Intraspecific Competitive Ability Research Articles

Performance and Secondary Chemistry of Two Hybrid Aspen (Populus tremula L. x Populus tremuloides Michx.) Clones in Long-Term Elevated Ozone Exposure

The effects of moderately elevated ozone (ca. 35 ppb) on the growth and secondary chemistry of the leaves of two soil-grown Finnish hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) clones with different ozone sensitivities were studied at an open-air exposure field in Kuopio, Finland. Stomatal conductance, photosynthetic rate, and chlorophyll fluorescence were measured during the third growing season. Foliar phenolic concentrations, ergosterol concentration of fine roots, and final dry mass of the trees were determined at the end of the third growing season. Elevated ozone increased the ectomycorrhizal status of the fine roots but had no effect on gas exchange or on the final biomass of either of the clones, indicating equal sensitivity to ozone and no effect of elevated ozone on the intraspecific competitive ability of the clones after three growing seasons. However, in agreement with the data from potted plants of the same clones after two growing seasons, significant differences between the clones were found in all parameters measured. A negative correlation between growth and high concentrations of foliar phenolics indicated that allocation to secondary chemistry also was costly in terms of growth under high resource availability.

Clothes maketh the new consultant

<h3>Abstract</h3> Abiotic stress is a major force of selection that organisms are constantly facing. While the evolutionary effects of various stressors have been broadly studied, it is only more recently that the relevance of interactions between evolution and underlying ecological conditions, that is, eco-evolutionary feedbacks, have been highlighted. Here, we experimentally investigated how populations adapt to pH-stress under high population densities. Using the protist species <i>Tetrahymena thermophila</i>, we studied how four different genotypes evolved in response to stressfully low pH conditions and high population densities. We found that genotypes underwent evolutionary changes, some shifting up and others shifting down their intrinsic rates of increase (<i>r</i>₀). Overall, evolution at low pH led to the convergence of <i>r</i>₀ and intraspecific competitive ability (<i>α</i>) across the four genotypes. Given the strong correlation between <i>r</i>₀ and <i>α</i>, we argue that this convergence was a consequence of selection for increased density-dependent fitness at low pH under the experienced high density conditions. Increased density-dependent fitness was either attained through increase in <i>r</i>₀, or decrease of <i>α</i>, depending on the genetic background. In conclusion, we show that demography can influence the direction of evolution under abiotic stress.

Dietary Overlap and Foraging Competition Between Mourning Doves and Eurasian Collared-Doves

The recent Eurasian collared-dove (Streptopelia decaocto, hereafter collared-dove) invasion of North America has raised concerns regarding its effects on native species, particularly mourning doves (Zenaida macroura). Our goal was to assess the potential for, and possible consequences of, foraging competition between mourning doves and collared-doves. We compared diet selection and measured degree of dietary overlap between the 2 dove species in a captive cafeteria experiment, and we measured aggression and competitive ability of these species in 3 captive competition experiments. We evaluated the effects of body size, temperature, and food distribution on aggression and competitive ability of both species, and we measured intraspecific aggression levels and competitive ability within each species. We found a high degree of dietary overlap between species; food preferences were similar between species, and Pianka's index of foraging-niche overlap was 0.95. Neither aggression nor competitive ability varied between species, however. Distribution of food in cafeteria experiments did not affect seed-preference results. Ambient temperature and body size of competitors had little effects on aggression or competitive ability of either species. Distribution of food in the environment affected aggression between species, however; aggression was greater in single (0.50 ± 0.10 interactions/min) than in multiple-patch (0.12 ± 0.02 interactions/min) trials. Collared-doves do not appear behaviorally more aggressive or competitively successful than mourning doves. Our results suggest that the potential for negative effects of collared-doves on mourning dove populations may be less than previously suspected, so control measures for collared doves may be unnecessary. However, competitive dynamics involving larger flocks of collared-doves and other potentially limiting resources must be considered.

Evolution of advantageous alleles affecting population ecological characteristics in partially inbreeding populations.

The fate of advantageous alleles affecting intrinsic growth rate, carrying capacity or intra-specific competitive ability was examined in a partially inbreeding population. Generally, inbreeding had an effect on the evolution of advantageous alleles affecting population ecological characteristics. For example, in a specific underdominant case the number of stable internal equilibria decreased from two to one with only a slight degree of inbreeding. Equilibrium frequencies of stable internal equilibria and stability of fixation equilibria were also affected by the degree of inbreeding. For strictly advantageous alleles, inbreeding had the same qualitative effect on the fixation probability and mean fixation time as predicted in simpler selection models.

Evolutionary and population dynamics of host–parasitoid interactions

AbstractThe role of evolutionary dynamics in understanding host–parasitoid interactions is interlinked with the population dynamics of these interactions. Here, we address the problems in coupling evolutionary and population dynamics of host–parasitoid interactions. We review previous theoretical and empirical studies and show that evolution can alter the ecological dynamics of a host–parasitoid interaction. Whether evolution stabilizes or destabilizes the interaction depends on the direction of evolutionary changes, which are affected by ecological, physiological, and genetic details of the insect biology. We examine the effect of life history correlations on population persistence and stability, embedding two types, one of which is competitively inferior but superior in encapsulation (for parasitoid, virulence), in a Nicholson–Bailey model with intraspecific resource competition for host. If a trade‐off exists between intraspecific competitive ability and encapsulation (or virulence, as a countermeasure) in both the host and parasitoid, the trade‐off or even positive correlation in the parasitoid is less influential to ecological stability than the trade‐off in the host. We comment on the bearing this work has on the broader issues of understanding host–parasitoid interactions, including long‐term biological control.

INTERACTING PHENOTYPES AND THE EVOLUTIONARY PROCESS: I. DIRECT AND INDIRECT GENETIC EFFECTS OF SOCIAL INTERACTIONS.

Interacting phenotypes are traits whose expression is affected by interactions with conspecifics. Commonly-studied interacting phenotypes include aggression, courtship, and communication. More extreme examples of interacting phenotypes-traits that exist exclusively as a product of interactions-include social dominance, intraspecific competitive ability, and mating systems. We adopt a quantitative genetic approach to assess genetic influences on interacting phenotypes. We partition genetic and environmental effects so that traits in conspecifics that influence the expression of interacting phenotypes are a component of the environment. When the trait having the effect is heritable, the environmental influence arising from the interaction has a genetic basis and can be incorporated as an indirect genetic effect. However, because it has a genetic basis, this environmental component can evolve. Therefore, to consider the evolution of interacting phenotypes we simultaneously consider changes in the direct genetic contributions to a trait (as a standard quantitative genetic approach would evaluate) as well as changes in the environmental (indirect genetic) contribution to the phenotype. We then explore the ramifications of this model of inheritance on the evolution of interacting phenotypes. The relative rate of evolution in interacting phenotypes can be quite different from that predicted by a standard quantitative genetic analysis. Phenotypic evolution is greatly enhanced or inhibited depending on the nature of the direct and indirect genetic effects. Further, unlike most models of phenotypic evolution, a lack of variation in direct genetic effects does not preclude evolution if there is genetic variance in the indirect genetic contributions. The available empirical evidence regarding the evolution of behavior expressed in interactions, although limited, supports the predictions of our model.

Absence of Differential Fitness Between Giant Foxtail (Setaria faberi) Accessions Resistant and Susceptible to Acetyl-Coenzyme A Carboxylase Inhibitors

Experiments were conducted to determine the productivity, inter-, and intraspecific competitive ability of giant foxtail accessions resistant (PCW1) and susceptible (AC1) to acetyl-coenzyme A carboxylase (ACCase) inhibitors. Under noncompetitive conditions in the field, shoot dry biomass, plant height, and leaf area over time were similar between the PCW1 and AC1 accessions. The instantaneous relative growth rate and instantaneous net assimilation rate did not differ between the accessions; however, the instantaneous leaf area ratio was slightly greater for the AC1 accession than the PCW1 accession. The seed yield was similar between the PCW1 and AC1 accessions. Addition series experiments were conducted in the field to determine the intraspecific competitive ability of the PCW1 and AC1 accessions. Regression surface analysis of reciprocal mean shoot dry biomass and seed yield indicated that the relative competitive ability of the PCW1 and AC1 accessions was similar. Replacement series experiments were conducted in the greenhouse with or without corn to determine the interspecific competitive ability of the accessions. The relative growth rate, shoot dry biomass, and seed yield of the PCW1 and AC1 accessions were greater without than with corn competition. The relative competitive ability of the PCW1 and AC1 accessions was similar with or without corn competition based on shoot dry biomass or seed yield. These results indicated that the intra- and interspecific competitive ability of the PCW1 and AC1 giant foxtail accessions are similar and suggest that resistance to ACCase inhibitors has not reduced the relative fitness of the PCW1 accession.

Influence of cucumber mosaic virus infection on the intraspecific competitive ability and fitness of purslane (Portulaca oleracea).

Plants of purslane (Portulaca uleracea L.), either inoculated with cucumber mosaic virus CMV) or not, were grown to seeding at a range of densities up to 628 plants m-2 , both in monoculture and together. CMV infection reduced vegetative growth and flower production in monoculture and in mixture. However, in mixture, unequal interference amplified the effect of infection on growth. At high densities, intraphenotypic interference between healthy plants grown in monoculture was very important, whereas it did not affect infected plants. When grown in mixture, infected plants had a weaker competitive ability than did healthy plants. Healthy plants showed a plasticity which allowed them to compensate for she reduced growth of infected plants when in mixture. It is concluded that partial infection of a natural population causes no overall decrease in production, but might induce a qualitative change owing to the reduced participation of infected plants in providing seed for the next generation.

Productivity and Intraspecific Competitive Ability of a Velvetleaf (Abutilon theophrasti) Biotype Resistant to Atrazine

Studies were conducted to determine the relative competitive ability and productivity of an atrazine-resistant biotype (WRB1) and an atrazine-susceptible accession (WSA1) of velvetleaf from Wisconsin. Noncompetitive productivity studies conducted in the field and greenhouse during 1992 and 1993 showed that instantaneous relative growth rate, net assimilation rate, and leaf area ratio of the WRB1 biotype and WSA1 accession were similar. There were no consistent differences between the WRB1 biotype and WSA1 accession in plant height, shoot dry biomass, and leaf area over time. The WRB1 biotype and WSA1 accession did not differ in noncompetitive seed yield. A replacement series experiment conducted in the field in 1992 and 1993 showed that the WRB1 biotype and WSA1 accession did not differ in shoot dry biomass or seed yield at densities of 36, 64, and 100 plants m−2. Results suggest that resistance to atrazine has not reduced the noncompetitive productivity or the intraspecific competitive ability of the WRB1 biotype relative to the WSA1 accession. Thus, even in the absence of atrazine selection intensity, the frequency of atrazine resistance gene(s) is unlikely to decrease in the velvetleaf population from which the WRB1 biotype was selected.

Sexual Differences in Life History and Resource Utilization by the Hermit Crab

Evolutionary causes and ecological consequences of sexual differences in hermit crabs were examined through their life history patterns, habitats, behavioral responses to seasonal and size differences in the utilization of vacant shell resources and intraspecific competitive ability. The sand—dwelling hermit crab, Diogenes nitidimanus, and the sand snail, Umbonium moniliferum, were extremely abundant in the study area, comprising a virtually single resource and utilizer system. Size—frequency distributions of the crabs were nearly flat for males but had a prominent peak for larger females, suggesting different patterns of growth and mortality for the two sexes. No conspicuous peak in the size range of small crabs was obvious in size—frequency histograms for each sex from September to the following June. Males were larger than females. The number and size of vacant shells produced by the population of U. moniliferum, estimated from their survivorshop and growth curves, indicated that both extremely small and large shells were readily available, whereas medium—sized shells suitable for small crabs were relatively rare. This resulted in small crabs using shells that were damaged, much smaller than the sizes they preferred, and of less common species or species that did not inhabit the sand flat. Habitat use by makes was more diverse than by females. The males used shells of U. moniliferum and other snails inhabiting the sand flat and shells of preferred size more frequently than did females, and also tended to use shells that were larger and more variable in size. Shells that were broken or epibiota covered were less preferred by both sexes, although epibiota—covered shells were used more frequently by males. Broken shells were used at similar frequency both sexes. Crabs supplied with shells of a similar size distribution to that available in the field were cultured for 1. yr. Males tended to grow faster than females, exhausting the limited supply of medium—sized shells. The smaller crabs restrained their growth until such shells became available, explaining the absence of a peak of small crabs in the field population. Although the sex ratio of the field population was female biased, that of the laboratory population. Although the sex ratio of the field population was female biased, that of the laboratory population was 1:1, indicating that males had a higher mortality rate in the field. These results suggest that males are better intraspecific competitors than females in the acquisition of vacant shells. The quality enables males to grow larger, which is advantageous in the acquisition of females. However, shell scarcity produced greater stress in males resulting in their higher mortality rate.

NATURAL SELECTION ON HYDROID COLONY MORPHOLOGY BY INTRASPECIFIC COMPETITION.

Previous work on colonial hydroids in the genus Hydractinia has demonstrated that colony morphology is highly variable and determines intraspecific competitive ability. Competitive encounters are known to be common in nature, suggesting that intraspecific competition may be a major selective force acting on morphological variation. A replicated common garden experiment demonstrated a genetic basis to morphological variation and two data sets provided correlative support for the hypothesis of selection by intraspecific competition. First, morphologies inferior in competitive ability were less abundant in two adult, postcompetition, samples than in juvenile, precompetition, samples from the same populations. Second, among eight populations, the relative frequency of different morphologies was correlated with the frequency of intraspecific competition observed in each population. The direction of selection by competition on the morphological variation present in this species conflicts with recent predictions based on surveys across diverse taxa, suggesting limitations to the inference of competition as a past selective agent on the basis of present day correlations among species.

Population structure of the colonial hydroid Hydractinia sp. nov. C in the Gulf of Maine

The spatial and temporal variation of population parameters were investigated in eight populations of the colonial hydroid Hydractinia sp. nov. C in the Gulf of Maine. Reproduction, juvenile recruitment, and intraspecific competition varied seasonally and the recruitment positions of juvenile colonies, size frequency distribution of colonies, and sex ratio of adults exhibited little interpopulation variation. One gastropod shell species, Littorina littorea (L.), and one host crab species, Pagurus acadianus (Benedict), predominated in all populations. Hydroid colonies were more abundant on shells inhabited by P. acadianus than on those occupied by P. arcuatus (Squires) and laboratory experiments demonstrated that larvae are more likely to settle on shells inhabited by P. acadianus. Hydroid densities and the frequency of intraspecific competition varied widely among populations, but exhibited little variation from year to year within populations. Hydractinia colonies are known to exhibit genetically based variation in colony morphology and morphology has been shown to determine intraspecific competitive ability. Consequently, the frequency of competition in a population should correspond to the intensity of natural selection on morphological variation and interpopulation variation in this parameter may result in variation in the relative abundance of different colony morphologies among populations. kw]Hermit crab; Histocompatibility; Hydractinia; Intraspecific competition; Morphology; Recruitment

ON THE EVOLUTIONARY REVERSAL OF COMPETITIVE DOMINANCE.

The relationship between ecology and genetics is investigated, in part, by coevolutionary biology. One aspect of this relationship is the effect that genetic variability and natural selection have on determining the outcome of competitive interactions. This paper investigates the evolutionary reversal of competitive dominance, a particular mechanism whereby genetic considerations may influence competitive outcomes and population abundances. The formulation of these models was motivated by the laboratory experiments of Pimentel et al. (1965) with houseflies and blowflies and by the experiments of Ayala (1966a, 1969) and Moore (195 2a) with Drosophila. Arthur (1982) critically reviews these experiments. Pimentel et al. (1965) give a description of the dynamic interplay between the intensity of selection for competitive ability and the relative abundances of two competing species that forms the basis of the reversal of dominance. Suppose a rare and an abundant species are engaged in a competitive interaction. Members of the rare species will encounter the abundant species more often than they will encounter individuals of their own kind, hence superior interspecific competitive ability will be selected for. Conversely, individuals of the abundant species will encounter their own type more frequently, hence superior intraspecific competitive ability will be selected for. As the interspecific competitive ability of the originally rare species increases, the density of that species will increase and eventually overtake that of the originally abundant species, thereby producing a reversal of dominance. The most common experimental result of the authors cited above was for only one, or sometimes no, reversals of dominance to be observed, after which either the experiment was terminated, or one of the competitors went extinct. For example, in the experiment of Pimentel et al. (1965) houseflies and blowflies competed in a multicell laboratory cage. In nature, the houseflies and blowflies coexist on refuse and dung, while the laboratory populations oviposited on the same mixture of liver, yeast, agar and dried milk. During the first 50 weeks, or about 25 generations, houseflies comprised approximately 92% of the total fly population, and blowflies 8%. At about the twenty-fifth generation the blowfly abundance increased rapidly, with a concurrent decrease in the housefly population, which went extinct shortly thereafter. In single cell cages the blowfly usually went extinct, presumably before it could evolve increased interspecific competitive ability. Pimentel et al. therefore postulated that the spatial structure of the multicell cage allowed the blowfly to coexist sufficiently long for evolution, and hence a reversal of dominance, to occur. In this paper, I will use a quantitative genetical approach to modeling the evolution of species interactions and the reversal of dominance. The outline of the remainder of the paper is as follows: Three related mathematical models will be derived and discussed. In these models a distinction is made between the constant, nonevolutionary component of the competition coefficients, with a genetic variance of zero, and the part of the competition coefficients which is subject to evolutionary change, and therefore has a positive genetic variance. The first model is neutrally stable, and is used as a foundation on which the remaining two more

"GRANDFATHER EFFECTS": THE GENETICS OF INTERPOPULATION DIFFERENCES IN OFFSPRING SIZE IN THE MOSQUITO FISH.

The variety of life history patterns exhibited by animals and plants has become a major focus of interest in evolutionary biology because the traits involved are so clearly related to fitness. To interpret life history patterns as adaptations, one must assume that differences between populations derive at least in part from genetic differences. Few studies have been concerned with determining whether such genetic differences exist. Most of these studies confound maternally mediated environmental influences with genetically determined interpopulation differences (e.g., Ramakrishnan, 1960; Law et al., 1977; Naevdal et al., 1978; Berven et al., 1979; exceptions include Alm, 1949; Birch et al., 1963). Offspring size is a critical life history parameter because it limits the number of offspring a female can produce without increasing reproductive effort (Svardson, 1949; Salthe, 1969; Salthe and Duellman, 1973; Smith and Fretwell, 1974; Wilbur, 1977; Kaplan and Salthe, 1979), and because it is associated with intraspecific competitive ability of the offspring (Harper, 1977), growth and development rates (Gall, 1974; Kaplan, 1980), duration of survival in the absence of food (Blaxter and Hempel, 1963; Hirshfield, 1977), and the probability of survival through early stages of development (Bagenal, 1969). These qualities influence the fitness of offspring and hence the parent; they should therefore be subject to natural selection. Before considering the evolutionary causes of changes or interpopulation differ-

SELECTION FOR COMPETITIVE ABILITY: NEGATIVE RESULTS IN DROSOPHILA.

The occurrence of competition between populations may result in selection pressures favoring increases in competitive ability. Competition is used in the following sense: competition occurs when two or more species or genotypes experience depressed fitness (growth rate or carrying capacity) attributable to their mutual presence in an area. This is a modification of a definition given by Emlen (1973, p. 307). Competitive ability is used to mean the sum of all those characteristics of an individual relevant to competition. It is likely in most species that the components of competitive ability are polygenic traits, and as such, affected by epistasis, geneenvironment interactions, and correlations with other traits. Consequently, the evolutionary effects of the occurrence of competition are not straightforward. In particular, the question, Does competitive ability change rapidly and predictably in response to selection pressures arising from the occurrence of competition? has no obvious answer. This is a matter of some interest, as failure to improve rapidly could result in exclusion by superior competitors and, if this were the case, preadaptation would be important to the structure of natural communities and the success of invasions by new species. In efforts to examine the evolution of competitive ability, laboratory investigations of the effect of intraspecific competition have been done for some short-lived insect species. The results of these studies were dissimilar. In some populations significant improvements were found within a small number of generations (Seaton and Antonovics, 1967; Bryant and Turner, 1972; Sulzbach and Emlen, 1979), while in others no change was detected (Sokal et al., 1970; Dawson, 1972; Sulzbach and Emlen, 1979). Cases of rapid evolution may be dependent upon giving the larvae or ovipositing females of one of the competing populations a temporal headstart in mixtures (Bryant and Turner, 1972). However, such initial asymmetries have been found to be insufficient in themselves to cause rapid increases in competitive ability, and it was proposed that rapid changes may be exceptional and highly dependent on the particular populations used (Sulzbach and Emlen, 1979). Most studies of the evolution of intraspecific competitive ability have employed samples from single laboratory populations, and the period of selection under constant conditions was short, generally less than ten generations. The present experiments employ hybrid populations and a selection protocol maintained for a longer period.

ESTIMATION OF AVERAGE FITNESS OF POPULATIONS OF DROSOPHILA MELANOGASTER AND THE EVOLUTION OF FITNESS IN EXPERIMENTAL POPULATIONS.

In the companion paper (Jungen and Hartl, 1979) we proposed a method for estimating average fitness of entire populations of Drosophila melanogaster by introducing the population into cultures containing compound-autosome flies and measuring, in the next generation, the proportion of wild-type flies among the offspring. (In these papers we operationally define the fitness of a population as the intraspecific competitive ability of that population.) Because compound-autosome flies have both left arms of an autosome attached to one centromere and both right arms attached to another centromere, they are unable to produce viable offspring when mated with wild-type flies. Thus the genomes of the wild-type and compound-autosome flies cannot intermix. Although major components of fitness such as fecundity, viability, and mating activity can be assessed in one-generation experiments (Jungen and Hartl, 1979), and each component is automatically given weight according to its actual importance under the experimental conditions, certain other components of fitness such as longevity and length of fertile period are not amenable to inclusion in one-generation experiments. In the same way, one-generation experiments may give undue weight to certain components of fitness that, in multigeneration tests, are not so important. In this paper we extend the compoundautosome method of Jungen and Hartl (1979) to multigeneration experiments. The method proves to be more sensitive to detecting small differences in fitness than is the one-generation method. Moreover, although fitness rankings of strains are generally consistent with what one would expect based on one-generation tests, the inclusion of more fitness components in the multigeneration test does produce some differences. Although the wild-type strain Swedish-c performs slightly but consistently better than does Oregon R-C in one-generation tests, for example, the use of a multigeneration design shows Oregon R-C to have a higher fitness than Swedish-c. Being the more powerful and inclusive method, multigeneration experiments would seem to provide the best measure of average fitness. Experiments are not as easily replicated as they are in one-generation experiments, however, and there are other problems as well. By recombination in the centromeric heterochromatin in females, the compound autosomes can detach, becoming structurally normal. When an egg carrying a detached chromosome is fertilized by a normal sperm, the resulting zygote will be viable, karyotypically normal, and able to reproduce. Thus, due to detachment of the compounds, the wild and compound-autoI Supported by NSF grant PCM77-00886. D.L.H. is recipient of Research Career Award GM0002301. This paper is dedicated to the memory of Spencer W. Brown.

FACTORS RESPONSIBLE FOR A CHANGE IN INTERSPECIFIC COMPETITIVE ABILITY IN DROSOPHILA.

Classical interspecific competition theory assumes all individuals of a particular species are genetically alike in their ability to compete with another species. Recently, a number of studies have demonstrated the importance of the genotypes of competing species on the outcome of interspecific competition. A study by Lerner and Ho (1961) in Tribolium first showed that the genetic constitution of competing species was an important factor in determining competitive outcome. Other studies by Moore (1952), Pimentel et al. (1965) and Ayala (1966) have observed reversals of numerical dominance in competing species and suggested that genetic changes in competitive ability were responsible. If there is genetic variation for competitive ability, natural selection for increased competitive ability should occur when two competitors are artificially maintained in association. Such experiments for both intraand interspecific competition have had mixed results, although they appear to have been more successful when selection was for intraspecific competitive ability. Seaton and Antonovics (1967) in Drosophila and Bryant and Turner (1972) in Musca found significant increases in intraspecific competitive ability while van Delden (1970) observed increased interspecific competitive ability only after 65 generations in Drosophila. Selection for intraspecific competitive ability did not lead to a demonstrable change in experiments by Sokal et al. (1970) in Tribolium and Musca and for interspecific competitive ability in studies by Park and Lloyd (1955) in Triboliumn and by Futuyma (1970) and Barker (unpubl.) in Drosophila. Selection pressure for interspecific competitive ability for both competitors should be greatest when the two species are nearly equal in competitive ability and maintain similar numbers since at this point interspecific competition is maximized for both species relative to intraspecific competition. In an effort to find a strain of Drosophila melanogaster which could maintain nearly equal numbers to a v (vermilion eye) strain of D. siinulans, in 1968 J. S. F. Barker tested a number of mutant Drosophila melanogaster strains maintained by W. G. Baker. In a one generation test, he found that a yw (yellow body, white eye) strain of melanogaster produced a similar number of progeny to his simulans strain. Specifically, in mixed culture the melanogaster strain produced 5 9.0% of the progeny when 50% of the parents were melanogaster (Barker, 1971). The present study was initiated to examine interspecific competition between these two strains over an extended period of time using a serial transfer approach (Buzzati-Traverso, 1955; Ayala, 1966). Unexpectedly, melanogaster quickly eliminated simulans in interspecific competition. As a result the focus of this work became to (1) demonstrate whether or not there was a genetic change in the competitive ability of the melanogaster strain and then upon finding that there was, (2) determine in what respects the melanogaster strain changed such that its competitive ability was dramatically altered.