PUNCTUATED EQUILIBRIUM © 2016 Paul Billiet ODWS

NEO-DARWINISM Evolutionary change is both slow and gradual Accumulation of many small genetic changes favoured by natural selection Other effects occasionally making small contributions Gradualism. © 2016 Paul Billiet ODWS

Evolution of new species Two possible ways from gradualism PHYLETIC TRANSFORMATION ALLOPATRIC or SYMPATRIC SPECIATION. © 2016 Paul Billiet ODWS

PHYLETIC TRANSFORMATION Gradual accumulation of small genetic variations preserved by natural selection A whole population imperceptibly evolves into a new species Impossible to draw a clear line between species Long period of intermediate forms. © 2016 Paul Billiet ODWS

ALLOPATRIC or SYMPATRIC SPECIATION Geographical or reproductive isolation of a part of the population Allows it to evolve in a different direction Possibly more rapidly than the main population If the isolated population is small, it might be very difficult to find fossils of the intermediate stages. © 2016 Paul Billiet ODWS

Evolution Time Species X Species Z Species Y Species Y Species W Allopatric speciation of Species W into Species X due to the isolation of a small population of Species W Evolution Time Species X Species Z Species Y Sympatric speciation of Species Y into Species Z due to the reproductive isolation of members of Species Y Species Y Phyletic transition of Species W into Species Y due to the slow gradual accumulation of mutations in Species W Species W © 2016 Paul Billiet ODWS

THE PUNCTUATED EQUILIBRIUM MODEL The fossil record gives a different picture for the evolution There were long periods of stasis (4-10 million years) involving little evolutionary change Then occasional rapid formation of new species As little as 5,000 - 50,000 years. Stephen J Gould Niles Eldredge © 2016 Paul Billiet ODWS

Stasis and change A species resists evolutionary change A species would rather move to a new area where it can find its niche than adapt to a new one If a small population of a species should get isolated in an area where its habitat does not exist Or a major change occurs in their native environment (e.g. climate change)… …rapid change could take place to bring the population back to equilibrium (stasis) But it is no longer the same species because the niche is different… so the selective pressures are different. © 2016 Paul Billiet ODWS

Time Evolution Species Y Species Z Species X Species W Rapid speciation Stasis Species W Species X Species Y Species Z © 2016 Paul Billiet ODWS

Mechanisms Rapid natural selection in isolated populations or after a crisis Genetic drift in small isolated populations Hopeful monsters. © 2016 Paul Billiet ODWS

Criticisms What is a species? How rapid is rapid? The incomplete fossil record. © 2016 Paul Billiet ODWS