Week 6.1 - Gene Flow Drift & Non Random Mating

Gene flow

Gene flow is the movement of alleles between populations, which can change allele frequencies and act as a mechanism of non-adaptive evolution.

Gene flow and population differentiation

Gene flow is typically a homogenizing force, reducing genetic differences between populations by introducing new alleles.

Example of gene flow opposing natural selection

In Lake Erie water snakes, selection favors unbanded snakes on islands (lower predation), but gene flow from the mainland continuously reintroduces banded alleles.

Non-random gene flow

Migration is often biased toward certain phenotypes, affecting adaptation. For example, more exploratory or active individuals may migrate more, altering allele frequencies disproportionately.

Genetic drift

Genetic drift is the random change in allele frequencies due to sampling error, affecting smaller populations more significantly.

Impact of genetic drift on small populations

More random fluctuations in allele frequencies. Loss of genetic variation over time. Fixation of alleles (one allele becomes 100% prevalent). Increased homozygosity, which can lead to inbreeding depression.

Causes of genetic drift

Bottlenecks - A drastic reduction in population size reduces genetic diversity (e.g., cheetahs). Founder effects - A small group starts a new population, leading to genetic differences from the original population (e.g., Polynesian crickets).

Heterozygosity

Heterozygosity is the proportion of individuals in a population that are heterozygous at a given locus. Genetic drift reduces heterozygosity over generations.

Equation for heterozygosity due to drift

Hg+1 = Hg(1−1/2N) where N is the population size. This shows heterozygosity declines faster in small populations.

Effective population size (Ne)

The size of an idealized population that would lose heterozygosity at the same rate as the actual population. It accounts for factors like sex ratios, reproductive variance, and population structure.

Consequences of genetic drift for conservation

Loss of genetic diversity reduces adaptive potential. Small populations struggle to survive environmental changes. Fixation of harmful alleles leads to inbreeding depression. Example: Bighorn sheep populations below 50 individuals had a 100% extinction rate within 50 years.

Minimum viable population (MVP) concept

The smallest population size needed to have a 99% chance of surviving for 1,000 years, despite genetic drift, environmental changes, and catastrophes.

Non-random mating

Non-random mating occurs when individuals preferentially mate with certain genotypes, affecting genotypic frequencies but not necessarily allele frequencies.

Key assumptions of Hardy-Weinberg equilibrium

No mutation, Infinitely large population (no drift), No selection, No migration (no gene flow), Random mating. Violations of these assumptions indicate evolution is occurring.

Inbreeding

Inbreeding is mating among genetic relatives, which reduces heterozygosity and increases homozygosity, leading to inbreeding depression.

Coefficient of inbreeding (F)

F measures the probability that two alleles in an individual are identical by descent. It quantifies the impact of inbreeding. Hf = H0 (1−F) where: Hf = heterozygosity in an inbred population, H0 = heterozygosity in a randomly mating population.

Inbreeding depression

A reduction in fitness due to the increased expression of deleterious recessive alleles in homozygous form.

Behavior of dominant and recessive deleterious alleles

Dominant deleterious alleles are quickly eliminated by selection. Recessive deleterious alleles persist because they are hidden in heterozygotes.

Historical examples of inbreeding depression

Tutankhamun (Egyptian Pharaoh) - Severe genetic defects due to royal inbreeding. Charles II of Spain - Extreme deformities and infertility caused by Habsburg inbreeding. Florida Panthers - Suffered severe inbreeding depression until outbreeding was introduced.