Adaptive and non-adaptive convergent evolution in feather reflectance of California Channel Islands songbirds.

Convergent evolution is widely regarded as a signature of adaptation. However, testing the adaptive consequences of convergent phenotypes is challenging, making it difficult to exclude non-adaptive explanations for convergence. Here, we combined feather reflectance spectra and phenotypic trajectory analyses with visual and thermoregulatory modelling to test the adaptive significance of dark plumage in songbirds of the California Channel Islands. By evolving dark dorsal plumage, island birds are generally less conspicuous to visual-hunting raptors in the island environment than mainland birds. Dark dorsal plumage also reduces the energetic demands associated with maintaining homeothermy in the cool island climate. We also found an unexpected pattern of convergence, wherein the most divergent island populations evolved greater reflectance of near-infrared radiation. However, our heat flux models indicate that elevated near-infrared reflectance is not adaptive. Analysis of feather microstructure suggests that mainland-island differences are related to coloration of feather barbs and barbules rather than their structure. Our results indicate that adaptive and non-adaptive mechanisms interact to drive plumage evolution in this system. This study sheds light on the mechanisms driving the association between dark colour and wet, cold environments across the tree of life, especially in island birds.

Parameterizing mechanistic niche models in biophysical ecology: a review of empirical approaches.

Mechanistic niche models are computational tools developed using biophysical principles to address grand challenges in ecology and evolution, such as the mechanisms that shape the fundamental niche and the adaptive significance of traits. Here, we review the empirical basis of mechanistic niche models in biophysical ecology, which are used to answer a broad array of questions in ecology, evolution and global change biology. We describe the experiments and observations that are frequently used to parameterize these models and how these empirical data are then incorporated into mechanistic niche models to predict performance, growth, survival and reproduction. We focus on the physiological, behavioral and morphological traits that are frequently measured and then integrated into these models. We also review the empirical approaches used to incorporate evolutionary processes, phenotypic plasticity and biotic interactions. We discuss the importance of validation experiments and observations in verifying underlying assumptions and complex processes. Despite the reliance of mechanistic niche models on biophysical theory, empirical data have and will continue to play an essential role in their development and implementation.

Seasonal patterns of dietary partitioning in vertebrates.

Dietary partitioning plays a central role in biological communities, yet the extent of partitioning often varies dramatically over time. Food availability may drive temporal variation in dietary partitioning, but alternative paradigms offer contrasting predictions about its effect. We compiled estimates of dietary overlap between co-occurring vertebrates to test whether partitioning is greater during periods of high or low food abundance. We found that dietary partitioning was generally greatest when food abundance was low, suggesting that competition for limited food drives partitioning. The extent of dietary partitioning in birds and mammals was also related to seasonality in primary productivity. As seasonality increased, partitioning increased during the nonbreeding season for birds and the breeding season for mammals. Although some hypotheses invoke changes in dietary breadth to explain temporal variation in dietary partitioning, we found no association between dietary breadth and partitioning. These results have important implications for the evolution of dietary divergence.

Performance Trade-Offs and Resource Availability Drive Variation in Reproductive Isolation between Sympatrically Diverging Crossbills.

AbstractTheoretical models indicate that speciation, especially when the scope for gene flow is great (e.g., sympatric speciation), is most likely when strong performance trade-offs coincide with reproduction. We tested this classic hypothesis using measures of the strength of three prezygotic reproductive isolating barriers (habitat isolation, reduced immigrant fecundity, and behavioral isolation) between two young (~2,000 years) and sympatric red crossbill (Loxia curvirostra) ecotypes. All three isolating barriers increased with increases in performance trade-offs, with total reproductive isolation varying between 0.72 and 1 (0 represents random mating, and 1 represents complete reproductive isolation). Strong trade-offs led to strong habitat isolation, an inability to breed in the "wrong" habitat, and more assortative flocks, with the latter leading to stronger behavioral isolation. Reproductive isolation decreased as resource availability increased relative to the demands of breeding, with higher resource availabilities eliminating the positive relationship between reproductive isolation and performance trade-offs. This latter result is consistent with previous work suggesting that increasing resource abundance dampens the effect of strong performance trade-offs on evolutionary divergence. Because many organisms, with the notable exception of host-specific phytophagous insects, rely on abundant food resources with weak performance trade-offs while breeding, our results may explain why sympatric speciation is uncommon.

Strong yet incomplete reproductive isolation in Vermivora is not contradicted by other lines of evidence: A reply to Toews et al.

Toews et al. assert that strong reproductive isolation in Vermivora is inconsistent with other lines of evidence. Here, we discuss how strong yet incomplete reproductive isolation is consistent with other results from this system.

Character displacement of a learned behaviour and its implications for ecological speciation.

Cultural evolution may accelerate population divergence and speciation, though most support for this hypothesis is restricted to scenarios of allopatric speciation driven by random cultural drift. By contrast, the role of cultural evolution in non-allopatric speciation (i.e. speciation with gene flow) has received much less attention. One clade in which cultural evolution may have figured prominently in speciation with gene flow includes the conifer-seed-eating finches in the red crossbill (Loxia curvirostra) complex. Here we focus on Cassia crossbills (Loxia sinesciuris; an ecotype recently split taxonomically from red crossbills) that learn social contact calls from their parents. Previous work found that individuals modify their calls throughout life such that they become increasingly divergent from a closely related, sympatric red crossbill ecotype. This open-ended modification of calls could lead to character displacement if it causes population-level divergence in call structure that, in turn, reduces (maladaptive) heterospecific flocking. Heterospecific flocking is maladaptive because crossbills use public information from flockmates to assess resource quality, and feeding rates are depressed when flockmates differ in their ability to exploit a shared resource (i.e. when flockmates are heterospecifics). We confirm the predictions of character displacement by documenting substantial population-level divergence in Cassia crossbill call structure over just two decades and by using field experiments to demonstrate that Cassia and red crossbills differentially respond to these evolved differences in call structure, reducing heterospecific flock formation. Moreover, because crossbills choose mates from within flocks, a reduction in heterospecific flocking should increase assortative mating and may have been critical for speciation of Cassia crossbills in the face of ongoing gene flow in as few as 5000 years. Our results provide evidence for a largely neglected yet potentially widespread mechanism by which reproductive isolation can evolve between sympatric lineages as a byproduct of adaptive cultural evolution.

An Alternative to Adaptation by Sexual Selection: Habitat Choice.

Adaptation in mating signals and preferences has generally been explained by sexual selection. We propose that adaptation in such mating traits might also arise via a non-mutually exclusive process wherein individuals preferentially disperse to habitats where they experience high mating performance. Here we explore the evolutionary implications of this process.

Comment on Van Belleghem et al. 2016: Habitat choice mechanisms in speciation and other forms of diversification.

Several mechanisms of habitat choice can contribute to speciation. Empirical studies of habitat choice mechanisms provide important insights into the relative roles of these mechanisms in speciation. A recent paper by Van Belleghem and colleagues characterizes the mechanistic basis of a component of habitat choice-departure behavior-in two salt marsh beetle ecotypes that inhabit different environments. The authors compare the departure behavior between the two ecotypes in response to an environmental cue and find that ecotypes differ in their tendency to depart in response to this cue and that the environment experienced by immature beetles affects the departure behavior of adult beetles. The authors conclude that such plastic behavioral differences between ecotypes should reduce gene flow and thereby facilitate reproductive isolation between ecotypes. We question whether such a mechanism of departure behavior would effectively reduce gene flow between ecotypes. Furthermore, their study highlights the need for some clarification of habitat choice mechanisms and related concepts, as conceptual inconsistencies are common in the literature. Here, we clarify major mechanisms of habitat choice and discuss how each mechanism might facilitate speciation. We emphasize that future empirical work should be guided by careful consideration of the natural history of species under study.

Assessing the Potential Contributions of Reduced Immigrant Viability and Fecundity to Reproductive Isolation.

Reduced fitness of immigrants from alternative environments is thought to be an important reproductive isolating barrier. Most studies evaluating the importance of the relative fitness of immigrants to speciation have focused on reduced survival of immigrants (i.e., immigrant inviability). However, variation in fecundity appears to have a greater impact on variation in fitness than does variation in viability, suggesting that reduced fecundity of immigrants could act as an important yet largely overlooked reproductive isolating barrier. Using a model and a survey of studies of local adaptation, we evaluate the relative strength of reduced immigrant viability and fecundity as potential causes of reproductive isolation. We found that reduced fecundity as compared to reduced viability as a reproductive isolating barrier should increase in importance as the relative costs of reproduction increase. Consistent with the elevated demands of reproduction reported in the literature, we found that reproductive isolation from reduced immigrant fecundity was of similar magnitude or greater than that from reduced immigrant viability, particularly in the early stages of speciation. These results suggest that the important role of differential fecundity in local adaptation extends to speciation.