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Speciation Notes

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University of Pittsburgh
Biological Sciences
BIOSC 0160

4/3 Chapter 26: Speciation • Production of two or more new species from a single ancestral species What is a species? • Biological Species Concept o Pioneered by Ernst Mayr o Groups of co-specifics (same species) o Potential to interbreed o Cannot interbreed with other such groups o Problems with this theory  Doesn’t apply to asexual species  Species don’t overlap geographically  Takes too much time and energy  Can’t be done for all species in the world—there are too many • Other Concepts o Morphological  Uses anatomical differences to establish a species  Most common method in practice o Cohesion  Phenotypic cohesion resulting from: • Integrated complexes of genes • Integrated sets of adaptations o Different from species to species o Species cannot interbreed because the adaptations that one have may not work together with another species 4/3 o Ecological  Natural selection yields adaptations  Hybrids are at a great selective disadvantage • Isolating Mechanisms o Species are sexually isolated—why? o Prezygotic Mechanisms: before fertilization  Isolation via… • Habitat; two different species never have the opportunity to interact and mate • Behavior; species-specific mating habits • Time; different reproductive seasons between species • Incompatible reproductive anatomy • Gamete incompatibility o Postzygotic Mechanisms: after fertilization  Via reduced: • Hybrid viability; it cannot develop • Hybrid fertility; it cannot reproduce  Hybrid “breakdown” • No initial problems • Later, problems in genetics become evident o When pre-/postzygotic mechanisms fail:  Hybrids are successful  Introgression can occur • The transfer of genetic information from one species to another as a result of hybridization between them and repeated backcrossing. 4/3 Within­Lineage Change • Anagenesis o Evolutionary change within a lineage o Change that does not involve lineage splitting o Adaptive change is anagenetic o Speciation requires anagenesis to occur first • Phyletic Species: when enough anagenesis has occurred that a single species looks different o Dividing line between ancestral and descendant species is arbitrary o No splitting, not a form of speciation  Doesn’t cause an increase in the number of species at any given time  Used for the purpose of paleontologists • Fossil evidence o Good fossil sequences document anagenesis  Marine bivalves • Observed o Direct observations are frequent o Galapagos finches, peppered moths, cliff swallows Lineage Splitting • Cladogenesis o Splitting evolution; two or more descendant species o Genesis of clades through successive speciation events • Fossil evidence o Splitting seen in the marine bivalve record o A few other such cases are known 4/3 • Inferences from distributions o Many geographic distributions suggest splitting o Antelope Squirrel  Look different between the north and south rim of the Grand Canyon  Isolated geographically o Snapping Shrimp  Became isolated when the Isthmus of Panama formed  Each
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