Microevolution Notes

Redefining Microevolution

  • Microevolution is defined in terms of genes, focusing on allele frequencies.

Mechanisms of Microevolution

  • Change in allele frequencies can occur due to:

    • Natural selection

    • Genetic drift

    • Gene flow

    • Mutation

Natural Selection

  • Survival and reproduction of individuals best suited to the environment.

  • It is the main mechanism of evolution.

Allele Frequency

  • Example 1:

    • Initial frequencies:

    • $Freq(A1) = \frac{10}{20} = 0.5$

    • $Freq(A2) = \frac{10}{20} = 0.5$

    • 10 years later:

    • $Freq(A1) = \frac{4}{20} = 0.2$

    • $Freq(A2) = \frac{16}{20} = 0.8$

Origin of New Alleles

  • New alleles can arise through:

    • Mutation (random, heritable changes in DNA)

    • Horizontal gene transfer

Mutation Types

  • Point Mutation: Change in single base pair affecting a single amino acid

  • Gene Duplication: Accumulation of mutations in duplicated genes.

  • Genome Duplication: Extra copies of all chromosomes.

Genetic Drift

  • Changes in allele frequencies due to chance events, especially in small populations.

    • Founder effect: New population started by a small number of individuals.

    • Bottleneck effect: Rapid decrease in population size leading to loss of genetic diversity.

Gene Flow

  • Transfer of alleles between populations via migration.

Modes of Natural Selection

  1. Directional Selection: Favors one extreme phenotype.

    • Example: size of salmon.

  2. Disruptive Selection: Favors extreme phenotypes over intermediate.

    • Example: bill size in Galapagos finches.

  3. Stabilizing Selection: Favors intermediate phenotypes, removing extremes.

    • Example: human birth weight.

Microevolution focuses on changes in allele frequencies through mechanisms such as natural selection, genetic drift, gene flow, and mutation. Natural selection favors survival and reproduction of best-adapted individuals. For instance, allele frequencies can shift over time as seen in the example of $Freq(A1)$ going from $0.5$ to $0.2$ while $Freq(A2)$ goes from $0.5$ to $0.8$. New alleles can arise via mutation or horizontal gene transfer. Mutation types include point mutations, gene duplication, and genome duplication. Genetic drift affects allele frequencies due to chance, often evident in small populations through founder and bottleneck effects. Gene flow involves allele transfer through migration. Natural selection occurs in three main modes: directional selection (favoring one extreme), disruptive selection (favoring extremes), and stabilizing selection (favoring intermediates).