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Chapter 25 Speciation - Chapter 25 Speciation-Darwin...

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Chapter 25: Speciation --Darwin concentrated on changed within populations, and not at all between populations --Populations of the same species can diverge from each other (evolve independently) when there is no gene flow between them. (Remember, gene flow equalizes allele frequencies between populations) -If mutation, selection, genetic drift causes enough divergence, then speciation takes place. -Speciation —creation of 2 or more distinct species from a single ancestral group.) -results from genetic isolation and genetic divergence: Isolation = no gene flow Divergence = selection, genetic drift, mutation I. Defining and identifying species A. Species —distinct and evolutionary independent group of organisms. i. Each species has different genetics because mutation, drift, and selection act on each species separately. ii. Evolutionary independence —if gene flow between populations stops, then the other 3 things being to act on each population independently and this can lead to speciation if it happens long enough. Each species evolves independently of each other. 1. ex) if a mutation occurs in one population and there is no gene flow, then the other population cannot have this mutated allele, therefore the two populations differ and evolve independently. . 2. species can also be defined as an evolutionary independent population, but this can be hard to back with experimental data, so we use 3 criteria for identifying species: 3 Concepts to define and identify species: B. Biological Species Concept —reproductive isolation between species (they don’t breed and don’t make viable offspring) i. Mechanisms that stop gene flow b/t populations: 1. Prezygotic isolation —prevents organisms of different species from mating, or stops fertilization if mating occurs. (Reproductive isolation occurs before mating.) A. Temporal isolation B. Habitat isolatoin C. Behavioral isolation D. Gametic isolation E. Mechanic isolation 2. Postzygotic isolation —offspring of parents from 2 different species has low
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fitness. Results in fetal death or infertile offspring. A. Hybrid viability reduced B. Hybrid sterility reduced C. Hybrid breakdown ii. Advantages: reproductive isolation = evolutionary independence iii. disadvantages: reproductive isolation can’t be evaluated in fossils or asexual organisms C.
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