1	NEUTRAL THEORY of EVOLUTION There is much genetic variation within almost all species. The level of genetic variation is too much to be maintained by selection.
2	Reading Assignment The sections relevant to this lecture are 24.3 Genetic Drift, 24.3 Gene Flow & 24.5 Mutation. Box 26.1 (p. 566) on the Molecular Clock is relevant. Phylogeny which is currently a major activity of biologists implicitly assumes that neutral allelic substitutions accumulate with time.
3	Natural Selection view of Evolution Mutations arise by chance (meaning the mutations are not directed to match environmental needs). Mutations can be favorable or unfavorable. Favorable mutations increase in frequency via selection. Deleterious mutations are reduced in frequency by selection. Genetic variation in populations should be low as holding two alleles (a stable polymorphism) is possible only via heterozygote advantage.
4	Amounts of Genetic Variation When molecular techniques allowed scientists to measure genetic variation they found 1) many polymorphic loci and 2) some loci with many alleles. The amount of genetic variation was much greater than scientists expected from the population genetic models that existed. Selection models were ‘tweaked’, but ….
5	New Idea The neutral theory of evolution developed by Motoo Kimura (1^st publication 1968). The neutral theory departed from all existing models by using N, the population size, as the most important population parameter. What is the neutral theory of evolution?
6	The Neutral Theory 1) There are no fitness differences between almost all of the molecular variation that is detected in populations. Neutral is the word chosen to describe the lack of fitness differences (functionally equivalent alleles). 2) Amount of genetic variation in a population is determined by a balance between an increase due to mutation, rate =μ, and a decrease due to finite population size (=genetic drift).
7	What is genetic drift? The allele frequency p is equal to p’ (stays the same) only if the population is NOT finite, i.e., population is infinite. All real populations have only N members (with 2N genes) and even if mating is independent of genotype, the allele abundance goes upward or downward by chance (those fluctuations are called drift). With coin flips you expect 50% heads and 50% tails, but the numbers are rarely equal (and never equal for an odd number of flips).
8	Allele frequency in finite populations In finite populations the frequency of an allele in the next generation will NOT generally be equal to its current frequency. One way to see this is to calculate an allele frequency based on Hardy-Weinberg. If 1 of 10 snails have the recessive phenotype, the estimated allele frequency is 0.316, but 0.30 & 0.35 are the closest possible actual frequencies in population with only 20 (=2N) genes in ten diploid snails.
9	Fixation (of alleles) For allele frequencies the values of 0 and 1 are especially important, because a previously polymorphic population that had both A and a alleles now has only one kind, i.e. has no variation. Alleles that reach 100% are said to be fixed. Once lost, the only way genetic variation can be regained is through mutation.
10	Rate of loss of genetic variation in one generation If there are N individuals in a population, there are 2N genes. Only if every gene were different could there be as many as 2N kinds. Sampling from a infinite gamete pool with 2N types, means there is a probability of 1/2N that the second allele drawn is identical to the first one drawn. F, a measure of genetic variation loss, = 1/2N
11	Genetic Variability Loss over time Proportion of variability retained from one generation to the next is: (1 - 1/2N) = 100% minus that which is expected to be lost. After t generations remaining variability = (1 - 1/2N)^twhich goes to zero as t increases. If N = 50 then 2N = 100 so in one generation (1 - 1/2N) = 0.99 = 99% retained (0.99)⁴⁰ = 0.67, so in 40 generations 1/3rd of initial variability is lost in population of 50 individuals (each generation).
12	Losses and gains of polymorphism Each polymorphic population has a possibility of becoming monomorphic (= completely homozygous) in the next generation. The rate that variability is lost is inversely proportional to population size. Once monomorphic (=only one allele) the only return to polymorphism is via mutation.
13	Bottlenecks When population is now large, but was very much smaller in the past we say it has gone thru a “genetic bottleneck”. The population will have less genetic variation than expected from its current population size, if, in the past, its population size was much smaller.
14	Probability of eventual fixation of neutral alleles Each neutral allele has an equal chance of being the one that will eventually become “fixed” via drift, i.e. reach 100% frequency. If the frequency of a is 0.15, then the probability it will eventually become the only allele in the population is 0.15. If the initial frequency of a is 0.7, then the probability it will eventually become fixed is 0.7.
15	GENETIC DRIFT recapitulation All populations have genetic drift. The average change in allele frequency (drift) from the value of the last generation is greater the smaller the population size. Thus drift is inversely proportional to population size.
16	Neutral Theory recapitulation Most evolution (change in allele frequency) is the result of: Biological populations have a size, N. Functionally equivalent alleles are expected to rise or fall by chance in finite populations. Genetic drift results in a loss of genetic variation Mutation is the only ‘mechanism’ to increase genetic variation.
17	Dynamics of Genetic Variation Natural selection and drift are the main forces leading to allele frequency change. Both are expected to lead to genetic uniformity. Mutation generates new alleles, variety. Separate populations is vary independently, except if migration between the populations is occurring. Migration makes the populations less differentiated.
18	Geographic structure & migration Species are composed of populations that are at least partially isolated from one another. Most of the geographical genetic structure that is seen in populations is the consequence of genetic drift. Migration between populations reduces the differences that tend to build up thru drift.
19	Phenotypic Patterns of Selection Applies to a measurable trait. Describes how selection changes the mean and the variability in the population. DIRECTIONAL Selection moves the mean in a direction. STABILIZING Selection leaves mean the same but reduces variability.
20	Directional Selection
21	Stabilizing selection
22	Vocabulary Genetic variation Mutation Neutral theory Selectively equivalent Drift Fixation Loss of variability N 1/2N Monomorphic Genetic bottleneck Directional selection Stabilizing selection inversely proportional