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biological species concept Bio153: lecture 8 Rates of Evolution microevolution = evolutionary changes within species natural selection sexual selection genetic drift can lead to speciation interbreeding population reproductively isolated species = gene pool Modes of speciation: • e.g. island archipelagos: • Allopatric speciation model: time geographic barrier separate gene pools 1) invasion of new islands isolation 2) over time, divergence of pop’s 3) “re-invasion” 1 • population genetics: Wright (1977) “shifting balance”: drift gene flow (local pop’s) (among pop’s) anagenesis and cladogenesis: anagenesis + selection evolution may lead to large, adaptive changes in a widespread population - directional change in characters in a lineage cladogenesis - branching by speciation evolutionary changes cladogenesis without anagenesis sibling species: similar species that do not interbreed gray flycatcher anagenesis without cladogenesis: e.g. mimetic butterflies -much geographic variation without speciation dusky flycatcher Empidonax spp. cladogenesis + anagenesis : speciation events: interpreting the history of evolution: • in fossil record, taxonomy defined by morphological characters • difficult to separate anagenesis from cladogenesis divergence along X axis: morphological change how do you define a species?? 2 1) rate of ∆ of character in a lineage (anagenesis) 2) replacement of “species” origination and extinction (cladogenesis) morphology rapid anagenesis slow anagenesis time enough change to look like new species morphological change without speciation • chronospecies: anagenesis makes descendents look like separate species → taxonomic pseudoextinction no speciation looks like speciation! morphology rates of evolution : time many speciation events; little morphological change problem of stasis: fossil deposits: 50 - 100 my apart; short term ∆’s are lost however: 1) long periods without change (stasis) 2) rapid appearance of new forms 3) no transitional forms Changes in the shape of molar teeth of the early Eocene mammal Hypsodus real or artifact? 3 two hypotheses: 1. Phyletic Gradualism - constant anagenetic change - speciation gradual - transitional forms lost in fossil record basic tenets of PE: 1) stasis; rapid change is real 2) speciation by peripheral isolates 3) adaptive ∆ mostly during speciation 2. Punctuated Equilibrium - stasis is real - evolution occurs during speciation - long-term trends in morphology due to species selection • Peripheral isolate model: - species selection (species sorting) environment homogeneous population small founded by few individuals low genetic diversity genetic drift must be no gene flow • related species overlap in niche space competition → displacement; extinction • 2 main processes leading to anagenetic change: differential speciation differential extinction differential speciation differential extinction t2 time time t2 t1 body size t1 body size 4 • examples: comparisons of speciation rates African antelopes: impalas: slow wildebeest : fast marine snails : planktonic larva: slow non-planktonic : fast next lecture: from 2 kingdoms to 3 domains why? genetic isolation same in: burrowing rodents: coruros: slow tuco-tuco : fast 5