Scale: 1 cm. Key Concepts Adaptive diversification and speciation resulting from competition for a broadly distributed resource is thus likely if the niche width is fixed and relatively narrow or free to evolve but subject to costs. However, so far, all genomic reanalyses of classic examples of sympatric speciation indicate secondary gene flow occurred. Copyright © 2020 Elsevier B.V. or its licensors or contributors. It is likely that other traits in freshwater stickleback populations are the result of selection of standing genetic variation brought from the sea. Furthermore, I summarize empirical evidence that disruptive selection in stickleback drives other forms of evolutionary diversification (plasticity, increased trait variance, and sexual dimorphism) instead of speciation. Felsenstein (21) pointed out that in the absence of very strong reproductive isolation between hybridizing populations recombination between genes governing assortative mating and genes under divergent natural selection will cause existing levels of assortative mating between populations to decay and inhibit the evolution of even stronger assortative mating. We present a hypothesis for the repetitive origin of freshwater stickleback species involving natural selection of standing genetic variation. Furthermore, it is unclear if and how speciation-promoting mechanisms such as magic traits, phenotype matching, and physical linkage of coadapted alleles promote rapid bursts of speciation. Theoretical models suggest that resource competition can lead to the adaptive splitting of consumer populations into diverging lineages, that is, to adaptive diversification. Individual marine ecotypes displaying freshwater-adaptive building blocks were discovered at a frequency of less than 2% . Although numerous cases now indicate the importance of ecological speciation in nature, very little is known about the genetics of the process. We acknowledge that the limnetic and benthic habitats in a lake are not discrete and treating them as two distinct habitats is certainly an oversimplification, yet this distinction is a common and fairly reasonable assumption. Here, we summarize the genetics of premating and postzygotic isolation and the role of standing genetic variation in ecological speciation. Today, sticklebacks exist in many different shapes and sizes (fig. Mutations were assumed to be pleiotropic, which means that while they change the population mean for the trait under directional selection in each environment, they have the side-effect of changing the mean in other traits, too. (d) Speciation can occur when disruptive selection and assortment are near the upper end of plausible empirical values (2γ=0.4, r=0.8). Results from this research (formatted as data sheets included in the teaching notes) are provided to students who design experiments and then compare actual data to investigate why benthic and limnetic sticklebacks seldom interbreed in Paxton Lake. 3B). Genome-wide Patterns of Adaptation and Speciation and Demographic Histories of Young Radiations of Cichlids, Divergence and reproductive isolation in the bushcricket Mecopoda elongata, Exploring the origin and maintenance of biodiversity : insights from the bilaterally asymmetrical cichlid fish Perissodus microlepis, Comparing Adaptive Radiations Across Space, Time, and Taxa, Resource diversity promotes among‐individual diet variation, but not genomic diversity, in lake stickleback. Not as many populations have been surveyed as in the case of Eda, but the low-pigmentation alleles present in 3 freshwater populations in British Columbia, Washington state, and California form a monophyletic clade (Fig. One of the most glaring deficiencies is the almost complete absence of information on the genetics of ecological speciation. Because egg isotopes are derived from females, this correlation reflects assortative mating between males and females by diet.