Neuronal circuits and evolution.
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Adaptation, Biological↗
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The evolution of mutation rates.
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Adaptation, Biological↗
Meanderings in nature, our world, its creation and evolution--a personal interpretation.
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Adaptation, Biological↗
Cross-generational environmental effects and the evolution of offspring size in the Trinidadian guppy Poecilia reticulata.
The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort.
Animals↗
Mitochondrial mutations may drive Y chromosome evolution.
The human Y chromosome contains very low levels of nucleotide variation. It has been variously hypothesized that this invariance reflects historic reductions in the human male population, a very recent common ancestry, a slow rate of molecular evolution, an inability to evolve adaptively, or frequent selective sweeps acting on genes borne on the Y chromosome. We propose an alternative theory in which human Y chromosome evolution is driven by mutations in the maternally inherited mitochondrial genome, which impair male fertility and ultimately lead to a reduction in the effective population size (N(e)) and consequently the variability of the Y chromosome.
Animals↗
A role for phenotypic plasticity in the evolution of aposematism.
The evolution of warning coloration (aposematism) has been difficult to explain because rare conspicuous mutants should suffer a higher cost of discovery by predators relative to the cryptic majority, while at frequencies too low to facilitate predator aversion learning. Traditional models for the evolution of aposematism have assumed conspicuous prey phenotypes to be genetically determined and constitutive. By contrast, we have recently come to understand that warning coloration can be environmentally determined and mediated by local prey density, thereby reducing the initial costs of conspicuousness. The expression of density-dependent colour polyphenism is widespread among the insects and may provide an alternative pathway for the evolution of constitutive aposematic phenotypes in unpalatable prey by providing a protected intermediate stage. If density-dependent aposematism can function as an adaptive intermediate stage for the evolution of constitutive aposematic phenotypes, differential reaction norm evolution is predicted among related palatable and unpalatable prey populations. Here, I present empirical evidence that indicates that (i) the expression of density-dependent colour polyphenism has differentially evolved between palatable and unpalatable populations of the grasshopper Schistocerca emarginata (= lineata) (Orthoptera: Acrididae), and (ii) variation in plasticity between these populations is commensurate with the expected costs of conspicuousness.
Animals↗
Photosynthetic and respiratory rates of two psychrophilic diatoms.
The photosynthetic rates in two psychrophilic diatoms, Chaetoceros sp. strain K3-10 and Nitzschia sp. K3-3 for cells grown at 0 degrees C were 8 to 10 microliters O(2) evolved per milligram dry weight per hour, and 10-fold higher, about 80 for cells grown at 10 degrees C. The respiration rates followed the same pattern, with a value of around 1 microliter dark uptake per milligram dry weight per hour for both organisms grown at 0 degrees C, and 6 to 10 for cells grown at 10 degrees C. When cells grown at 0 degrees C were immediately shifted to 10 degrees C or cells grown at 10 degrees C were shifted to 0 degrees C, the respiratory rates quickly adapted to values characteristic of cells grown at the shift temperature. On the other hand, the light-saturated rate of O(2) evolution showed much less immediate adaptation, especially on the up shift, 0 degrees to 10 degrees C. The chlorophyll a content of 0 degrees C grown cells was about 0.5% of dry weight, in 10 degrees C grown cells 1.3% (strain K3-10) and 2.2% (strain K3-3). In addition to a diminished chlorophyll a content in 0 degrees C grown cells, there seemed proportionally (by absorbance and calculation) less c to a than in 10 degrees C grown cells. The relative fluorescence excitation spectra of 680-nm emission also showed a lower contribution by both chlorophyll c and fucoxanthin in 0 degrees C grown cells of Chaetoceros sp. strain K3-10 as compared to 10 degrees C grown cells. The data at hand suggest that in psychrophilic diatoms continuously growing at 0 degrees C there may be problems associated with synthesis of an effective accessory pigment system, and as a working hypothesis it is suggested this is related to restriction of synthesis of one or several accessory pigment proteins.
Journal Article↗
Impact of founder population, drift and selection on the genetic diversity of a recently translocated tree population.
Recently established, temperate tree populations combine a high level of differentiation for adaptive traits, suggesting rapid genetic evolution, with a high level of genetic diversity within population, suggesting a limited impact of genetic drift and purifying selection. To study experimentally the evolutionary forces in a recently established population, we assessed the spatial and temporal patterns of genetic diversity within a disjunct population of Cedrus atlantica established 140 years ago in south-eastern France from a North African source. The population is expanding through natural regeneration. Three generations were sampled, including founder trees. We analysed 12 isozyme loci, three of which were previously found in tight association with selected genes, and quantitative traits. No bottleneck effect was detected in the founder generation, but a simple test of allelic association revealed an initial disequilibrium which disappeared in the following generations. The impact of genetic drift during secondary evolution was limited, as suggested by the weak temporal differentiation. The genetic load was not reduced after 3 generations, and the quantitative variation for adaptive traits did not change either. Thus, initial genetic changes first proceed from a rapid re-organisation of the diversity through mating and recombination, whereas genetic erosion through drift and selection is delayed due to temporal and spatial stochasticity. Two life-history traits of trees contribute to slowing down the processes of genetic erosion: perenniality and large spatial scale. Thus, one would expect recently established tree populations to have a higher diversity than older ones, which seems in accordance with experimental surveys.
Biological Evolution↗
Evolution of the interleukins.
As the adaptive immune system arose in jawed vertebrates, a reasonable working hypothesis is that cytokines involved exclusively in controlling the adaptive immune system, e.g. T1 and T2 cytokines, will only be found in jawed vertebrates whilst those with a role in innate responses, e.g. type I IFNs or pro-inflammatory cytokines, will be universal, or have functional equivalents in 'lower' animals. Progress to date in cloning cytokines, including interleukins, in vertebrates outside of mammals is limited and non-existent in invertebrates. T1 and T2 interleukins have only been cloned in birds. Receptors for T1 and T2 interleukins have been cloned in fish, however, suggesting the presence of the corresponding interleukins. Pro-inflammatory interleukins have been cloned in birds, fish and amphibia, but not reptiles. Cross-reactive bioassays, polyclonal antisera and mAbs suggest that an IL-6-like factor exists in starfish, as yet the only evidence for innate immune response cytokines in 'lower' animals.
Amino Acid Sequence↗
Frequent false detection of positive selection by the likelihood method with branch-site models.
Positive Darwinian selection promotes fixations of advantageous mutations during gene evolution and is probably responsible for most adaptations. Detecting positive selection at the DNA sequence level is of substantial interest because such information provides significant insights into possible functional alterations during gene evolution as well as important nucleotide substitutions involved in adaptation. Efficient detection of positive selection, however, has been difficult because selection often operates on only a few sites in a short period of evolutionary time. A likelihood-based method with branch-site models was recently introduced to overcome such difficulties. Here I examine the accuracy of the method using computer simulation. I find that the method detects positive selection in 20%-70% of cases when the DNA sequences are generated by computer simulation under no positive selection. Although the frequency of such false detection varies depending on, among other things, the tree topology, branch length, and selection scheme, the branch-site likelihood method generally gives misleading results. Thus, detection of positive selection by this method alone is unreliable. This unreliability may have resulted from its over-sensitivity to violations of assumptions made in the method, such as certain distributions of selective strength among sites and equal transition/transversion ratios for synonymous and nonsynonymous substitutions.
Adaptation, Biological↗
Distinct types of selection and genetic architecture shape molecular variation during the domestication of vegetable crops.
Humans select vegetable crops with desirable traits via a complex evolutionary process called domestication, generating a variety of cultivars worldwide. With advances in sequencing technologies, genomic scans for "signatures of selection" are widely used to identify target loci of selection. In the early phases of domestication, humans tended to favor similar sets of phenotypes in diverse crops, resulting in "domestication syndrome" and parallel evolution in multiple species. Subsequently, adaptation to distinct environments or different consumer preferences has diversified crop cultivars. Here, we review molecular and population genetic studies on genes affecting trait evolution during this complex process. We emphasize that, depending on interactions among different types of selection (directional selection within or divergent selection between groups), the genetic architecture of the target trait (Mendelian or polygenic), and the origin of the causal variant (new mutation or standing variation), the resulting molecular patterns of variation can be highly diverse. Situations in which the typical hard selective sweep model could be applied may be limited. Therefore, it is crucial to obtain a thorough understanding of the target species' historical, environmental, and ecological contexts.
Domestication↗
Minor quantitative trait loci underlie floral traits associated with mating system divergence in Mimulus.
The genetic basis of species differences provides insight into the mode and tempo of phenotypic divergence. We investigate the genetic basis of floral differences between two closely related plant taxa with highly divergent mating systems, Mimulus guttatus (large-flowered outcrosser) and M. nasutus (small-flowered selfer). We had previously constructed a framework genetic linkage map of the hybrid genome containing 174 markers spanning approximately 1800 cM on 14 linkage groups. In this study, we analyze the genetics of 16 floral, reproductive, and vegetative characters measured in a large segregating M. nasutus x M. guttatus F2 population (N = 526) and in replicates of the parental lines and F1 hybrids. Phenotypic analyses reveal strong genetic correlations among floral traits and epistatic breakdown of male and female fertility traits in the F2 hybrids. We use multitrait composite interval mapping to jointly locate and characterize quantitative trait loci (QTLs) underlying interspecific differences in seven floral traits. We identified 24 floral QTLs, most of which affected multiple traits. The large number of QTLs affecting each trait (mean = 13, range = 11-15) indicates a strikingly polygenic basis for floral divergence in this system. In general, QTL effects are small relative to both interspecific differences and environmental variation within genotypes, ruling out QTLs of major effect as contributors to floral divergence between M. guttatus and M. nasutus. QTLs show no pattern of directional dominance. Floral characters associated with pollinator attraction (corolla width) and self-pollen deposition (stigma-anther distance) share several pleiotropic or linked QTLs, but unshared QTLs may have allowed selfing to evolve independently from flower size. We discuss the polygenic nature of divergence between M. nasutus and M. guttatus in light of theoretical work on the evolution of selfing, genetics of adaptation, and maintenance of variation within populations.
Biological Evolution↗
Molecular evolution of the vertebrate immune system.
Adaptive immunity is unique to the vertebrates, and the molecules involved (including immunoglobulins, T cell receptors and the major histocompatibility complex molecules) seem to have diversified very rapidly early in vertebrate history. Reconstruction of gene phylogenies has yielded insights into the evolutionary origin of a number of molecular systems, including the complement system and the major histocompatibility complex (MHC). These analyses have indicated that the C5 component of complement arose by gene duplication prior to the divergence of C3 and C4, which suggests that the alternative complement pathway was the first to evolve. In the case of the MHC, phylogenetic analysis supports the hypothesis that MHC class II molecules evolved before class I molecules. The fact that the MHC-linked proteasome components that specifically produce peptides for presentation by class I MHC appear to have originated before the separation of jawed and jawless vertebrates suggests that the MHC itself may have been present at this time. Immune system gene families have evolved by gene duplication, interlocus recombination and (in some cases) positive Darwinian selection favoring diversity at the amino acid level.
Animals↗
Peripheral variability and central constancy in mammalian visual system evolution.
Neural systems are necessarily the adaptive products of natural selection, but a neural system, dedicated to any particular function in a complex brain, may be composed of components that covary with functionally unrelated systems, owing to constraints beyond immediate functional requirements. Some studies support a modular or mosaic organization of the brain, whereas others emphasize coordination and covariation. To contrast these views, we have analysed the retina, striate cortex (V1) and extrastriate cortex (V2, V3, MT, etc.) in 30 mammals, examining the area of the neocortex and individual neocortical areas and the relative numbers of rods and cones. Controlling for brain size and species relatedness, the sizes of visual cortical areas (striate, extrastriate) within the brains of nocturnal and diurnal mammals are not statistically different from one another. The relative sizes of all cortical areas, visual, somatosensory and auditory, are best predicted by the total size of the neocortex. In the sensory periphery, the retina is clearly specialized for niche. New data on rod and cone numbers in various New World primates confirm that rod and cone complements of the retina vary substantially between nocturnal and diurnal species. Although peripheral specializations or receptor surfaces may be highly susceptible to niche-specific selection pressures, the areal divisions of the cerebral cortex are considerably more conservative.
Animals↗
A co-evolution perspective of the TNFSF and TNFRSF families in the immune system.
Within the 'success story' of vertebrate evolution, the acquisition and refinement of the adaptive immune system is far from the least impressive example of co-evolution. Members of the tumour necrosis factor superfamily (TNFSF) and TNF receptor superfamily (TNFRSF) have crucial roles in both innate and adaptive immunity. Here, we propose a detailed description of the phylogenetic relations of the TNFSF and TNFRSF members and offer evidence that the divergence of the TNFSF and TNFRSF families paralleled the emergence of the adaptive immune system, at least partly through en bloc duplication. Unexpectedly, TNFSF subfamilies form monophyletic groups with shared functions, including TNFRSF usage. Finally, the mechanisms of (co-)evolution of TNFSF and TNFRSF are discussed.
Animals↗
Remarkably high rate of molecular evolution of ruminant placental lactogens.
The sequences of ovine and bovine placental lactogens (based on published cDNA sequences) are remarkably different, indicating a very rapid rate of evolution. Analysis of the cDNA sequences indicates that the rate of nonsynonymous substitution in these proteins is considerably greater than the rate of synonymous substitution. This is an unusual situation, which suggests that the observed rapid rate of evolution is due to incorporation of adaptive rather than neutral mutations.
Animals↗
Developmental dynamics and contemporary evolutionary psychology: status quo or irreconcilable views? Reply to Bjorklund (2003), Krebs (2003), Buss and Reeve (2003), Crawford (2003), and Tooby et Al. (2003).
The authors address commentaries by D. F. Bjorklund (2003); D. M. Buss and H. K. Reeve (2003); C. B. Crawford (2003); D. L. Krebs (2003); and J. Tooby, L. Cosmides, and H. C. Barrett (2003) on their analysis of the underlying assumptions of contemporary evolutionary psychology (R. Lickliter & H. Honeycutt, 2003). The authors argue that evolutionary psychology currently offers no coherent framework for how to integrate genetic, environmental, and experiential factors into a theory of behavioral or cognitive phenotypes. The authors propose that this absence is due to a lack of developmental analysis in the major works of evolutionary psychology, resulting in an almost exclusive focus on adaptationist accounts of evolution by natural selection rather than a more broad-based focus on the process and products of evolution by epigenetic developmental dynamics.
Adaptation, Physiological↗
Deterministic and stochastic regimes of asexual evolution on rugged fitness landscapes.
We study the adaptation dynamics of an initially maladapted asexual population with genotypes represented by binary sequences of length L. The population evolves in a maximally rugged fitness landscape with a large number of local optima. We find that whether the evolutionary trajectory is deterministic or stochastic depends on the effective mutational distance d(eff) up to which the population can spread in genotype space. For d(eff) = L, the deterministic quasi-species theory operates while for d(eff) < 1, the evolution is completely stochastic. Between these two limiting cases, the dynamics are described by a local quasi-species theory below a crossover time T(x) while above T(x) the population gets trapped at a local fitness peak and manages to find a better peak via either stochastic tunneling or double mutations. In the stochastic regime d(eff) < 1, we identify two subregimes associated with clonal interference and uphill adaptive walks, respectively. We argue that our findings are relevant to the interpretation of evolution experiments with microbial populations.
Adaptation, Biological↗