Evolution of Populations

The Evolution of Populations

Allele Frequencies

• In the early 1900s, biologists began to study how allele frequencies change in populations after the rediscovery of Mendel's work.

Hardy-Weinberg Principle

• Developed in 1908 by G.H. Hardy and Wilhelm Weinberg.
• Demonstrates that dominant alleles do not automatically replace recessive alleles.
• Proves that allele frequencies in a population remain constant unless evolutionary forces act upon them.
• Applicable only to large populations with minimal inbreeding.
• Five evolutionary forces invalidate the principle: mutation, non-random mating, natural selection, gene flow, and genetic drift.
• Expressed as an equation to predict genotype frequencies in a population.

Evolutionary Forces

Mutation

• Slow rate of mutation in nature leads to insignificant allele frequency changes, except over long periods.
• Not all mutations result in phenotypic changes.
• Mutation is the source of variation, making evolution possible.

Gene Flow

• Movement of individuals (migration) causes genetic change.
• Immigrants add alleles; emigrants remove alleles.

Non-Random Mating

• Individuals prefer mating with nearby individuals or those of their own phenotype.
• Inbreeding is a type of non-random mating that lowers heterozygote frequency.
• Inbreeding increases homozygote proportion without changing allele frequencies.
• Mate selection based on traits like size, color, or ability to gather food is an example.

Genetic Drift

• Chance events significantly affect allele frequencies in small populations.
• Examples: fires or landslides reducing population size.
• Losing even one individual can permanently change allele frequency.
• Change in allele frequency occurs randomly (drifting).
• Isolated small populations diverge due to genetic drift.
• Cheetahs: population greatly reduced, leading to genetic uniformity and increased extinction risk due to lower disease resistance.

Natural Selection

• Increases or decreases allele frequencies, as seen with sickle cell anemia.
• In the US, the allele for sickle cell anemia is gradually decreasing because homozygous individuals rarely choose to have children.
• Natural selection plays a major role in genetic change.

Action of Natural Selection on Phenotypes

• Natural selection acts on phenotypes, not genotypes.
• Individuals with favorable traits reproduce and pass traits to offspring.
• Selection acts when alleles become common enough for heterozygous individuals to produce homozygous offspring.
• Natural selection does not occur if characteristics are not expressed (e.g., rare recessive or mutant alleles).

Why Genes Persist

• Cystic fibrosis illustrates limitations on natural selection.
• About 1 in 2,500 individuals are homozygous recessive for cystic fibrosis.
• 1 in 25 Caucasians carry the defective gene without showing symptoms.
• Natural selection cannot eliminate these genetic conditions because very few individuals with the cystic fibrosis gene express the recessive phenotype.

Natural Selection and the Distribution of Traits

• Populations shaped by natural selection acting on polygenic traits (influenced by several genes).
• Examples: human skin color and height.
• Natural selection changes allele frequencies of genes governing a single trait, influencing genes contributing most to the phenotype.
• Polygenic traits exhibit a range of phenotypes clustered around an average value, forming a normal distribution (hill-shaped curve).

Directional Selection

• Elimination of one extreme from a range of phenotypes.
• Alleles promoting this extreme diminish in the population.
• Trait frequency leans toward one direction.

Stabilizing Selection

• Reduction of extremes in a range of phenotypes.
• Increase in intermediate phenotype frequencies.
• Common in nature, supports the average by increasing the proportion of similar individuals.

Hardy-Weinberg Equation

• $p^2 + 2pq + q^2 = 1$
• $p + q = 1$

Where:

• $p$ is the frequency of the dominant allele.
• $q$ is the frequency of the recessive allele.
• $p^2$ is the frequency of the homozygous dominant genotype.
• $2pq$ is the frequency of the heterozygous genotype.
• $q^2$ is the frequency of the homozygous recessive genotype.