Anole Annals

The invasive brown anole A. sagrei is a territorially polygynous species, and male aggressive behavior is an important trait that affects male fitness. Aggressive behavior is quite variable across individuals and populations, and can differ based on intra- and inter-specific community context. As AA regulars know, A. sagrei is also a very successful invasive species; it has been established in southern Florida for decades, and has been steadily spreading north along the gulf coast, colonizing new regions of the US. Populations at the leading edge of the range expansion experience different biotic and abiotic environments than established populations, which can lead to different selective pressures and divergence in relevant traits. Invasive populations of A. sagrei thus provide a good opportunity to explore variation in aggressive display behavior across different ecological contexts.

Julie Wiemerslage decided to take that opportunity and explore the variation in aggressive behavior across different populations of A. sagrei. In her poster “Population Differences in Territorial Aggression in the Invasive Brown Anoles, Anolis sagrei” she proposes the following two hypotheses: 1) Lizards at the leading edge of the range expansion will be more aggressive, allowing them to outcompete other species in their new range 2) Lizards at the leading edge will be less aggressive, because population densities will be lower than areas with established populations.

To test these hypotheses, Wiemerslage collected male lizards from a) native populations, b) well-established invasive populations, and c) recent invasive populations and brought them to the lab for behavioral trials. For each population, she placed pairs of males together in a cage and quantified aggressive behavioral traits including pushups, head bobs, lunges, and dewlap flashes (don’t worry, the lizards were tethered so they couldn’t actually harm one another). She found that aggression was lowest in the leading edge populations, supporting hypothesis 2. Interestingly, the most aggressive populations were the well-established invasive populations, while individuals from the native range showed an intermediate level of aggression. The cause of this pattern is unclear, though Wiemerslage suggests that more information about these source populations (such as density, community composition) will improve our understanding of the factors affecting aggressive behavior.

Researchers that are interested in ecological and evolutionary dynamics through time often make inferences about past patterns and processes using modern data, such as DNA sequences and geographic distributions of extant taxa. But this is not the only possible approach. Studies of extinct taxa and populations using fossils can provide direct measures of species distributions and abundances in the past, which are often impossible to accurately infer with modern data alone.

In her talk titled “Extinction biases and their ramifications on Caribbean lizard communities,” Melissa Kemp described her research using fossil data to characterize the former herpetofaunal community of several islands in the Caribbean. She explored the following questions linking extinction to community ecology: 1) how has extinction proceeded in the Caribbean lizard community? 2) what is the impact of species extinction on the whole community? 3) can we predict future patterns of extinction using fossil data?

To characterize past extinction patterns, Kemp measured species abundance and morphological traits of fossil remains through time in lizard communities in the Caribbean. She sought to determine whether certain taxa underwent more local extinctions, and whether extinctions were correlated with certain morphological traits. She also quantified community evenness to see how extinction events affect the whole lizard community. She found that one family, the Leiocephalidae, has gone extinct more often than others. Interestingly, in a four-species community in which Leiocephalidae went extinct, anoles went from relatively average abundance to becoming the dominant taxa, a pattern which continues to this day. Modern Leiocephalids have been shown to predate on anoles, so this community shift may have been a result of predator release. In addition, anole body sizes increased after Leiocephalid extinction, lending further support to the predator release conclusion.

After looking at historical patterns of extinction and diversity, Kemp explored whether fossil data might give us insight into current and future patterns of extinction. For example, are species that have gone extinct in some areas vulnerable to extinction in other parts of their range? And if so, what traits are causing this vulnerability? To address these questions, Kemp compared traits of extinct taxa to traits of modern successful introduced species,  which are likely to have a very low risk of extinction. She found that extinct species tend to have different reproductive modes and habitats from introduced species, suggesting that these traits may have played a role in their extinctions. In addition, modern species with similar suites of traits as the extinct taxa may be more vulnerable to extinction in the future.

Kemp’s research shows that it’s not always best to leave the past behind. Fossil data enhances our understanding not only of extinct species, but of modern ecological and evolutionary processes as well.

Species divergence is driven by a wide variety of forces, but two of the strongest predictors of speciation are the amount of time a lineage has persisted in a landscape, and the ability of lineages to move through a landscape. Lineages are more likely to diverge when they have occupied a landscape for a long time, and/or if their ability to move is restricted, thus limiting gene flow.

In his talk titled “Geographical factors promoting diversification of the northern Andes and Brazilian Cerrado regions: the case of frogs and Anole lizard species,” Carlos Guarnizo described his efforts to test whether these patterns hold true in both different landscapes and different taxa. He surveyed two herpetofaunal communities in two diversity hotspots in South America: frogs in the northern Andes mountain range and Anolis lizards in the Brazilian Cerrado. The montane Andean landscape is structurally complex and covers a range of altitudes, while the Cerrado region is a more uniform savannah-like environment, with intermediate structural complexity. Guarnizo used species distributions and genetic data to look at patterns of diversification across these landscapes to explore which landscape characteristics lead to higher levels of divergence and speciation.

He found that in both areas, topography was a strong predictor of divergence; specifically, more structurally complex landscapes led to higher levels of genetic divergence between sister lineages. These genetic breaks are also often deeper than previously realized, likely representing cryptic species. Despite these strong genetic splits, the niches occupied by sister taxa are generally well-conserved, lending support to the conclusion that landscape structure – rather than adaptive divergence – is responsible for the genetic divergence observed. Interestingly, in Andean frogs, Guarnizo found that the strongest genetic breaks did not occur across mountain peaks as previously thought. Instead, valleys appear to be the strongest geographic barrier to dispersal.

These cases show that landscape topography is a strong factor determining genetic divergence across different landscapes and taxa (including anoles), and may lead to high levels of cryptic speciation.

Photo by Nsimean

It’s true, they’re not anoles, but lizards of the genus Liolaemus form another extremely diverse clade, occupying one of the broadest climatic and elevational niche ranges of any vertebrate. Whether the ecological and phenotypic diversity of this genus are correlated, as is the case in adaptive radiation, remains an open question. Studies of the whole genus have shown that body size diversification is consistent with expansion into different ecophysiological niches, but other morphological traits don’t show the same pattern. Yet much of the ecology of the genus is unknown, so it is difficult to draw any definite conclusions.

In her talk “Evolution of niche and ecomorphological traits in a phylogenetic context in lizards of the Liolaemus bibroni complex,” Dan Edwards sought to address this gap in understanding of Liolaemus by focusing on one species complex within the genus, L. bibroni. The L. bibroni species group is composed of 26 species that occupy a broad range of habitats representative of those occupied by the genus as a whole. To explore their history of genetic and morphological diversification, Edwards constructed a phylogeny of the group, characterized rates of diversification, and measured a suite of relevant morphological traits. She found that there has been an increase in trait diversification over time, consistent with the colonization of new habitat types. In addition, she found that ecology and body size are significantly correlated, supporting previous results from studies of the genus as a whole. Other morphological traits were not as clearly associated with habitat type, but there do appear to be possible patterns of ecomorphological divergence in response to divergence in habitat. Edwards plans to further characterize the evolutionary relationships and explore more ecomorphological traits of Liolaemus species to resolve this question.

Many exaggerated phenotypic traits, such as the large and colorful dewlaps of male anoles, increase fitness of individuals who possess them. But these traits are often energetically costly. Too high an investment in showy or extreme traits can come at the cost of an individual’s health and performance. Such traits are therefore said to be condition-dependent; that is, individuals will not develop them unless they are already in a healthy condition.

John David Curlis and colleagues explored  several potential condition-dependent traits in two closely related Central American Anolis species, A. limifrons and A. humilis. He quantified a number of sexually and naturally selected traits and tested whether they varied by body condition to see whether any of them were condition dependent, and whether the degree of condition dependence varied between two closely related species. None of the traits he tested were condition dependent in A. limifrons, but two traits – jaw width and dewlap size – were condition dependent in A. humilis. He therefore concluded that the degree of condition dependence of these traits is evolutionarily labile. In addition, A. humilis dewlaps are generally larger than A. limifrons, which suggests that condition dependence may be a more important force affecting traits that are subjected to stronger sexual selection. Taken together, these results suggest that condition-dependence of sexually-selected traits may be playing a role in dewlap diversity (and perhaps other phenotypic traits) throughout Anolis lizards.

Studies of adaptive radiation often focus on two main axes of divergence: the structural niche (e.g., where a species lives) and resource niche (e.g., what a species eats). In his SSE Symposium talk titled “The physiology of adaptive radiation,” Alex Gunderson explained the importance of a third, under-appreciated axis of species diversification: the thermal niche. Gunderson and colleagues tested whether different approaches to estimate the rates of evolution of the thermal niche lead to different conclusions, and whether thermal traits evolve at similar rates to classic ecomorphological traits like body size and limb length.

Scientists generally use three main approaches to quantify the thermal niche and estimate rates of thermal niche evolution: ecological niche modeling (ENM), organismal body temperatures, and physiological data (tolerance/sensitivity to different temperatures). Different studies use different approaches, but few use all three. Each of these metrics addresses a different scale of thermal biology, from broad environmental variables (ENM) to individual organisms (physiology). Gunderson and colleagues therefore predicted that estimated rates of evolution would vary based on the metrics used, and they used data from a number of Anolis species to test this prediction.

Specifically, the authors predicted that: a) ecological niche modeling approaches would estimate greater rates of thermal niche evolution, because environmental factors like temperature and precipitation used in ENM are very broad metrics, and are not necessarily directly correlated with individual thermal niche; b) organismal temperature data would estimate intermediate rates of thermal niche evolution, while it is a measure of individual thermal niche, it is also quite plastic; c) physiological measures would estimate the most conservative/low  rates of evolution, because measures of thermal maxima and minima most accurately reflect the possible tolerance and sensitivity of individuals to thermal environments. They found that physiological data does indeed produce the most conservative estimates of thermal trait evolution, but their predictions about the performance of ENM and body temperature differed. Estimates of thermal niche evolution were highest when using body temperature data, and were intermediate when based on ENM. The fact that body temperature-based estimates of evolution rates were higher than ENM-based estimates suggests that researchers are generally underestimating error in body temperature measurements in the field.

After evaluating the results of these three different approaches in relation to thermal niche evolution, the researchers then compared rates of evolution of thermal traits to those of classical ecomorphological traits. When they used ENM, thermal traits seemed to evolve much more rapidly than morphological traits. In contrast, when they used physiological data, they found the opposite. Clearly, different metrics of climatic niche lead to different conclusions about evolutionary patterns. Gunderson therefore recommends incorporating aspects of multiple ecological and physiological scales when studying divergence of the thermal niche.