Lori Stevens

Contextual analysis of models of group selection, soft selection, hard selection, and the evolution of altruism

Contextual analysis is used to examine models of group, hard, and soft selection and the evolution of altruism. We extend the methodology for measuring phenotypic selection to multiple levels in structured populations by analyzing selection acting on a trait at the individual level and its mean at the group level. With contextual analysis, we partition phenotypic selection into group and individual components using partial regressions These analyses identify the level(s) at which selection is acting and distinguish indirect from direct selection acting at other levels Contextual analysis of group selection in the absence of individual selection indicates that indirect selection is acting on individuals Under soft selection, though all groups have the same relative fitness, contextual analysis detects equal and opposite levels of group and individual selection resulting from frequency-dependent selection acting within groups. Under hard selection, groups vary in relative fitness, but there is no group selection Instead, indirect selection acts on the group mean phenotype. Thus, contextual analysis reveals that group, kin, frequency-dependent, and soft selection are related phenomena. Finally, we rederive Hamiltons rule for the evolution of altruism and determine when group selection is expected to be more powerful than individual selection

Experimental Studies of Group Selection: What Do They Tell US About Group Selection in Nature?

The study of group selection has developed along two autonomous lines. One approach, which we refer to as the adaptationist school, seeks to understand the evolution of existing traits by examining plausible mechanisms for their evolution and persistence. The other approach, which we refer to as the genetic school, seeks to examine how currently acting artificial or natural selection changes traits within populations and focuses on current evolutionary change. The levels of selection debate lies mainly within the adaptationist school, whereas the experimental studies of group selection lie within the genetic school. Because of the very different traditions and goals of these two schools, the experimental studies of group selection have not had a major impact on the group selection debate. We review the experimental results of the genetic school in the context of the group selection controversy and address the following questions: Under what conditions is group selection effective? What is the genetic basis of a response to group selection? How common is group selection in nature?

Why should parasite resistance be costly

Parasite resistance is sometimes associated with fitness costs. Costs of resistance are fundamentally important in epidemiology, and in the ecology and evolution of host-parasite interactions. The cost of resistance is often envisioned as the cost of re-allocating limiting resources to resistance machinery from other traits. This popular paradigm has resulted in a spate of research that assumes a fitness cost to resistance. We comment on this trend and propose a working framework of various resistance means and mechanisms. Within these means and mechanisms, we suggest that many are not likely to incur significant fitness costs.

Male-killing Wolbachia in a flour beetle

The bacteria in the genus Wolbachia are cytoplasmically inherited symbionts of arthropods. Infection often causes profound changes in host reproduction, enhancing bacterial transmission and spread in a population. The reproductive alterations known to result from Wolbachia infection include cytoplasmic incompatibility (CI), parthenogenesis, feminization of genetic males, fecundity enhancement, male killing and, perhaps, lethality. Here, we report male killing in a third insect, the black flour beetle Tribolium madens, based on highly female–biased sex ratios of progeny from females infected with Wolbachia. The bias is cytoplasmic in nature as shown by repeated backcrossing of infected females with males of a naturally uninfected strain. Infection also lowers the egg hatch rates significantly to approximately half of those observed for uninfected females. Treatment of the host with antibiotics eliminated infection, reverted the sex ratio to unbiased levels and increased the percentage hatch. Typically Wolbachia infection is transmitted from mother to progeny, regardless of the sex of the progeny; however, infected T. madens males are never found. Virgin females are sterile, suggesting that the sex–ratio distortion in T. madens results from embryonic male killing rather than parthenogenesis. Based on DNA sequence data, the male–killing strain of Wolbachia in T. madens was indistinguishable from the CI–inducing Wolbachia in Tribolium confusum, a closely related beetle. Our findings suggest that host–symbiont interaction effects may play an important role in the induction of Wolbachia reproductive phenotypes.

MULTILEVEL SELECTION IN NATURAL POPULATIONS OF IMPATIENS CAPENSIS

This study partitions selection in natural populations of jewelweed, Impatiens capensis, into group- and individual-level components. Group selection has been a subject of controversy for decades, yet this is the first study to partition phenotypic selection in a natural population. Using contextual analysis combined with path analysis, we measured the correlation between fitness components (survival rate to first reproduction, chasmogamous [open-pollinated] seed production, and cleistogamous [selfed] seed production) and several group- and individual-level traits. Survival rate was studied for 2 yr, and the reproductive traits were studied for 1 yr. For survival rate and cleistogamous seed production, both group and individual selection occurred, and the two types of selection were in opposition. For chasmogamous seed production, only individual selection was detected. Group selection may be responsible for the constant yield law in plants. It may be more common than previously believed because it may be mistaken for frequency-dependent selection. Evolutionary theory suggests different components of genetic variation are available to different levels of selection. Thus, the demonstration of group-level selection in nature challenges evolutionary biologists to consider new components of variation as raw material for selection. The results are discussed with respect to the evolution of altruism and the use of multiple regression versus path analysis in studies of selection.

INBREEDING: ITS EFFECT ON RESPONSE TO SELECTION FOR PUPAL WEIGHT AND THE HERITABLE VARIANCE IN FITNESS IN THE FLOUR BEETLE, TRIBOLIUM CASTANEUM

We report our studies of the effect of inbreeding on the response to selection for increased pupal weight in the flour beetle, Tribolium castaneum. We also report the effects of inbreeding and selection for pupal weight on the heritable variation in fitness and fitness components. We created replicate and independent inbred lines with F‐values of 0.00, 0.375, and 0.672, by 0, 2, and 5 generations, respectively, of brother‐sister mating of adult beetles from an outbred stock population. Subsequently, we imposed artificial within‐family selection for increased pupal weight in each of 15 inbred lines for eight generations; each line had its own paired, unselected control. We compared the response to selection across the three levels of inbreeding with theoretical expectation, and investigated the effects of inbreeding and selection on fitness variation among families within all 30 selected and control lines. Among‐line variation in pupal weight increased with increased inbreeding prior to selection but diminished with directional selection. Inbreeding reduced the realized heritability of pupal weight concordant with quantitative predictions of additive theory. Mean fitness, measured in several ways, declined with inbreeding and declined further with selection. In contrast, the genetic variation for fitness in the inbred and selected lines lines equalled or exceeded that of the outbred controls. Our results suggest that inbreeding and selection may affect traits in different ways depending on the relative amounts of additive and nonadditive genetic variation.

CONSEQUENCES OF INBREEDING ON INVERTEBRATE HOST SUSCEPTIBILITY TO PARASITIC INFECTION

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THE GENETICS AND EVOLUTION OF CANNIBALISM IN FLOUR BEETLES (GENUS TRIBOLIUM )

Cannibalism plays a major role in population regulation in Tribolium confusum, accounting for up to tenfold differences in population size between different genetic strains. I characterized the within‐ and between‐strain genetic variation for cannibalism using standard quantitative‐genetic methods. The four laboratory strains studied have similar birth and death rates but differ in their strain‐specific cannibalistic tendencies. The cannibalism rates of the strains were stable for more than 60 generations of laboratory husbandry. I found considerable genetic variation for cannibalism within each strain. A genetic analysis of the between‐strain differences in each of three types of cannibalism (larvae eating eggs, adults eating eggs, and adults eating pupae) showed that all three cannibalism pathways are autosomally inherited and exhibit minor degrees of dominance. Adult cannibalism of eggs and larval cannibalism of eggs appear to be genetically correlated. The differences between the “high” and “low” cannibalism strains appear to be polygenic for two kinds of cannibalism, larvae eating eggs and adults eating pupae. However, strain differences in adult cannibalism of eggs may be due to only two loci. The stability of the between‐strain differences for more than 60 generations, the additive nature of inheritance, and the demonstration of considerable within‐strain genetic variation suggest that cannibalism may be selectively neutral or under stabilizing selection with many adaptive peaks.

A New Method for Forensic DNA Analysis of the Blood Meal in Chagas Disease Vectors Demonstrated Using Triatoma infestans from Chuquisaca, Bolivia

Background Feeding patterns of the vector are important in the epidemiology of Chagas disease, the leading cause of heart disease in Latin America. Chagas disease is caused by the parasite, Trypanasoma cruzi, which is transmitted by blood feeding insects. Historically, feeding behaviours of haematophagous insects have been investigated using serological reactions, which have detection limits in terms of both taxonomic resolution, and quantity and quality of the blood meal. They are labor intensive, require technical expertise, need fresh or frozen samples and antibodies often are either not available commercially or the resources for synthesis and purification are not available. We describe an assay to identify vertebrate blood meal sources, and the parasite T. cruzi using species-specific PCR assays from insect vectors and use the method to provide information regarding three questions: (1) Do domestic and peri-domestic (chicken coop and animal corral) habitats vary in the blood meals detected in the vectors? (2) What is the pattern of multiple blood meals? (3) Does the rate of T. cruzi infection vary among habitats and is it associated with specific blood meal types? Methodology/Principal Findings Assays based on the polymerase chain reaction were evaluated for identification of the blood meal source in the heamatophagous Chagas disease vector Triatoma infestans. We evaluate a technique to identify 11 potential vertebrate food sources from the complex mixture extracted from the vectors abdomen. We tested the assay on 81 T. infestans specimens collected from the Andean highlands in the department of Chuquisaca, located in central Bolivia, one of the regions in South America where sylvatic T. infestans have been reported. This area is suggested to be the geographic origin of T. infestans and has very high human infection rates that may be related to sylvatic vector populations. Conclusion/Significance The results of the assays revealed that a high percentage of insects collected in human dwellings had fed on peri-domestic animals. In contrast, one insect from a chicken coop but no bugs from corrals tested positive for human blood. Forty-eight percent of insects tested positive for more than one vertebrate species. T. cruzi infection was detected in 42% of the specimens. From the epidemiological point of view, the results reveal an overall pattern of movement from peri-domestic structures to human habitations for T. infestans in this region of Bolivia as well as the important role of pigs, dogs, chickens and guinea pigs in the dynamics of T. cruzi infection.

THE EFFECT OF POPULATION SIZE ON EFFECTIVE POPULATION SIZE : AN EMPIRICAL STUDY IN THE RED FLOUR BEETLE TRIBOLIUM CASTANEUM

Despite the increasing number of studies on the magnitude of N e / N ratios, much remains unknown about the effects of demographic and environmental variables on N e / N . We determined N e / N for seven population size treatments, ranging from N = 2 to N = 960, in the red flour beetle Tribolium castaneum. N e / N decreased with increasing N , as evidenced by a significant negative relationship between log N and N e / N . Our results are consistent with other published data on the relationship between N e / N and N . Effective population sizes in large populations may be much smaller than previously recognized. These results have important implications for conservation and evolutionary biology.