Ecological opportunity – through entry into a new environment, the origin of a key innovation or extinction of antagonists – is widely thought to link ecological population dynamics to evolutionary diversification. The population‐level processes arising from ecological opportunity are well documented under the concept of ecological release. However, there is little consensus as to how these processes promote phenotypic diversification, rapid speciation and adaptive radiation. We propose that ecological opportunity could promote adaptive radiation by generating specific changes to the selective regimes acting on natural populations, both by relaxing effective stabilizing selection and by creating conditions that ultimately generate diversifying selection. We assess theoretical and empirical evidence for these effects of ecological opportunity and review emerging phylogenetic approaches that attempt to detect the signature of ecological opportunity across geological time. Finally, we evaluate the evidence for the evolutionary effects of ecological opportunity in the diversification of Caribbean Anolis lizards. Some of the processes that could link ecological opportunity to adaptive radiation are well documented, but others remain unsupported. We suggest that more study is required to characterize the form of natural selection acting on natural populations and to better describe the relationship between ecological opportunity and speciation rates.

Comparative studies of codistributed taxa test the degree to which historical processes have shaped contemporary population structure. Discordant patterns of lineage divergence among taxa indicate that species differ in their response to common historical processes. The complex geologic landscape of the Isthmus of Central America provides an ideal setting to test the effects of vicariance and other biogeographic factors on population history. We compared divergence patterns between two codistributed Neotropical frogs (Dendropsophus ebraccatus and Agalychnis callidryas) that exhibit colour pattern polymorphisms among populations, and found significant differences between them in phenotypic and genetic divergence among populations. Colour pattern in D. ebraccatus did not vary with genetic or geographic distance, while colour pattern co-varied with patterns of gene flow in A. callidryas. In addition, we detected significant species differences in the phylogenetic history of populations, gene flow among them, and the extent to which historical diversification and recent gene flow have been restricted by five biogeographic barriers in Costa Rica and Panama. We inferred that alternate microevolutionary processes explain the unique patterns of diversification in each taxon. Our study underscores how differences in selective regimes and species-typical ecological and life-history traits maintain spatial patterns of diversification.

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Aim  Studies of the spatial and temporal patterns of phenotypic diversity help to elucidate the fine‐scale evolutionary and ecological mechanisms underlying geographical differentiation. The red‐eyed tree frog, Agalychnis callidryas, is a widespread Neotropical frog that exhibits a broad range of polymorphism of coloration and flank‐stripe pattern. The goal of this study was two‐fold: first, to investigate the stability of polymorphisms over a 38‐year period; and second, to evaluate biogeographical hypotheses of diversification between lower Central American populations through quantification of phenotypic diversity on a fine geographical scale. Location  This study was conducted at 12 sites across four biogeographical regions in Costa Rica and Panama. Methods  We quantified colour, categorized flank‐stripe pattern from digital photos taken during field sampling, and measured body size for each individual. We compared the regional frequency of each flank‐stripe pattern in 2005 with the frequency distribution from a previous study of the same sites in 1967 using logistic regression analyses. We determined the geographical signal of leg coloration by employing linear discriminant function analyses to generate a classification matrix based on covariance similarities, and by comparison of the average hue values within and between regions. Results  We found a temporal shift in the frequency of flank‐stripe patterns in three of four regions over 38 years. Based on measures of leg coloration, the frequency distribution of flank‐stripe patterns and body size, we conclude that A. callidryas populations are easily distinguishable at a regional scale. Main conclusions  Agalychnis callidryas exhibits regional differentiation in all phenotypic traits measured in this study, supporting the role of three major biogeographical barriers to gene exchange. We found evidence of a putative contact zone between polytypic regions in Costa Rica. In addition, we report temporal instability of the relative frequency of stripe patterns located on the flanks. The ecological and evolutionary mechanisms that may underlie this variation include sexual selection and avoidance of predators.

We used mitochondrial DNA sequence comparisons to assess range-wide population structure and historical patterns of differentiation among populations of the bog turtle (Glyptemys muhlenbergii). This species is one of North America's smallest and most endangered pond turtles, and is currently found in three largely disjunct groups of populations: in the southern U.S., in the northeast, and in the Finger Lakes and Lake Ontario Plains region of western and central New York State. All the New York sites and most of the northeastern sites were glaciated during the Pleistocene. We surveyed 2793 bases pairs of mitochondrial DNA spanning three genes (cytb, nd4, and d-loop) in 41 individuals from 21 populations throughout most of the bog turtle's distribution. We found surprisingly low levels of divergence among populations, even in southern populations that have been hypothesized as refugia during times of climate change. Our data suggest populations of bog turtle's suffered a bottleneck, followed by a rapid postPleistocene expansion into northern segments of the species' range. We discuss historical changes in habitat availability and climate that may have influenced the historical deployment of lineages in this species, and possible life history traits and habitat dynamics that might also contribute to the overall low genetic diversity across its range.

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Ecological opportunity is any change that allows populations to escape selection from competition and predation. After encountering ecological opportunity, populations may experience ecological release: enlarged population size, broadened resource use, and/or increased morphological variation. We identified ecological opportunity and tested for ecological release in three lizard colonists of White Sands, New Mexico (Sceloporus undulatus, Holbrookia maculata, and Aspidoscelis inornata). First, we provide evidence for ecological opportunity by demonstrating reduced species richness and abundance of potential competitors and predators at White Sands relative to nearby dark soils habitats. Second, we characterize ecological release at White Sands by demonstrating density compensation in the three White Sands lizard species and expanded resource use in White Sands S. undulatus. Contrary to predictions from ecological release models, we observed directional trait change but not increased trait variation in S. undulatus. Our results suggest that ecological opportunity and ecological release can be identified in natural populations, especially those that have recently colonized isolated ecosystems.

Spatial patterns of heritable phenotypic diversity reflect the relative roles of gene flow and selection in determining geographic variation within a species. We quantified color differentiation and genetic divergence among 20 populations of the red-eyed tree frog (Agalychnis callidryas) in lower Central America. Phylogenetic analyses revealed 5 well-supported mitochondrial DNA clades, and we infer from our phylogeny that geographic barriers have played a large role in structuring populations. Two phenotypic characters varied independently among isolated population groups: Flank coloration distinguished Caribbean from Pacific individuals, whereas leg coloration exhibited a more complex geographic pattern. We detected 3 generalized spatial patterns of genetic and phenotypic diversity: 1) phenotypic differentiation in the presence of historical connectivity, 2) phenotypic uniformity across genetically differentiated regions, and 3) codistribution of genetic and phenotypic characters. These patterns indicate that phenotypic diversification is highly regionalized and can result from spatial variation in localized adaptations, geographic isolation, genetic drift, and/or evolutionary stasis. Although the mode of selection underlying color variation was not the focal objective of this study, we discuss the possible roles of natural and sexual selection in mediating population differentiation. Our study underscores the fact that selection gradients vary across relatively small spatial scales, even in species that occupy relatively homogeneous environments.