1. Population Genetics 2
Micro-evolution is changes in the genetic structure of a population
Last lecture described populations in Hardy-Weinberg equilibrium
We will now consider the 4 main forces that cause a population to evolve
They are Migration, Mutation, Drift and Selection

2. Mutation Mutation is the means by which new alleles are created
Mutation rates are very low - about 10-6 (1 in a million) for a gene, 10-9 (1 in a billion) for a particular basepair in DNA
This can generate a lot of potential variation in a population - 6,000,000,000 humans would produce about 12,000,000,000 new alleles per generation
However in population genetics we usually study “old” alleles that have been in the population for some time
The probability of any particular allele mutating is very low, so mutation does not usually make a detectable difference to Hardy-Weinberg equilibrium

3. Migration
If individuals migrate from a population into another population, with different allele frequencies, this will cause a deviation from H-W equilibrium (until the migrants have randomly interbred with the natives)
Example: population 1 has p = q = 0.5, and population 2 has p = 0.9, q = 0.1. Both populations are individually in H-W equilibrium. Mix together 100 from each population…..

4. Migration (2)
This result is a statistically-significant deviation from H-W

5. Drift
In a population, some individuals may by chance not pass on their alleles to next generation, others by chance pass on more than their “fair share”
This effect causes changes in allele frequency between generations - genetic drift
The effect is particularly pronounced in small populations
Given enough time, any allele frequency can drift to 1 (fixation) or 0 (extinction)
Drift is the major cause of genetic differences between sub-populations

6. Drift (2)
Many populations go through “bottlenecks” where the population is reduced (migration, disease, famine, climate)
See Fig 23.8 in Purves. A small sample from a population may have a non-random distribution of alleles
When the population grows, it will have different allele frequencies from the population before bottleneck
A few individuals colonising a new region can cause a “founder effect” whereby some genes are more common in the colony than the population they came from
Example is myotonic dystrophy (inherited muscular disease) - much more common in a region of French Canadian immigrants than in Europe, because some of the original settlers were carrying the gene

7. Selection
Selection results when fitness (the probability that an organism will pass on its genes) depends on genotype
Certain alleles can increase or decrease fitness
These effects can be dominant, recessive, co-dominant or additive
Whether an allele is selective will also depend on the genetic background (other genes) and environment

8. A Selection Experiment
Bacteria are a good system to use because of short generation time (30 minutes)

9. Selection in diploid organisms
Selection on recessive alleles is very inefficient because when the allele arises, it is virtually always in the heterozygotes

10. Heterozygote Advantage
In some cases the fitness of the heterozygote can be more than that of either homozygote
Example - sickle cell anaemia, a lethal recessive disorder of blood
Homozygotes with SCA are less fit, but carriers are more fit because of resistance to malaria
This causes the SCA allele to be maintained in populations where malaria is common

11. Distribution of SCA and malaria

12. Random Mating AA: p4 + 2p3q + p2q2 = p4 + 2p3(1-p) + p2(1-p)2
= p4 + 2p3 - 2p4 + p2 - 2p3 + p4 = p2

13. Assortative Mating - a type of selection
AA: p2 + pq/2 Aa: pq aa: q2 + pq/2