Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Sympatric speciation Evolution Biol. 4974/5974 D.F. Tomback Sympatric Speciation Biol. 4974/5974 Evolution Figures from Hall and Hallgrimsson, 2014, Strickberger’s Evolution, Jones and Bartlett Learning goals Know and understand: • How populations with phenotypic plasticity are potential candidates for sympatric speciation. • How the process of sympatric speciation differs from that of allopatric speciation. • How sympatric speciation works by the process of disruptive selection, and how assortative mating can lead to reproductive isolation. • Examples illustrating sympatric speciation, including the North American fly (apple maggot), cichlid fish diversification, and redwood salamander forming a “ring species”. Reaction norms and phenotypic plasticity For some species, phenotype varies with the environment, which means that the genotype has more than one reaction norm. This leads to the possibility of sympatric speciation. • Different phenotypes or morphs are produced in response to different abiotic or biotic conditions. • This response is referred to as phenotypic plasticity. Examples discussed in the text: • Rotifers and water fleas develop protective spines in the presence of predators. • When food is restricted, tadpoles of spadefoot toads develop a cannabilistic morph with bigger heads and jaws. • In three-spined stickleback fish, the pelvic girdle and spines may be reduced in the absence of predators. 1 Sympatric speciation Evolution Biol. 4974/5974 D.F. Tomback Example: North American emerald moth Caterpillars feed on oak trees. They vary in morphology, depending on time of hatching: • Spring hatching: Caterpillars feed on catkins, which they resemble, the catkin morphs. • Summer hatching: Caterpillars feed on oak leaves and mimic twigs, the twig morphs. The difference in morphology is triggered by the tannins in oak leaves. Fig. 22.4 Sympatric speciation Defined as speciation without geographical isolation. Phenotypic plasticity within a population can lead to speciation. Reproductive isolation evolves between different phenotypes. Sympatric speciation by disruptive selection Most sympatric speciation models rely on disruptive selection: Individuals sort by phenotype (morph) into different microhabitats or on different resources. • Intermediate phenotypes are not well-adapted and selected against. • Natural selection increases an individual’s ability to discriminate between phenotypes (“us” vs. “them”). • Leads to assortative mating: like mating with like. • This reduces gene flow between morphs. Assortative mating may result from imprinting, pheromone differences, host plant differences--where mating occurs, or fertility differences. Example: Pea aphids suck sap from plants in the pea family. • Aphids raised on red clover prefer red clover, and aphids raised on alfalfa prefer alfalfa. • Hybrids do poorly. • Populations are now diverging (Via 1991). 2 Sympatric speciation Evolution Biol. 4974/5974 D.F. Tomback Sympatric speciation in the North American fly The North American fly or apple maggot is undergoing sympatric speciation. • Natural diet of the maggot (larva) is the fruit of the hawthorn bush, which is a distant relative of apples. • Cultivated apples were introduced to North America around 1800. • Within 50 years some maggots fed on apples. • The hawthorn-feeding morph and apple-feeding morph would not switch between foods. • The adults of these different morphs emerge at different times, coinciding with fruit ripening. • The different morphs are genetically differentiated. • Speciation is not entirely complete, but changes have occurred in the genome. Sympatric speciation in cichlid fishes Adaptive radiation occurs when a lineage rapidly diversifies, producing many species adapted to a similar ecological lifestyle or niche. Rapid diversification in cichlid fish in the African Rift Valley and in Nicaragua and in the Amazon. • Since the end of the ice age (14,000 years ago in Africa), 300 new cichlid species arose in Lake Victoria. • Diversification also in other Rift Valley lakes. • Adaptive radiation by sympatric speciation, as a consequence of a new adaptation, pharyngeal jaws. • With two two sets of jaws, the fish adapted to different, specialized diet. • The outer set of jaws used to capture prey; the pharyngeal set used to grind up prey. Cichlid diversification Fig. 22 B2.1 Fig. 22.7 3 Sympatric speciation Evolution Biol. 4974/5974 D.F. Tomback Ring species Fig. 22.10 Sympatric speciation in progress in ring species. Known from several groups: e.g., Greenish Warblers, Black-headed gulls, and the redwood salamander. • Redwood salamander, Ensatina escholtzii comprises 7 subspecies in California Central valley. • All overlap and interbreed, except for several southernmost populations of E. s. klauberi and E. s. escholtzii, which do not hybridize. • Speciation appears complete in these southern populations. Study questions • What is phenotypic plasticity? Why are populations with phenotypic plasticity good candidates for sympatric speciation? • Explain phenotypic plasticity in the North American Emerald moth, and how it is triggered by a biotic factor. Why is phenotypic plasticity adaptive for the moth? • How does the process of sympatric speciation differ from that of allopatric speciation? Explain how both disruptive selection and assortative mating can lead tosympatric speciation. • Explain the process of sympatric speciation, as it is now occurring in the North American fly. What started this process? • Cichlid fish in African lakes have experienced explosive sympatric speciation. How rapidly has this occurred? Explain how a new adaptation may have contributed to this tremendous adaptive radiation of fish. • Explain how the redwood salamander “ring species” represents a gradient in the degree of completion of sympatric speciation. The subspecies appear to be in the process of speciating. 4