Evolutionary Ecology

Intraspecific floral color variation as perceived by pollinators and non-pollinators: evidence for pollinator-imposed constraints? Abstract Pollinator-mediated selection is expected to constrain floral color variation within plant populations, yet populations with high color variation (at least in human visual space) are common in nature. To explore this paradox, we collected floral reflectance spectra for 34 populations of 14 putatively bee-pollinated plant species of north-central New Mexico, USA, and translated them into three different animal visual spaces. We asked, (1) is intrapopulation variation in flower color constrained to be lower than the discrimination threshold of the putative dominant pollinators? And, (2) is perceived intrapopulation variation in flowers higher for non-pollinating animals than it is for the presumed dominant pollinator group? We found evidence consistent with some pollinator-imposed constraints on floral color in our species, with the majority (70.6%) of populations having > 95% of flower–flower or flower-centroid comparisons (where the centroid represents the mean phenotype) estimated to be visually indiscriminable to bees. We also found that perceived floral color variation was significantly greater for two non-pollinating groups—birds and humans—than for bees. Our results suggest that a large portion of human-perceived floral color variation within populations persists because it is effectively invisible to pollinators. In this scenario, human-perceived color may evolve neutrally (via drift) or via indirect selection on correlated characters such as herbivore- or drought-resistance, consistent with previous studies identifying non-pollinator agents of selection on flower color.

Rapid changes in dispersal on a small spatial scale at the range edge of an expanding population Abstract In expanding populations, the allocation of resources to life-history traits is expected to change rapidly after the colonization of a new area. Understanding these changes is of crucial importance to predict the future changes in distribution ranges, and the possible impacts of expanding species on the colonized environments. Both theoretical and empirical studies have provided some evidence for an increased dispersal at the range edge of expanding populations. Because this phenomenon has been documented on large spatial scales, it seems difficult to determine what proportion of the phenotypic variation is due to the expansion process (spatial sorting) versus spatial variation in the environment (local adaptation). The aim of our study is to determine whether, in a small and recent expanding population, the dispersal capacity is increased at the range edge compared to the range core despite a minimal role of local adaptation. We studied the modification of dispersal capacity at the range edge of an expanding population of the African clawed frog, Xenopuslaevis recently settled in Western France. To do so, we used experiments to compare movements of individuals depending on their location in the population, and capture–mark–recapture field surveys to estimate variation in traits related to dispersal across the invasive range. Both methods consistently showed significantly higher dispersal rates and distances, as well as in the relative length of the hind limb, at the range edge, for both sexes. This result is the first report suggesting that changes in allocation to dispersal, and not only traits associated with dispersal, can occur at a small geographical and temporal scale, where the role of local adaptation is likely minimal. This study fills a gap in an issue where it is currently crucial, for conservation purposes, to understand the mechanisms involved in range expansions.

How far do tadpoles travel in the rainforest? Parent-assisted dispersal in poison frogs Abstract Parents can influence offspring dispersal through breeding site selection, competition, or by directly moving their offspring during parental care. Many animals move their young, but the potential role of this behavior in dispersal has rarely been investigated. Neotropical poison frogs (Dendrobatidae) are well known for shuttling their tadpoles from land to water, but the associated movements have rarely been quantified and the potential function of tadpole transport in dispersal has not been addressed. We used miniature radio-transmitters to track the movements of two poison frog species during tadpole transport, and surveyed pool availability in the study area. We found that parental males move farther than expected by the distance to the nearest pool and spread their offspring across multiple pools. We argue that these movement patterns cannot be fully explained by pool quality and availability, and suggest that adaptive benefits related to offspring dispersal also shape the spatial behavior of parental frogs.

Floral isolation and pollination in two hummingbird-pollinated plants: the roles of exploitation barriers and pollinator competition Abstract Differences in feeding performance and aggressive abilities between species and sexes of hummingbirds are often associated with the partitioning of their food sources, but whether such partitioning results in floral isolation (reproductive isolation at the stage of pollination) has received little attention. We examined components of floral isolation and pollinator effectiveness of Heliconia caribaea and H. bihai on the island of Dominica, West Indies. The short flowers of H. caribaea match the short bills of male Anthracothorax jugularis, its primary pollinator, whereas the long flowers of H. bihai match the long bills of female A. jugularis, its primary pollinator. In pollination experiments, both sexes of A. jugularis were equally effective at pollinating the short flowers of H. caribaea, which they preferred to H. bihai, whereas females were more effective at pollinating the long flowers of H. bihai. Moreover, an average difference in length of 12 mm between H. caribaea and H. bihai flowers did not prevent heterospecific pollen transfer, and both sexes transported pollen between the two plant species. In field studies using powdered dyes as pollen analogs, however, heterospecific pollen transfer was minimal, with only 2 of 168 flowers receiving dye from the other species. The length of H. bihai flowers acted as an exploitation barrier to male A. jugularis, which were unable to completely remove nectar from 88% of the flowers they visited. In contrast, interference competition combined with high floral fidelity through traplining prevented female A. jugularis from transferring pollen between the two Heliconia species. A combination of exploitation barriers, interference and exploitative competition, and pollinator preferences maintains floral isolation between these heliconias, and may have contributed to the evolution of this hummingbird-plant system.

Herbivory and inbreeding affect growth, reproduction, and resistance in the rhizomatous offshoots of Solanum carolinense (Solanaceae) Abstract Resource sharing within clonal plant networks can occur via the translocation of water, nutrients, and photoassimilates through rhizomes and stolons. Similar mechanisms may mediate the sharing of information (e.g., about herbivory or other environmental stressors) among ramets via molecular or biochemical signals. The storage of such information in belowground structures could facilitate the transmission of appropriate phenotypic responses across growing seasons in perennial species. However, few previous studies have explored the potential transfer of ecologically relevant information within such networks. This study addresses the effects of foliar herbivory and belowground overwintering on the growth and flowering, physical defenses, and herbivore resistance in the clonally spreading species Solanum carolinense L. (Solanaceae). We used rhizomes from inbred and outbred plants that were repeatedly exposed to feeding damage by Manduca sexta L. (Sphingidae) caterpillars and rhizomes from undamaged control plants. These rhizomes were either planted immediately or exposed to overwintering conditions and allowed to produce new ramets (rhizomatous offshoots). We then assessed offshoot emergence, flowering, physical defense traits, and herbivore performance. Relative to controls, offshoots of herbivore-damaged plants exhibited greater spine and trichome densities, and reduced performance of M. sexta larvae. However, they also emerged and flowered significantly later, and produced fewer flowers than offshoots of undamaged plants. Inbreeding also negatively affected offshoot emergence, flowering, trichome production, and herbivore resistance. These effects of parental herbivory were more pronounced in outbred offshoots, indicating that inbreeding may compromise the trans-seasonal induction of plant defenses. Finally, exposure to overwintering conditions increased trichome production and reduced caterpillar performance on offshoots. Together, these results show that induced defenses can be transmitted through rhizomes and affect offshoot growth, flowering, defensive traits, and herbivore resistance. They also document fitness-related costs associated with defense induction in offshoots and suggest that the transfer of defenses across seasons can be compromised by inbreeding.

Evolutionary quantitative genetics of juvenile body size in a population of feral horses reveals sexually antagonistic selection Abstract Inter-individual variation in juvenile body size can have important consequences for individual fitness, population dynamics, and adaptive evolution. In wild vertebrate populations, larger juvenile size is usually expected to be selected for. However, understanding how such selection may translate into adaptive evolution requires an understanding of the genetic underpinnings of early development and the factors modulating selection. In this study, we characterised the genetic basis of and selection pressures acting upon juvenile body size in a large insular population of feral horses on Sable Island, Canada, to gain insights into the evolution of juvenile body size in wild vertebrate populations. We used pedigree-based quantitative genetic ‘animal models’ to quantify the sources of phenotypic variation in withers-knee length, and assessed the influence of maternal age, sex, and temporal (birth year) and spatial environmental heterogeneity in modulating overwinter survival selection. We found that withers-knee length is moderately heritable and that there was a significant positive genetic correlation between males and females. There was no indication of directional selection in a pooled-sex analysis, but we did find evidence for significant sexually antagonistic selection, with a tendency for smaller body size to be favoured in males and larger body size to be favoured in females. These results suggest that juvenile body size has the potential to evolve in this population, and that selection on juvenile size may play an important role in modulating sex-specific contributions to population dynamics. However, our results also suggest that there is unlikely to be evolutionary change in the mean body size of Sable Island foals.

Competitive responses based on kin-discrimination underlie variations in leaf functional traits in Japanese beech ( Fagus crenata ) seedlings Abstract Identifying how variations in functional traits arise is important to predicting the effects of such variations within communities and ecosystems. Here, we evaluated the effects of kin-discrimination on the uptake of soil resources and leaf traits in seedlings of Fagus crenata (Fagaceae). We grew F. crenata seedlings either alone or paired with a sibling or non-sibling, and examined the total biomass, shoot-to-root ratio, leaf traits (chlorophyll content, leaf mass per area, and total contents of phenolic compounds and condensed tannins), and uptake of soil nitrogen, phosphor, and water. In all neighbour treatments, seedlings grew similarly and had similar shoot-to-root ratios. Chlorophyll content and leaf mass per area were higher in plants grown with non-siblings than in those grown alone or with siblings. The total content of phenolic compounds was highest in single seedlings and lowest in non-siblings. Soil moisture was lowest and thus water uptake was highest in plants grown with non-siblings. Our findings suggest that differences in the intensity of competition for soil resources based on kin-discrimination result in differences in leaf traits.

Differential selection pressures result in a rapid divergence of donor and refuge populations of a high conservation value freshwater fish Coregonus lavaretus (L.) Abstract As a conservation measure to protect European whitefish in Scotland, a translocated population was established in Loch Sloy from Loch Lomond stock between 1988 and 1990. Previous study has assumed that current morphological differences between adults from the donor and refuge lakes have arisen through phenotypic plasticity. The present study compared the morphologies of whitefish at three life stages: alevins and fry raised in a common garden, and wild-caught adults. Alevins were clearly distinguishable by their lake of origin. Loch Sloy alevins were distinguishable also by family, although this was not the case for Loch Lomond. Differential allometric trajectories facilitated the persistence of morphological differences associated with lake of origin through the fry stage into adulthood. Overall, the whitefish from Loch Lomond displayed morphologies associated with pelagic feeders, while the more robust heads and ventrally positioned snouts of the Loch Sloy whitefish conformed to expectations for more benthic feeding habits. That differences between populations were present not only in wild adults, but also in alevins and fry from a common garden setup, strongly suggests that the divergence between populations is due to inheritance mechanisms, rather than differential plastic responses, and questions the effectiveness of translocation as a conservation measure.

Investigating the role of body size, ecology, and behavior in anuran eye size evolution Abstract Vertebrate eye size typically scales hypoallemetrically with body size—as animals grow larger their eyes get relatively smaller. Additionally, eye size is highly variable across species, and such variability often reflects functional adaptations to differences in behavior and/or ecology. The selective pressures underlying the evolution of eye size are especially well studied in birds, mammals, and fishes. However, whether similar scaling rules and selective pressures also underlie the evolution of eye size in amphibians remains enigmatic. Variation in eye size is intimately linked with variation in brain anatomy, as the retina is ontogenetically part of the brain. Eye size may therefore coevolve with brain size. Here we use phylogenetic comparative methods to study interspecific variation in eye volume across 44 species of anurans from 8 families from the Hengduan Mountains, China. We relate this variation to key factors known to impact eye size evolution in other vertebrate taxa such as body mass, habitat use, defense strategy and foraging mobility. We found that also in anurans eyes size scaled hypoallometrically with body mass. However, neither of the behavioral or ecological factors explained any variation in relative eye size in our sample. Whether this is representative for other frog species needs to be clarified. We therefore conclude that eye size in frogs is tightly linked to body mass evolution but that, at least in the species investigated here, none of our tested ecological and behavioral factors have a strong influence on eye size evolution.

Selection on growth rates via a trade-off between survival to sexual maturity and longevity in the swordtail fish Xiphophorus multilineatus Abstract There is growing evidence across taxa for a mortality-growth rate tradeoff, however the extent to which individuals experience selection both for and against growing faster in the wild can be difficult to assess. We used otoliths to assess growth rates in the wild for Xiphophorus multilineatus, a species of swordtail fish with genetically influenced male alternative reproductive tactics. The ARTs are thought to be maintained due to a trade-off between a higher probability of reaching sexual maturity (smaller sneaker males) and maturing later as a large male with higher mating success (larger courter males). We show that the early juvenile growth rates of the subadult courter males were faster as compared to the other juveniles, potentially suggesting selection for growing faster to increase the probability of reaching sexual maturity. However, variance in the early juvenile growth rates of adult courter males was reduced (truncating high values) as compared to the subadult courter males and overall growth rates were faster in the subadult courter males than the adult courter males. These results support a cost of growing faster as juveniles: reduced longevity as an adult. We also detected a negative relationship between the early growth rates of females and their age, further supporting a longevity cost to growing faster. While both male ARTs are likely to experience stabilizing selection on growth rates, the differences between the morphs in how they optimize this tradeoff in relation to overall fitness (courters optimize growing faster to increase probability of reaching sexual maturity, sneakers optimize longevity as adults) has the potential to lead to disruptive selection between the ARTs on growth rates. We show that the data collected here is consistent with the “growth-mortality optimization” hypothesis and highlight some of the challenges that stabilizing selection presents for examining adaptive variation of growth rates in a natural environment.