polymorphism and an unusual degree of variation in plumage are adaptations in parasitic cuckoos which reduce the probability of recognition by hosts. Polymorphism in Cuculus canorus and other parasitic cuckoos is considered to be the result of apostatic selection for deviant phenotypes. Polymorphism of...
TL;DR: In this paper, the authors made observations in Elens-Wattiaux observation chambers of matings in groups of Drosophila pseudoobscura with different proportions of individuals with CH or with AR chromosomes.
Abstract: Observations have been made in Elens-Wattiaux observation chambers of matings in groups of Drosophila pseudoobscura with different proportions of individuals with CH or with AR chromosomes. Minority males mate more frequently, and majority less frequently, than expected from their relative abundance. When the two kinds of males are equally frequent, their matings are also equally frequent. This is true when the numbers of the females and males per chamber are the same, and when the females outnumber the males. Attempts to identify the signals which allow the females to perceive the relative frequency of the males in the chamber have thus far not been successful.
TL;DR: The results show a negative correlation between ocellar size and the light intensity threshold for foraging, and are compatible with the observation that ocelli serve as enhancers of the sensitivity of compound eyes.
Abstract: The occurrence of enlarged ocelli in nocturnal foraging bees has led to a quantitative study of the relation between ocellar size and foraging activity in a number of species of bees. The results show a negative correlation between ocellar size and the light intensity threshold for foraging. Ocelli also increase with head size, but this relationship is not as good a predictor of ocellar size as is the light intensity threshold. No nocturnal foraging bees have small head sizes, and it is suggested that this may be the result of selection for larger ocelli for bees that forage at very low light intensities. The results are all compatible with the observation that ocelli serve as enhancers of the sensitivity of compound eyes.
TL;DR: Albino females of the ST/bJ strain were allowed to mate first with albino males of their own strain and then to black-agouti males of the CBA/J strain and a reverse sequence of matings were arranged.
Abstract: One group of 14 albino females of the ST/bJ strain were allowed to mate first with albino males of their own strain and then to black-agouti males of the CBA/J strain. In a second group of 14 albino females, a reverse sequence of matings were arranged. All females were sacrificed and dissected 18 days later. When the albino males had mated first, there were found 113 (95%) albino and 6 (5%) pigmented fetuses. When the black-agouti males had mated first, there were found 88 (70%) albino and 38 (40%) pigmented fetuses. In each experiment the excess of albino fetuses over pigmented fetuses was statistically significant. Various aspects of selective fertilization and its relation to sexual selection are discussed.
TL;DR: The mating habits of Drosophila pseudoobscura have been studied by exposing females of known chromosomal constitution to males whose chromosomes were also known.
Abstract: The mating habits of Drosophila pseudoobscura have been studied by exposing females of known chromosomal constitution to males whose chromosomes were also known. Although the females repel all males for some hours after a copulation, they often mate again within 5 to 15 days. A batch of eggs deposited by a female may be fertilized by the sperms of more than a single male.
TL;DR: That numbers of insular species and endemics are respectively predicted by average and nearest-neighbor isolation, and not by area, only in the Darwin Finches demonstrates emphatically the importance of isolation in regulating endemism and species abundance in the adaptive radiation of monophyletic bird groups within archipelagos.
Abstract: For interisland variation in number of Darwin Finches in the Galapagos Archipelago, endemism is predicted by nearest-neighbor isolation; and species abundance or sympatry is predicted by average isolation. Nearest-neighbor isolation is measured by distance from the nearest island, and average isolation is the average distance to all other islands in the archipelago. The two measures of isolation are of little predictive value when tested for the avifaunas of six other archipelagos or oceanic island groupings. In these situations, area is a better predictor of species abundance or endemism; and the role of isolation appears only when measured as distance from the major avifaunal source region (e.g., New Guinea for islands of the East-central Pacific; African mainland for islands in the Gulf of Guinea). Insular isolation in these instances is, however, a relatively small contributor to variance of species number when contrasted with the greater contributions made by insular area. That numbers of insular spe...
TL;DR: Under these conditions, only the trivial equilibria can result in which one allele is eliminated and the population becomes homozygous for the alternative color allele, and imprinting can act as a powerful sympatric speciation mechanism.
Abstract: A model is presented to determine the effects of absolute imprinting on population structure when individuals will only select mates the same color as their parents. Under these conditions, only the trivial equilibria can result in which one allele is eliminated and the population becomes homozygous for the alternative color allele or in which the population is subdivided into two moieties, each moiety being homozygous for one of the color alleles and between which there is no gene flow. Therefore imprinting can act as a powerful sympatric speciation mechanism. When imprinting is partial, it can act as a mechanism, independent of heterosis, to establish a balanced polymorphism in a population.
TL;DR: In this chapter, four genetic mechanisms, "habitat selection, pleiotropic genes, modifying genes, and assortative mating genes" which could result in sympatric speciation are presented.
Abstract: Opposed to this concept is the suggestion that the effects of dispersal and the bridging effects of heterzygotes would prevent sympatric populations from diverging genetically. Hanson (1966) reviewed how emigration can mask pockets of differential fitness in populations. Bigelow (1965) discussed the proposal that hybridization will make sympatric populations more similar. He stated "genes from one population will be incorporated into the gene pool of the other if they improve the well integrated harmony of the 'foreign' gene pool, but if they tend to disrupt that harmony their frequency will be reduced." Bigelow does not indicate how disruptive the conditions must be or how intense a selective pressure is necessary to overcome swamping due to gene flow. Wilson (1965), discussing reproductive character displacement, stated that when two "Mendelian populations with imperfect intrinsic isolating mechanisms come together they will, given enough time, either fuse completely as one species or else displace reproductively to become more perfectly isolated." No other result appears possible according to Wilson. In addition, Smith (1966) presented four genetic mechanisms, "habitat selection, pleiotropic genes, modifying genes, and assortative mating genes" which could result in sympatric speciation.
TL;DR: A method by which the niche hypervolume concept may be used to construct a model useful in the analysis of ecological processes and the proposed indices of versatility and their use in ecological analysis are valid and appropriate is suggested.
Abstract: A method is suggested by which the niche hypervolume concept may be used to construct a model useful in the analysis of ecological processes. If a series of separate habitats, in which there is inter-habitat heterogeneity, is examined, the frequency of occurrence of sufficiently or equally vagile species will tend to be correlated with two characteristics: (1) the size of the species' niche hypervolume and, (2) the degree to which hypervolume shape tends to be cubical (spherical). Two methods of computation of versatility indices which provide estimates of relative hypervolumes are used. Both are based on versatility coefficients such that each coefficient represents the (relative) dimension of one of the factors which makes up a species' niche hypervolume. One is based on a ranking procedure and the other on the fraction of the "range of life" of each factor which is included within the toleration limits of the species. Rank difference correlation coefficients comparing the frequency of occurrence of 20 ...
TL;DR: Strains of Drosopbila pseudoobscura and D. persimilis vary in their reactions to light and to gravity; some are positive, others neutral, and still others negative.
Abstract: Strains of Drosopbila pseudoobscura and D. persimilis vary in their reactions to light and to gravity. Some are positive, others neutral, and still others negative. D. persimilis is, on the average, more photopositive than D. pseudoobscura; but there is a considerable overlap. Males tend to be more photopositive than females, but there are again some exceptions. The modal behavior with respect to gravity is in both species geotactic neutrality.
TL;DR: In spite of the extensive pollen exchange between confluent populations, such exchange was limited by differences in the species' floral mechansims and floral pigmentation.
Abstract: Phlox pilosa subsp. pilosa and P. glaberrima subsp. interior are sympatric over extensive areas, but have different flowering periods. The potential for pollen exchange exists, nevertheless, because of the presence of late-flowering race of P. pilosa whose scope of ecological tolerance permits contact or proximity with P. glaberrima. The pollinating systems of the two Phlox do not constitute a barrier to pollen exchange. The floral morphology of the phloxes is so similar that most pollinators may feed on either species. The pollinators appear to be a somewhat indiscriminate lot as evidenced by the extensive pollen flow between confluent or contiguous populations of the species. Twenty-five per cent of the stigma of P. glaberrima and 16% of the stigma of P. pilosa from confluent populations bore alien pollen. The average pollen load on a contaminated stigma of the former was 10.0 grains as compared to 1.3 grains on a stigma of the latter. Annual pollen exchange between such populations ostensibly amounts t...
TL;DR: An analysis was made of the effects of reciprocal crosses, male and female age, number of flies per mating bottle, and sex ratio on the degree of sexual isolation, and the frequency of interspecific hybridization increased with increase in the proportion of males.
Abstract: An analysis was made of the effects of reciprocal crosses, male and female age, number of flies per mating bottle, and sex ratio on the degree of sexual isolation. When the isolation was measured as the percentage of fertile mating bottles it was higher (as expected) than when measured as the percentage of females fertilized. This resolves the apparent discrepancies between the results of earlier investigators (using mass matings, generally with an excess of males, and measuring percentage of fertile mating bottles), and those of Barker (1962, using pair matings, and measuring percentage of fertilized females). The frequency of interspecific hybridization increased with increase in the proportion of males (1:5, 1:1, 5:1), and was higher for the reputedly less successful D. melanogaster [female]~ x D. simulans ~[male] than for the reciprocal cross. Further, the effects of male and female age were not as expected from previous studies, particularly for the cross D. simulans ~[female] x D. melanogaster [male...
TL;DR: The somatic chromosomes of a black African rhinoceros, known to occur in the southern and eastern parts of Africa, were examined while the specimen came from Kenya and was in the collection of the Philadelphia Zoological Society.
Abstract: We have recently had an opportunity to examine the somatic chromosomes of a black African rhinoceros [Perissodactyla:Mammalia]; to our knowledge the chromosomes of none of the five species of rhinoceros usually recognized have been studied heretofore. This species is known to occur in the southern and eastern parts of Africa. This adult female specimen came from Kenya and was in the collection of the Philadelphia Zoological Society. Death was attributed to pulmonary abscess.
TL;DR: Exposure in the egg stage to a temperature of 5 C also appeared to have an adverse effect on fertility, and decreases of 10 to 20% in fertility were noted after sub-zero exposures, but these were not analyzed statistically.
Abstract: Adults, pupae, larvae, and eggs of Drosophila pseudoobscura were exposed to temperatures of 16, 5, -3, and -10 C for periods of 1, 7, and 21 days. Resistance to cold temperature was measured in terms of survival of adults or survival and subsequent development of nonadult stages. The life cycle stages were ranked in terms of decreasing resistance as follows: adults > pupae, larvae > eggs. The -3 and -10 C adult survival curves were transformed into probit regression lines for use in future comparisons and planning of selection experiments. No important sexual differences in resistance to cold temperature were observed. Decreases of 10 to 20% in fertility were noted after sub-zero exposures, but these were not analyzed statistically. Exposure in the egg stage to a temperature of 5 C also appeared to have an adverse effect on fertility.
TL;DR: The difference in chromosome size between Cyphomandra betacea and the various Solanum species was equally apparent in preparations of both gametic and somatic cells, suggesting that the two genera may indeed be phylogenetically distinct.
Abstract: Recent cytological work on various Solanaceae has revealed a significant difference between Cyphomandra betacea and several species of Solanum with respect to chromosome size. Since these two genera are considered to be very closely related if notcongeneric, this difference may be important to our understanding of their taxonomy and phylogeny. Meiotic figures were prepared with 1%. acetocarmine from buds fixed in Farmer's fixative. For root-tip smears, the lacto-propionic orcein procedure used by Kawano (1965) was employed. The 2,4 dinitrophenol was omitted from Ostergren and Heneen's fixative as these workers suggested (1962). The range in size between the longest and shortest chromosome of each species, based upon at least two metaphase figures, is recorded in Table 1. In the case of Solanum, the respective section to which each species belongs is indicated in parentheses. The difference in chromosome size between Cyphomandra betacea and the various Solanum species was equally apparent in preparations of both gametic and somatic cells. The only taxonomic character separating Cyphomandra from Solanum is the supposedly thickened anther connectives in Cyphomandra, though an intensive taxonomic revision of the latter genus might reveal other distinguishing characters. The difference in chromosome size, however, suggests that the two genera may indeed be phylogenetically distinct. Although it cannot be assumed that all other species in both genera have similar sized chromosomes, the inclusion here of Solanum species belonging to several distantly related subdivisions of that genus supports the contention that Solanum chromosomes are rather uniformly small. Examination of additional Cyphomandra species will be necessary to determine if the larger chromosome size is a general rule in this genus. If the difference in chromosome size between Solanum and Cyphomandra does hold true, it will fit the generalization advanced by Darlington (1937) and Stebbins (1938; 1950) that woody angiosperms usually have small sized chromosomes, whereas in herbaceous members of this class all sizes of chromosomes exist. Cyphomandra is a genus of only somewhat woody treelike shrubs. A large chromosome size in this genus would suggest its origin from herbaceous ancestors. Solanum on the other hand includes herbs, lianas (herbaceous and woody), subshrubs, shrubs and trees. A small chromosome size in all subdivisions of this genus would be indicative of an origin from woody ancestors, the present herbaceous species having evolved later but their chromosomes never having undergone subsequent phloygenetic increase
TL;DR: It is concluded that the inter-species interactions observed in studies on the flour beetles Tribolium castaneum and T. confusum in the standard medium whole wheat flour plus yeast are primarily mutual predator-prey interactions in which CS and CF act both as predator and prey when introduced into a limited universe.
Abstract: 1. Under the narrow definition of competition adopted here, drawing upon the available evidence, it is concluded that the inter-species interactions observed in studies on the flour beetles Tribolium castaneum (CS) and T. confusum (CF) in the standard medium whole wheat flour plus yeast are primarily mutual predator-prey interactions in which CS and CF act both as predator and prey when introduced into a limited universe. 2. In other media such as corn, where certain nutritional requirements are in short supply, it is possible that the increase in cannibalism on the part of CS may reflect competition for required nutrients. 3. Data are presented which show that the kind of food supplied may reverse the outcome of predator-prey relationships between CS and CF. In these experiments the CS strain used was superior to CF in whole wheat flour plus yeast but inferior to it in corn or in corn plus yeast. CS was able to survive four times longer in corn plus yeast than in corn without a yeast additive. 4. Recent ...
TL;DR: A number of the points which arise in the mathematical analysis of Fisher's theory of the evolution of dominance are discussed, which are perhaps of minor genetical interest in the overall problem of dominance.
Abstract: It is the purpose of this short note to discuss a number of the points which arise in the mathematical analysis of Fisher's theory of the evolution of dominance. Most of these points have been made previously (Ewens, 1965a, 1965b, 1966); they are, however, to some extent hidden in the mathematical treatment in those papers, and it appears useful to collect them in a nonmathematical context. The arguments are also illustrated here by a numerical example. This is done because the process under consideration is rather more complex mathematically than seems to be supposed, and a number of seemingly reasonable intuitive statements turn out to be wrong and in some cases misleading. The present author's interest lies entirely within the mathematical treatment, which is perhaps of minor genetical interest in the overall problem of dominance. Despite this admitted subordinate part, some implications will be drawn from the use of the correct mathematical theory.
TL;DR: The purpose of this paper is to describe this widespread condition, which will refer to as the M-V linkage, which is found in interspecific hybrids in many plant groups and is generalize from the evidence in the various particular cases.
Abstract: Studies of interspecific hybrids in many plant groups provide evidence for linkage between genes determining certain morphological features and genes affecting viability. It is the purpose of this paper to describe this widespread condition, which we will refer to as the M-V linkage. Previous authors have presented the data obtained in separate hybridization experiments, and our task here is to generalize from the evidence in the various particular cases.
TL;DR: It was concluded that, although the predators can be considered to provide space for the additional species of more diverse systems, additional factors must be invoked to explain the existence of the resources which support these additional species.
Abstract: The predators of ecological systems have been delegated an active role in determining the number of species in their systems. This paper has explored one possible influence by predators on their prey which would correlate with diversity: the regulation of prey density, and the consequent regulation of the distribution of space among the different prey species. A hypothesis has been presented as an explicit statement, and suggestions have been made for estimating the parameters of the statement. The limitations of the hypothesis were explored; and it was concluded that, although the predators can be considered to provide space for the additional species of more diverse systems, additional factors must be invoked to explain the existence of the resources which support these additional species.