Stumped by trees? A generalized null model for patterns of organismal diversity

Journal ArticleEvolutionary biologists increasingly have become interested in the factors determining the structure of phylogenetic trees. For example, highly asymmetric trees seem to suggest that the probability of extinction and/or speciation differs among lineages

Anolis Newsletter VII

Newsletter for the 7th Anolis Symposium, Fairchild Tropical Botanic Gardens, Miami, Florida, 17-18 March 2018. It had been nearly a decade since the previous Anolis symposium was held in Cambridge, MA, at the Museum for Comparative Zoology, Harvard. A reunion of anole biologists en masse was long past due and it was decided that this symposium would be slightly different – we were going to hold it somewhere with anoles! And so, on the weekend of 17-18th March, 2018, nearly 70 anole biologists traveled to sunny south Florida to attend the 7th Anolis Symposium held at the beautiful Fairchild Tropical Botanic Gardens in Miami. In the grounds of the botanical gardens, attendees were presented with a diverse community of six (!) species of anole, both native and non-native, representing four distinct ecomorphs

Adaptation, Speciation, and Convergence: A Hierarchical Analysis of Adaptive Radiation in Caribbean Anolis Lizards

This is the publisher's version, also available electronically from http://www.bioone.org/doi/abs/10.3417/0026-6493%282006%2993%5B24%3AASACAH%5D2.0.CO%3B2.Caribbean Anolis lizards are a classic case of adaptive radiation, repeated four times across islands of the Greater Antilles. On each island, very similar patterns of evolutionary divergence have occurred, resulting in the evolution of the same set of ecological specialists—termed ecomorphs—on each island. However, this is only part of the story of the Caribbean anole radiations. Indeed, much of the species diversity of Caribbean Anolis occurs within clades of ecomorphs, which contain as many as 14 ecologically-similar species on a single island. We ask to what extent the classic model of ecological interactions as the driving force in adaptive radiation can account for this aspect of anole evolutionary diversity. Our answer is that it can in part, but not entirely. More generally, the most complete understanding of evolutionary diversification and radiation is achieved by studying multiple hierarchical evolutionary levels from clades to populations

Predator-driven natural selection on risk-taking behavior in anole lizards

Biologists have long debated the role of behavior in evolution, yet understanding of its role as a driver of adaptation is hampered by the scarcity of experimental studies of natural selection on behavior in nature. After showing that individual Anolis sagrei lizards vary consistently in risk-taking behaviors, we experimentally established populations on eight small islands either with or without Leiocephalus carinatus, a major ground predator. We found that selection predictably favors different risk-taking behaviors under different treatments: Exploratory behavior is favored in the absence of predators, whereas avoidance of the ground is favored in their presence. On predator islands, selection on behavior is stronger than selection on morphology, whereas the opposite holds on islands without predators. Our field experiment demonstrates that selection can shape behavioral traits, paving the way toward adaptation to varying environmental contexts

Do the relationships between hindlimb anatomy and sprint speed variation differ between sexes in Anolis lizards?

The ability of an animal to run fast has important consequences on its survival capacity and overall fitness. Previous studies have documented how variation in the morphology of the limbs is related to variation in locomotor performance. Although these studies have suggested direct relations between sprint speed and hindlimb morphology, few quantitative data exist. Consequently, it remains unclear whether selection acts in limb segment lengths, overall muscle mass or muscle architecture (e.g. muscle fiber length and cross-sectional area). Here, we investigate whether muscle architecture (mass, fiber length and physiological cross-sectional area), hindlimb segment dimensions, or both, explain variation in sprint speed across 14 species of Anolis lizards. Moreover, we test whether similar relationships exist between morphology and performance for both sexes, which may not be the case given the known differences in locomotor behavior and habitat use. Our results show that the main driver of sprint speed is the variation in femur length for both males and females. Our results further show sexual dimorphism in the traits studied and, moreover, show differences in the traits that predict maximal sprint speed in males and females. For example, snout vent length and overall muscle mass are also good predictors of sprint speed in males, whereas no relationships between muscle mass and sprint speed was observed in females. Only a few significant relationships were found between muscle architecture (fiber length, cross-sectional area) and sprint speed in male anoles, suggesting that overall muscles size, rather than muscle architecture, appears to be under selection

Head Size of Male and Female Lizards Increases with Population Density Across Island Populations in the Bahamas

In polygynous lizards, male–male competition is an important driver of morphologic and behavioral traits associated with intraspecific dominance. The extent to which females engage in aggressive behavior and thus contribute to competition-driven morphologic variation is not well studied. We used injury frequencies of brown anoles (Anolis sagrei) in 16 island populations to test the hypothesis that injury-inducing aggressive encounters increase with population density in both male and female lizards. We further asked whether intraspecific competition is a potential driver of phenotypic traits related to dominance by using population density as proxy for intraspecific competition. We found that the proportion of individuals with injuries was greater in populations with higher densities, suggesting that agonistic competitive interactions increase with population density. Size-adjusted head length of male and female lizards increased with population density, suggesting that larger heads might be advantageous when intraspecific competition is strong. We detected differences in morphology and injury frequency among islands for both males and females, which suggests that agonistic competitive interactions among females may be stronger than previously appreciated. Further research is needed to determine whether aggressive encounters involving females are restricted to intrasexual competition or whether they also involve males, and how morphologic traits of females are related to competitive dominance and reproductive success

Favorable Climate Change Response Explains Non-Native Species' Success in Thoreau's Woods

Invasive species have tremendous detrimental ecological and economic impacts. Climate change may exacerbate species invasions across communities if non-native species are better able to respond to climate changes than native species. Recent evidence indicates that species that respond to climate change by adjusting their phenology (i.e., the timing of seasonal activities, such as flowering) have historically increased in abundance. The extent to which non-native species success is similarly linked to a favorable climate change response, however, remains untested. We analyzed a dataset initiated by the conservationist Henry David Thoreau that documents the long-term phenological response of native and non-native plant species over the last 150 years from Concord, Massachusetts (USA). Our results demonstrate that non-native species, and invasive species in particular, have been far better able to respond to recent climate change by adjusting their flowering time. This demonstrates that climate change has likely played, and may continue to play, an important role in facilitating non-native species naturalization and invasion at the community level

Determinants of spread in an urban landscape by an introduced lizard

Context: Urban landscapes are a mixture of built structures, human-altered vegetation, and remnant semi-natural areas. The spatial arrangement of abiotic and biotic conditions resulting from urbanization doubtless influences the establishment and spread of non-native species in a city. Objectives: We investigated the effects of habitat structure, thermal microclimates, and species coexistence on the spread of a non-native lizard (Anolis cristatellus) in the Miami metropolitan area of South Florida (USA). Methods: We used transect surveys to estimate lizard occurrence and abundance on trees and to measure vegetation characteristics, and we assessed forest cover and impervious surface using GIS. We sampled lizard body temperatures, habitat use, and relative abundance at multiple sites. Results: At least one of five Anolis species occupied 79 % of the 1035 trees surveyed in primarily residential areas, and non-native A. cristatellus occupied 25 % of trees. Presence and abundance of A. cristatellus were strongly associated with forest patches, dense vegetation, and high canopy cover, which produced cooler microclimates suitable for this species. Presence of A. cristatellus was negatively associated with the ecologically similar non-native A. sagrei, resulting in reduced abundance and a shift in perch use of A. cristatellus. Conclusions: The limited spread of A. cristatellus in Miami over 35 years is due to the patchy, low-density distribution of wooded habitat, which limits dispersal by diffusion. The presence of congeners may also limit spread. Open habitats—some parks, yards and roadsides—contain few if any A. cristatellus, and colonization of isolated forest habitat appears to depend on human-mediated dispersal

An incipient invasion of brown anole lizards (\u3cem\u3eAnolis sagrei\u3c/em\u3e) into their own native range in the Cayman Islands: a case of cryptic back-introduction

Human-mediated dispersal has reshaped distribution patterns and biogeographic relationships for many taxa. Long-distance and over-water dispersal were historically rare events for most species, but now human activities can move organisms quickly over long distances to new places. A potential consequence of human-mediated dispersal is the eventual reintroduction of individuals from an invasive population back into their native range; a dimension of biological invasion termed “cryptic back-introduction.” We investigated whether this phenomenon was occurring in the Cayman Islands where brown anole lizards (Anolis sagrei) with red dewlaps (i.e., throat fans), either native to Little Cayman or invasive on Grand Cayman, have been found on Cayman Brac where the native A. sagrei have yellow dewlaps. Our analysis of microsatellite data shows strong population-genetic structure among the three Cayman Islands, but also evidence for non-equilibrium. We found some instances of intermediate multilocus genotypes (possibly 3–9% of individuals), particularly between Grand Cayman and Cayman Brac. Furthermore, analysis of dewlap reflectance data classified six males sampled on Cayman Brac as having red dewlaps similar to lizards from Grand Cayman and Little Cayman. Lastly, one individual from Cayman Brac had an intermediate microsatellite genotype, a red dewlap, and a mtDNA haplotype from Grand Cayman. This mismatch among genetic and phenotypic data strongly suggests that invasive A. sagrei from Grand Cayman are interbreeding with native A. sagrei on Cayman Brac. To our knowledge, this is the first evidence of cryptic back-introduction. Although we demonstrate this phenomenon is occurring in the Cayman Islands, assessing its frequency there and prevalence in other systems may prove difficult due to the need for genetic data in most instances. Cryptic back-introductions may eventually provide some insight into how lineages are changed by the invasion process and may be an underappreciated way in which invasive species impact native biodiversity

Adaptive radiation along a deeply conserved genetic line of least resistance in \u3cem\u3eAnolis\u3c/em\u3e lizards

On microevolutionary timescales, adaptive evolution depends upon both natural selection and the underlying genetic architecture of traits under selection, which may constrain evolutionary outcomes. Whether such genetic constraints shape phenotypic diversity over macroevolutionary timescales is more controversial, however. One key prediction is that genetic constraints should bias the early stages of species divergence along “genetic lines of least resistance” defined by the genetic (co)variance matrix, G. This bias is expected to erode over time as species means and G matrices diverge, allowing phenotypes to evolve away from the major axis of variation. We tested for evidence of this signal in West Indian Anolis lizards, an iconic example of adaptive radiation. We found that the major axis of morphological evolution was well aligned with a major axis of genetic variance shared by all species despite separation times of 20–40 million years, suggesting that divergence occurred along a conserved genetic line of least resistance. Further, this signal persisted even as G itself evolved, apparently because the largest evolutionary changes in G were themselves aligned with the line of genetic least resistance. Our results demonstrate that the signature of genetic constraint may persist over much longer timescales than previously appreciated, even in the presence of evolving genetic architecture. This pattern may have arisen either because pervasive constraints have biased the course of adaptive evolution or because the G matrix itself has been shaped by selection to conform to the adaptive landscape