Microevolution and Natural Selection

Microevolution

Definition

  • Microevolution: Refers to the change in allele frequency in a population over time (generations).

Key Concepts

  • Population: A localized group of individuals all belonging to the same species.

  • Species: Individuals that can interbreed and produce fertile offspring.

Population Genetics

Outline of Population Genetics

  • Population Genetics encompasses the following topics:

    • Population structure and dynamics

    • Causes of Microevolution

    • Genetic Variation and Natural Selection

    • Natural Selection as the Mechanism of Adaptive Evolution

Causes of Microevolution

Sources of Microevolution

  1. Mutation

  2. Genetic Drift

  3. Gene Flow

  4. Natural Selection

Mutation

  • Approximately 60 new mutations per generation are observed.

  • Mutations can dramatically alter gene frequencies over longer time scales.

  • Example of mutation:

    • Normal hemoglobin DNA: CTT

    • Mutant hemoglobin DNA: CAT

    • mRNA Conversion:

    • Normal hemoglobin mRNA: GAA

    • Sickle-cell hemoglobin mRNA: GA

  • Phenotypic Effect:

    • Normal hemoglobin -> Glutamic acid (Glu)

    • Sickle-cell hemoglobin -> Valine (Val)

How Mutation Affects Fitness

  • Fitness: The ability to survive and reproduce.

    1. No effect on fitness

    2. Reduced fitness

    3. Increased fitness

Genetic Drift

Overview

  • Changes in the gene pool of a small population due to chance.

  • Most populations are large enough to avoid genetic drift, but smaller populations are susceptible.

Types of Genetic Drift

  1. Bottleneck Effect

    • Occurs when a population's size is significantly reduced for at least one generation, leading to a loss of genetic diversity.

  2. Founder Effect

    • Occurs when a small number of individuals colonize a new area, potentially leading to different allele frequencies compared to the original population.

Gene Flow

Definition

  • Gene flow refers to the transfer of alleles into and out of a population.

  • This flow is accomplished through the gain or loss of fertile individuals and/or gametes, which can lead to increased genetic similarity between neighboring populations.

Natural Selection

Definition

  • Natural selection is defined as differences in reproductive success among individuals in a population.

  • Alleles are not passed along in frequencies proportional to their population frequencies.

  • If possessing a specific allele increases the likelihood of reproduction, the frequency of that allele will rise in subsequent generations.

  • Natural selection can act as an agent of evolutionary change, as well as maintaining stability within a population.

Modes of Selection

1. Stabilizing Selection

  • Definition: Acts against extreme phenotypes and favors intermediate varieties.

  • Graphical Representation: Shows a higher frequency of individuals with intermediate phenotypes compared to extreme phenotypes in a population.

2. Directional Selection

  • Definition: Shifts the frequency curve in one direction, favoring one extreme phenotype over others.

  • Graphical Representation: A change in allele frequency whereby one extreme becomes more common in the population over time.

3. Diversifying (Disruptive) Selection

  • Definition: Acts against intermediate phenotypes and favors extreme phenotypes.

  • Graphical Representation: Shows two peaks in phenotype frequency corresponding to the favored extreme traits, with reduced representation of intermediates.

Non-Random Mating (Sexual Selection)

Definition

  • Non-random mating occurs when individuals select their partners based on specific phenotypic features or appearances, deviating from random mating expectations.

  • This process can create deviations from expected genotype frequencies and set the stage for natural selection to drive evolutionary changes.

Examples of Sexual Selection

  • Male peacocks exhibit colorful and elaborate plumage, whereas female peacocks have more drab coloration.

  • Male lions possess manes, while females lack this feature.

Types of Sexual Selection

  1. Intersexual Selection (Between sexes)

    • This type typically involves female choice in mate selection.

  2. Intrasexual Selection (Within sex)

    • Typically involves male-male competition for access to mating opportunities.

References

  • Copyright © 2009 Pearson Education, Inc.

  • Image credits for visual examples in relation to different modes of selection and peacock sexual selection are attributed to Barjumin Cummings.