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The Origin of Species Speciation = origin of species Microevolution: changes within a single gene poolMacroevolution: evolutionary change above the species level -cumulative effects of speciation over long periods of time Biological Species Concept-Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring (reproductively compatible)-Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids Types of Reproductive Barriers-Prezygotic – impede mating/fertilizationtypes: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation-Postzygotic – prevent hybrid zygote from developing into viable adult types: reduced hybrid viability, reduced hybrid fertility, hybrid breakdown Other definitions of species:-Morphological – by body shape, size, and other structural features-Ecological – niche/role in community-Phylogenetic – share common ancestry, branch on tree of life Two main models of speciation:-Allopatric Speciation – a population forms a new species while geographically isolated from its parent population-Sympatric Speciation – a subset of a population forms a new species without geographic separation Allopatric-geographically isolated-caused by geologic events or processes-evolves by natural selection and genetic drift-ex: squirrels on N/S rims of Grand Canyon Sympatric-overlapping populations within a homerange-gene flow between subpopulations blocked by: polyploidy, sexual selection, habitat differentiation-ex: polyploidy in crops (oats, cotton, potatoes, wheat)-Autopolyploid – extra sets of chromosomes – failure of cell division (2n à 4n) ex: strawberries are 4n, 6n, 8n, 10n-Allopolyploid – 2 species produce a hybrid – species A (2n = 6) + species B (2n = 4) à hybrid (2n = 10) Adaptive Radiation-Many new species arise from a single common ancestor-Occurs when -a few organisms make way to new, distant areas (allopatric speciation)-environmental change à extinctions à new niches for survivors (ex: Hawaiian archipelago – plants descended form ancestral tarweed from north America 5 million years ago) Hybrid Zones-Incomplete reproductive barriers-Possible outcomes: reinforcement (more distinct), fusion (together), stability (remain the same) Tempo of Evolution-Gradualism: common ancestor. Slow, constant change.-Punctuated Equilibrium: Long periods of stasis punctuated by short bursts of significant change
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