PUNCTUATED EQUILIBRIUM MODEL OF EVOLUTION
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Darwin’s Theory of Evolution worked on the premise that big changes occur due to the
accumulation of many small changes over long periods of time (gradualism).
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Problem: Paleontologists rarely find transitional fossils, but often observe species appearing as
new forms suddenly in the rock layers.
o Forms persist virtually unchanged for years and then disappear
Punctuated equilibrium = theory that evolution occurs in spurts of relatively rapid change followed
by long periods of stasis
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Proposed by Niles Eldredge and Stephen Jay Gould in 1972
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Species undergoing most of their morphological modification as they first separate from the
parent species, then show little change
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There are long periods of stasis punctuated with episodes of change
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Ex. The origin of new polyploidy plants can occur in 1 generation
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Ex. Allopatric speciation of a splinter population
o For a population facing new environmental conditions, genetic drift and natural
selection can cause significant change in only a few hundred or thousand generations.
o Successful species survive for a few million years
o Considered rapid in reference to the geological time scale

Ex. If a species survives for 5 million years and most of its morphological
changes occur in the first 50,000 years, the speciation episode occurred in just
1% of species’ lifetime.
o With this time scale, a species will appear “suddenly” in rocks of a certain age.
o While forming, the species may have gradually accumulated modifications, but with
reference to its overall history, its formation was sudden
The Reason for the Absence of Transitional Fossils:BAD Odds
In order for a transitional fossil to be found by a scientist:
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The organism has to die under the right conditions to be trapped in sediments
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It has to be preserved instead of decomposed
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It has to be a transitional form (small percentage, see notes above)
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The scientist has to know where to dig
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It has to not be destroyed by current life or geology

The scientist has to dig out the fossil without destroying it
o When put together, the odds of finding a transitional fossil are very slim!
The Origin of Evolutionary Novelty
Most evolutionary novelties are modified versions of older structures;
Developmental Genes play a major role in Macroevolution

Changes at several gene loci are probably involved
o Ex. Changes in Homeotic (Hox) genes in animals alters the location of repeated
structures in all vertebrates
Development of the Eye

Animals have eyes that vary from compound to single-lens

Pax6 gene is required for eye formation in all animals
Development of Limbs

The Tbx5 gene is necessary for limb bud development in embryos

Birds and humans both express this gene, but differ in which genes turn it on

Subtle changes in gene control can have profound effects on the shape of the body, which
could explain the tremendous variation we see in organisms of all kinds
Development of Overall Shape

Hox genes control the # and appearance of repeated structures along the main body axes in
vertebrates

Changes in Hox gene expression in embryos alters the appearance of the baby/organism
o Ex. Chick v. Snake vertebrae
Pelvic-Fin Genes

Pitx1 gene timing results in either long or short stickleback pelvic spines

Hindlimb reduction has occurred during the evolution of some vertebrates, like whales and
manatees. Both evolved from land-dwellers.

Stickleback study showed how natural selection can lead to major skeletal changes in a
relatively short time
Human Evolution
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All vertebrates have ̴ 23,000 genes
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Scientists do not expect to find new genes to account for humans, rather they expect to find
differential gene regulation

A slight alteration of development becomes compounded in its effect on adult Allometic
growth (differences in growth rates & timing of various parts of the body) which helps to shape
an organism.
An Evolutionary Trend Does NOT Mean That Evolution is Goal-Oriented
Ex. Horse Evolution – many paths led to “dead ends”

Each ancestral species was well-adapted to its environment, but over time as their environment
changed, different variations were selected for