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

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

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…..

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

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

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

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

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

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

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

Distribution of SCA and malaria

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

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