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. (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
- 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: 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
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 : a. • A species with a large geopgraphic range is likely to undergo local adaptation,
especially at the edges of its range where outlying populations may face novel
conditions. Gene flow from the centre of a species range may prevent local
adaptations from persisting and spreading.
b. • However, local populations that get separated by geographic barriers to gene flow
may adapt to local conditions without homogenizing geneflow and as a result
c. • Once they speciated, the new adaptation would be preserved even if the
geographic barrier disappeared.
d. • This would produce a pattern of punctuated equilibrium where morphological
change appears to be correlated with speciation.Species with large geographic
range is likely to undergo local adaptation to that range.
e. All the character traits that evolve in the periphery are more prone to change like
green and red and purple. So they all try to evolve to the blue. Gene swamping keep
t genetically homogenized to the BLUE
f. If we get a fking river all of a sudden (black rectangle) . purple will locally adapt to
that little periphery area but it might evolve reproductive isolation.
g. Morphological change and punctuated = formation of new speciation = phylogenetic
rates and taxanomic rates.
4) Niche conservation:
a. is the long-continued dependence of related species on the same resources and
environmental conditions and may result from any of these 3 processes that result
in stasis: genetic constraints, stabilizing selection or gene flow swamping b. As the degree of adaptation to any one environment increases, the differential in
fitness between that environment and a new environment also increases so that
adaptation to an alternative environment may become steadily less likely.
i. In other words: as species become more specialized the chances they will
evolve to a new niche decrease. Generalist species are more likely to
undergo niche swapping.
ii. If we have a specialized species: it gets more and more adaptation to that
niche. And lets say that the niche doesn’t change it becomes more and
more specialized => specialist => making it difficult to evolve into something
iii. On the other hand a generalist that is not specialized to one adaptive
landscape, doesn’t utilize both niches perfectly but utilizes both
iv. Generalists species will undergpe gene swamping than niche conservativism
• Example: the red-flowered clade of Morning glory (hummingbird specialist) is evolutionarily derived
and uses a Pelargonidin pigment. Purple/blue is the ancestral colour, based on a Cyanidin pigment.
- In the red clade the F3’H enzyme initiates the Cyanidin pathway. It has undergone
inactivation so that only the Pelargonidin pathway is activated.
- P evolved so much that its hard for it to go back to C if their niches change.
- Red = pelargonidin activated: P = Cyanidin activiated
- One change in DHK was enough to change the pigment by adding a hydroxyl group
- To go back to C we require two mutation. That alter two pathways. So its difficult for them
to go back
- Probably because the Cyanidin pathway is no longer functional, a second enzyme, DFR, has
also mutated so that it no longer recognizes DHQ and only works on DHK.
- Thus at least 2 restorative mutations would be needed to reactivate the Cyanidin pathway.
• In general, if you don’t use it, you will loose it.
• Unused (nonfunctional) genes acquire disabling mutations such as stop codon
- signals and end up becoming pseudogenes.
• The loss of gene function is one likely basis of Dollo’s Law: the generalization that a complex
character, if lost, is seldom regained in its original form.
• For example, often a trait will be lost. At a latter point in time a new trait evolves that serves the
same function as the lost trait.
• Birds (e.g. Archaeopteryx and other early birds) originally had proper teeth in their bills.
• Tooth development in modern birds is initiated but stops at a very early stage of development.
A mutation is known that allows tooth development to proceed further. Nevertheless,
mergansers have evolved substitutes to teeth, serrations at the edge of the bill which function
like teeth allowing them to grip slippery fish. Similar system found in bears and digits:
• The earliest land tetrapods had more than 6 digits on each limb, but soon settled on 5 digits.
Since then almost no tetrapod has evolved more digits.
• The Giant Panda however, has evolved an additional 6th finger which functions somewhat like a
human thumb to manipulating the bamboo on which it feeds.
• This thumb however, is not a true digit but rather a sesamoid bone that develops from
Gradualism versus Saltation
Darwin said constant rates of
A) History of thought
• Darwin proposed that evolution proceeds gradually, by small steps. In The Origin of Species he
wrote, “if it could be demonstrated that any complex organ existed, which could not possibly
have been formed by numerous, successive, slight modifications, my theory would absolutely
o In 1950 Schinderwolf proposed just this. He proposed that differences amongst higher
taxa have “arisen discontinuously, by saltation” and that “the first bird [i.e.
Archaeopteryx] hatched from a reptilian egg”.
o Saltation means “a jump”
o Saltation results from the fixation of a single mutation of very large effect.
o This concept is different from punctuated equilibrium, which allows that evolutionary
changes in morphology might have passed through many gradual intermediate steps,
but such changes occurred over a short time period.
• In 1940 the accomplished geneticist Richard Goldschmidt argued that species and higher taxa
arise not from the genetic variation that resides within species, but instead “in single
evolutionary steps as completely new genetic systems,”
o Thought that major changes of the chromosomal material, would give rise to highly
altered creatures. :
o Most such creatures would have little chance of survival, but a few would be “hopeful
monsters” adapted to new ways of life.
o He poin