Genetic Drift and Microevolution

Problem in Mendelian Genetics: According to Mendelian principles, if there are two alleles (dominant/incompletely dominant), one would expect $25\%$ homozygous dominant, $25\%$ homozygous recessive, and $50\%$ heterozygous individuals in a population.
Mendel's Limitation: This mathematical expectation often does not hold true for most real populations.
The Discrepancy: The Mendelian model works in theory but breaks down in real life because it assumes all alleles are equally common in a population.
Reality of Allele Distribution: In most instances, one allele is significantly more common than the other. For example, recessive genetic disorders are rare, meaning their alleles are rare.
Hardy-Weinberg Solution: Developed equations to account for unequal allele distribution.
- Allele Frequencies:
  - $p$ = proportion of dominant alleles.
  - $q$ = proportion of recessive alleles.
  - Definition: $p + q = 1$ (representing $100\%$ of alleles in the population).
- Genotype Frequencies: Extended to calculate genotype frequencies from allele frequencies.
  - $p^2$ = expected frequency of homozygous dominant genotype.
  - $q^2$ = expected frequency of homozygous recessive genotype.
  - $2pq$ = expected frequency of heterozygous genotype.
  - Definition: $p^2 + 2pq + q^2 = 1$ (representing $100\%$ of genotypes in the population).

Allele Frequency Stability: Unless a population is under evolutionary pressure, allele frequencies are not expected to change over time.
- Example (Red/White Alleles): A population starting with $80\%$ red and $20\%$ white alleles will maintain these frequencies across generations if no evolutionary forces are acting.
Hardy-Weinberg Equilibrium (HWE):
- Definition: A population is in HWE when allele frequencies are not changing over time at a particular locus.
- Implication: A population in HWE is not evolving at that locus.
Five Key Conditions for HWE (Absence of Evolutionary Forces): For a population to remain in HWE, five specific conditions must be met. If any of these are violated, the population will evolve (allele frequencies will change).
1. No Mutation
2. Random Mating
3. No Natural Selection
4. Extremely Large Population Size (approaches infinity)
5. No Gene Flow (no migration)
Focus of this Lecture: Mutation and Population Size (Genetic Drift).

Definition: A change in the genetic material, capable of generating new variation.
Violation of HWE: Introduces new alleles, thereby changing allele frequencies.
- Example: A system with $80\%$ red and $20\%$ white flowers. A mutation introduces yellow flowers (even at $1\%$ ), changing frequencies to $79\%$ red, $1\%$ yellow, $20\%$ white. This is an allele frequency change.
Sickle Cell Anemia Example: Often arises from a point mutation in the hemoglobin gene. While usually inherited, new mutations can still occur, though rarely.

Definition: Any change in allele frequencies in a population over time.
- Scale: Happens at the population level (not individual or community).
- Driving Force: Does not matter what drives the change; any change is microevolution.
Significance: Microevolutionary processes, given enough time, lead to larger-scale evolution (macroevolution) and the formation of new species.
- Analogy (Hutton and Lyell's Uniformitarianism): Similar to how a small rivulet can carve out a massive canyon (Grand Canyon) over millions of years.
Rate: Microevolutionary processes can happen much faster than often perceived.

Hardy-Weinberg Assumption: Requires a hypothetically infinite population size for equilibrium.
Reality: No natural population is truly infinite.
- Implication: All populations, especially smaller ones, are prone to evolving due to violations of this condition.

Definition: Random changes or random variation in who lives, who dies, who reproduces, and whose genetic input is passed on. These random fluctuations result in drastic variations in allele frequencies.
Key Characteristic: Genetic drift is a random process, driven by chance, like coin tosses. It is not a directed process like natural selection.
**Hypothetical Flower Example (Small Population: 10 Individuals, $p=0.7$ , $q=0.3$ ):
- Generation 1: Initial frequencies: Red allele = $0.7$ , White allele = $0.3$ . (10 individuals)
- Reproduction: Assume reproductive success is completely random (coin toss).
- Generation 2: Only 5 individuals randomly survive and reproduce. Allele frequencies change, e.g., Red allele = $0.5$ , White allele = $0.5$ . This is microevolution.
- Generation 3: Only 2 individuals randomly survive and reproduce (e.g., both red flowers).
- Result: The red allele becomes $100\%$ ( $p=1$ ) and the white allele disappears ( $q=0$ ).

Definition: The phenomenon where heterozygosity (allelic variation) disappears from a population, and one allele becomes $100\%$ prevalent.
Consequence: Once fixed, all subsequent generations will only carry the fixed allele (e.g., $100\%$ red flowers).

Once an allele is lost through genetic drift (fixation), there are only two primary ways it can re-enter the population:
1. Mutation: A new mutation arises that produces the previously lost allele. This is rare and a slow process.
2. Gene Flow (Migration): Individuals carrying the lost allele migrate from an adjacent population with different genetic makeup, introducing the allele back into the population (e.g., wind-blown pollen, human intervention).
Hybridization (Clarification): Not the same as hybridization, which refers to offspring of two different species (e.g., horse + donkey = mule). The flower example involves reproduction within the same species.

Purpose of Models: Mathematical models (simulations) help demonstrate the power of particular concepts by programming behaviors that mimic nature.
'Genie' Model (Reed Cartwright):
- Setup: An arena with individuals (boxes) carrying different alleles (colors).
- Simulation: Random