7 Population genetics Evolution

Population Genetics and Evolution

  • Diversity from the gene to the community

    • Genes, Individuals, Species, Communities

    • Equation: S = 4Nu

Population Genetics vs Transmission Genetics

  • Population Genetics: Concerned with the frequency of particular genes and genotypes within a population.

  • Transmission Genetics: Deals with expected ratios of offspring genotypes based on parent genotypes.

Definition of a Population

  • A group of interbreeding individuals localized in time and space belonging to a single species.

    • Example: Population 1, Population 2.

Understanding Frequency

  • Frequency: The proportion of a specific type of object within a group.

    • Frequencies are properties of populations, not individuals.

    • Frequencies sum to 1 and range from 0 to 1.

    • Example Calculation:

      • Frequency[dimpled individuals] = number of dimpled individuals / total number of individuals = 92 / 150 = 0.613.

Gene Transmission to Offspring

  • Genes are inherited from parents to offspring.

    • Genes combine to form genotypes, which determine phenotypes (observable traits).

    • If certain individuals do not breed due to various reasons, they cannot pass on their genes, influencing population genetics.

    • Example: Natural selection as a restricting factor.

Defining Evolution

  • Evolution: A change in gene frequency from one generation to the next in a population.

    • Influences on gene frequency include migration and mutation.

    • New types of genes can enter the population.

Evolutionary Change Factors

  • Components that may cause evolution include:

    • Mutation: New gene variations introduced over generations.

    • Migration: Movement of individuals into or out of a population.

    • Selection Pressure: Factors preventing individuals from reproducing, can be either random (genetic drift) or non-random (natural selection).

Representation of Population Evolution

  • Population Evolution Illustration: Changes in phenotypic traits over generations illustrated by colored dots:

    • Generation 1: High variation in phenotypes.

    • Generation 150: Reduced genetic variation (more red dots).

Evolutionary Forces Producing Change

  • Forces that produce evolutionary change include:

    • Mutation: Changes across generations, albeit less likely.

    • Migration: Influence of new individuals entering or leaving the population.

    • Natural Selection: Non-random survival and reproduction based on fitness.

    • Genetic drift: Random changes in gene frequency due to chance events.

Example of Natural Selection

  • Variation in coat color in rabbits influenced by predation:

    • Dark red rabbits survive better against predation due to camouflage.

    • Over generations, higher frequency of dark coloring due to selection pressure from wolves.

Genetic Drift

  • Genetic Drift: Random changes in gene frequency due to chance events.

    • It can lead to random loss or predominance of traits and can be especially pronounced in small populations.

    • “Genetic” refers to the frequency

    • “Drift” refers to the fact that the genes at a particular locus will change in frequency at whim—drift around.

    • The founder effect: Rare individuals (and the genes they carry) become prevalent due to disproportionate frequencies in an initial breeding population

Complications in Genotype-Phenotype Relationships

  • Interaction between genotype and environmental variables influences phenotypic expression:

  • Environment can enhance or constrain traits; examples include nutrition and sunlight exposure.

  • Pleiotrophy: When a single locus controls more than one trait

Continuous Traits and Their Variation

  • Continuous Traits: Traits without clear categories influenced by multiple loci:

    • Example: Height is a continuous trait typically distributed in a bell curve fashion.

    • Continuous traits are sometimes referred to as polygenic traits.

    • The key is to note that each gene at a locus contributes to the same trait—and with several loci contributing to the same trait.  More genes = more variation in the trait and with lots of variation, the trait takes on a bell-curve shape.

    • They get their bell-shaped distribution because many loci all contribute to the trait.

    • Continuous traits are also called polygenic traits.  They tend to be more influenced by the environment (meaning for a given genotype, the phenotype (trait) will have slightly different values as shown with the dimple/sunlight example)

Selection on Continuous Traits

  • Directional Selection: Favoring individuals at one extreme of a trait distribution.

    • Example: The distribution of height will shift to a higher value over time because they are stronger(over generations)

  • Stabilizing Selection: Favoring average individuals in a trait distribution.

    • The distribution of height will shift toward the average value over time (over generations). Fewer and fewer shirt and tall people

  • Disruptive Selection: Favoring extreme phenotypes, disregarding average ones.

    • Selection eliminates the unfit “average” individuals.

    • The distribution of height will shift toward eachextreme value over time (over generations)