Evolutionary Mechanisms and Population Genetics

Evolutionary Mechanisms

Learning Objectives

This section introduces the fundamental concepts of how allele frequencies in a population's gene pool change over time, which is the basis of evolution or descent with modification. Students should be able to:

• Explain how changes in allele frequencies relate to evolution.

• Identify when evolution has occurred in a population and determine the likely evolutionary mechanism based on evidence.

• Describe how allele frequencies would change under different evolutionary mechanisms.

• Compare and contrast random and selective forces of evolution.

• Explain sexual selection and the roles of mate choice and competition for mates.

Hardy-Weinberg Equilibrium and Its Assumptions

Overview

The Hardy-Weinberg equilibrium provides a null model for population genetics, describing a population in which allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary influences. Evolution occurs when any of the five key assumptions are violated.

Assumption of H-W Equilibrium	Possible Outcomes if Broken (Cause of Evolution)
Large population size	Genetic drift is especially relevant in small populations, where random chance events can lead to loss of alleles.
Random mating	Non-random mating due to inbreeding (not an evolutionary process itself) or due to sexual selection via mate choice.
No mutations	Mutations are the only evolutionary mechanisms that generate new alleles in a population by chance.
No gene flow	Gene flow tends to equalize allele frequencies in populations but also brings in new alleles via migration of individuals.
No natural selection	Natural selection favors certain alleles, removing others, and can lead to adaptations.

Population Genetics: Detecting Evolution

Allele and Genotype Frequencies

To determine if evolution has occurred, scientists compare allele frequencies across generations. If allele frequencies change, evolution has occurred. If only genotype frequencies change but allele frequencies remain constant, the population is in Hardy-Weinberg equilibrium.

Example: Calculating Allele Frequencies

• Suppose a population of plants has the following genotypes:

  • AA: 20 individuals

  • Aa: 50 individuals

  • aa: 30 individuals

• Total individuals = 100; total alleles = 200

• Frequency of A ($p$): $p = \frac{2 \times 20 + 50}{200} = 0.45$

• Frequency of a ($q$): $q = \frac{2 \times 30 + 50}{200} = 0.55$

Tracking Changes Over Generations

• Generation 1: AA = 20, Aa = 50, aa = 30; $p = 0.45$, $q = 0.55$

• Generation 10: AA = 10, Aa = 70, aa = 20; $p = 0.45$, $q = 0.55$

• Conclusion: No change in allele frequencies ($p$ and $q$ remain the same), so the population is in Hardy-Weinberg equilibrium and no evolution has occurred, even though genotype frequencies changed.

Key Points

• Allele frequency is the proportion of a specific allele among all alleles for a gene in a population.

• Genotype frequency is the proportion of a specific genotype among all individuals in a population.

• Evolution is defined as a change in allele frequencies over time.

• The Hardy-Weinberg equilibrium serves as a baseline to detect evolutionary change.

Formulas

• Allele frequency for A: $p = \frac{2N_{AA} + N_{Aa}}{2N_{total}}$

• Allele frequency for a: $q = \frac{2N_{aa} + N_{Aa}}{2N_{total}}$

• Hardy-Weinberg equation: $p^2 + 2pq + q^2 = 1$

Example Application

• If a population's allele frequencies remain constant over generations, it is not evolving at that gene locus.

• If allele frequencies change, one or more evolutionary mechanisms are at work (e.g., selection, drift, gene flow, mutation, non-random mating).

Additional info: The Hardy-Weinberg equilibrium is a foundational concept in population genetics, providing a framework for understanding how populations evolve and for identifying the forces that drive evolutionary change.