Evolutionary Processes and Hardy-Weinberg Equilibrium: Study Notes for General Biology

Evolutionary Processes: Selection

Natural Selection

Natural selection is a fundamental mechanism of evolution, where individuals with advantageous traits survive and reproduce more successfully than others. This process leads to changes in allele frequencies within a population over time.

• Definition: Natural selection is the process by which certain heritable traits become more common in a population because they confer a survival or reproductive advantage.

• Key Point: Selection changes allele frequencies in populations, driving evolutionary change.

• Example: The peppered moth (Biston betularia) in England changed coloration due to industrial pollution, demonstrating natural selection in response to environmental change.

Selection Changes Allele Frequencies

Allele frequencies refer to how common different alleles are in a population. Selection can increase or decrease the frequency of specific alleles depending on their impact on survival and reproduction.

• Key Point: Selection acts on phenotypic variation, which is often caused by genetic variation. The environment determines which traits are advantageous.

• Example: Antibiotic resistance in bacteria increases in frequency when antibiotics are present.

Modes of Selection

Directional Selection

Directional selection shifts the distribution of a trait in one direction, favoring individuals at one extreme of the trait range.

• Definition: Directional selection occurs when individuals with one extreme phenotype have higher fitness than others.

• Key Point: This mode of selection changes the average value of a trait in the population.

• Example: Selection for increased beak size in finches during drought conditions.

Stabilizing Selection

Stabilizing selection reduces variation by favoring intermediate phenotypes and selecting against extremes.

• Definition: Stabilizing selection compresses variation, increasing the frequency of average traits and reducing extremes.

• Key Point: Does not change the average trait value, but decreases variation.

• Example: Human birth weight: very small and very large babies have higher mortality, so average birth weights are favored.

Disruptive Selection

Disruptive selection favors individuals at both extremes of a trait, increasing variation and potentially leading to speciation.

• Definition: Disruptive selection increases variation by selecting against intermediate phenotypes.

• Key Point: Can result in two distinct phenotypes within a population.

• Example: Selection for small and large beak sizes in finches when food resources favor both extremes.

Testing for Hardy-Weinberg Equilibrium (HWE)

Steps to Test HWE

The Hardy-Weinberg Equilibrium provides a mathematical model to study genetic variation in populations. It predicts genotype frequencies under certain assumptions.

1. Calculate genotype frequencies: Count the number of individuals with each genotype and divide by the total number of individuals.

2. Calculate observed allele frequencies: Use genotype counts to determine allele frequencies.

3. Calculate expected genotype frequencies: Use allele frequencies to calculate expected genotype frequencies under HWE.

4. Compare observed and expected genotype frequencies: Assess whether the population is in HWE.

Example: Heterozygote Excess/Deficiency

Testing for HWE can reveal whether a population has more or fewer heterozygotes than expected, which may indicate selection, non-random mating, or other evolutionary forces.

• Hypothesis: If a population is in HWE, the observed genotype frequencies should match the expected frequencies.

• Key Point: Deviations from HWE suggest evolutionary processes are acting on the population.

Main Points

• Natural selection is one of several evolutionary processes.

• Not all traits in a population are adaptations; some may be neutral or due to genetic drift.

• Genetic drift, gene flow, mutation, and selection can all change allele frequencies.

• Natural selection results from the increase in fitness due to traits that enhance survival or reproduction (adaptations).

Key Vocabulary

• Chromosome: Structure of nucleic acids and protein carrying genetic information.

• Gene: Unit of heredity transferred from parent to offspring.

• Allele: Different forms of a gene found at a locus.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Genotype: Genetic makeup of an individual.

• Phenotype: Observable traits of an individual.

• Adaptation: A trait that increases fitness in a particular environment.

Hardy-Weinberg Equilibrium: Calculating Allele Frequencies

Definition and Purpose

The Hardy-Weinberg Equilibrium is a principle stating that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences.

• Key Point: If a population is in HWE, the observed allele frequencies will not change over time.

Assumptions of Hardy-Weinberg Equilibrium

• No natural selection

• No genetic drift (large population size)

• No gene flow (no migration)

• No mutation

• Random mating

Hardy-Weinberg Principle as a Null Hypothesis

The HWE principle is used as a null hypothesis to test whether a population is evolving at a particular gene locus.

Key Vocabulary

• Null hypothesis: The assumption that no evolutionary process is acting on the population.

• Genotype frequency: Proportion of each genotype in the population.

• Allele frequency: Proportion of each allele in the population.

Hardy-Weinberg Equation

The Hardy-Weinberg equation is used to calculate expected genotype frequencies:

• $p^2 + 2pq + q^2 = 1$

• Where $p$ is the frequency of one allele, $q$ is the frequency of the other allele, $p + q = 1$

Summary Table: Modes of Selection

Mode of Selection	Effect on Trait Distribution	Example
Directional	Shifts mean trait value in one direction	Finch beak size during drought
Stabilizing	Reduces variation, favors average traits	Human birth weight
Disruptive	Favors extremes, increases variation	Finch beak size with two food sources

Additional info: Academic context and examples have been expanded for clarity and completeness.