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Assumptions of the Hardy-Weinberg Principle
1. No mutation
2. Infinitely large population
3. Closed breeding colony
4. Mating is random
5. Equal probability of survival & reproduction
Review
Forces of evolution
These change genotype frequencies over generations and thereby change the average phenotype of the population.
Genetic drift
New genetic variation through mutation
Gene flow
Sexual selection
Natural selection
Finite population size
Random Walk
A guy walks out of a bar…
Signs That You're Too Drunk ...
-You can focus better with one eye closed. -You lose arguments with objects.-Mosquitoes catch a buzz after
biting you -You fall off the floor... www.ahajokes.com/fp043.html
Everybody write down either X or Q & either R or L
, aka “Drunkard’s Walk
Finite population size
Coin example:
Two alleles: H and T
Population Size = 10
Freq. of H
Trial number
1 2 3 4 5 6 7 8 9 10
1.0
0.4
0.6
0.2
0.8
windwardskies.com
2 neutral alleles = Random Walk
Flobberworms
http://hp.upperwood.ru/
Adapted from the Biology Project at the University of Arizona.
Given conditions:
• 4 alleles for color (R, W, G, B)
• haploid individuals
• constant population size
• asexual reproduction
• one offspring for each worm
R
W
G
B
Flobberworms
What if the number of offspring varied?
R
W
G
B
Generation
0 4321 5 76 98 10
This is where chance comes in.
Add chance to the Flobberworm example
Same given conditions but now chance determines the number of offspring.
One color for each individual1. Roll die (dX for population size, d4 for a population of 4 individuals) to
select which organism reproduces by one offspring (keep population constant)
2. Repeat step 1 for several generations
Genetic drift can be thought of as sampling error – the next generation’s allele frequencies are not representative of the previous generation’s.
R
W
G
B
Generation
0 4321 5 76 98 10
How do these changes by chance come about?
For example, plants destroyed by a rockslide.
http://www.tabloidcolumn.com
Fixation- one allele remains in the population.
Probability of fixation of an allele is its frequency at that time.
What’s the probability of fixation?
1 2 3 4 5 6 7 8 9 10
1.0
0.4
0.6
0.2
0.8Freq. of allele
Generation
Calculate the probability of fixation for each generation for a single allele.
What’s the probability of losing the allele?
How does population size affect genetic drift?
Generations
R
R
W
G
W
G
B
Y
B
Y
1
2
3
4
56
7
8
9
10
1 2 3 4 5 6 7 8 9 10
Multiple populations
What about a collection of independent populations?
R
W
G
B
Generation
0 4321 5 76 8
R
W
G
B
A
B
Genetic drift tends to cause the two populations to diverge.
Fixation- one allele remains in the population.
Probability of fixation of an allele is its frequency at that time.
What’s the probability of fixation?
1 2 3 4 5 6 7 8 9 10
1.0
0.4
0.6
0.2
0.8Freq. of allele
Generation
Calculate the probability of fixation for each generation for a single allele.
What’s the probability of losing the allele?
Important points about evolution by genetic drift
1. Allele frequencies fluctuate at random but one allele eventually becomes fixed.
2. The population eventually loses its genetic variation.
3. The probability, at time t, that an allele will eventually become fixed equals the frequency of the allele at that time.
4. The smaller the population, the greater the rate at which these events occur.
5. Initially similar populations diverge in allele frequency and may become fixed for different alleles.
Neutral theory
Most variations are neutral
How much of a role has drift played in the evolution of morphological traits?
cstl-csm.semo.edu
At the level of DNA sequences, genetic drift has been a major factor in evolution.
www.nyssmoh.org
GCUGCC GCAGCG
Redundancy in the genetic code
All code for Alanine
psc.edu/~deerfiel
Non-binding site regions
It’s also possible to get a Y-chromosome
“Adam”
- most-recent common ancestor of all humans alive on Earth today with respect to matrilineal descent.
Mitochondrial “Eve”
Pix: Wikipedia & www.argusbio.com/mtDNA.html
Bottlenecks
Founder Effect
http://www.greatbasingallery.com
- the almost complete lack of blood type B in Native Americans.
http://easyweb.easynet.co.uk/~iany
- so little genetic variation that you can graft skin from one animal to another with no immunosupressant drugs.
jacquesdeshaies.com
BiogeographyEvolutionary theory expects that:
a) Each new species has a specific site or region of origin
b) A species receives broader distribution via migration
c) A species becomes modified, especially though genetic drift and natural selection, when exposed to new environments and leaves modified descendents
BiogeographyDarwin’s finches are similar to those found on the
mainland of S America, but show signs of evolutionary differentiation and local adaptation
Plate Tectonics
Problem for Darwin’s view:
The trilobite Paradoxides was named for its paradoxical distribution (only Europe and New England).
Problem solved:
Europe & New England were part of the same continent at the time Paradoxides lived.
Biogeography
Plate TectonicsFor more info see:
http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
Plate Tectonics12 major plates
Plate Tectonicshttp://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
Plate Tectonicshttp://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
200mya Pangea started to split into Laurasia &
Gondwanaland
Plate Tectonicshttp://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
Plate Tectonicshttp://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
Plate Tectonicshttp://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Pangea/Pangea1.html
*Note: This picture does not take global warming into account
BiogeographyMost biogeographical data support
Darwin’s predictions. We find new world vs old world species of some families; relict populations, etc
For example:Prosimians occur first in N America,
then spread to the old world, then go extinct in N Am, then in most of the old world. But a few species persist in Asia & mainland Africa. And relict populations of numerous prosimians persist on the island of Madagascar.
Marsupials originate in the Mesozoic, spread out all over, then are replaced in most areas by
placental mammals. But on the isolated continent of Australia,
many marsupial species persist. These relict populations occupy only a subset of their ancestors’
home ranges.
BiogeographyMost biogeographical data support
Darwin’s predictions. We find new world vs old world species of some families; relict populations etc
Prosimians & New World Monkeys 2-1-3-3 dental formula
Old World Monkeys 2-1-2-3
Apes 2-1-2-3
Biogeography
Humans of recent African descent have 17 microsatellite DNA polymorphisms (Kidd & Kidd, 1986); Humans from the rest of the world have only 2 of those.
Suggests a long history for our species in Africa & a relatively recent diaspora out of Africa (ca 100kya)
BiogeographyDarwin felt that biogeography had a lot to tell us about
the history of life on earth. But today it is rather neglected.
Why?
a) Expensive to mount expeditions & bring back & store specimens
b) Widespread habitat destruction, sport hunting, and industrial pollution make studying ecosystems a nightmare.
c) Hi-tech genetic research is in vogue. Nobody wants to fund expensive “old fashioned” biogeographic projects. So, what biogeography does get done is usually in the context of collecting blood samples or looking for viruses.
d) Plate Tectonics
Modern biodiversity
Modern biodiversity
• Does complexity increase with evolution?
• Left wall?
• Right wall?
7.3(1) Taxonomic diversity of skeletonized marine animal families during the Phanerozoic
7.3(2) Taxonomic diversity of 25,049 skeletonized marine animal genera
7.4(1) Changes in the number of known families of insects
7.4(2) Changes in the number of known species of vascular land plants
7.4(3) Changes in the number of known families of families of nonmarine tetrapod vertebrates
7.5 Rates of origination of marine animal genera in 107 stages of the Phanerozoic
7.8(1) The history of diversity of the three “evolutionary faunas” in the
marine fossil record
7.8(2) The history of diversity of the three “evolutionary faunas” in the marine fossil record
7.8(3) The history of diversity of the three “evolutionary faunas” in the
marine fossil record
7.9(2) Taxonomic survivorship curves
7.11(1) Changes in the proportions of marine animal genera classified by three functional criteria
7.13 Models of competitive displacement and replacement
7.15(1) Probable competitive replacement of brachiopods by bivalves
7.15(2) Probable competitive replacement of brachiopods by bivalves
7.17 Echinoid diversity increased during the Mesozoic and Cenozoic
A predatory moth caterpillar in the Hawaiian islands
7.18 Two replicated sister-group comparisons of herbivorous clades of insects with sister clades
7.19(1) Changes in taxonomic diversity of Cenozoic mammals in North America
