Chapter 23 - LO & PPt: Microevolution and Population Genetics

Microevolution

• Definition: Microevolution refers to the change in allele frequencies within a population over generations.

• Key Mechanism: The primary mechanism driving microevolution is natural selection.

  • Natural Selection:

  • This is the only mechanism that causes adaptive evolution, which refers to improvements in the match between organisms and their environment.

  • Requirements for Natural Selection:

    • Variation in heritable traits must be present in a population.

    • Natural selection only acts on phenotypic variations that have a genetic basis.

  • Adaptive evolution is described as a continuous, dynamic process observed in many organisms.

  • Phenotypic variation often exists on a continuous spectrum.

Genetic Drift

• Definition: Genetic drift is defined as chance events that affect survival and reproduction, altering allele frequencies within a population.

• Effects of Genetic Drift:

  • It tends to reduce genetic variation by causing the loss of alleles, especially pronounced in small populations.

Founder Effect

• Definition: The Founder effect occurs when a few individuals from a larger population become isolated.

  • Consequence: The new allele frequencies in the isolated population may not reflect those of the original larger population.

Bottleneck Effect

• Definition: The Bottleneck effect is a significant reduction in population size due to environmental events or human activities, resulting in a gene pool that no longer represents the genetic diversity of the larger original population.

  • Impact on Small Populations:

  • The Bottleneck effect is more pronounced in small populations.

  • Some alleles can become disproportionately represented, leading to random changes in allele frequencies.

  • Resulting impact on genetic variation can lead to harmful alleles becoming fixed in the population.

  • Ultimately, this may influence a species' adaptation to its environment.

Gene Flow

• Definition: Gene flow refers to the transfer of alleles between populations.

  • Implications: Gene flow can either increase or decrease the fitness of a population.

Population and Gene Pool

• Definition of a Population: A population is a group of individuals of the same species that live in the same area and interbreed.

• Gene Pool: Refers to the total collection of alleles in a population at any one time.

Hardy-Weinberg Principles

• Importance: Understanding the Hardy-Weinberg principles is essential for determining if a population is evolving.

  • Calculation of Allele Frequencies:

  • The sum of two alleles for one genetic locus (designated as A and a) can be expressed as: $p + q = 1$.

  • Calculation of Genotype Frequencies:

  • Use the Hardy-Weinberg equilibrium formula: $p^2 + 2pq + q^2 = 1$

    • Where:

    • $AA = p imes p$

    • $Aa = 2 imes p imes q$

    • $aa = q imes q$

• Conditions for Non-Evolving Populations:

  • There are five conditions that must be met for a population to be in Hardy-Weinberg equilibrium and thus non-evolving:

  1. No mutations must occur.

  2. Mating must be random.

  3. No natural selection occurring.

  4. The population must be very large.

  5. No gene flow (migration into or out of the population).

• Real-World Implications: In actual populations, allele frequencies do change over time, indicating that while some genes can remain in Hardy-Weinberg equilibrium, others may be subject to selection and evolution.

Microevolution and Population Genetics

Definitions and Overview

• Microevolution: Refers to changes in allele frequencies in a population over generations.

  • Allele Frequency Change Mechanisms:

  • Natural Selection: Only mechanism that causes adaptive evolution, improving the fit between organisms and their environment.

  • Genetic Drift: Refers to chance events that alter allele frequency, especially significant in small populations.

  • Gene Flow: Transfer of alleles between populations.

Types of Natural Selection

• Natural Selection Examples:

  • Ancestor Finch: Examples of insect-eating versus seed-eating adaptations.

Experimental Methods to Assess Adaptation

• Reciprocal Transplant Experiment: Used to determine if a coping strategy in a population is a result of acclimation (plasticity) or true adaptation.

  • E.g., organisms from different altitudes are moved to check adaptability.

• Common Garden Experiment: Another method to study phenotypic plasticity, where organisms are raised in the same environment.

Sources of Genetic Variation

• Genetic Variation Sources:

  • New genes and alleles arise through:

  1. Mutation

     • Types of Mutations:

       • Mutations in noncoding regions of DNA are often harmless.

       • Mutations in coding genes can be neutral due to redundancy in genetic code.

       • Phenotypic altering mutations are often harmful.

       • Beneficial mutations can enhance protein production.

  2. Gene Duplication: Results in additional gene copies that can evolve new functions.

• Sexual Reproduction: Shuffles existing alleles through:

  1. Crossing Over: Exchange of genetic material during meiosis.

  2. Independent Assortment: Random distribution of chromosomes to gametes during meiosis.

  3. Fertilization: Union of gametes from two parents increases genetic diversity.

Genetic Drift

• Types of Genetic Drift:

  • Bottleneck Effect: Occurs when a large portion of a population is drastically reduced, leading to loss of genetic diversity.

    • Example: Original population undergoes a bottlenecking event; only a small surviving population continues.

  • Founder Effect: When a small group from a larger population establishes a new population, often carrying a limited genetic diversity.

Hardy-Weinberg Principle

• The Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces.

• Hardy-Weinberg Equation: Can be used to test if a population is evolving.

  • $p + q = 1$: Total allele frequencies.

  • Genotype frequencies can be expressed as:

  • $AA = p^2$ (homozygous dominant)

  • $AB = 2pq$ (heterozygous)

  • $BB = q^2$ (homozygous recessive)

• In a population where gametes contribute randomly, allele frequencies remain stable.

Application of Hardy-Weinberg Principle

• Population Definition: A population consists of localized individuals capable of interbreeding and producing fertile offspring.

• Gene Pool: The sum of all alleles for every locus in a population.

• An allele is said to be fixed if all individuals at a locus are homozygous for the same allele.

Conditions for Hardy-Weinberg Equilibrium

• For a population to be in Hardy-Weinberg equilibrium (not evolving), five conditions must be met:

  1. No mutations

  2. Random mating (absence of inbreeding)

  3. No natural selection (equal survival/reproductive success of all individuals)

  4. Extremely large population size

  5. No gene flow (migration of individuals in or out)

Calculation of Allele Frequency Example

• Example Calculation: A population of wildflowers with incomplete dominance:

  • 320 Red flowers (CRCR)

  • 160 Pink flowers (CRCW)

  • 20 White flowers (CWCW)

  • Total = 500 flowers

  • Total alleles at a locus = 1000 (500 individuals * 2 alleles)

  • Frequency of alleles:

  • $p = ext{frequency of } CR = 0.8$

  • $q = ext{frequency of } CW = 0.2$

• Genotype Frequency Calculations:

  • $CRCR = p^2 = (0.8)^2 = 0.64$

  • $CRCW = 2pq = 2(0.8)(0.2) = 0.32$

  • $CWCW = q^2 = (0.2)^2 = 0.04$

Summary of Microevolution Processes

• Microevolution is driven by the following processes:

  1. Natural Selection: Increases the frequency of adaptive alleles.

  2. Genetic Drift: Significant in small populations, includes founder and bottleneck effects.

  3. Gene Flow: Migration effects between populations.

  4. Introduction of New Mutations: Adds new genetic diversity.