NATURAL SELECTION ON THERMOGENIC CAPACITY OF HIGH-ALTITUDE DEER MICE

Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole-animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [VO₂max] elicited via cold exposure) of high-altitude (~3800 m) deer mice (Peromyscus maniculatus). A high VO₂max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high VO₂max, at least in one year. The selection for increased VO₂max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for VO₂max that we observed suggests that—given ample genetic variation—aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.     

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: PREDICTION AND MEASUREMENT OF NATURAL SELECTION

A classic example of natural selection, that of color-pattern variation in Lake Erie island populations of water snakes, was reexamined to overcome shortcomings resulting from classification of snakes into discrete color-pattern categories and use of cross-sectional data. Four continuously varying color-pattern components (DB, the number of dorsal blotches; LB, the number of lateral blotches; ROWS, the height of lateral blotches measured in scale rows; and VEXT, the extent of ventral pigmentation) were analyzed. Patterns of natural selection were predicted from the relationship between color-pattern scores and independent measures of relative crypsis. Tests for natural selection were carried out using longitudinal data on neonate to juvenile-aged snakes and cross-sectional data on juvenile to adult-aged snakes. As predicted, the form of selection differed between younger and older age classes of snakes: selection resulted in a reduction in DB and LB among neonate and juvenile snakes but had little influence on color-pattern components in older snakes. The correspondence between observed patterns of natural selection and predictions based on the relationship between color-pattern scores and relative crypsis supports the hypothesis that differential predation by visual predators on younger age classes of snakes is the mechanism of selection. Gene flow from mainland populations or the initial lack of an allele necessary for reduced pattern may explain why selection has not resulted in greater differentiation between island and mainland populations.     

PATTERN OF PHENOTYPIC VIABILITY AND FECUNDITY SELECTION IN A NATURAL POPULATION OF IMPATIENS PALLIDA

Estimates of viability and fecundity selection of 13 phenotypic characters for 1,536 individuals of Impatiens pallida growing in 24 locations within a single natural population were compared. Directional viability selection of cotyledon area, day of initial leaf production, number of leaves, and stem length was detected throughout this population. Directional fecundity selection of cotyledon area, day of initial flower production, number of leaves present on day of initial flower production, stem length on day of initial flower production, number of leaves, and stem length was also detected. Phenotypic selection of these characters was strong in some cases, and the strength of selection was significantly heterogeneous among locations. For several of the characters, directional phenotypic selection within the population was significantly positive in some locations and significantly negative in others separated by only a few meters. Fecundity selection was stronger than viability selection for some characters, implying that fecundity selection was at least as important as viability selection within this population. Soil moisture levels and light intensities played a larger role than soil nutrient levels in determining the patterns of both viability and fecundity selection, and differences in directional viability selection were more strongly related to environmental variation than were differences in fecundity selection. The pattern of phenotypic selection could not be reliably inferred from the patterns of mortality and reproduction alone.     

MALE GAMETOPHYTE SELECTION IN A NATURAL POPULATION OF HAPLOPAPPUS GRACILIS (COMPOSITAE)

Distributions of two forms of the A chromosome of Haplopappus gracilis in a hybrid natural population and in artificial crosses are analyzed. Pollen bearing the standard type A chromosome is generally superior to that bearing the Mexican type A when they compete on homozygous pistils of either type, but there is no selection on heterozygous pistils. The distribution of sporophyte karyotypes in the natural population is not in equilibrium, and it is surmised that the standard type A chromosome is increasing in frequency in the population.     

MUTATION,SELECTION, AND THE MAINTENANCE OF LIFE-HISTORY VARIATION IN A NATURAL POPULATION

In an effort to provide insight into the role of mutation in the maintenance of genetic variance for life-history traits, we accumulated spontaneous mutations in 10 sets of clonal replicates of Daphnia pulex for approximately 30 generations and compared the variance generated by mutation with the standing level of variation in the wild population. Mutations for quantitative traits appear to arise at a fairly high rate in this species, on the order of at least 0.6 per character per generation, but have relatively small heterozygous effects, changing the phenotype by less than 2.5% of the mean. The mean persistence time of a new mutation affecting life-history/body-size traits is approximately 40 generations in the natural population, which requires an average selection coefficient against new mutations of approximately 3% in the heterozygous state. These data are consistent with the idea that the vast majority of standing genetic variance for life-history characters may be largely a consequence of the recurrent introduction of transient cohorts of mutations that are at least conditionally deleterious and raise issues about the meaning of conventional measures of standing levels of variation for fitness-related traits.     

POPULATION BIOLOGY OF AVENA. VIII. COLONIZATION EXPERIMENT AS A TEST OF THE ROLE OF NATURAL SELECTION IN POPULATION DIVERGENCE

Artificially founded colonies of Avena barbata were utilized in two experiments in order to examine the role of natural selection in the evolution of regional differentiation in Central California. One experiment involved a total of 41 colonies founded in three different areas and scored over a 10-yr period; these colonies, started with known genotypes, showed that although both Valley and Bay region genotypes establish successful colonies, their relative survivorship and fecundity suggests weak selective forces consistent with those predicted from the observed patterns in natural populations. The second experiment involved two localities, representing two climatic regimes of temperature, in each of which ten colonies were started from identical seed sources. These colonies also showed relatively higher fitness of the genotypes matching with those sampled from the Valley and Coastal regions, respectively, again in line with their regional pattern of distribution. Colonization experiments appeared to be useful even as short-term in situ tests of certain specific evolutionary hypotheses on the role and intensity of natural selection.     

TETRODOTOXIN RESISTANCE IN GARTER SNAKES: AN EVOLUTIONARY RESPONSE OF PREDATORS TO DANGEROUS PREY

The use of the “arms race” analogy as a conceptualization of evolutionary predator-prey interactions has been criticized because of the lack of evidence that predators can and do adapt to increased antipredator ability of prey. We present evidence that the garter snake Thamnophis sirtalis has evolved resistance to tetrodotoxin (TTX) in response to the toxicity of the newt Taricha granulosa on which the snake feeds. A bioassay (locomotor performance before and after injection of TTX) was used to obtain repeated measures of resistance for individual snakes. We studied interpopulation and interspecific variation by comparing resistance in Thamnophis sirtalis from populations occurring sympatrically and allopatrically with Taricha granulosa, and in Thamnophis ordinoides (which does not feed on the newt) occurring sympatrically with Taricha granulosa. We also examined intrapopulation variation in TTX resistance using snakes from a population known to feed on Taricha granulosa. Resistance differed significantly among individuals and litters; repeatability and heritability estimates of the assay were significantly different from zero, demonstrating the potential for response to selection. The population of Thamnophis sirtalis that occurs with Taricha granulosa exhibited levels of resistance much greater than either of the other groups. These results suggest that the predator-prey arms race analogy may be applicable to this system.     

EVOLUTIONARY RESPONSE OF PREDATORS TO DANGEROUS PREY: PREADAPTATION AND THE EVOLUTION OF TETRODOTOXIN RESISTANCE IN GARTER SNAKES

Coevolutionary interactions typically involve only a few specialized taxa. The factors that cause some taxa and not others to respond evolutionarily to selection by another species are poorly understood. Preadaptation may render some species predisposed for evolutionary response to new pressures, whereas a lack of genetic variation may limit the evolutionary potential of other taxa. We evaluate these factors in the predator-prey interaction between toxic newts (Taricha granulosa) and their resistant garter snake predators (Thamnophis sirtalis). Using a bioassay of resistance to tetrodotoxin (TTX), the primary toxin in the prey, we examined phenotypic evolution in the genus Thamnophis. Reconstruction of ancestral character states suggests that the entire genus Thamnophis, and possibly natricine snakes in general, has slightly elevated TTX resistance compared to other lineages of snakes. While this suggests that T. sirtalis is indeed predisposed to evolving TTX resistance, it also indicates that the potential exists in sympatric congeners not expressing elevated levels of TTX resistance. We also detected significant family level variation for TTX resistance in a species of Thamnophis that does not exhibit elaborated levels of the trait. This finding suggests that evolutionary response in other taxa is not limited by genetic variability. In this predator-prey system, species and population differences in resistance appear to be largely determined by variation in the selective environment rather than preadaptation or constraint.     

HOST-PARASITE COEVOLUTION: EVIDENCE FOR RARE ADVANTAGE AND TIME-LAGGED SELECTION IN A NATURAL POPULATION

In theory, parasites can create time-lagged, frequency-dependent selection in their hosts, resulting in oscillatory gene-frequency dynamics in both the host and the parasite (the Red Queen hypothesis). However, oscillatory dynamics have not been observed in natural populations. In the present study, we evaluated the dynamics of asexual clones of a New Zealand snail, Potamopyrgus antipodarum, and its trematode parasites over a five-year period. During the summer of each year, we determined host-clone frequencies in random samples of the snail to track genetic changes in the snail population. Similarly, we monitored changes in the parasite population, focusing on the dominant parasite, Microphallus sp., by calculating the frequency of clones in samples of infected individuals from the same collections. We then compared these results to the results of a computer model that was designed to examine clone frequency dynamics for various levels of parasite virulence. Consistent with these simulations and with ideas regarding dynamic coevolution, parasites responded to common clones in a time-lagged fashion. Finally, in a laboratory experiment, we found that clones that had been rare during the previous five years were significantly less infectible by Microphallus when compared to the common clones. Taken together, these results confirm that rare host genotypes are more likely to escape infection by parasites; they also show that host-parasite interactions produce, in a natural population, some of the dynamics anticipated by the Red Queen hypothesis.     

THE EVOLUTIONARY HISTORY OF DROSOPHILA BUZZATII. VIII. EVIDENCE FOR ENDOCYCLIC SELECTION ACTING ON THE INVERSION POLYMORPHISM IN A NATURAL POPULATION

The pattern of selection acting in nature on the chromosomal polymorphism of the cactophilic species Drosophila buzzatii was investigated by comparing inversion and karyotypic frequencies through four different life-cycle stages: adult males, eggs, third-instar larvae, and immature adults. All population samples were obtained in June 1981 at an old Opuntia ficus-indica plantation near Carboneras, Spain. The analysis rests on several assumptions which are explicitly set forth and discussed. The results, if these assumptions prove true, indicate strong directional selection for larval viability acting on the second-chromosome karyotypes and also suggest selective differences in fecundity and longevity. Heterotic selection, however, cannot be ruled out for other fitness components such as male mating success. This kind of selection could be operating on the fourth-chromosome polymorphism as well. Some gene arrangements showed significant and opposite changes in frequency at different parts of the life cycle, thus demonstrating endocyclic selection.