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.

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Newly formed polyploid lineages must contend with several obstacles to avoid extinction, including minority cytotype exclusion, competition, and inbreeding depression. If polyploidization results in immediate divergence of phenotypic characters these hurdles may be reduced and establishment made more likely. In addition, if polyploidization alters the phenotypic and genotypic associations between traits, i.e. the P and G matrices, polyploids may be able to explore novel evolutionary paths, facilitating their divergence and successful establishment. Here we report results from a study of the perennial plant Heuchera grossulariifolia in which the phenotypic divergence and changes in phenotypic and genotypic covariance matrices caused by neopolyploidization have been estimated. Our results reveal that polyploidization causes immediate divergence for traits relevant to establishment and results in significant changes in the structure of the phenotypic covariance matrix. In contrast, our results do not provide evidence that polyploidization results in immediate and substantial shifts in the genetic covariance matrix.

The prevalence of polyploidy among flowering plants is surprising given the hurdles impeding the establishment and persistence of novel polyploid lineages. New polyploid lineages are doomed to extinction through minority cytotype exclusion except in cases of strong assortative mating, reproductive assurance through selfing or vegetative reproduction, or large intrinsic fitness advantages. Thus, much work has focused on a search for adaptive advantages associated with polyploidy, such as increased competitive ability, enhanced ecological tolerances, or increased resistance to pathogens. Yet, no consistent adaptive advantages of polyploidy have been identified. Here we develop a model of a diploid population which sporadically gives rise to novel polyploids to investigate the potential for polyploid establishment and persistence in the absence of any intrinsic fitness advantages, and with only very small levels of initial assortative mating or niche differentiation generated by dosage effects of genome duplication. Our results show that by allowing assortative mating and competitive interactions to evolve, establishment of novel polyploid lineages becomes common. Additional scenarios where adaptive optima change over time reveal that rapid environmental change promotes the fixation of polyploid lineages. These results help to explain recent empirical findings that suggest many contemporary polyploid lineages arose during the Cretaceous-Tertiary extinction without invoking adaptive advantages of polyploidy.

Despite the well-documented historical importance of polyploidy, the mechanisms responsible for the establishment and evolutionary success of novel polyploid lineages remain unresolved. One possibility, which has not been previously evaluated theoretically, is that novel polyploid lineages are initially more resistant to pathogens than the diploid progenitor species. Here, we explore this possibility by developing and analysing mathematical models of interactions between newly formed polyploid lineages and their pathogens. We find that for the genetic mechanisms of pathogen resistance with the best empirical support, newly formed polyploid populations of hosts are expected to be more resistant than their diploid progenitors. This effect can be quite strong and, in the case of perennial species with recurrent polyploid formation, may last indefinitely, potentially providing a general explanation for the successful establishment of novel polyploid lineages.