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Chapter 26

Chapter 26- Speciation.docx

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Fiona Rawle

Chapter 26- Speciation -Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to selection, genetic drift, or mutation. 26.1 How Are Species Defined And Identified? -If gene flow between populations stops, then mutation, 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.As a result, allele frequencies and other characteristics in the populations diverge. When allele frequencies change sufficiently over time, populations become distinct species. Species: evolutionarily independent population or group of populations. 3 different sets of criteria to identify them: 1. The Biological Species Concept -The critical criterion for identifying species is reproductive isolation. This is a logical measuring stick because no gene flow occurs between populations that are reproductively isolated from each other. -prezygotic isolation: prevents individuals of different species from mating -postzygotic isolation: offspring of mating between members of different species do not survive or reproduce. -Although the biological species concept has a strong theoretical foundation, it has disadvantages. This criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually. In addition, It is hard to apply when closely related populations do not happen to overlap with each other geographically. 2. The Morphospecies Concept -The logic behind this concept is that distinguishing features are most likely to arise if populations are independent and isolated from gene flow. (size, shape, other morphological features) -Its disadvantage is that the features used to distinguish species are subjective. 3. The Phylogenetic Species Concept -Based on reconstructing evolutionary history of populations. -On a tree of populations, each tip is a phylogenetic species. Monophyletic group/clade/lineage: consists of an ancestral population, all of its descendants, and only those descendants Advantages: Can be applied to any population (fossil, asexual, sexual), it is logical because populations are distinct enough to be monophyletic only if they are isolated from gene flow and have evolved independently Disadvantage: Relatively few well-estimated phylogenies are currently available Species Definitions inAction: The Case of the Dusky Seaside Sparrow Subspecies: populations that live in discrete geographic areas and have distinguishing features, such as coloration or calls, but are not considered distinct enough to be called a separate species. 26.2 Isolation and Divergence in Allopatry -Genetic isolation happens routinely when populations become physically separated. Physical isolation, in turn, occurs in one of two ways: dispersal or vicariance. Vicariance: A physical splitting of habitat Allopatric speciation: Speciation that behins with physical isolation via either dispersal of vicariance Allopatry: Populations that live in different areas Biogeography: study of how species and populations are distributed geographically. Colonization events are likely to trigger speciation for two reasons: 1. The physical separation between populations reduces or eliminates gene flow 2. Genetic drift will cause the old and new populations to diverge rapidly, The characteristics of a colonizing population are likely to be different from the characteristics of the source populations due to founder effects. Subsequent natural selection may extend the rapid divergence that begins with genetic drift. Physical isolation of populations via dispersal or vicariance produces genetic isolation- the first requirement of speciation. When genetic isolation is accompanied by genetic divergence due to mutation, selection, and genetic drift, speciation results. 26.3 Isolation and Divergence in Sympatry Sympatry: When populations or species live in the same geographic area or at least close enough for interbreeding Sympatric speciation: Speciation that occurs even though gene flow is possible Even though sympatric populations are not physically isolated, they may be isolated by preference for different habitats How Can Polyploidy lead to Speciation? Mutation reduces gene flow between mutant and normal, or wild-type, individuals. It do
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