THE RATE OF EVOLUTION: TWO THEORIES
1. Gradualism
 Darwin and Lyell shared the belief that changes on the Earth, both biological
and geological, are the result of small incremental changes
(uniformitarianism).
 Like Lyell, Darwin believed that changes in biological life are the result of
small dissimilarities that we can see in populations today.
 Darwin’s theory of gradualism was based on the idea that new species
originate from the gradual accumulation of small changes over long periods of
time will result in a qualitative change.
 Based on this theory one would expect to find a long series of intermediate
forms in the fossil record, but this is not the case. Darwin’s theory of
gradualism began to be questioned when evidence from the fossil record did
not support Darwin’s hypothesis.
*** See Graph
2. Punctuated Equilibrium
 Proposed in 1972 by Stephen Jay Gould (Harvard) to explain the
discontinuities of the fossil record
 The theory of punctuated equilibrium is based on the idea that considerable
evolutionary change takes place during short periods of time that “punctuate”
the general state of “equilibrium” or stasis.
 According to this theory...
o Speciation occurs in the event of an environmental crisis (extinction
event) – many species die off leaving empty niches.
o During this period of strong selection, surviving species evolve rapidly
to become adapted to their new environment.
o Once again stabilizing selection prevails, and evolutionary change slows
to become more gradual.
Today it is generally accepted that both theories of evolution, gradualism and
punctuated equilibrium, are represented in the fossil record. *** See Graph
The slope of the
line represents
Darwinian gradual
speciation.
Gradualism vs. Fossil Record
4500
4000
3500
3000
Geologic Time Period
Cenozoic
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Silurian
Devonian
Ordovician
500
0
Precambrian
Cambrain
Number of 2500
Genera 2000
1500
1000
GRADUALISM VS. PUNCTUATED EQUILIBRIUM
Two of the four rock layers contain fossils. The sudden presence of a new species in rock layer three seems to
indicate a rapid burst of evolution...or does it?
Gradualism: “Bursts” of Evolution as a Geologic Illusion
It is possible that the layers of rock containing the transitional forms has not been preserved, creating the
illusion that a new species developed during a “burst” of evolution.
Transitional forms
not preserved
Punctuated Equilibrium: Real “Burst” of Evolution
It is possibly that a new species did evolve quickly as a result of the strong selective powers of a speciation
event.
Speciation Event