Evolution and Population Genetics Notes

Microevolution is defined as the change in allele frequencies within a population over generations.
Population genetics provides the mathematical framework for studying microevolution.
The central question is how 19th-century evolution theories fit with microevolution.

Darwin recognized:
- Genetic variation exists within populations.
- There is overproduction of offspring.
- There is a struggle for existence.
- Differential survival and reproduction occur (unequal reproductive success).
Natural Selection is the mechanism for evolution.

Microevolution involves changes in the allelic composition of a gene pool over time.
Five processes drive microevolution:
- Mutation
- Selection (natural and artificial)
- Gene migration/gene flow
- Genetic drift
  - Founder effect
  - Bottleneck

Antibiotic resistance in bacteria is an example of microevolution driven by natural selection.
- A population of mainly susceptible bacteria is exposed to antibiotics.
- Resistant bacteria survive and reproduce, leading to a population of mainly resistant bacteria.

Examples of microevolution in humans:
- Lactase Persistence: Ability to digest milk, common in European, African, and Middle Eastern populations.
- Sickle Cell mutations: Ability to survive malaria, common in African populations.
- Thalassemia mutations: Ability to survive malaria, common in African, Mediterranean, and Southeast Asian populations.
- Cystic Fibrosis mutations: Ability to survive diarrheal diseases, common in European populations.

A population is a group of individuals of the same species living in the same area and interbreeding.
The gene pool includes all copies of every type of allele at every locus in all members of the population.

Genetic drift refers to chance events causing unpredictable fluctuations in allele frequencies from one generation to the next.
The smaller the population, the greater the impact of genetic drift.
- Founder effect
- Bottleneck

The change in allele frequency is more drastic in smaller populations.
- In a large population of 10,000 individuals, if 50% survive, including 900 allele carriers, the allele frequency changes from $\frac{2000}{20000} = 10%$ to $\frac{900}{10000} = 9%$ .
- In a small population of 10 individuals, if 50% survive with no allele carriers, the allele frequency changes from $\frac{2}{20} = 10%$ to $\frac{0}{10} = 0%$ .
Genetic Drift can have a profound effect on small populations!

Advantages of using allelic frequencies to describe the gene pool of a population instead of using genotypic frequencies:
- Fewer parameters are needed as there are fewer alleles than genotypes.
- Genotypes are temporary assemblages of alleles that break down each generation, while alleles are passed from generation to generation.

The founder effect occurs when a few individuals colonize a new area.
The smaller the group, the less likely the genetic makeup of the founders will represent the larger gene pool they left.

In Venezuela, there is a high incidence of Huntington's Disease.
The mutation can be traced back to one common ancestor who likely inherited it from her father, a European sailor.

The bottleneck effect occurs when a catastrophe kills off the majority of a population.
The small surviving population is unlikely to have the same genetic makeup as the original population.

Achromatopsia (total color blindness and extreme sensitivity to light) affects about 0.003% of the US population.
In Pingelap, a Micronesian island, it affects about 5% of the population.
In 1775, a typhoon killed 90% of the population in Pingelap, but one of the survivors had Achromatopsia.

Similarities:
- Genetic variation in the population.
- Overproduction of offspring.
- Competition for survival.
Differences:
- Natural selection involves population-limiting factors that are selective pressures, differentially affecting individuals with different gene-based phenotypes.
  - Selection is non-random and leads to a better-adapted population over time.
- Genetic drift involves population-limiting factors that are chance events, affecting any individual equally, regardless of gene-based phenotype.
  - Selection is random and may have no predictable effect on the level of adaptation of the population.

Deviations in expected allele frequencies can indicate a population is evolving.
(i.e., if the population is not in Hardy-Weinberg Equilibrium)

Assumptions:
- The population is large.
- Mating is random.
- No new mutations.
- No migration.
- No natural selection.
Predictions:
- Allelic frequencies do not change.
- Genotypic frequencies stabilize after one generation.
$p^2$ : homozygous dominant allele pair frequency
$q^2$ : homozygous recessive allele pair frequency
$2pq$ : heterozygous allele frequency

The statement that allelic frequencies (p and q) are equal is NOT an assumption of the Hardy-Weinberg law.

Allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences.
If the frequency of the dominant allele W (no web) is designated p, and the frequency of the recessive allele w (web) is designated q, and they are the only alleles for this gene in the population, then:
- $p + q = 1$
No webbing
Webbing

The expected frequency of heterozygotes in a population with allelic frequency x and y that is in Hardy–Weinberg equilibrium is 2xy.

In sickle-cell anemia, normal homozygous individuals (SS) have normal blood cells that are easily infected with the malarial parasite.
Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily sickle when deoxygenated.
Heterozygous individuals (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. Malaria cannot survive well within these