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Chapter 26

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Jon Stone

Chapter 26 SpeciationIf mutation selection and genetic drift cause isolated populations to diverge sufficiently distinct types or species form a process called speciationSpeciation is a splitting event that creates two or more distinct species from a single ancestral groupWhen speciation is complete a new branch has been added to the tree of lifeIn essence speciation results from genetic isolation and genetic divergenceoIsolation results from lack of gene flow and divergence occurs because selection genetic drift and mutation proceed independently in the isolate populationsHow Species are Defined and IdentifiedSpecies are distinct types of organisms and represent evolutionarily independent groupsSpecies are distinct from one another in appearance behaviour habitat use or other traitsThese characteristics differ among species because their genetic characteristics differGenetic distinctions occur because mutation selection and drift act on each species independently of what is happening in other populationsAllele frequencies in populations and thus the populations characteristics become more alike when gene flow occurs between themIf gene flow between populations is extensive and continues over time it eventually causes even highly distinct populations to coalesce into speciesConversely if gene flow between populations stops then mutation selection and drift begin to act on the populations independentlyIf 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 populationAs a result allele frequencies and other characteristics in the populations divergeWhen allele frequencies change sufficiently over time populations become distinct speciesA species is then defined as an evolutionarily independent population or group of populationsThe three criteria for identifying species areoThe biological species conceptoThe morphospecies conceptoThe phylogenetic species conceptThe Biological Species ConceptoAccording to the biological species concept the critical criterion for identifying species is reproductive isolationoNo gene flow occurs between populations that are reproductively isolated from each otheroSpecifically if two different populations do not interbreed in nature or if they fail to produce viable and fertile offspring when matings take place then they are considered distinct species oGroups that naturally or potentially interbreed and that are reproductively isolated from other groups belong to the same species oReproductive isolation can result from a wide variety of events and processesoTo organize the various mechanisms that stop gene flow between populations biologists distinguishPrezygotic isolationPrevents individuals of different species from matingReproductive isolation occurs before mating can occurTemporal isolationoPopulations are isolated because they breed at different timesoEx Bishop pines and Monterey pines release their pollen at different times of the yearHabitat isolationoPopulations are isolated because they breed in different habitats
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