Genetic variation
 All of the genetic information carried by members of a population make up the gene pool of that population
 Part of genetic variation is due to nucleotide sequence differences o In one study, 43 nucleotide variations were detected within the
2721 base pairs of the gene o Of these, 14 were in exon-coding regions o Of these, only one leads to an amino acid replacement - giving rise to the two alleles (Adh-f and Adh-s)
Genotypic frequencies
 Allelic frequencies o If there are only two alleles (A and a) o p + q = 1
 Allelic frequencies can also be calculated from the genotypic frequencies o p = f(A) = f(AA) + 1/2 f(Aa) o q = f(a) = f(aa) + 1/2 f(Aa)
Hardy-Weinberg law
 Provides us with insight into the answer to“How do the segregation of alleles in gamete formation and the combining of alleles in fertilization influence the gene pool?”
 Assumptions: o The population must be large o Mating must be random o The alleles must not be affected by mutation or natural selection
Prediction 1 - The allelic frequencies of a population do not change
Prediction 2 - The genotypic frequencies will stabilize after one generation in the proportions p 2 [f(AA)], 2pq [f(Aa)] and q 2 [f(aa)]
The population is in Hardy-Weinberg equilibrium
 So the Hardy-Weinberg law indicates that, if the assumptions are met, reproduction does not alter allelic or genotypic frequencies
 But how often are the assumptions met?
 The Hardy-Weinberg law only refers to a single locus at a time
 Therefore, the conditions need only be met for that one locus
Implications
1. If a population meets the Hardy-Weinberg assumptions, evolution for that one locus will not occur
2. Genotypic frequencies are determined by allelic frequencies
Human populations
 Consider the CCR5 gene - C-C chemokine receptor-5
 The CCR5 protein is a receptor for HIV, allowing infection of cells
 A mutant allele - CCR5-D32 prevents infection
 Direct analysis of the CCR5 gene was performed on one population
 79 individuals were (1/1), 20 were (1/D32), and 1 was (D32/D32)

Testing for Hardy-Weinberg equilibrium
 To test whether a population is in Hardy-Weinberg equilibrium, you can compare the expected genotypic frequencies with the actual frequencies using the chi-square test
Practical applications
 Consider cystic fibrosis o Incidence rate among North American Caucasians = 1/2500
Since the Hardy-Weinberg law tells us that segregation and reproduction alone will not lead to changes in allelic frequencies, any changes that are seen must be due to other causes
Mutation
 Mutation can directly affect the allelic frequencies
 Although the overall effect of mutation is small, over many generations, the small effects of mutation can amount to a substantial effect if it is the only force involved
Migration
 Gene flow (migration) can o prevent divergence between populations o increase genetic variation within a population
 If migration continues, eventually both populations will exhibit the same allelic frequencies - they will be homogeneous
Genetic drift o The effects of random occurrences are amplified with small populations o Essentially, this is sampling error o Genetic drift refers to changes in allelic frequencies due to sampling error o Causes of genetic drift
1. Natural limitations may simply keep the population size small for many generations
2. Founder effect - when there are a limited number of founders of a population, the allelic variation may not reflect the overall original population o Human examples include albinism among the Navajo and polydactyly among the Amish
3. Bottleneck effect - when a population suffers a severe reduction in size
Effects of genetic drift
1. Changes in allelic frequencies
Random matings within small populations can often lead to fixation of one allele or the other
2. Reduction in genetic variation
When an allele goes to fixation, the other alleles at the locus are lost
3. Different isolated populations will diverge genetically over time

Nonrandom mating
Nonrandom mating will affect the combinations of alleles in the offspring
Positive assortative mating - like individuals mate
Negative assortative mating - unlike individuals mate
Inbreeding
Positive assortative mating for relatedness that affects all genes
Inbreeding coefficient F - probability that two alleles are “identical by descent” as opposed to “identical by state”
F can range from 0 to 1
When inbreeding occurs, the genotypic frequencies are altered
The proportion of heterozygotes is decreased and the proportion of homozygotes is increased
Natural selection
Differential reproduction of certain genotypes
Individuals with adaptive traits produce a greater number of offspring than those with less adaptive traits
Natural selection promotes adaptation
Fitness is defined as the relative reproductive success of a genotype
It is calculated as the average number of offspring produced by a particular genotype divided by the mean number of offspring produced by the most prolific genotype
Even a lethal recessive allele can hang around in the population
The rate of change in allelic frequency depends heavily on the intensity of the selection
Types of selection o Directional selection o Stabilizing selection o Disruptive selection
Changes in allelic frequencies will occur over time
Reduced gene flow, genetic drift, and selection can lead to speciation
Speciation o Due to the evolution of reproductive isolation
Reproductive isolating mechanisms o Can fall into two categories
1. Prezygotic
2. Postzygotic

Modes of speciation
1. Allopatric
2. Sympatric o Rhagoletis pomonella - apple maggot fed on the hawthorn fruit o In the period 1800-1850, a race of flies spontaneously emerged that preferred apples o This coincided with the introduction of apples to North America o The apple feeding race does not normally feed on the hawthorn; the hawthorn feeding race does not normally feed on apples o Flies usually choose their mates around their preferred fruits o With the Lake Malawi cichids, sexual selection and food choices have led to an amazing array of species
3. Speciation through polyploidy o In general, plants are much more tolerant of polyploidy than are animals o Any event causing polyploidy can produce a new species o However, that new species must be able to reproduce o A nondisjunction event or two can take care of that o Sequence analysis can be extended to determine molecular phylogenies o leading to a phylogenetic tree o There are two possible approaches
1. Distance approach - based on overall degree of similarly
2. Parsimony approach - based on minimum number of evolutionary changes that must have taken place since two organisms last shared a common ancestor o Sometimes these yield similar results; sometimes not
Molecular clocks can be used to date evolutionary events
This is based on the rate at which a protein evolves
Assumes a relatively constant rate of substitution