Showing papers by "Jonathan B. Losos published in 1997"

Adaptive differentiation following experimental island colonization in Anolis lizards

Abstract: If colonizing populations are displaced into an environment that is often very different from that of their source1, they are particularly likely to diverge evolutionarily, the more so because they are usually small and thus likely to change by genetic restructuring or drift2,3. Despite its fundamental importance, the consequence of colonization for traits of founding populations have primarily been surmised from static present-day distributions1,2,4,5, laboratory experiments6 and the outcomes of haphazard human introductions7–9, rather than from replicated field experiments. Here we report long-term results of just such an experimental study. Populations of the lizard Anolis sagrei, introduced onto small islands from a nearby source, differentiated from each other rapidly over a 10–14-year period. The more different the recipient island's vegetation from that of the source, the greater the magnitude of differentiation. Further, the direction of differentiation followed an expectation based on the evolutionary diversification of insular Anolis over its entire geographic range. In addition to providing a glimpse of adaptive dynamics in one of the most extensive generic radiations on earth, the results lend support to the general argument that environment determines the evolution of morphology.

A comparison of evolutionary radiations in mainland and caribbean anolis lizards

Abstract: Comparisons between closely related radiations in different environments provide a unique window into understanding how abiotic and biotic factors shape evolu- tionary pathways. Anolis lizards have radiated extensively in the West Indies, as well as mainland Central and South America. In the Caribbean, similar communities of anole species specialized for different habitats (ecomorphs) have evolved independently on each Greater Antillean island. We examined ecological and morphological data on 49 Anolis species (33 Caribbean, 16 mainland) to investigate whether the same set of ecomorphs has arisen in mainland regions. More generally, we investigated whether the relationship between ecology and morphology was similar among anoles in the two regions. Radiations in the two regions are very different. The majority of mainland anole species exhibit morphological characteristics unlike any Caribbean ecomorph. Furthermore, rela- tionships between ecology and morphology are very different between the two sets of anole species. Among mainland anole species, toepad size is positively correlated with perch height, whereas tail length is negatively related to perch diameter. In contrast, among Caribbean anole species, both forelimb length and body mass are positively associated with perch diameter, and both tail length and hindlimb length are negatively related to perch diameter. Biomechanical considerations provide a functional basis for some of these cor- relations, but much variation remains to be explained. These findings demonstrate that factors that caused anole species to converge repeatedly in the West Indies are not present in mainland regions, and that environmental factors can strongly influence the shape of evolutionary radiations.

Evolutionary consequences of ecological release in caribbean anolis lizards

Abstract: On the large islands of the Greater Antilles, multi-species communities ofAnolislizards are composed of species specialized to use particular habitats; similar sets of specialized species have evolved independently on each island. We studied species of anoles found on small Caribbean islands. Because these islands contain at most only one other species of anole, we predicted that species on these islands should not be as specialized as Greater Antillean species; rather, they might be expected to exhibit a generalized morphology and a greater breadth of habitat use. Our findings, however, do not confirm these predictions. Lesser Antillean species do not exhibit greater breadth of habitat use than Greater Antillean species, nor do they exhibit a generalized morphology. Most species are ecologically and morphologically similar to specialized trunk-crown anoles of the Greater Antilles, although some species exhibit morphologies unlike those seen in Greater Antillean species. Among descendants of specialized Greater Antillean species occurring on one- or two-species islands, most descendants of trunk-crown species have diverged relatively little, whereas several descendants of trunk-ground anoles have diverged considerably. Consequently, we propose that ancestral species in the Greater Antilles may have been trunk-crown anoles.

A Comparative Study of Population Density and Sexual Size Dimorphism in Lizards

Abstract: Two hypotheses predict a positive relationship between population density and sex- ual size dimorphism (SSD) among species of Anolis lizards; these hypotheses focus on food competition and male-male competition for breeding territories, respectively. We first tested an underlying assumption of the food competition hypothesis, using data on the prey consumed by Anolis aeneus. This assumption is that SSD reduces intersexual food competition relative to the amount of competition expected if males and females are the same size. Contrary to this assumption, estimates of food competition were lower if males and females were the same size than if males were larger than females or vice versa. Next, we tested the prediction from both hypotheses that SSD should be positively related to female density, using data from 25 taxa (24 species) of anoles. Statistically significant relationships between these two variables were ob- tained in the vast majority of potentially correct phylogenies for the species in this data set, based on either a gradual or a speciational model of evolution. In addition to documenting a relationship between SSD and density, this study shows how comparative questions can be pursued in taxa that currently lack a definitive phylogeny.

Ecological correlates of number of subdigital lamellae in anoles

Abstract: We investigated whether a relationship exists between the number of lamellae and the height and diameter of perches utilized by a diverse set of anoline lizards. Although the number of lamellae on both the fore- and hindfoot is related to both ecological variables, these results are confounded by body size. When the effect of size is removed, the number of lamellae is related to perch height, but not perch diameter. When the data are analyzed in a phylogenetic framework, results are similar, but only the number of lamellae on the forefoot is related to perch height. Consequently, although inter- and intraspecific correlations exist among the number of lamellae on different toes of the fore- and hindfeet, examining the number of lamellae on a single toe may be insufficient for ecomorphological analyses.

Testing for unequal amounts of evolution in a continuous character on different branches of a phylogenetic tree using linear and squared-change parsimony: an example using lesser antillean anolis lizards.

Abstract: Although a large body of work investigating tests of correlated evolution of two continuous characters exists, hypotheses such as character displacement are really tests of whether substantial evolutionary change has occurred on a particular branch or branches of the phylogenetic tree. In this study, we present a methodology for testing such a hypothesis using ancestral character state reconstruction and simulation. Furthermore, we suggest how to investigate the robustness of the hypothesis test by varying the reconstruction methods or simulation parameters. As a case study, we tested a hypothesis of character displacement in body size of Caribbean Anolis lizards. We compared squared-change, weighted squared-change, and linear parsimony reconstruction methods, gradual Brownian motion and speciational models of evolution, and several resolution methods for linear parsimony. We used ancestor reconstruction methods to infer the amount of body size evolution, and tested whether evolutionary change in body size was greater on branches of the phylogenetic tree in which a transition from occupying a single-species island to a two-species island occurred. Simulations were used to generate null distributions of reconstructed body size change. The hypothesis of character displacement was tested using Wilcoxon Rank-Sums. When tested against simulated null distributions, all of the reconstruction methods resulted in more significant P-values than when standard statistical tables were used. These results confirm that P-values for tests using ancestor reconstruction methods should be assessed via simulation rather than from standard statistical tables. Linear parsimony can produce an infinite number of most parsimonious reconstructions in continuous characters. We present an example of assessing the robustness of our statistical test by exploring the sample space of possible resolutions. We compare ACCTRAN and DELTRAN resolutions of ambiguous character reconstructions in linear parsimony to the most and least conservative resolutions for our particular hypothesis.

A comparison of evolutionary radiations in Mainland and West Indian Anolis lizards. Ecology

Abstract: Comparisons between closely related radiations in different environments provide a unique window into understanding how abiotic and biotic factors shape evolutionary pathways. Anolis lizards have radiated extensively in the West Indies, as well as mainland Central and South America. In the Caribbean, similar communities of anole species specialized for different habitats (ecomorphs) have evolved independently on each Greater Antillean island. We examined ecological and morphological data on 49 Anolis species (33 Caribbean, 16 mainland) to investigate whether the same set of ecomorphs has arisen in mainland regions. More generally, we investigated whether the relationship between ecology and morphology was similar among anoles in the two regions. Radiations in the two regions are very different. The majority of mainland anole species exhibit morphological characteristics unlike any Caribbean ecomorph. Furthermore, relationships between ecology and morphology are very different between the two sets of anole species. Among mainland anole species, toepad size is positively correlated with perch height, whereas tail length is negatively related to perch diameter. In contrast, among Caribbean anole species, both forelimb length and body mass are positively associated with perch diameter, and both tail length and hindlimb length are negatively related to perch diameter. Biomechanical considerations provide a functional basis for some of these correlations, but much variation remains to be explained. These findings demonstrate that factors that caused anole species to converge repeatedly in the West Indies are not present in mainland regions, and that environmental factors can strongly influence the shape of evolutionary radiations.

Notes on Jumping Ability and Thermal Biology of the Enigmatic Anole Chamaelinorops barbouri

Abstract: Hispaniola, is remarkable. Unlike other anoles, C. barbouri is almost strictly terrestrial, lives in leaf litter, moves primarily by short hops, and has no stereotyped display behavior (Jenssen and Feely, 1991; Flores et al., 1994). Morphologically, C. barbouri possesses interzygapophysial wings that are unique among tetrapods and the function of which is poorly understood (Forsgaard, 1983). The phylogenetic relationships of C. barbouri are also uncertain. Williams (1969) proposed that Chamaelinorops is the last vestige of a pre-Anolis radiation and thus constitutes the sistergroup to the genus Anolis. By contrast, recent biochemical studies suggest that C. barbouri arose from within Anolis (Wyles and Gorman, 1980; Hedges and Burnell, 1990; Hass et al., 1993; Jackman et al., in press; but see Case and Williams, 1987) and, consequently, that novel features of C. barbouri are relatively recently derived.