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Population Genetics 2 • Micro-evolution is changes in the genetic structure of a population • Last lecture described populations in HardyWeinberg 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) Genotype From popn 1 From popn 2 Total Expected 2 observed H-W AA 25 81 106 98 64/98 Aa 50 18 68 84 256/84 aa 25 1 26 18 64/18 p1 = 0.5 q1 = 0.5 p2 = 0.9 q2 = 0.1 pt = 0.7 qt = 0.3 Total 2 = 7.25 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 subpopulations 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, codominant 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 (p2) Aa (2pq) aa (q2) AA (p ) AA (p4) AA (p2pq) Aa (p2pq) Aa (p2q2) Aa (2pq) AA (p2pq) Aa (p2pq) AA (p2q2) Aa (2p2q2) aa (p2q2) Aa (q2pq) aa (q2pq) Aa (q2pq) aa (q2pq) 2 2 aa (q ) Aa (p2q2) aa (q4) 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) AA Aa (2pq) aa (q2) AA (p2) Aa AA (pq/2) Aa (pq) aa (pq/2) aa (q2) aa AA: p2 + pq/2 Aa: pq aa: q2 + pq/2