A genetic paradox exists in invasion biology: how do introduced populations, whose genetic variation has probably been depleted by population bottlenecks, persist and adapt to new conditions? Lessons from conservation genetics show that reduced genetic variation due to genetic drift and founder effects limits the ability of a population to adapt, and small population size increases the risk of extinction. Nonetheless, many introduced species experiencing these same conditions during initial introductions persist, expand their ranges, evolve rapidly and become invasive. To address this issue, we studied the brown anole, a worldwide invasive lizard. Genetic analyses indicate that at least eight introductions have occurred in Florida from across this lizard's native range, blending genetic variation from different geographic source populations and producing populations that contain substantially more, not less, genetic variation than native populations. Moreover, recently introduced brown anole populations around the world originate from Florida, and some have maintained these elevated levels of genetic variation. Here we show that one key to invasion success may be the occurrence of multiple introductions that transform among-population variation in native ranges to within-population variation in introduced areas. Furthermore, these genetically variable populations may be particularly potent sources for introductions elsewhere. The growing problem of invasive species introductions brings considerable economic and biological costs. If these costs are to be mitigated, a greater understanding of the causes, progression and consequences of biological invasions is needed.
Sympatric speciation is often proposed to account for species-rich adaptive radiations within lakes or islands, where barriers to gene flow or dispersal may be lacking. However, allopatric speciation may also occur in such situations, especially when ranges are fragmented by fluctuating water levels. We test the hypothesis that Miocene fragmentation of Cuba into three palaeo-archipelagos accompanied species-level divergence in the adaptive radiation of West Indian Anolis lizards. Analysis of morphology, mitochondrial DNA (mt DNA) and nuclear DNA in the Cuban green anoles (carolinensis subgroup) strongly supports three pre dictions made by this hypothesis. First, three geographical sets of populations, whose ranges correspond with palaeo-archipelago boundaries, are distinct and warrant recognition as independent evolutionary lineages or species. Coalescence of nuclear sequence fragments sampled from these species and the large divergences observed between their mtDNA haplotypes suggest separation prior to the subsequent unification of Cuba ca. 5 Myr ago. Second, molecular phylogenetic relationships among these species reflect historical geographical relationships rather than morphological similarity. Third, all three species remain distinct despite extensive geographical contact subsequent to island unification, occasional hybridization and introgression of mtDNA haplotypes. Allopatric speciation initiated during partial island submergence may play an important role in speciation during the adaptive radiation of Anolis lizards.
The role of behaviour in evolutionary change has long been debated. On the one hand, behavioural changes may expose individuals to new selective pressures by altering the way that organisms interact with the environment, thus driving evolutionary divergence. Alternatively, behaviour can act to retard evolutionary change: by altering behavioural patterns in the face of new environmental conditions, organisms can minimize exposure to new selective pressures. This constraining influence of behaviour has been put forward as an explanation for evolutionary stasis within lineages and niche conservatism within clades. Nonetheless, the hypothesis that behavioural change prevents natural selection from operating in new environments has never been experimentally tested. We conducted a controlled and replicated experimental study of selection in entirely natural populations; we demonstrate that lizards alter their habitat use in the presence of an introduced predator, but that these behavioural shifts do not prevent patterns of natural selection from changing in experimental populations.
Phenotypic differences among species are known to have functional consequences that in turn allow species to use different habitats. However, the role of behaviour in this ecomorphological paradigm is not well defined. We investigated the relationship between morphology, ecology and escape behaviour among 25 species of the lizard clade Liolaemus in a phylogenetic framework. We demonstrate that the relationship between morphology and characteristics of habitat structure shows little or no association, consistent with a previous study on this group. However, a significant relationship was found between morphology and escape behaviour with the distance a lizard moved from a potential predator correlated with body width, axilla-groin length, and pelvis width. A significant relationship between escape behaviour and habitat structure occupation was found; lizards that occupied tree trunks and open ground ran longer distances from predators and were found greater distances from shelter. Behavioural strategies used by these lizards in open habitats appear to have made unnecessary the evolution of limb morphology that has occurred in other lizards from other clades that are found in open settings. Understanding differences in patterns of ecomorphological relationships among clades is an important component for studying adaptive diversification.
Two recent hurricanes passed directly over the northern Bahamas 2 years apart, allowing a comparison of their effects on lizard populations inhabiting exactly the same islands. The hurricanes differed in two ways: one struck during the reproductive season and was relatively severe; the other struck after most reproduction had taken place and was milder. The late-season hurricane produced a significant relation between population reduction and lowness of the island that lasted at least through two seasons; the earlier hurricane produced no such relationship. The late-season hurricane wiped out populations of lizards on two islands (two of the three lowest) that the earlier hurricane failed to exterminate even though it was stronger. We relate these effects to the fact that the study lizards regenerated from the earlier hurricane only via the egg stage, whereas eggs were unavailable when the later storm struck and regeneration was via hatched lizards. We discriminate and illustrate four kinds of hurricanes, cross-classified by two contrasts: earlier vs. later and stronger vs. weaker. A later, stronger hurricane completely exterminated lizard populations at a second Bahamian site, whereas an earlier, weaker hurricane had no detectable effect at a third Bahamian site. We suggest that, in addition to severity, the timing of a hurricane as it coincides with reproductive scheduling or other phenological aspects may determine the magnitude of its effect on a variety of organisms.
Studies on the lizard, Anolis sagrei, revealed that after Hurricane Floyd devastated the Bahamas in 1999, some populations consisted only of hatchlings. Because the storm surge of the hurricane completely inundated these islands, apparently for up to 6 h, survival of anole eggs in salt water for such periods is implied. To test this hypothesis directly, we placed A. sagrei eggs in saltwater for 3 or 6 h with unimmersed eggs serving as the control. Hatching success and incubation time did not differ among the three treatments. These findings help explain the persistence of anole populations on small islands vulnerable to hurricanes.
Niche conservatism--the tendency for closely related species to be ecologically similar--is widespread. However, most studies compare closely related taxa that occur in allopatry; in sympatry, the stabilizing forces that promote niche conservatism, and thus inhibit niche shifts, may be countered by natural selection favouring ecological divergence to minimize the intensity of interspecific interactions. Consequently, the relative importance of niche conservatism versus niche divergence in determining community structure has received little attention. Here, we examine a tropical lizard community in which species have a long evolutionary history of ecological interaction. We find that evolutionary divergence overcomes niche conservatism: closely related species are no more ecologically similar than expected by random divergence and some distantly related species are ecologically similar, leading to a community in which the relationship between ecological similarity and phylogenetic relatedness is very weak. Despite this lack of niche conservatism, the ecological structuring of the community has a phylogenetic component: niche complementarity only occurs among distantly related species, which suggests that the strength of ecological interactions among species may be related to phylogeny, but it is not necessarily the most closely related species that interact most strongly.
Identification of general properties of evolutionary radiations has been hindered by the lack of a general statistical and phylogenetic approach applicable across diverse taxa. We present a comparative analytical framework for examining phylogenetic patterns of diversification and morphological disparity with data from four iguanian-lizard taxa that exhibit substantially different patterns of evolution. Taxa whose diversification occurred disproportionately early in their evolutionary history partition more of their morphological disparity among, rather than within, subclades. This inverse relationship between timing of diversification and morphological disparity within subclades may be a general feature that transcends the historically contingent properties of different evolutionary radiations.
The study of adaptive radiations has played a fundamental role in understanding mechanisms of evolution. A recent resurgence in the study of adaptive radiations highlights a gap in our knowledge about determining whether a clade constitutes adaptive diversification. Specifically, no objective criteria exist to judge whether a clade constitutes an adaptive radiation. Most clades, given enough time, will diversify adaptively to some extent; therefore, we argue that the term "adaptive radiation" should be reserved for those clades that are exceptionally diverse in terms of the range of habitats occupied and attendant morphological adaptations. Making such a definition operational, however, requires a comparative analysis of many clades. Only by comparing clades can one distinguish those that are exceptionally diverse (or nondiverse) from those exhibiting a normal degree of adaptive disparity. We propose such a test, focusing on disparity in the ecological morphology of monophyletic groups within the lizard family Iguanidae. We find that two clades, the Polychrotinae and Phrynosomatinae, are exceptionally diverse and that two others, the Crotaphytinae and Oplurinae, are exceptionally nondiverse. Potential explanations for differences in diversity are discussed, as are caveats and future extensions of our approach.
We report the results of phylogenetic analyses of 1447 bases of mitochondrial DNA sequence for 21 populations representing seven species of the Anolis grahami series (A. conspersus, A. garmani, A. grahami, A. lineatopus, A. opalinus, A. reconditus, and A. valencienni), six of which occur on Jamaica. These data include 705 characters that are phylogenetically informative according to parsimony. A parsimony analysis of these data combined with previously published allozymic data yields a single most parsimonious tree with strong support for monophyly of the A. grahami series, the sister-group relationship between Anolis lineatopus and A. reconditus and a clade composed of Anolis garmani, A. grahami, and A. opalinus. Based on DNA data alone, A. conspersus is nested within A. grahami. Haplotypes sampled from geographic populations of A. grahami, A. lineatopus, and A. opalinus are highly divergent (approximately 12-15% sequence difference on average for each species) and show similar phylogeographic patterns, suggesting that each of these currently recognized species may be a complex of species. Anolis valencienni also shows high sequence divergence among haplotypes from different geographic populations (approximately 8% sequence difference) and may contain cryptic species. Divergence among haplotypes within A. garmani is substantially lower (approximately 3% sequence difference), and phylogeographic patterns are significantly different from those observed in A. grahami, A. lineatopus and A. opalinus.
Why convergent evolution occurs among some species occupying similar habitats but not among others is a question that has received surprisingly little attention. Caribbean Anolis lizards, known for their extensive convergent evolution among islands in the Greater Antilles, are an appropriate group with which to address this question. Despite the well-documented pattern of between-island convergence, some Greater Antillean anoles are not obviously part of the convergence syndrome. One example involves aquatic anoles--species that are found near to and readily enter streams-which have evolved independently twice in the Caribbean and also twice on mainland Central America. Despite being found in similar habitats, no previous study has investigated whether aquatic anoles represent yet another case of morphological convergence. We tested this hypothesis by collecting morphological data for seven aquatic anole species and 29 species from the six convergent types of Greater Antillean habitat specialists. We failed to find evidence for morphological convergence: the two Caribbean aquatic species are greatly dissimilar to each other and to the Central American species, which, however, may be convergent upon each other. We suggest two possible reasons for this lack of convergence in an otherwise highly convergent system: either there is more than one habitat type occupied by anoles in the proximity of water, or there is more than one way to adapt to a single aquatic habitat. We estimate that almost all of the 113 species of Greater Antillean anoles occupy habitats that are also used by distantly related species, but only 15% of these species are not morphologically similar to their distantly related ecological counterparts. Comparative data from other taxa would help enlighten the question of why the extent of convergence is so great in some lineages and not in others.