Prey capture rates of two species of salmonids (Salmo trutta and Thymallus thymallus) in an artificial stream: effects of temperature on their functional response

The foraging success of predators depends on how their consumption of prey is affected by prey density under different environmental settings. Here, we measured prey capture rates of drift-feeding juvenile brown trout and European grayling at different prey densities in an artificial stream channel at 5 and 11?°C. Capture rates were lower at 5 than at 11?°C, and the difference was most pronounced at high prey densities. At high prey densities, we also observed that European grayling had higher capture rates than brown trout. Type III functional response curves, i.e. sigmoidal relationships between capture rates and prey densities, fitted the data better than type I (linear) and II (hyperbolic) curves for all four combinations of temperatures and species. These results may explain the dominance of grayling in stream habitats with low water velocities and results such as these may be of use when developing foraging-based food web models of lotic ecosystems that include drift-feeding salmonids.     

Drift effects on the multivariate floral phenotype of Calceolaria polyrhiza during a post‐glacial expansion in Patagonia

Quaternary environmental changes substantially impacted the landscape and promoted rapid evolutionary changes in many species; however, analyses of adaptive phenotypic variation in plants have usually neglected the underlying historical context. Here, we associate phylogeography and phenotypic evolution by analysing the divergence of Calceolaria polyrhiza multivariate floral phenotype after a Pleistocene post‐glacial expansion in Patagonia. Phenotypic matrix ( P ) properties (size, shape, orientation and phenotypic integration) of six refugium and six recent populations from two different phylogroups were compared following different approaches. We found that P ‐matrix shape and orientation remained stable despite the strong phylogeographic footprint of post‐glacial expansion. However, average proportional reductions in matrix size supported the expectation that drift had a significant effect on the floral phenotype in the northern phylogroup. When phylogeographic history was not included in the analyses, the results overestimated phenotypic differences, whereas under explicit phylogeographic control, drift appeared as the best explanation for matrix differences. In general, recent populations showed a larger phenotypic divergence among them, but a lower overall phenotypic variation than refugium populations. Random Skewers analyses indicated a lower potential response to selection in recently colonized populations than in refugium populations. We discuss that the combination of phylogeographic analyses with geographical distribution of functional phenotypic (genotypic) variation is critical not only to understand how historical effects influence adaptive evolution, but also to improve field comparisons in evolutionary ecology studies.     

NICHE DIMENSIONALITY AND THE GENETICS OF ECOLOGICAL SPECIATION

Niche dimensionality is suggested to be a key determinant of ecological speciation (“multifarious selection” hypothesis), but genetic aspects of this process have not been investigated theoretically. We use Fisher's geometrical model to study how niche dimensionality influences the mean fitness of hybrids formed upon secondary contact between populations adapting in allopatry. Gaussian selection for an optimum generates two forms of reproductive isolation (RI): an extrinsic component due to maladaptation of the mean phenotype, and an intrinsic variance load resulting from what we term transgressive incompatibilities between mutations fixed in different populations. We show that after adaptation to a new environment, RI increases with (1) the mean initial maladaptation of diverging population, and (2) niche dimensionality, which increases the phenotypic variability of fixed mutations. Under mutation selection drift equilibrium in a constant environment, RI accumulates steadily with time, at a rate that also increases with niche dimensionality. A similar pattern can be produced by successive shifts in the optimum phenotype. Niche dimensionality thus has an effect per se on postzygotic isolation, beyond putative indirect effects (stronger selection, more genes). Our mechanism is consistent with empirical evidence about transgressive segregation in crosses between divergent populations, and with patterns of accumulation of RI with time in many taxa.     

SPEED OF ADAPTATION AND GENOMIC FOOTPRINTS OF HOST–PARASITE COEVOLUTION UNDER ARMS RACE AND TRENCH WARFARE DYNAMICS

Coevolution between hosts and their parasites is expected to follow a range of possible dynamics, the two extreme cases being called trench warfare (or Red Queen) and arms races. Long‐term stable polymorphism at the host and parasite coevolving loci is characteristic of trench warfare, and is expected to promote molecular signatures of balancing selection, while the recurrent allele fixation in arms races should generate selective sweeps. We compare these two scenarios using a finite size haploid gene‐for‐gene model that includes both mutation and genetic drift. We first show that trench warfare do not necessarily display larger numbers of coevolutionary cycles per unit of time than arms races. We subsequently perform coalescent simulations under these dynamics to generate sequences at both host and parasite loci. Genomic footprints of recurrent selective sweeps are often found, whereas trench warfare yield signatures of balancing selection only in parasite sequences, and only in a limited parameter space. Our results suggest that deterministic models of coevolution with infinite population sizes do not predict reliably the observed genomic signatures, and it may be best to study parasite rather than host populations to find genomic signatures of coevolution, such as selective sweeps or balancing selection.     

On luck and sex

Sex has many costs with respect to asexual reproduction, so its ubiquity is a puzzle. There has been a continuing effort to identify general circumstances in which aspects of sex generate an evolutionary advantage over asexual reproduction. Here we focus on the generality that individuals can experience good and bad "luck" at various stages of their life history regardless of genotype, and on the interindividual nature of sex. Sexual outcrossing combines genetic information from individuals with potentially different experiences, so it is conceivable that sex might reduce the contribution of individual luck to noise in inheritance. In a simple way, we derive expressions for noise in inheritance in terms of some sources of within-generation ecological noise. We demonstrate that interindividual reproduction can indeed dampen the effects of ecological noise better than lone-individual modes, but there are conditions under which it does not. Empirical and theoretical work on plants, modeled here, suggest noise dampening conditions. Ecological noise dampening operates alongside other features of sex such as recombination and segregation and, because noise in inheritance weakens the role of selection in genetic change, we speculate that noise dampening may offer a benefit to be deducted from the costs of sex. We also suggest that the amount of selfing relative to outcrossing observed in natural populations may be influenced by the amount of individual-level ecological noise in a given habitat.     

Microgeographical breeding structure of the tsetse fly, Glossina pallidipes in south-western Kenya

The origins of extant Glossina pallidipes Austen (Diptera: Glossinidae) populations in the ecologically well-studied Lambwe and Nguruman valleys in Kenya are controversial because populations have recovered after seemingly effective attempts to achieve high levels of control. The microgeographical breeding structure of the tsetse fly, G. pallidipes, was investigated by analysing spatial and temporal variation at eight microsatellite loci to test hypotheses about endemism and immigration. Samples were obtained at seasonal intervals from trap sites separated by 200 m to 14 km and arranged into blocks. G. pallidipes populations nearest to Lambwe and Nguruman also were sampled. Spatial analysis indicated that genetic differentiation by genetic drift was much less among trapping sites within Lambwe and Nguruman (F(ST) < or = 0.049) than between them (F(ST) = 0.232). F(ST) between Serengeti and Nguruman was 0.16 and F(ST) between Kodera Forest and Lambwe was 0.15. The genetic variance in G. pallidipes explained by dry and wet seasons (0.33%) was about one-fifth the variance among collection dates (1.6%), thereby indicating reasonable temporal stability of genetic variation. Gene frequencies in Kodera and Serengeti differed greatly from Lambwe and Nguruman, thereby falsifying the hypothesis that Lambwe and Nguruman were repopulated by immigrants. Harmonic mean effective (= breeding) population sizes were 180 in Lambwe and 551 in Nguruman. The genetic data suggest that G. pallidipes in Lambwe and Nguruman have been endemic for long intervals.     

Habitat geometry, population viscosity and the rate of genetic drift

In all natural populations, individuals located close to one another tend to interact more than those further apart. The extent of population viscosity can have important implications for ecological and evolutionary processes. Here we develop a spatially explicit population model to examine how the rate of genetic drift depends upon both spatial population structure and habitat geometry. The results show that the time to fixation for a new and selectively neutral mutation is dramatically increased in viscous populations. Furthermore, in viscous populations the time to fixation depends critically on habitat geometry. Fixation time for populations of identical size increases markedly as landscape width decreases and length increases. We suggest that similar effects will also be important in metapopulations, with the spatial arrangement of subpopulations and their connectivity likely to determine the rate of drift. We argue that the recent increases in computer power should facilitate major advances in our understanding of evolutionary landscape ecology over the next few years, and suggest that the time is ripe for a unification of spatial population dynamics theory, landscape ecology and population genetics.     

Clonal erosion and genetic drift in cyclical parthenogens – the interplay between neutral and selective processes

The occurrence of alternating phases of clonal and sexual reproduction may strongly impact the interplay between neutral and selective genetic variation in populations. Using a physiologically structured model of the life history of Daphnia, we investigated to what extent clonal erosion associated with selection during the clonal phase affects the genetic structure as observed by neutral markers. Incorporating conservative levels of quantitative genetic variation at 11 physiological and life history traits induces strong clonal erosion, reducing clonal diversity (CD) near the end of the simulations (1000 days) to a level between 1 and 5, even in habitats with high initial CD (10⁸ clones). This strong clonal erosion caused by selection can result in reduced genetic diversity, significant excess of heterozygotes and significant genetic differentiation between populations as observed by neutral markers. Our results indicate that, especially in relatively small habitats, clonal selection may strongly impact the genetic structure and may contribute to the often observed high level of neutral genetic differentiation among natural populations of cyclical parthenogens.