Population Genetics

Populations and Demes

  • Definition of Population: Broadly defined, the term population encompasses any set of items. In biological sciences, it is refined to describe aggregates of similar individuals. Population geneticists further narrow this definition to denote groups of sexual forms that associate for reproduction and ecological reasons.

  • Genetic Definition: A genetic population is a spatially and temporally restricted group of interbreeding conspecific individuals. It maintains continuity across generations due to reproductive interconnections and exhibits spatial unity facilitated by interbreeding.

    • Population Dynamics: A population can increase or decrease in size due to:

    • Immigration and emigration

    • Changes in birth rates and death rates

    • Fusion with other populations

    • Extinction through total elimination or complete dispersion of members.

    • Temporal Persistence: This definition allows for the persistence of a population over time despite the continual death and replacement of individual members.

  • Gene Exchange: The concept focuses primarily on gene exchange among individuals rather than on individual populations. All genetic information across interbreeding groups forms a collective gene pool.

    • Gene Pool Concept:

    • The gene pool is a temporary collection of genotypes per generation.

    • Zygotes (genotypes) are derived from gametes produced by the preceding generation and are influenced by meiosis, gamete combination during fertilization, and with every generation, a reconstituted gene pool emerges.

    • Isolated or semi-isolated colonies that interbreed randomly showcase this gene pool management.

    • Terminology: Such populations can be termed as panmictic units (Wright, 1931) or local Mendelian populations (Dobzhansky, 1951), commonly referred to as genetic populations or simply "deme."

Change in Gene Frequencies

  • Definition of Evolution: Evolution can be understood as a change in gene frequencies, particularly measurable at single or multiple loci in a genome.

  • Genetic Similarity:

    • Assessment of genetic similarity between populations is conditional on the organisms and specific genes surveyed.

    • Metrics: If two populations have identical alleles at all loci, their genetic identity is valued at 1.0. Any deviations result in a value of less than 1.0; a total lack of shared alleles gives a genetic identity value of 0.

Hardy-Weinberg Equilibrium

  • Key Concept: In the absence of evolutionary processes, allelic frequencies remain constant over time. This principle can be analyzed through Hardy-Weinberg equations while understanding the assumptions of the model.

  • Hardy-Weinberg Equations:

    • Basic Definitions:

    • Let p = frequency of allele A

    • Let q = frequency of allele a

    • Relation: p+q=1.0p + q = 1.0

    • Predicting Genotype Frequencies:

    • For two alleles, allele frequencies can predict genotype frequencies using the binomial expansion:

    • p2+2pq+q2=1.0p^2 + 2pq + q^2 = 1.0

    • Observations: Different populations may exhibit the same gene frequencies with varied genotypic frequencies, but after a generation of random mating, they will reach the Hardy-Weinberg expected genotype frequencies under model assumptions.

  • Assumptions of the Hardy-Weinberg Model:

    1. Large Populations: Assumption of infinitely large populations minimizes sampling error that can introduce changes in gene frequencies. Smaller populations are subject to genetic drift and founder effects.

    2. Random Mating: Individuals have an equal chance of mating, meaning inbreeding or assortative mating can skew gene frequencies.

    3. No Mutation: Mutation alters gene frequencies unless backward mutation counteracts the new allele at equivalent rates.

    4. No Migration: Gene flow introduced by immigration and emigration alters gene frequencies in both the source and recipient populations.

    5. No Selection: Natural selection leads to differential perpetuation of genotypes, affecting evolutionary trajectories based on fitness advantages of certain alleles.

Types of Selection

  1. Stabilizing Selection: Favors individuals in the center of the normal distribution of phenotypes, increasing their representation and diminishing extremes in phenotype distributions.

  2. Directional Selection: Favors one end of the phenotype range, shifting distributions positively or negatively toward the chosen phenotype.

  3. Disruptive Selection: May result in a population developing two high-frequency phenotypes instead of maintaining a single peak, particularly if a species adapts to differing environments.

Genetic Drift

  • Definition: The Hardy-Weinberg model often makes unrealistic assumptions about population sizes and mating patterns, leading to sampling biases that can result in gene frequency changes.

  • Conditions Influencing Genetic Drift:

    1. Continuous Drift: Occurs in small populations over generations, consistently affecting gene frequencies.

    2. Intermittent Drift: In larger populations, temporary reductions in population size can introduce large sampling errors.

    3. Founder Effect: Occurs when new populations are established by few individuals, leading to a gene pool that is not reflective of the parental population, thus differing gene frequencies significantly with respect to the original population.