Lectures 22-24.Speciation Study Questions.2013

Partial isolation and reinforcement selection acts to

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Unformatted text preview: ALL of the taxa/species descended from a specific common ancestor. BIS 2B, Spring 2013 Speciation Page 3 4) Paraphyletic clade: a taxonomic group that excludes some of the descendants from a specific common ancestor. 5) Polyphyletic clade: a taxonomic group that does not contain the most recent common ancestor of its members. 4. Three geographic scenarios that could reduce/eliminate gene flow (geographic classification): a) Geographic Isolation/Allopatric Scenarios: Populations have discrete ranges ⇒ geographical barriers limit gene flow. 1) Vicariant Models: some kind of geographic barrier arises that subdivides a formerly interconnected set of populations • Diversifying or other forms of selection could then promote differentiation between populations living in different habitats. • Example: Speciation in snapping shrimp across the Isthmus of Panama 2) Dispersal Models: a few “lucky” dispersing propagules cross a geographic barrier, and successfully colonize a new habitat. • Founder effects and genetic drift in small populations promote population differentiation • Example: Speciation and adaptive radiation in Hawaiian Drosophila 3) Secondary contact: What happens when a geographic barrier disappears and two geographically isolated populations come back into contact? • Complete isolation and no hybridization ⇒ • Hybridization, introgression, and the merging of populations. • Partial isolation and reinforcement: selection acts to reduce matings between the parental populations when they produce hybrids with low relative fitness b) Parapatric Scenarios: Populations have abutting ranges ⇒ Environmental discontinuities can limit gene flow. 1) Example: Speciation on mine tailings in pasture grass (Agrostis tenuis) • Secondary evolution of divergent flowering times (late vs. early flowering) c) Sympatric Scenarios: Populations/incipient species have overlapping ranges 1) Primary sympatry vs. secondary sympatry • Primary sympatry: Species differentiated in sympatry (true sympatric speciation). • Secondary sympatry: Species differentiated in allopatry or parapatry. ⇒ After becoming reproductively isolated, the populations moved into sympatry (not the condition under which reproductive isolation evolved). • Using phylogenies for distinguishing between primary vs. secondary sympatry 2) Mechanisms of sympatric speciation. • Multiple niche polymorphism: Sympatric speciation in cr...
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