Wyatt W. Anderson

Competition for Pollinators between Simultaneously Flowering Species

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

Polytene Chromosomal Maps of 11 Drosophila Species: The Order of Genomic Scaffolds Inferred From Genetic and Physical Maps

The sequencing of the 12 genomes of members of the genus Drosophila was taken as an opportunity to reevaluate the genetic and physical maps for 11 of the species, in part to aid in the mapping of assembled scaffolds. Here, we present an overview of the importance of cytogenetic maps to Drosophila biology and to the concepts of chromosomal evolution. Physical and genetic markers were used to anchor the genome assembly scaffolds to the polytene chromosomal maps for each species. In addition, a computational approach was used to anchor smaller scaffolds on the basis of the analysis of syntenic blocks. We present the chromosomal map data from each of the 11 sequenced non-Drosophila melanogaster species as a series of sections. Each section reviews the history of the polytene chromosome maps for each species, presents the new polytene chromosome maps, and anchors the genomic scaffolds to the cytological maps using genetic and physical markers. The mapping data agree with Mullers idea that the majority of Drosophila genes are syntenic. Despite the conservation of genes within homologous chromosome arms across species, the karyotypes of these species have changed through the fusion of chromosomal arms followed by subsequent rearrangement events.

The Frequency of Heteroplasmy in the HVII Region of mtDNA Differs across Tissue Types and Increases with Age

An immobilized sequence-specific oligonucleotide (SSO) probe system consisting of 16 SSO probes that detect sequence polymorphisms within five regions of the mtDNA control region was used to investigate the frequency of heteroplasmy in human mtDNA. Five regions of hypervariable region II (HVII) of the control region were studied in blood-, muscle-, heart-, and brain-tissue samples collected from 43 individuals during autopsy. An initial search for heteroplasmy was conducted by use of the SSO probe system. Samples in which multiple probe signals were detected within a region were sequenced for the HVII region, to verify the typing-strip results. The frequency of heteroplasmy was 5 of 43 individuals, or 11.6%. The frequency of heteroplasmy differed across tissue types, being higher in muscle tissue. The difference in the frequency of heteroplasmy across different age groups was statistically significant, which suggests that heteroplasmy increases with age. As a test for contamination and to confirm heteroplasmy, the samples were sequenced for the HVI region and were typed by use of a panel of five polymorphic nuclear markers. Portions of the tissues that appeared to be heteroplasmic were extracted at least one additional time; all gave identical results. The results from these tests indicate that the multiple sequences present in individual samples result from heteroplasmy and not from contamination.

The hypothesis of reproductive compensation and its assumptions about mate preferences and offspring viability

The Compensation Hypothesis says that parents and prospective parents attempt to make up for lowered offspring viability by increasing reproductive effort to produce healthy, competitive offspring and by increasing investment in less viable, but still-living progeny (parental effects). The hypothesis assumes that offspring viability is lower when individuals are constrained (often through sexual conflict) to breed with individuals they do not prefer. We review results of experimental tests of the offspring-viability assumption in Tanzanian cockroaches, fruit flies, pipefish, wild mallards, and feral house mice. Experimental constraints on mating preferences lowered offspring viability in each of the studies. Females breeding under constraints laid more eggs or gave birth to more young than females breeding without or with fewer constraints on their mating preferences, and males mating under constraints on their mate preferences ejaculated more sperm than males mating without constraints. The number of eggs laid or offspring born was higher when female choosers were experimentally constrained to reproduce with males they did not prefer. Constrained females may increase fecundity to enhance the probability that they produce adult offspring with rarer phenotypes with survival benefits against offspring generation pathogens. Similarly, ejaculation of more sperm when males are paired with females they do not prefer may be a mechanism that provides more variable sperm haplotypes for prospective mothers or that may provide nutritional benefits to mothers and zygotes.

A theoretical assessment of recombinational speciation

Using a computer simulation, we have examined the dynamics of recombinational speciation, a potentially rapid mode of evolution dependent on chromosomal reassortment in populations of partially sterile interspecific hybrids. We describe how various parameters affect the time required for a new recombinant species to become established within the setting of a spatially structured hybrid zone. Our results indicate that recombinational speciation is most likely to occur where (1) the hybrid zone interface is long, (2) the organisms involved are predominantly selfing, (3) the hybrids are relatively fertile, and (4) the number of differences in chromosomal structure between the parental species is small. The speciation dynamics are characterized by long-term stasis followed by an abrupt transition to a new reproductively isolated type. The results are largely the same whether the nascent recombinant species is favoured by a fertility or a viability advantage. Recombinational speciation, like polyploidy, appears to be a feasible mechanism for sympatric speciation in plants.

Evolutionary genomics of inversions in Drosophila pseudoobscura: Evidence for epistasis

Drosophila pseudoobscura harbors a rich polymorphism for paracentric inversions on the third chromosome, and the clines in the inversion frequencies across the southwestern United States indicate that strong natural selection operates on them. Isogenic inversion strains were made from isofemale lines collected from four localities, and eight molecular markers were mapped on the third chromosome. Nucleotide diversity was measured for these loci and formed the basis of an evolutionary genomic analysis. The loci were differentiated among inversions. The inversions did not show significant differences among populations, however, likely the result of extensive gene flow among populations. Some loci had significant reductions in nucleotide diversity within inversions compared with interspecies divergence, suggesting that these loci are near inversion breakpoints or are near targets of directional selection. Linkage disequilibrium (LD) levels tended to decrease with distance between loci, indicating that some genetic exchange occurs among gene arrangements despite the presence of inversions. In some cases, however, adjacent genes had low levels of interlocus LD and loosely linked genes had high levels of interlocus LD, suggesting strong epistatic selection. Our results support the hypothesis that the inversions of D. pseudoobscura have emerged as suppressors of recombination to maintain positive epistatic relationships among loci within gene arrangements that developed as the species adapted to a heterogeneous environment.

Frequent Multiple Insemination in a Natural Population of Drosophila pseudoobscura

Gadgil, M., and W. H. Bossert. 1970. Life historical consequences of natural selection. Amer. Natur. 104:1-24. Hairston, N. G., D. W. Tinkle, and H. M. Wilbur. 1970. Natural selection and the parameters of population growth. J. Wildlife Manage. 34:681-690. Lewontin, R. C. 1965. Selection for colonizing ability. Pages 77-94 in H. G. Baker and G. L. Stebbins, eds. The genetics of colonizing species. Academic Press, New York. Lotka, A. J. 1956. Elements of mathematical biology. Dover, New York. Mertz, D. B. 1971a. The mathematical demography of the California condor population. Amer. Natur. 105:437-453. 1971b. Life history phenomena in increasing and decreasing populations. Pages 361-399 in G. P. Patil, E. C. Pielou, and W. E. Waters, eds. Statistical ecology. Vol. 2. Sampling and modeling biological population dynamics. Pennsylvania State University Press, University Park. Wiley, R. HI. 1974. Evolution of social organization and life-history patterns among grouse (Aves: Tetraonidae). Quart. Rev. Biol. (in press). Williams, G. C. 1966. Natural selection, the costs of reproduction, and a refinement of Lacks nrincinle. Amer. Natur. 100:687-692. R. HAVEN WILEY DEPARTMENT OF ZOOLOGY UNIVERSITY OF NORTH CAROLINA CHAPEL HILL, NORTH CAROLINA 27514 March 19, 1974

AGE-SPECIFIC SELECTION. II. THE INTERACTION BETWEEN r AND K DURING POPULATION GROWTH

The interplay between density-independent (r) and density-dependent (K) components of selection is investigated using models of Mendelian populations with breeding at discrete intervals and overlapping generations. Selection within each genotype is age-specific, while the contribution of each genotype to population growth is logistic. The interaction between the r- and K-characteristics of the genotypes, in conjunction with the initial population structure, determines the patterns of population growth and gene frequency change. Moreover, these patterns may be strikingly different from the sigmoid curves obtained with simpler models. In a constant environment, K-characteristics alone determine the ultimate outcome of selection. Heterozygote advantage with respect to K is necessary for a balanced genetic polymorphism. The situation is different if the environment varies. We have investigated the effects of two kinds of environmental variation on the interaction between r- and K-selection. In the first of these, population size is reduced at the end of each environmental cycle. As the number of breeding intervals in each cycle is reduced, the r-characteristics become progressively more important and ultimately determine both equilibrium gene frequencies and population size. In the second, the genotypic Ks are treated as seasonal variables. The relative amplitude of oscillations in K defines a genotypes ability to buffer the effects of environmental variation. In turn, the patterns of oscillation strongly influence changes in population size and gene frequency. Examples are presented to illustrate these points. Finally, the meaning of fitness under this model of age-specific, density-regulated selection is discussed.

Genetic Equilibrium and Population Growth Under Density-Regulated Selection

A maximization principle relating gene frequency changes and population growth is formulated for a simple model of density-regulated selection. The selective value of each genotype is altered by density in a way analogous to logistic population growth. The growth of the population as a whole is logistic, its maximal rate of increase being a weighted arithmetic mean of the genotypic rates, and its carrying capacity a weighted harmonic mean of the genotypic carrying capacities. A stable genetic polymorphism occurs at a maximum, and an unstable equilibrium at a minimum, of the mean genotypic carrying capacity.

Experimental constraints on mate preferences in Drosophila pseudoobscura decrease offspring viability and fitness of mated pairs

Using Drosophila pseudoobscura, we tested the hypothesis that social constraints on the free expression of mate preferences, by both females and males, decrease offspring viability and reproductive success of mating pairs. Mate preference arenas eliminated intrasexual combat and intersexual coercion. The time female and male choosers spent in arena tests near either of two opposite-sex individuals measured the preferences of choosers. We placed choosers in breeding trials with their preferred or nonpreferred discriminatee when they met the minimum criteria for showing the same preference in two consecutive tests. There was no statistically significant difference in the frequency of female and male choosers meeting minimal preference criteria. There was a significant difference between female and male choosers for offspring viability, with female choice having the greater effect, but there was not a significant difference in the overall reproductive success of male and female choosers. There were significant differences in fitness between matings to preferred and nonpreferred partners. Female and male choosers paired with their nonpreferred discriminatees had offspring of significantly lower viability, as predicted by the constraints hypothesis. Reproductive success, our measure of overall fitness, was greater when males or females mated with the partner they preferred rather than the one they did not prefer.