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PUNCTUATED EQUILIBRIUM http://www.saburchill.com/IBbiology/chapters05/017.html 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 ODWS Evolution of new species Two possible ways from gradualism PHYLETIC TRANSFORMATION ALLOPATRIC SPECIATION © 2008 Paul Billiet ODWS 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 ODWS 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 ODWS Allopatric speciation of Species W into species X due to the isolation of a small population of Species W Species X Species Z Species Y Species Y Time 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 Species W © 2008 Paul Billiet ODWS Evolution 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 (410 million years) involving little evolutionary change Then occasional rapid formation of new species As little as 5,000 - 50,000 years © 2008 Paul Billiet ODWS Stephen J Gould Niles Eldredge 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 ODWS Species Y Species Z Species X Stasis Species W Stasis Time Stasis © 2008 Paul Billiet ODWS Rapid speciation Rapid speciation Evolution Rapid speciation Mechanisms 1. 2. 3. 4. Rapid natural selection in isolated populations Genetic drift in small isolated populations. Hopeful monsters Breakdown of developmental homeostasis © 2008 Paul Billiet ODWS Criticisms What is a species? How rapid is rapid? The incomplete fossil record © 2008 Paul Billiet ODWS
