1. PUNCTUATED EQUILIBRIUM
© 2016 Paul Billiet ODWS

2. 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

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

4. 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

5. 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

6. 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

7. 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 years.
Stephen J
Gould
Niles Eldredge
© 2016 Paul Billiet ODWS

8. 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

9. 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

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

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