Repression of competition within groups joins kin selection as the second major force in the history of life shaping the evolution of cooperation. When opportunities for competition against neighbors are limited within groups, individuals can increase their own success only by enhancing the efficiency and productivity of their group. Thus, characters that repress competition within groups promote cooperation and enhance group success. Leigh first expressed this idea in the context of fair meiosis, in which each chromosome has an equal chance of transmission via gametes. Randomized success means that each part of the genome can increase its own success only by enhancing the total number of progeny and thus increasing the success of the group. Alexander used this insight about repression of competition in fair meiosis to develop his theories for the evolution of human sociality. Alexander argued that human social structures spread when they repress competition within groups and promote successful group-against-group competition. Buss introduced a new example with his suggestion that metazoan success depended on repression of competition between cellular lineages. Maynard Smith synthesized different lines of thought on repression of competition. In this paper, I develop simple mathematical models to illustrate the main processes by which repression of competition evolves. With the concepts made clear, I then explain the history of the idea. I finish by summarizing many new developments in this subject and the most promising lines for future study.

We investigated the role of the number of loci coding for a neutral trait on the release of additive variance for this trait after population bottlenecks. Different bottleneck sizes and durations were tested for various matrices of genotypic values, with initial conditions covering the allele frequency space. We used three different types of matrices. First, we extended Cheverud and Routman's model by defining matrices of “pure” epistasis for three and four independent loci; second, we used genotypic values drawn randomly from uniform, normal, and exponential distributions; and third we used two models of simple metabolic pathways leading to physiological epistasis. For all these matrices of genotypic values except the dominant metabolic pathway, we find that, as the number of loci increases from two to three and four, an increase in the release of additive variance is occurring. The amount of additive variance released for a given set of genotypic values is a function of the inbreeding coefficient, independently of the size and duration of the bottleneck. The level of inbreeding necessary to achieve maximum release in additive variance increases with the number of loci. We find that additive-by-additive epistasis is the type of epistasis most easily converted into additive variance. For a wide range of models, our results show that epistasis, rather than dominance, plays a significant role in the increase of additive variance following bottlenecks.

The primary rationale for the use of phylogenetically based statistical methods is that phylogenetic signal, the tendency for related species to resemble each other, is ubiquitous. Whether this assertion is true for a given trait in a given lineage is an empirical question, but general tools for detecting and quantifying phylogenetic signal are inadequately developed. We present new methods for continuous-valued characters that can be implemented with either phylogenetically independent contrasts or generalized least-squares models. First, a simple randomization procedure allows one to test the null hypothesis of no pattern of similarity among relatives. The test demonstrates correct Type I error rate at a nominal α = 0.05 and good power (0.8) for simulated datasets with 20 or more species. Second, we derive a descriptive statistic, K, which allows valid comparisons of the amount of phylogenetic signal across traits and trees. Third, we provide two biologically motivated branch-length transformations, one based on the Ornstein-Uhlenbeck (OU) model of stabilizing selection, the other based on a new model in which character evolution can accelerate or decelerate (ACDC) in rate (e.g., as may occur during or after an adaptive radiation). Maximum likelihood estimation of the OU (d) and ACDC (g) parameters can serve as tests for phylogenetic signal because an estimate of d or g near zero implies that a phylogeny with little hierarchical structure (a star) offers a good fit to the data. Transformations that improve the fit of a tree to comparative data will increase power to detect phylogenetic signal and may also be preferable for further comparative analyses, such as of correlated character evolution. Application of the methods to data from the literature revealed that, for trees with 20 or more species, 92% of traits exhibited significant phylogenetic signal (randomization test), including behavioral and ecological ones that are thought to be relatively evolutionarily malleable (e.g., highly adaptive) and/or subject to relatively strong environmental (nongenetic) effects or high levels of measurement error. Irrespective of sample size, most traits (but not body size, on average) showed less signal than expected given the topology, branch lengths, and a Brownian motion model of evolution (i.e., K was less than one), which may be attributed to adaptation and/or measurement error in the broad sense (including errors in estimates of phenotypes, branch lengths, and topology). Analysis of variance of log K for all 121 traits (from 35 trees) indicated that behavioral traits exhibit lower signal than body size, morphological, life-history, or physiological traits. In addition, physiological traits (corrected for body size) showed less signal than did body size itself. For trees with 20 or more species, the estimated OU (25% of traits) and/or ACDC (40%) transformation parameter differed significantly from both zero and unity, indicating that a hierarchical tree with less (or occasionally more) structure than the original better fit the data and so could be preferred for comparative analyses.

The geographic range of many parasites is restricted relative to that of their hosts. We study possible evolutionary mechanisms for this observation using a simple model that couples coevolution and demography. The model assumes that the environment consists of two habitats connected by movement and that coevolution is governed by quantitative traits. Our results demonstrate that host gene flow is an important determinant of parasite geographic range. Fluctuations in the rate of host gene flow cause shifts in parasite population densities and associated range expansions or contractions. In extreme cases, changing the rate of host gene flow can lead to global extinction of the parasite. Through a process we term demographic compensation, these shifts in parasite density may occur with little or no change in parasite adaptation to the host. As a consequence, reciprocal adaptation between host and parasite can become uncoupled from the rate of host gene flow.

The evolution of virulence is a rapidly growing field of research, but few reports deal with the evolution of virulence in natural populations of parasites. We present here an observational and experimental analysis of the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) during an epidemic on tomato in eastern Spain. Three types of CMV isolates were found that caused in tomato plants either a systemic necrosis (N isolates), stunting and a severe reduction of leaf lamina (Y isolates), or stunting and leaf curl (A isolates). These phenotypes were due to the presence of satellite RNAs (satRNAs) necrogenic (in N isolates) or attenuative (in A isolates) of the symptoms caused by CMV without satRNA (Y isolates). For these three types of isolates, parameters of virulence and transmission were estimated experimentally. For virulence the ranking of isolates was N > Y > A, for transmissibility, Y > A > N. The predictions of theoretical models for the evolution of virulence were analyzed with these parameters and compared with observations from the field. A single-infection model predicted adequately the observed long-term evolution of the CMV population to intermediate levels of virulence. A coinfection model that considered competition between isolates with an effect on transmission explained the invasion of the CMV population by N isolates at the beginning of the epidemic, and its predictions also agreed with field data on the long-term evolution of the CMV population. An important conclusion from both models was that the density of the aphid vector's population is a major factor in the evolution of CMV virulence. This may be relevant for the design of control strategies for CMV-induced diseases.

We studied intraspecific competition and assortative mating between strains of the anther smut fungus Microbotryum violaceum from two of its host species, Silene latifolia and S. dioica. Specifically, we investigated whether strains from allopatric host populations have higher competitive ability on their native host species and show positive assortative mating. In general, strains isolated from S. latifolia outcompeted strains isolated from S. dioica on both host species, but in female hosts, heterotypic dikaryons (i.e., dikaryons composed of a haploid strain originating from S. latifolia and a haploid strain originating from S. dioica) were most successful in competition. Furthermore, the latency period was significantly shorter for heterokaryons that contained at least one strain originating from S. latifolia, compared to heterokaryons that only contained strains originating from S. dioica. The frequencies of conjugations between strains originating from S. latifolia were much higher than conjugation frequencies between strains originating from S. dioica. A significant positive correlation was detected between the relative success of strains in competition and in conjugation, suggesting that success of a strain in competition might be partly determined by its swiftness of mating. In addition, reciprocal differences within heterotypic crosses revealed a significant effect of fungal mating type, with mating type a₁ being the main determinant of mating pace. The observed differences in infection success, conjugation rate, and latency period in favor of strains from S. latifolia relative to strains from S. dioica on both host species are discussed in an evolutionary context of opportunities for the maintenance of differentiation between different formae speciales upon secondary contact.

It has been proposed that parasitic infections increase selection against inbred genotypes. We tested this hypothesis experimentally using pairs of selfed and outcrossed sibling lines of the freshwater crustacean Daphnia magna, which can be maintained clonally. We studied the performance of selfed relative to outcrossed sibling clones during repeated pairwise clonal competition in the presence and absence of two species of microsporidian parasites. In 13 of the 14 pairs, the selfed clones did worse than the outcrossed ones in the control treatment, but the presence of either parasite did not result in an overall increase in this difference. Rather, it decreased the performance of the selfed relative to the outcrossed sibling in some pairs and increased it in others. Moreover, the two parasite species did not have the same effect in a given pair. This indicates that, contrary to the hypothesis that parasites generally lead to a decreased performance of inbred genotypes, their effect may depend on the genetic background of the host as well as on the parasite species, and suggests that inbreeding can lead to reduced or increased resistance to parasites. Our findings also indicate that there is variation for specific resistance to different species of parasites in the metapopulation from which the hosts for this study were obtained.

Organisms are often confronted with multiple enemy species. Defenses against different parasite species may be traded off against each other. However, if resistance is based on (potentially costly) general defense mechanisms, it may be positively correlated among parasites. In an experimental study, we confronted 19 clones from one Daphnia magna population with two bacterial and three microsporidian parasite species. All parasites were isolated from the same pond as the hosts. Host clones were specific in their susceptibility towards different parasite species, and parasite species were host-clone specific in their infectivity, spore production, and virulence, resulting in highly significant host-parasite interactions. Since the Daphnia's resistance to different parasite species showed no obvious correlation, neither general defense mechanisms nor trade-offs in resistance explain our findings. None of the Daphnia clones were resistant to all parasite species, and the average level of resistance was quite similar among clones. This may reflect a cost of defense, so that the cumulative cost of being resistant to all parasite species might be too high.

Models accounting for genetic variation for resistance to herbivores within plant populations often postulate a balance between the costs of that resistance and its benefits. The production of glandular trichomes by Datura wrightii was shown to be costly in a previous one-year study because plants producing glandular trichomes (sticky plants), a factor conferring resistance to some insect herbivores, also produced 45% fewer seeds than plants producing nonglandular trichomes (velvety plants) when grown in a common garden. Because sticky plants tended to be larger than velvety plants but produced fewer seed capsules, we postulated an allocation trade-off in which velvety plants are more reproduction-dominated whereas sticky plants are more growth-dominated. If a greater commitment to vegetative growth eventually allows sticky plants to compensate for reduced seed production, we would expect a reduction or elimination of the cost of resistance over time in this perennial plant. We monitored growth, survival, and seed production of plants from defined crosses of local populations for three years in a common garden when exposed to and protected from herbivores, and with and without supplemental water. The majority of plants exposed to herbivores had died by the end of the study. We used standard life-table methods to determine the net reproductive rate (R₀) and the finite rate of increase (λ) of plants of each trichome type. After three years, when plants were protected from herbivores, sticky plants were 187–245% larger than velvety plants, depending upon irrigation treatment, but sticky plants continued to be less efficient in producing seeds per unit of canopy volume. Even though the total seed production of sticky plants eventually equaled that of velvety plants, the advantage of earlier reproduction by velvety plants increased λ by 55–230% over that of sticky plants, depending upon herbivore and irrigation treatment. Exposure to herbivores reduced λ by 69–83%, depending upon plant type and irrigation treatment, whereas supplemental irrigation increased λ by 29–175%, depending upon plant type and exposure to herbivores. Although there was a large allocation trade-off between growth and reproduction, the benefits of such a trade-off did not emerge before most plants were killed by herbivores. The cost of producing glandular trichomes strictly for herbivore resistance continued to exceed its benefits, and in the absence of other, unmeasured benefits from the suite of life-history characters associated with glandular trichome production, natural selection is expected to eliminate this costly resistance trait from D. wrightii populations.

According to the geographic mosaic theory of coevolution, selection intensity in interactions varies across a landscape, forming a selection mosaic; interaction traits match at coevolutionary hotspots where selection is reciprocal and mismatch at coldspots where reciprocity is not a factor. Chemical traits play an important role in the interaction between wild parsnip (Pastinaca sativa) and the parsnip webworm (Depressaria pastinacella). Furanocoumarins, produced as plant defenses, are detoxified by the webworms by cytochrome P450 monooxygenases; significant additive genetic variation exists for both furanocoumarin production in the plant and detoxification in the insect, making these traits available for selection. To test the hypothesis that differences in selection intensity affect the distribution of coevolutionary hotspots and coldspots in this interaction, we examined 20 populations of webworms and wild parsnips in Illinois and Wisconsin that varied in size, extent of infestation, proximity to woods (and potential vertebrate predators), and proximity to a chemically distinct alternate host plant, Heracleum lanatum (cow parsnip). Twelve of 20 populations displayed phenotype matching between plant defense and insect detoxification profiles. Of the eight mismatched populations, a logistic regression model related matching probability to two predictors: the presence of the alternate host and average content of xanthotoxin (one of the five furanocoumarins produced by P. sativa). The odds of mismatching were significantly increased by the presence of the alternate host (odds ratio = 15.4) and by increased xanthotoxin content (odds ratio = 6.053). Parsnips growing near cow parsnip displayed chemical phenotypes that were chemically intermediate between cow parsnip and parsnips growing in isolation. Rapid phenotype matching in this system is likely due in part to differential mortality every season; larvae transferred to a plant 30 m or more from the plant on which they developed tended to experience increased mortality over larvae transferred to another umbel on the same plant on which they had developed, and plant populations that mismatched in 2001 displayed a change in chemical phenotype distribution from the previous year. Trait mixing through gene flow is also a likely factor in determining mismatch frequency. Populations from which webworms were eradicated the previous year were all recolonized; in three of seven of these populations, infestation rates exceeded 90%. Our findings, consistent with the geographic mosaic theory, suggest that the presence of a chemically distinct alternate host plant can affect selection intensity in such a way as to reduce the likelihood of reciprocity in the coevolutionary interaction between wild parsnip and the parsnip webworm.

Relationships between the closely related island species of Phylica (Rhamnaceae) and a mainland species, P. paniculata, were elucidated using amplified fragment length polymorphisms (AFLPs). Parsimony, neighbor joining, and principal coordinate (PCO) analyses indicated that each of the species studied is distinct. AFLPs were also useful in elucidating the genetic relationships and possible infraspecific origins of different island populations in the Atlantic and Indian Oceans. Phylica nitida on Réunion is likely to have been derived from P. nitida on Mauritius. Although the sampling on New Amsterdam is not extensive, the data are also consistent with the hypothesis that P. arborea on New Amsterdam was derived from a single colonization of P. arborea from Gough Island. Similarly, the Gough Island population appears to have been derived from a single colonization event, but it is so distinct from those on Tristan da Cunha, that there may have been two separate dispersals to Gough and Tristan/Nightingale from different lines of the mainland progenitor. There is also evidence of a recolonization from Gough to Tristan da Cunha. Thus, Phylica arborea is capable of repeated long distance dispersal, up to 8000 km, even though the fruits and seeds are not of a type normally associated with this phenomenon.

Some astigmatic mites display dimorphic deutonymphs (hypopus) which are facultatively intercalated in their development cycle between protonymph and tritonymph. Such species, among them Glycyphagus privatus and Glycyphagus ornatus show three potential developmental pathways: (1) to bypass the hypopus stage and develop directly from the protonymph to the tritonymph and the subsequent reproductive stage when conditions are favorable; (2) to leave the original site and disperse by means of a phoretic hypopus morph; or (3) to survive inimical life conditions in the natal environment by means of a sedentary hypopus morph. By producing both dispersing (and afterwards at the arrival site reproducing) and sedentary (drought-hardy and dormancy-prone) progeny each single parent attains a selective advantage through a risk-reducing insurance against irregularly fluctuating and often fatal life conditions of their temporary patch habitats. Both genetic heterogeneity and ecological plasticity for hypopus production adapt the Glycyphagus species to cope with variation in the environment. Both traits (for dispersal and survival) are extremely polymorphic with genotypes ranging from low to high propensities for production of each hypopus type. There is a substantial environmental effect on genetic expression such that expression of both morphs depends on the quality of food. This ecological response allows a fast reaction of the mite to the current trophic environment. Phoretic morphs are predominantly expressed at favorable trophic conditions and sedentary morphs at poor trophic conditions. Ecological influences may override genetic propensities and vice versa. Although selection imposed by changing environmental patterns adjusts the frequencies of genotypes over generations and provides for long-term adaptation, the short-term process of environmental induction adapts the population within a generation to transient-habitat disturbances. The interaction of genetic and ecological determinants explains the varying proportions of directly developing mites, phoretic hypopodes, and sedentary hypopodes, in a population at any moment.

Allozyme loci are frequently found non randomly associated to the chromosomal inversions in which they are included in Drosophila. Two opposite views compete to explain strong allozyme-by-inversion gametic disequilibria: they result from natural selection or, conversely, merely represent remnants of associations accidentally established at the origin of inversions. Empirical efforts aimed at deciding between adaptive and historical scenarios have focused on the spatial distribution of disequilibria. Yet, the evolutionary significance of these associations remains uncertain. I report here the results of a time-series analysis of the seasonal variation of alleles at six allozyme loci (Acph, Lap, Pept-1, Ao, Mpi, and Xdh) in connection with the O chromosomal polymorphisms of D. subobscura. The findings were: (1) in the segment I of the O chromosome, Lap and Pept-1 allozymes changed seasonally in a cyclical fashion within the ST gene arrangement, but they changed erratically within the 3 4 gene configuration; (2) the frequencies of Lap^1.11 and Pept-1^0.40 within ST dropped to their lowest values in early and late summer, respectively, when the seasonal level of the ST arrangement is lowest. Furthermore, Lap^1.11 and Pept-1^0.40 covary with ST only within these seasons, yet in a fashion inconsistent with these alleles having a major influence on the dynamics of the inversion; (3) seasonal cycling of alleles within inversions were not detected at Acph, Ao, Mpi, and Xdh, yet these loci are nearly monomorphic at the study population, and/or their sampled series were shorter than those for Lap and Pept-1; and (4) simply monitoring allozyme frequencies separately for each inversion proved to be superior, for evidencing the seasonal cycles of the disequilibria, to the use of the D′ coefficient of association. Observed seasonal cycles of allozymes within inversions likely reflect natural selection.

Tension zones are maintained by the interaction between selection against hybrids and dispersal of individuals. Investigating multiple hybrid zones within a single species provides the opportunity to examine differences in zone structure on a background of differences in extrinsic factors (e.g., age of the zone, ecology) or intrinsic factors (e.g., chromosomes). The New Zealand tree weta Hemideina thoracica comprises at least eight distinct chromosomal races with diploid numbers ranging from 2n = 11 (XO) to 2n = 23 (XO). Five independent hybrid zones were located that involve races differing from one another by a variety of chromosomal rearrangements. The predicted negative correlation between extent of karyotypic differentiation (measured in terms of both percent of genome and number of rearrangements) and zone width was not found. Conversely, the widest zones were those characterized by two chromosome rearrangements involving up to 35% of the genome. The narrowest zone occurred where the two races differ by a single chromosome rearrangement involving approximately 2% of the genome. The five estimates of chromosomal cline width ranged from 0.5 km to 47 km. A comparative investigation of cline width for both chromosomal and mitochondrial markers revealed a complex pattern of zone characteristics. Three of the five zones in this study showed cline concordance for the nuclear and cytoplasmic markers, and at two of the zones the clines were also coincident. Zones with the widest chromosomal clines had the widest mitochondrial DNA clines. It appears that, even within a single species, the extent of karyotypic differentiation between pairs of races is not a good predictor of the level of disadvantage suffered by hybrids.

Studies on arrival time to breeding areas show that high-quality males usually arrive first and gain the highest reproductive success. This is generally assumed to be due to phenotype-dependent costs and benefits of early arrival. We show that the opposite arrival order can occur, probably due to selection on poor-quality males to increase their chances of reproduction. In a fish species, the threespine stickleback, Gasterosteus aculeatus, small males arrived before larger males at the breeding grounds. Early arrival was costly because predation risk was at its highest at the start of the season and early territory establishment was selected against, as demonstrated by selection coefficients for territory maintenance and hatching success. Large males probably postponed arrival until females were available to decrease predation risk costs and increase offspring production. An experimental study showed that a delay in arrival of large males does not decrease their probability of reproduction, because large males are able to take over nest sites from small males. Small males, on the other hand, are less likely to establish territories in competition with large males but can pay the costs of early arrival in exchange for the benefit of access to territories. Thus, whereas natural selection favors later arrival, sexual selection through competition for breeding territories favors early arrival in small, competitively inferior males. This results in the benefits of early arrival depending on the competitive ability of the male, which favors size-dependent optimal arrival times.

Strategies for optimal metamorphosis are key adaptations in organisms with complex life cycles, and the components of the larval growth environment causing variation in this trait are well studied empirically and theoretically. However, when relating these findings to a broader evolutionary or ecological context, usually the following assumptions are made: (1) size at metamorphosis positively relates to future fitness, and (2) the larval growth environment affects fitness mainly through its effect on timing of and size at metamorphosis. These assumptions remain poorly tested, because data on postmetamorphic fitness components are still rare. We created variation in timing of and size at metamorphosis by manipulating larval competition, nonlethal presence of predators, pond drying, and onset of larval development, and measured the consequences for subsequent terrestrial survival and growth in 1564 individually marked water frogs (Rana lessonae and R. esculenta), raised in enclosures in their natural environment. Individuals metamorphosing at a large size had an increased chance of survival during the following terrestrial stage (mean linear selection gradient: 0.09), grew faster and were larger at maturity than individuals metamorphosing at smaller sizes. Late metamorphosing individuals had a lower survival rate (mean linear selection gradient: −0.03) and grew more slowly than early metamorphosing ones. We found these patterns to be consistent over the three years of the study and the two species, and the results did not depend on the nature of the larval growth manipulation. Furthermore, individuals did not compensate for a small size at metamorphosis by enhancing their postmetamorphic growth. Thus, we found simple relationships between larval growth and postmetamorphic fitness components, and support for this frequently made assumption. Our results suggest postmetamorphic selection for fast larval growth and provide a quantitative estimate for the water frog example.

“Good genes” models of sexual selection predict that male courtship displays can advertise genetic quality and that, by mating with males with extreme displays, females can obtain genetic benefits for their offspring. However, because the relative performance of different genotypes can vary across environments, these genetic benefits may depend on the environmental context; in which case, static mating preferences may not be adaptive. To better understand how selection acts on the preference that female gray tree frogs (Hyla versicolor) express for long advertisement calls, I tested for genetic benefits in two realistic natural environments, by comparing the performance of half-sibling offspring sired by males with long versus short calls. Tadpoles from twelve such maternal half-sibships were raised in enclosures in their natal pond at two densities. In the low-density treatment, offspring of long-call males were larger at metamorphosis than were offspring of short-call males, whereas in the high-density treatment, offspring of males with long calls tended to metamorphose later than offspring of males with short calls. Thus, although the genes indicated by long calls were advantageous under low-density conditions, they were not beneficial under all conditions, suggesting that a static preference for long calls may not be adaptive in all environments. Such a genotype-by-environment interaction in the genetic consequences of mate choice predicts that when the environment is variable, selection may favor plasticity in female preferences or female selectivity among environments to control the conditions experienced by the offspring.

Interactions between species can affect the evolution of their sexual signals, receiver selectivity, or both. One commonly expected outcome is reproductive character displacement, whereby adverse consequences of mismating select for greater differentiation of communication systems in areas of sympatry than in areas of allopatry. We found evidence of reproductive character displacement in the acoustic communication system of green tree frogs (Hyla cinerea). The strength of female preferences for the spectral properties of calls that distinguish conspecific calls from those of a closely related congener, H. gratiosa, was greater in areas of sympatry with H. gratiosa than in areas of allopatry. We also found subtle differences in advertisement calls and in the heights of male calling perches when we restricted our comparisons to localities in which H. gratiosa was also breeding (syntopy) with localities where this species was absent. Hyla cinerea and H. gratiosa show only weak genetic incompatibility, but the calls representative of interspecific hybrids were unattractive to females of both parental species. Hybrids might also be at an ecological disadvantage because of different habitat preferences of the two taxa. Thus, selection against production of less fit or less attractive hybrid or backcross offspring are probably the main causes responsible for the differences documented in this paper.

There are three main hypotheses that explain how the evolution of parasite virulence could be linked to the evolution of secondary sexual traits, such as bird song. First, as Hamilton and Zuk proposed a role for parasites in sexual selection, female preference for healthy males in heavily parasitized species may result in extravagant trait expression. Second, a reverse causal mechanism may act, if sexual selection affects the coevolutionary dynamics of host-parasite interactions per se by selecting for increased virulence. Third, the immuno-suppressive effects of ornamentation by testosterone or limited resources may lead to increased susceptibility to parasites in species with elaborate songs. Assuming a coevolutionary relationship between parasite virulence and host investment in immune defense we used measures of immune function and song complexity to test these hypotheses in a comparative study of passerine birds. Under the first two hypotheses we predicted avian song complexity to be positively related to immune defense among species, whereas this relationship was expected to be negative if immuno-suppression was at work. We found that adult T-cell mediated immune response and the relative size of the bursa of Fabricius were independently positively correlated with a measure of song complexity, even when potentially confounding variables were held constant. Nestling T-cell response was not related to song complexity, probably reflecting age-dependent selective pressures on host immune defense. Our results are consistent with the hypotheses that predict a positive relationship between song complexity and immune function, thus indicating a role for parasites in sexual selection. Different components of the immune system may have been independently involved in this process.

Among tuco-tucos, Ctenomys rionegrensis is especially amenable to the study of the forces driving population differentiation because of the restricted geographic range it occupies in Uruguay. Within this limited area, the Rio Negro tuco-tuco is limited to sandy soils. It nonetheless exhibits remarkable variation in pelage color, including melanic, agouti, and dark-backed individuals. Two hypotheses have been put forth to explain this pattern: (1) local differentiation and fixation of alternative pelage types by genetic drift under limited gene flow; or (2) fixation by natural selection that may take place even in the presence of gene flow. A previous allozyme study rejected the genetic drift hypothesis on the basis of high inferred levels of migration. New estimates of gene flow from microsatellites and mitochondrial cytochrome b sequences were obtained for C. rionegrensis populations to further test these hypotheses. Much lower levels of gene flow were estimated with these more sensitive markers. Microsatellite-based estimates of gene flow are close to zero and may come closest to estimating current levels of migration. A lack of equilibrium between migration and genetic drift is also strongly suggested by the absence of an isolation-by-distance pattern found in all three genetic datasets. The microsatellite genotype data show that the species is strongly structured geographically, with subpopulations constituting distinct genetic entities. If current levels of gene flow are very low, as indicated by the new data, the local fixation of alternative alleles, including those responsible for pelage color polymorphism, is possible by drift alone. A scenario is thus proposed in which the species expanded in the recent past from a more restricted geographic range and has subsequently differentiated in near isolation, with genetic drift possibly playing a primary role in overall genetic differentiation. The local fixation of pelage color types could also be due to drift, but selection on this trait cannot be ruled out without direct analysis.

In polytocous mammals, the sex ratio during gestation can influence a variety of morphological, physiological, and life-history traits because of steroid leakage between fetuses. Similar phenomena have also recently been described for a viviparous lizard. Some of these effects have important fitness consequences by influencing reproductive success later in life. Thus, biasing the sex ratio toward one sex may lead to a decreased fitness for the other sex, and therefore constrain the evolution of skewed sex ratios. By incorporating effects of sex ratio on offspring fitness in a simple sex-allocation model, I show that, under some circumstances (1) skewed sex ratios are predicted to evolve, and (2) this cost can constrain the evolution of skewed sex ratios.

We examined the evolutionary response of wing area (a trait highly correlated with other measures of body size) to relative humidity (RH), temperature, and their interaction in Drosophila melanogaster, using replicated lines that had been allowed to evolve at low or high humidity at 18°C or at 25°C. We found that after 20 weeks of selection (5–10 generations), low RH lines had significantly greater wing areas than high RH lines in both sexes. This evolutionary response may have resulted from selection of larger flies with a smaller surface area for water loss relative to their weight, or as a correlated response to selection on some other unidentified trait. There were no evolutionary effects of temperature on wing area or cell density. This may have been due to the short duration of the selection experiment, and/or counteracting selection pressures on body size at warm temperature.