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Biological Species Concept

• “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”

• reproductive isolation: genetic differences preventing successful interbreeding

- Ernst Mayr, 1940

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Species on Earth

• # species on Earth today: ~3-50 million• # of species that ever lived: ~75-2500

million• It has been estimated that ~99% of all

species that have ever lived are extinct

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Speciation • Speciation

– the process by which one species splits into two or more species

• How?– Subgroups of the species become

reproductively isolated• Classes of reproductive isolation

– Prezygotic: prevents zygote formation– Postzygotic: prevents survival or

reproduction of offspring

4

Classes of speciation

• Allopatric– geographic

separation• Parapatric

– divergence across a continous habitat

• Sympatric– No separation

“Female Fickleness May Split a Species”

Source: Matt Kaplan, ScienceNOW Daily News, 11/30/2007

• NY females prefer males with bright yellow bibs

• WI females prefer males with large black masks

• When mixed, preferences stayed the same

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Transitional fossils

• Fossils of organisms with morphological features intermediate between those of an ancestral species and its descendants

?

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Fish to tetropods

TiktaalikDiscovered in 2004 on Ellesmere Island, Canada.385-359 million

years ago

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Dinosaurs to Birds

Archaeopteryx

150 Million years ago

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Land mammals to sea mammals

Source: evolution.berkeley.edu

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Fossil DNA

• Remnants in a present-day genome of previously functional DNA that “eroded” due to an accumulation of mutations

…CAATGACGTTAGCATGAG…

…GAATGGCGTAAGCCTGAG…

ancient DNA

present-day DNA

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Human chromosome 2: A fusion

Human chromosome 2

Chimp chromosomes 2q and 2p

Remnants of old telomeresRemnants of old centromere