PUNCTUATED EQUILIBRIUM

NEO-DARWINISM  Evolutionary change is both slow and gradual  Resulting from the accumulation of many small genetic changes favoured by natural selection  Other effects occasionally making small contributions  Gradualism © 2008 Paul Billiet ODWSODWS

Evolution of new species  Two possible ways from gradualism  PHYLETIC TRANSFORMATION  ALLOPATRIC SPECIATION © 2008 Paul Billiet ODWSODWS

PHYLETIC TRANSFORMATION  Gradual accumulation of small genetic variations preserved by natural selection  A whole population imperceptibly to evolve in to a new species  Impossible to draw a clear line between the end of the first species and the beginning of its descendant species  There would be a long period of intermediate forms © 2008 Paul Billiet ODWSODWS

ALLOPATRIC SPECIATION  Geographical or reproductive isolation of a part of the population would allow 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 © 2008 Paul Billiet ODWSODWS

Species W Species X Species Y Species ZSpecies Y Allopatric speciation of Species W into species X due to the isolation of a small population of Species W Allopatric speciation of species Y into species Z due to the isolation of a small population of species Y Phyletic transition of species W into Species Y due to the slow gradual accumulation of mutations in Species W Evolution Time © 2008 Paul Billiet ODWSODWS

THE PUNCTUATED EQUILIBRIUM MODEL  They observed that the fossil record gives a different picture for the evolution  They claim that 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 years Stephen J Gould Niles Eldredge © 2008 Paul Billiet ODWSODWS

Stasis and change  A species resists evolutionary change  A species would rather move to a new area where it can find its habitat 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…  …rapid change could take place to bring the population back to equilibrium (stasis)  But it is no longer the same species © 2008 Paul Billiet ODWSODWS

Evolution Time Rapid speciation Stasis Species W Species X Species Y Species Z © 2008 Paul Billiet ODWSODWS

Mechanisms 1. Rapid natural selection in isolated populations 2. Genetic drift in small isolated populations. 3. Hopeful monsters 4. Breakdown of developmental homeostasis © 2008 Paul Billiet ODWSODWS

Criticisms  What is a species?  How rapid is rapid?  The incomplete fossil record © 2008 Paul Billiet ODWSODWS