Chapter 25

Chapter 25 - Chapter 25: Speciation Darwin said little on...

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Unformatted text preview: Chapter 25: Speciation Darwin said little on how species arise His data and analyses focused on the process of natural selection and the changes that occur within populations over time He spent much less time considering changes that occur between populations Biologists have realized that populations of the same species may diverge from each other when they are isolated in terms of gene flow If mutation, selection, and genetic drift cause isolated populations to diverge sufficiently, distinct types, or species, formthat is the process of speciation takes place When speciation is complete, a new branch has been added to the tree of life Speciation results from genetic isolation and genetic divergence Isolation results from lack of gene flow, and divergence occurs because selection, genetic drift, and mutation proceed independently in the isolated populations 25.1 DEFINING AND IDENTIFYING SPECIES Species are distinct types of organisms and represent evolutionarily independent groups Species are distinct from one another in appearance, behavior, habitat use, or other traits These characteristics differ among species because their genetic characteristics differ Genetic distinctions occur because mutation, selection, and drift act on each species independently of what is happening in other populations Lack of gene flow makes one species evolutionary independent of other species Gene flow eliminates genetic differences among populations Allele frequencies and the populations characteristics, become more alike when gene flow occurs between them If gene flow between populations is extensive and continues over time, it eventually causes even highly distinct populations to coalesce into the unit known as a species If gene flow between populations stops, then mutations, selection, and drift begin to act on the populations independently If a new mutation creates an allele that changes the phenotype of individuals in one population, there is no longer any way for that allele to appear in the other population When allele frequencies change sufficiently over time, populations become distinct species Formally, then, a species is defined as an evolutionary independent population or group of populations Three criteria for identifying species are in common use o The biological species concept o The morhpospecies concept o The phylogenetic species concept The Biological Species Concept According to the biological species concept, the critical criterion for identifying species is reproductive isolation...
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This note was uploaded on 04/17/2008 for the course BIOL 202 taught by Professor Kopeny during the Spring '08 term at UVA.

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Chapter 25 - Chapter 25: Speciation Darwin said little on...

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