Correlational selection and genetic architecture promote the leaf economics spectrum in a perennial grass

The leaf economics spectrum (LES) is hypothesized to result from a trade-off between resource acquisition and conservation. Yet few studies have examined the evolutionary mechanisms behind the LES, perhaps because most species exhibit relatively specialized leaf economics strategies. In a genetic mapping population of the phenotypically diverse grass Panicum virgatum, we evaluate two interacting mechanisms that may drive LES evolution: 1) genetic architecture, where multiple traits are coded by the same gene (pleiotropy) or by genes in close physical proximity (linkage), and 2) correlational selection, where selection acts non-additively on combinations of multiple traits. We found evidence suggesting that shared genetic architecture (pleiotropy) controls covariation between two pairs of leaf economics traits. Additionally, at five common gardens spanning 17 degrees of latitude, correlational selection favored particular combinations of leaf economics traits. Together, these results demonstrate how the LES can evolve within species.

Selection for two different mating systems in Aquilegia canadensis

Because plants are sessile, they depend on biotic and/or abiotic vectors to transfer pollen from the male pollen-producing anthers to the female pollen-receiving stigmas. As a result, plant mating systems evolve through selection on the floral traits that influence how much pollen is transferred from anthers to stigmas within flowers (self-pollination) vs. between flowers on different individuals (outcrossing). Thus, mating systems are influenced by the traits that dictate the relative abundance of self-versus outcrossed pollen on stigmas. Spatial separation between anthers and stigmas within flowers (herkogamy) is expected to regulate self-pollination yet there are few estimates of how natural selectin acts on this trait. Aquilegia canadensis (columbine, Ranunculaceae) is a short-lived herbaceous plant of rocky outcrops throughout eastern North America that makes seed through both self-fertilization which is influenced by herkogamy, and outcrossing, which is likely influenced by the plant’s floral display size (flower number and size). Selfing provides reproductive assurance in natural populations of columbine, whereas outcrossing appears to produce much fitter offspring, and there is a trade-off between thes two components of the mating system. We, therefore, predicted correlational selection between herkogamy and display size: selection would favour reduced herkogamy among individuals with small floral displays (to enhance reproductive assurance) and increased herkogamy among individuals with large floral displays (to reduce selfing when outcrossing is likely). We tested this prediction by using multivariate linear regression to estimate phenotypic selection through seeds/fruit and seeds/plant on floral traits and plant size for 1015 plants from nine populations of A. canadensis at the Queen’s University Biological Station. Although we detected positive direction selection on display size mostly through flower number, we did not detect selection on herkogamy or correlational selection between herkogamy and display size. As expected, large size is universally favoured yet selection of floral morphology is weak.

Genetic mechanisms and correlational selection structure trait variation in a coral snake mimic

Covariation among traits shapes both phenotypic evolution and ecological interactions across space and time. However, rampant geographical variation in the strength and direction of such correlations can be particularly difficult to explain through generalized mechanisms. By integrating population genomics, surveys of natural history collections and spatially explicit analyses, we tested multiple drivers of trait correlations in a coral snake mimic that exhibits remarkable polymorphism in mimetic and non-mimetic colour traits. We found that although such traits co-occur extensively across space, correlations were best explained by a mixture of genetic architecture and correlational selection, rather than by any single mechanism. Our findings suggest that spatially complex trait distributions may be driven more by the simple interaction between multiple processes than by complex variation in one mechanism alone. These interactions are particularly important in mimicry systems, which frequently generate striking geographical variation and genetic correlations among colour pattern traits.

Phenotypic Selection on Flower Color and Floral Display Size by Three Bee Species

Plants exhibit a wide array of floral forms and pollinators can act as agent of selection on floral traits. Two trends have emerged from recent reviews of pollinator-mediated selection in plants. First, pollinator-mediated selection on plant-level attractants such as floral display size is stronger than on flower-level attractant such as flower color. Second, when comparing plant species, distinct pollinators can exert different selection patterns on floral traits. In addition, many plant species are visited by a diverse array of pollinators but very few studies have examined selection by distinct pollinators. In the current study, we examined phenotypic selection on flower color and floral display size by three distinct bee species, the European honey bee, Apis mellifera, the common eastern bumble bee, Bombus impatiens, and the alfalfa leafcutting bee, Megachile rotundata, foraging on Medicago sativa. To estimate phenotypic selection by each bee species and for all bees combined simultaneously and on the same group of plants, we introduce a new method that combines pollinator visitation data to seed set and floral trait measurements data typical of phenotypic selection study. When comparing floral traits, all bee species selected on the number of racemes per stem and the number of stems per plant, two components of floral display size. However, only leafcutting bees selected on hue or flower color and only bumble bees selected on chroma or darkness of flowers. Selection on chroma occurred via correlational selection between chroma and number of open flowers per raceme and we examine how correlational selection may facilitate the evolution of flower color in plant populations. When comparing bee species, the three bee species exerted similar selection pattern on some floral traits but different patterns on other floral traits and differences in selection patterns were observed between flower-level and plant-level attractants. The trends detected were consistent with previous studies and we advocate the approach introduced here for future studies examining the impact of distinct pollinators on floral trait evolution.

Foraging behavior and extended phenotype independently affect foraging success in spiders

Multiple phenotypic traits often interact with each other to determine an individual’s fitness. Behavioral and extended phenotypic traits, such as architectural constructions, can contribute to fitness in an integrated way. The goal of this study was to understand how the interaction between behavioral and extended phenotypic traits can affect foraging success. We tested this question using black widow spiders, where spiders that are aggressive in a foraging context tend to build more gumfooted silk lines that aid in prey capture, while non-aggressive spiders build webs with fewer gumfooted lines. We repeatedly assessed behavior and web structure to quantify relationships between these traits, and then allowed spiders to forage for live prey on their own web or the web of a conspecific that differed in structure. Thus, we assessed how varying combinations of behavior and web structure affect foraging success, and if correlational selection might act on them. We confirmed that aggressiveness and number of gumfooted lines are positively correlated and found that capture success increased with both aggressiveness and the number of gumfooted lines. Yet, we did not find any evidence for correlational selection: aggressiveness and number of gumfooted lines appeared to affect foraging success independently of each other. These findings highlight that a correlation between traits that contribute towards the same ecological function does not necessarily imply correlational selection. Taking advantage of the experimental convenience afforded by extended phenotypic traits can provide insight into the functional consequences of phenotypic variation within and between individuals.

Pollinator-dependent evolution of floral trait combinations in an orchid herb

Aims As one of the most important agents driving floral evolution, pollinators shape the diversity of flowers in angiosperms. However, most previous studies have only quantified pollinators driving the evolution of a single floral trait, and experimental estimates of the potential role of pollinators in shaping the evolution of floral trait associations are relatively rare. Methods We experimentally identified and estimated the pollinator-mediated directional and correlational selection on single floral traits and trait combinations across 2 years in an orchid species, Spiranthes sinensis. Important Findings Pollinators mediated directional selection for an earlier flowering start date and larger corolla size. Pollinators mediated positive correlational selection on the combinations of floral display traits and negative correlational selection on the combinations of flowering phenology and floral display traits. In addition, the strength of selection differed over time. Our results highlight the potential role of pollinators in driving the evolution of floral trait combinations and suggest that it is necessary to consider floral character functional associations when seeking to understand and predict the evolutionary trajectory of flowers in angiosperms.

Bite performance surfaces of three ecologically divergent Iguanidae lizards: relationships with lower jaw bones

Traits that interact to perform an ecologically relevant function are expected to be under multivariate non-linear selection. Using the lower jaw morphology as a biomechanical model, we test the hypothesis that lower jaw bones of lizards are subjected to stabilizing and correlational selection, associated with mechanical advantage and maximum bite force. We used three closely related tropidurine species that differ in size, head shape and microhabitat: Eurolophosaurus nanuzae, Tropidurus hispidus and Tropidurus semitaeniatus. We predicted a common pattern of correlational selection on bones that are part of in-levers or part of the out-lever of the lower jaw. The predicted pattern was found in E. nanuzae and T. hispidus, but this could not be shown to be statistically significant. For T. semitaeniatus, we found significant disruptive selection on a contrast involving the surangular, and also significant directional selection on linear combinations of traits in all species. The results indicate that the non-linear selection on lower jaw bones does not reflect an optimum to enhance mechanical advantage in all species. Divergent functional demands and specific ecological contexts of species seem relevant in shaping patterns of selection on morphology.