Shawn R. Kuchta

Selective loss of polymorphic mating types is associated with rapid phenotypic evolution during morphic speciation

Polymorphism may play an important role in speciation because new species could originate from the distinctive morphs observed in polymorphic populations. However, much remains to be understood about the process by which morphs found new species. To detail the steps of this mode of speciation, we studied the geographic variation and evolutionary history of a throat color polymorphism that distinguishes the “rock-paper-scissors” mating strategies of the side-blotched lizard, Uta stansburiana. We found that the polymorphism is geographically widespread and has been maintained for millions of years. However, there are many populations with reduced numbers of throat color morphs. Phylogenetic reconstruction showed that the polymorphism is ancestral, but it has been independently lost eight times, often giving rise to morphologically distinct subspecies/species. Changes to the polymorphism likely involved selection because the allele for one particular male strategy, the “sneaker” morph, has been lost in all cases. Polymorphism loss was associated with accelerated evolution of male size, female size, and sexual dimorphism, which suggests that polymorphism loss can promote rapid divergence among populations and aid species formation.

Experimental Support for Aposematic Coloration in the Salamander Ensatina eschscholtzii xanthoptica: Implications for Mimicry of Pacific Newts

Abstract A field experiment was conducted to test the hypothesis that the plethodontid salamander Ensatina eschscholtzii xanthoptica benefits from aposematic coloration. Under field conditions, clay models painted to resemble E. e. xanthoptica were attacked significantly less often than models lacking the supposed aposematic colors. In addition, the head region of models was attacked significantly more frequently than random, suggesting the models were perceived as prey items. These results are consistent with the hypothesis that E. e. xanthoptica is a mimic of highly toxic Pacific Newts in the genus Taricha. The significance of the E. e. xanthoptica phenotype for the ring species biogeographic scenario and future research directions are briefly discussed.

ALTERNATIVE MATING STRATEGIES AND THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN THE SIDE-BLOTCHED LIZARD, UTA STANSBURIANA: A POPULATION-LEVEL COMPARATIVE ANALYSIS

Population‐level comparative analyses can link microevolutionary processes within populations to macroevolutionary patterns of diversification. We used the comparative method to study the evolution of sexual size dimorphism (SSD) among populations of side‐blotched lizards (Uta stansburiana). Uta stansburiana is polymorphic for different male mating and female life‐history strategies in some populations, but monomorphic in others. We tested whether intrasexual selection among males, fecundity selection on females, and the presence of polymorphic strategies affected levels of SSD. We first resolved a phylogeny for 41 populations across the range of the species and documented a substantial regional structure. Our intraspecific data had significant phylogenetic signal, and correcting for phylogeny using independent contrasts had large effects on our results. Polymorphic populations had male‐biased SSD and changes in male body size, levels of tail breaks, and SSD consistent with the intrasexual selection hypothesis. Monomorphic populations had changes in female size, clutch size, and SSD consistent with the fecundity selection hypothesis. Fecundity selection is a likely cause of some monomorphic populations having no SSD or female‐biased SSD. Our results suggest that changes in mating strategies are associated with phenotypic diversification and multiple evolutionary forces can shape SSD.

Pronounced phylogeographic structure on a small spatial scale: Geomorphological evolution and lineage history in the salamander ring species Ensatina eschscholtzii in central coastal California

The salamander Ensatina eschscholtzii is a classic example of a ring species, and has an intricate biogeographic history. Within a part of the ring distribution, earlier work using allozymes disclosed high levels of genetic structure in central coastal California, where the subspecies oregonensis, xanthoptica, and eschscholtzii meet. We used mitochondrial cytochrome b sequences to further examine patterns of divergence in this area, including data from 155 localities (309 individuals). Our focus is on the documentation of population-level haplotype lineages. We show that oregonensis is represented by two unrelated, phenotypically similar clades, both of which possess substantial substructure of their own. The subspecies xanthoptica includes two lineages that differ in phenotype, one of which has colonized the foothills of the Sierra Nevada. The subspecies eschscholtzii occurs mainly to the south, but some populations from a northern lineage extend into the Monterey Bay region, where they approach xanthoptica geographically. In sum, populations in the central coastal California region form a distributional patchwork, including three subspecies, three clades (which differ from the three subspecies), and ten haplotype lineages. We conclude that such striking levels of phylogeographic structure reflect interspersed episodes of spatial fragmentation, in part driven by the complex geomorphological evolution of the California Coast Range system.

WHY DOES THE YELLOW‐EYED ENSATINA HAVE YELLOW EYES? BATESIAN MIMICRY OF PACIFIC NEWTS (GENUS TARICHA) BY THE SALAMANDER ENSATINA ESCHSCHOLTZII XANTHOPTICA

Abstract Color patterns commonly vary geographically within species, but it is rare that such variation corresponds with divergent antipredator strategies. The polymorphic salamander Ensatina eschscholtzii, however, may represent such a case. In this species, most subspecies are cryptically colored, whereas E. e. xanthoptica, the Yellow eyed ensatina, is hypothesized to be an aposematic mimic of highly toxic Pacific newts (genus Taricha). To test the mimicry hypothesis, we conducted feeding trials using Western Scrub-Jays, Aphelocoma californica. In every feeding trial, we found that jays, following presentation with the presumed model (T. torosa), were more hesitant to contact the presumed mimic (E. e. xanthoptica) than a control subspecies lacking the postulated aposematic colors (E. e. oregonensis). The median time to contact was 315 sec for the mimic and 52 sec for the control. These results support the mimicry hypothesis, and we suggest that E. e. xanthoptica is likely a Batesian mimic, rather a Müllerian or quasi-Batesian mimic, of Pacific newts.

Predator-Mediated Natural Selection on the Wings of the Damselfly Calopteryx splendens: Differences in Selection among Trait Types

Traits that increase mating success in males may come at a cost, such as an increased risk of predation. However, predator-mediated selection is challenging to document in natural populations, hampering our understanding of the trade-offs between sexual selection and predation. Here we report on a study of predator-mediated natural selection on wing traits in the damselfly Calopteryx splendens, the males of which possess conspicuous wing patches. Wagtails (genus Motacilla) are important avian predators of C. splendens, capturing them in flight and removing the wings prior to consumption. Using geometric morphometric techniques, we quantified the strength and mode of selection on wing traits by comparing wings from depredated individuals with the standing variation present in the population. Our results reveal that predator-mediated selection is stronger on secondary sexual characters than on size and shape, suggesting that traits related to flight performance are closer to their adaptive peaks. This could be a consequence of the long-term evolutionary association with avian predators, whereas stronger selection on conspicuous secondary sexual traits may reflect trade-offs between sexual and natural selection. Finally, even though C. splendens possesses nearly identical fore- and hindwings, we found evidence for divergent selection between them.

Predator mediated selection and the impact of developmental stage on viability in wood frog tadpoles (Rana sylvatica)

BackgroundComplex life histories require adaptation of a single organism for multiple ecological niches. Transitions between life stages, however, may expose individuals to an increased risk of mortality, as the process of metamorphosis typically includes developmental stages that function relatively poorly in both the pre- and post-metamorphic habitat. We studied predator-mediated selection on tadpoles of the wood frog, Rana sylvatica, to identify this hypothesized period of differential predation risk and estimate its ontogenetic onset. We reared tadpoles in replicated mesocosms in the presence of the larval odonate Anax junius, a known tadpole predator.ResultsThe probability of tadpole survival increased with increasing age and size, but declined steeply at the point in development where hind limbs began to erupt from the body wall. Selection gradient analyses indicate that natural selection favored tadpoles with short, deep tail fins. Tadpoles resorb their tails as they progress toward metamorphosis, which may have led to the observed decrease in survivorship. Path models revealed that selection acted directly on tail morphology, rather than through its indirect influence on swimming performance.ConclusionsThis is consistent with the hypothesis that tail morphology influences predation rates by reducing the probability a predator strikes the head or body.

Wherefore and Whither the Ring Species

Ring species are widely recognized as one of the best natural illustrations of species formation. A ring species is a circular arrangement of populations with one boundary characterized by reproductive isolation, but intergradation among populations elsewhere. They form when populations disperse around a central barrier and form a secondary contact characterized by reproductive isolation. Ring species are often presented as a taxonomic conundrum, because the presence of a single boundary exhibiting reproductive isolation leaves the ring of populations uncomfortably situated between one and two species. Here we review the ring species concept, with a focus on the salamander Ensatina eschscholtzii and the Greenish Warbler, Phylloscopus trochiloides. We argue that ring species demonstrate the gradual nature of species formation, and thereby illustrate the model of species formation originally put forth by Darwin. We also argue that ring species have become overly idealized, with a focus on strict criteria to the detriment of evolutionary lessons. Like all models of evolutionary change, the ring species concept is an oversimplification, and an ideal ring species has never been found. Finally, we review ring species in light of the general lineage concept of species, and argue that ring species status, while nicely accommodated by recognizing a single species, is independent of taxonomy. The essential features of a ring species are a biogeographic history resulting in a ring-like distribution, and the presence of a single species border characterized by reproductive isolation. Under the general lineage concept, reproductive isolation is a contingent, but not necessary, property of evolutionary lineages. Whether one considers a ring species complex to be one species or many does not change the evolutionary message, and the problems (and lessons) presented by ring species do not go away with taxonomic changes.