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Lecture 18

BIOC50H3 Lecture Notes - Lecture 18: Cyanidin, Punctuated Equilibrium, Phyletic Gradualism

Biological Sciences
Course Code
Jason Weir

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Lecture 18: Macroevolution :
Rates of evolution: We can distinguish between:
1) Phylogentic rates: rates at which single characters ir complexes of character evolve.
2) Taxonomic rates: the rates at which species or higher taxa originate
Graph: time on y-axis and a character on x axis: (A) hypothetical
(A) You could say :Size increases linearly through time OR size increases in abrupt changes.
(B) Phyletic: Linear , traditional model. Evolution change is gradual and not associated with
(C) Punctuated equilibria : idea that we have statis (consistent) and punctuated rate of
evolution. Evolution happens in punctuated bursts.

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(D) Punctuated gradualism:same as equilibria . gradualism however doesn’t result in separate
species forming. NO SPECIATION
Phylogentic rates and taxanomic rates not associated : NO NEW SPECIES
1) Phyletic gradualism
2) Punctuated gradualism
Phylogentic rates and taxanomic rates are closely associated :
1) Punctuated equilibria
Punctuated equilibiria:
- Some morphological characters in the fossil record, such as the tooth dimensions of the
early horse Hyracotherium, appear to change little over large periods of time and then
abruptly change to a new type, which then appears to change little.
- This pattern is referred to as punctuated equilibrium: long periods of little change (which
are called stasis) interrupted by brief episodes of much more rapid change. The punctuated
bursts are often though to be associated with speciation.
- Associated with speciation events
- Look at horses and their molars . How the new species abruptly developed larger molars.
- Shape of forminerans:
o Till 500 we have linear stasis
o After 6 we have abrupt change then stasis
o After 5 abrupt change then stasis
- Rapid bursts of evolution, as occurs in punctuated equilibrium and punctuated gradualism,
are not hard to explain.
- What factors might drive a burst of rapid evolution?
o Change in predators , humidity, temperature : change in environment
o Nearby niche is empty and they want to fill up that niche by rapidly evolving
o New parasite evolves . rapid evolution to the resistance of the parasite
Stasis between punctuated bursts of evolution:
- Bivalve shells (Macrocallista maculata) look at shells now and at 4 MYA; NO CHANGE . It is
harder to understand stasis over long periods of time between punctuated bursts of
evolution. Its easier to understand change.
Three major hypothesis to explain long period of stasis in fossil record
during rapid change of evolution:

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1) Genetic or developmental constraints:
a. These would result in a lack of genetic variation, and thus nothing for directional
selection to act on.
b. Likewise, genetic correlations between different loci might hinder characters from
evolving independently to new optima.
c. Also some genes may have pleiotropic effects (i.e. alter multiple traits). (example:
pitch of bird song and body size).
d. Although such constraints may play a role in hindering evolution, they cannot
explain the constancy of size and shape of many quantitative characters, which are
almost always genetically variable and only imperfectly correlated with each other.
e. Marsupial digits: marsupials have the need to have grasping claws : it’s a
developmental constraint because these paws cant evolve into a flipper or hooves.
f. These would result in lack of genetic variation and this nothing for directional
selection to act on
g. Correlation of different loci: if two loci interact with one another and there is
suddenly a change in one loci . Nothing will happen. Both have to be stimulated
h. Pleiotropic effect: one gene controls different outcomes and there is selection for
one particular outcome. Small body size = high pitch. Big body size= low pitch. If
selection wanted a certain body size then we are constraining the pitch to that one
particular state
i. This doesn’t work for stasis for long periods
2) Stabilizing selection:
a. Stabilizing selection for a constant optimum phenotype results in long periods of
b. Its hard to image that over the course of millions of years climatic, environmental
and biotic factors have remained constant enough to result in stasis.
c. For example, over the past 2.5 million years, northern hemisphere ice ages
repeatedly blanketed Canada in a kilometer thick of ice.
d. Nevertheless, a species “effective environment” may be much more constant over
time than we might expect because of habitat tracking: the shifting of the
geographic distributions of species in concert with the distribution of their typical
habitat (e.g. boreal plants during ice ages).
e. In an ecological optimally place so evolution wont change you cause you are happy
where you are.
f. Punctuated bursts represent species that evolve to optimum but once they are
there they are happy = stasis
g. Argument: environment doesn’t stay the same over millions of years. So its not
realistic saying that the adaptive landscape is constant.
h. Counterargument to this: species can track their preferred habitats through time.
Rather than evolving to another optimum peak. This is true for plants and animals
that live in the boreal zone in Canada. Plants went south instead of evolving.
3) Gene flow swamping :
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