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- Oxidative damage increases with reproductive energy expenditure and is reduced by food-supplementation. [Journal Article]
- Evolution 2013 May; 67(5):1527-36.
A central principle in life-history theory is that reproductive effort negatively affects survival. Costs of reproduction are thought to be physiologically based, but the underlying mechanisms remain poorly understood. Using female North American red squirrels (Tamiasciurus hudsonicus), we test the hypothesis that energetic investment in reproduction overwhelms investment in antioxidant protection, leading to oxidative damage. In support of this hypothesis we found that the highest levels of plasma protein oxidative damage in squirrels occurred during the energetically demanding period of lactation. Moreover, plasma protein oxidative damage was also elevated in squirrels that expended the most energy and had the lowest antioxidant protection. Finally, we found that squirrels that were food-supplemented during lactation and winter had increased antioxidant protection and reduced plasma protein oxidative damage providing the first experimental evidence in the wild that access to abundant resources can reduce this physiological cost.
- Mating system, haldane's sieve, and the domestication process. [Journal Article]
- Evolution 2013 May; 67(5):1518-26.
Mating systems are expected to have a strong influence on both the dynamic of adaptation and the genetic architecture of adaptive traits. In particular, the bias toward the fixation of dominant or partially dominant beneficial mutations predicted under outcrossing (Haldane's sieve) is expected to be reduced under self-fertilization. To test this prediction in plants, we considered domestication as an example of adaptation. We compiled data from studies reporting the degree of dominance of quantitative trait loci (QTL) involved in the domestication syndrome. We found that adaptation to cultivation mostly proceeded through the selection of recessive and partially recessive genes in predominantly selfing species whereas a much larger fraction of domestication-related QTL were dominant or partially dominant in outcrossers, as expected under Haldane's sieve. Our study also showed that levels of dominance in mixed mating crop species resemble those observed in selfers, suggesting that recessive alleles can contribute to adaptation even under moderate selfing rates. Although these results rely on a particular example of adaptation, they constitute one of the first attempts to test theoretical expectations on the level of dominance of genes involved in plant adaptation.
- Sensitivity to phosphorus limitation increases with ploidy level in a new zealand snail. [Journal Article]
- Evolution 2013 May; 67(5):1511-7.
Evolutionary and ecological factors that explain natural variation in ploidy level remain poorly understood. One intriguing possibility is that nutrient costs associated with higher per-cell nucleic acid content could differentially influence the fitness of different ploidy levels. Here, we test this hypothesis by determining whether access to phosphorus (P), a main component of nucleic acids, differentially affects growth rate in asexual freshwater snails (Potamopyrgus antipodarum) that differ in ploidy. As expected if larger genomes generate higher dietary P requirements, tetraploid P. antipodarum experienced a more than twofold greater reduction in growth rate in low-P versus high-P conditions relative to triploids. Mirroring these results, tetraploid P. antipodarum also had a significant reduction in body P content under low P relative to high P, whereas triploid body P content was unaffected. Taken together, these results set the stage for the possibility that P availability could influence the distribution and relative frequency of P. antipodarum of different ploidy levels. These findings could be applicable to many other animal taxa featuring ploidy-level variation, which includes many mixed sexual/asexual taxa.
- Genetic drift in antagonistic genes leads to divergence in sex-specific fitness between experimental populations of Drosophila melanogaster. [Journal Article]
- Evolution 2013 May; 67(5):1503-10.
Males and females differ in their reproductive roles and as a consequence are often under diverging selection pressures on shared phenotypic traits. Theory predicts that divergent selection can favor the invasion of sexually antagonistic alleles, which increase the fitness of one sex at the detriment of the other. Sexual antagonism can be subsequently resolved through the evolution of sex-specific gene expression, allowing the sexes to diverge phenotypically. Although sexual dimorphism is very common, recent evidence also shows that antagonistic genetic variation continues to segregate in populations of many organisms. Here we present empirical data on the interaction between sexual antagonism and genetic drift in populations that have independently evolved under standardized conditions. We demonstrate that small experimental populations of Drosophila melanogaster have diverged in male and female fitness, with some populations showing high male, but low female fitness while other populations show the reverse pattern. The between-population patterns are consistent with the differentiation in reproductive fitness being driven by genetic drift in sexually antagonistic alleles. We discuss the implications of our results with respect to the maintenance of antagonistic variation in subdivided populations and consider the wider implications of drift in fitness-related genes.
- Ecological versus phylogenetic determinants of trophic associations in a plant-leafminer-parasitoid food web. [Journal Article]
- Evolution 2013 May; 67(5):1493-502.
Specialized trophic interactions in plant-herbivore-parasitoid food webs can spur "bottom-up" diversification if speciation in plants leads to host-shift driven divergence in insect herbivores, and if the effect then cascades up to the third trophic level. Conversely, parasitoids that search for victims on certain plant taxa may trigger "top-down" diversification by pushing herbivores into "enemy-free space" on novel hosts. We used phylogenetic regression methods to compare the relative importance of ecology versus phylogeny on associations between Heterarthrinae leafmining sawflies and their parasitoids. We found that: (1) the origin of leafmining led to escape from most parasitoids attacking external-feeding sawflies; (2) the current enemies mainly consist of generalists that are shared with other leafmining taxa, and of more specialized lineages that may have diversified by shifting among heterarthrines; and (3) parasitoid-leafminer associations are influenced more by the phylogeny of the miners' host plants than by relationships among miner species. Our results suggest that vertical diversifying forces have a significant-but not ubiquitous-role in speciation: many of the parasitoids have remained polyphagous despite niche diversification in the miners, and heterarthrine host shifts also seem to be strongly affected by host availability.
- Comparative analysis of animal growth: a primate continuum revealed by a new dimensionless growth rate coefficient. [Journal Article]
- Evolution 2013 May; 67(5):1485-92.
The comparative analysis of animal growth still awaits full integration into life-history studies, partially due to the difficulty of defining a comparable measure of growth rate across species. Using growth data from 50 primate species, we introduce a modified "general growth model" and a dimensionless growth rate coefficient β that controls for size scaling and phylogenetic effects in the distribution of growth rates. Our results contradict the prevailing idea that slow growth characterizes primates as a group: the observed range of β values shows that not all primates grow slowly, with galago species exhibiting growth rates similar or above the mammalian average, while other strepsirrhines and most New World monkeys show limited reduction in growth rates. Low growth rate characterizes apes and some papionines. Phylogenetic regressions reveal associations between β and life-history variables, providing tests for theories of primate growth evolution. We also show that primate slow growth is an exclusively postnatal phenomenon. Our study exemplifies how the dimensionless approach promotes the integration of growth rate data into comparative life-history analysis, and demonstrates its potential applicability to other cases of adaptive diversification of animal growth patterns.
- Experimental confirmation that body size determines mate preference via phenotype matching in a stickleback species pair. [Journal Article]
- Evolution 2013 May; 67(5):1477-84.
Mate choice by phenotype matching, whereby individuals prefer a mate whose phenotype is similar to their own, should facilitate speciation with gene flow. This is because the genes that control mate signal (the phenotype being matched) also determine the preferred mate signal ("mate preference"). Speciation is made even easier if phenotype matching is based on a trait under divergent natural selection. In this case, assortative mating should readily evolve as a byproduct of divergent selection on the trait. Previous observational studies of assortative mating between sympatric, hybridizing threespine stickleback species (Gasterosteus aculeatus complex) suggested that phenotype matching might occur by body size, a trait under divergent natural selection. To test this, we used experimental manipulation of body size to rule out the effects of confounding variables. We found that size-manipulated benthic and limnetic stickleback females prefer mates whose body size more closely matches their own. It is thus likely that assortative mating by phenotype matching has facilitated the origin and persistence of benthic and limnetic threespine sticklebacks in the face of gene flow.
- How does evolutionary variation in Basal metabolic rates arise? A statistical assessment and a mechanistic model. [Journal Article]
- Evolution 2013 May; 67(5):1463-76.
Metabolic rates are related to the pace of life. Hence, research into their variability at global scales is of vital importance for several contemporary theories in physiology, ecology, and evolution. Here we evaluated the effect of latitude, climate, primary productivity, habitat aridity, and species trophic habits, on mass-independent basal metabolic rates (BMRs) for 195 rodent species. The aims of this article were twofold. First, we evaluated the predictive power of different statistical models (via a model selection approach), using a dimensional reduction technique on the exogenous factor matrix to achieve a clear interpretation of the selected models. Second, we evaluated three specific predictions derived from a recently proposed hypothesis, herein called the "obligatory heat" model (OHM), for the evolution of BMR. Obtained results indicate that mean/minimum environmental temperature, rainfall/primary productivity and, finally, species trophic habits are, in this order, the major determinants of mass-independent BMR. Concerning the mechanistic causes behind this variation, obtained data agree with the predictions of the OHM: (1) mean annual environmental temperature was the best single predictor of residual variation in BMR, (2) herbivorous species have greater mass-independent metabolic rates, and tend to be present at high-latitude cold environments, than species in other trophic categories.
- Understanding the basis of diminished gene flow between hybridizing chromosome races of the house mouse. [Journal Article]
- Evolution 2013 May; 67(5):1446-62.
Speciation may be promoted in hybrid zones if there is an interruption to gene flow between the hybridizing forms. For hybridizing chromosome races of the house mouse in Valtellina (Italy), distinguished by whole-arm chromosomal rearrangements, previous studies have shown that there is greater interruption to gene flow at the centromeres of chromosomes that differ between the races than at distal regions of the same chromosome or at the centromeres of other chromosomes. Here, by increasing the number of markers along race-specific chromosomes, we reveal a decay in between-race genetic differentiation from the centromere to the distal telomere. For the first time, we use simulation models to investigate the possible role of recombination suppression and hybrid breakdown in generating this pattern. We also consider epistasis and selective sweeps as explanations for isolated chromosomal regions away from the centromere showing differentiation between the races. Hybrid breakdown alone is the simplest explanation for the decay in genetic differentiation with distance from the centromere. Robertsonian fusions/whole-arm reciprocal translocations are common chromosomal rearrangements characterizing both closely related species and races within species, and this fine-scale empirical analysis suggests that the unfitness associated with these rearrangements in the heterozygous state may contribute to the speciation process.
- On optimal learning schedules and the marginal value of cumulative cultural evolution. [Journal Article]
- Evolution 2013 May; 67(5):1435-45.
The age-dependent choice between expressing individual learning (IL) or social learning (SL) affects cumulative cultural evolution. A learning schedule in which SL precedes IL is supportive of cumulative culture because the amount of nongenetically encoded adaptive information acquired by previous generations can be absorbed by an individual and augmented. Devoting time and energy to learning, however, reduces the resources available for other life-history components. Learning schedules and life history thus coevolve. Here, we analyze a model where individuals may have up to three distinct life stages: "infants" using IL or oblique SL, "juveniles" implementing IL or horizontal SL, and adults obtaining material resources with learned information. We study the dynamic allocation of IL and SL within life stages and how this coevolves with the length of the learning stages. Although no learning may be evolutionary stable, we find conditions where cumulative cultural evolution can be selected for. In that case, the evolutionary stable learning schedule causes individuals to use oblique SL during infancy and a mixture between IL and horizontal SL when juvenile. We also find that the selected pattern of oblique SL increases the amount of information in the population, but horizontal SL does not do so.