Evolutionary Genetics

Evolutionary Genetics

Introduction to Evolutionary Genetics

  • Evolution: Heritable change in characteristics of a population/species over generations.
    • Microevolution:
    • Change in allele frequencies over time.
    • Mechanisms:
      • Mutation
      • Genetic drift
      • Migration
      • Natural selection
      • Inbreeding/non-random mating
    • Macroevolution:
    • Large-scale changes that create new species and higher taxa.
Key Finch Species
  • Common Finch
  • Warbler Finch
  • Woodpecker Finch
  • Ground Finch
  • These species have genes that control bill shape, illustrating phenotypic variation linked to genetic differences.

Part 1: Overview of Evolution and Mechanisms of Speciation

  • Defining a Species
    • Various methods to define species:
    • Morphological traits
    • Molecular features
    • Ecological factors
    • Evolutionary factors
    • Reproductive Isolation:
    • Essential for defining species, particularly in sexually reproducing species.
    • Key for non-sexually reproducing organisms.

Molecular Changes Underlying Speciation

  • Synteny Group: A group of genes found in the same order in different species, emphasizing shared ancestry.

Reproductive Isolation Mechanisms

  • Prezygotic Isolation:

    • Differences in mating behavior (e.g., song) prevent potential mates from recognizing each other.
    • Other types include:
    • Temporal differences in breeding seasons.
    • Anatomical incompatibility.
    • Gametes failing to unite.

  • Postzygotic Isolation:

    • Fertilization occurs, but the resulting organism is inviable or infertile:
    • Hybrid inviability: Fertilized egg does not develop.
    • Hybrid sterility: Surviving hybrid is sterile (e.g., mules).
    • Hybrid breakdown: F1 hybrids are viable/fertile, but subsequent generations show increasing inviability; often due to chromosomal incompatibility.

Chromosome Incompatibility and Species

  • Chromosomal differences lead to sterility or inviability in hybrids.
    • Example:
    • Horses (64 chromosomes, 2n = 64)
    • Donkeys (62 chromosomes, 2n = 62)
    • Mules (63 chromosomes) resulting from horse-donkey mating:
      • 32 from horse, 31 from donkey.
      • Resulting aneuploid gametes due to odd chromosome count.

Speciation and Chromosome Changes

  • Polyploidy: Can lead to reproductive isolation, resulting from changes in chromosome number.
    • Example: A tetraploid species arising from two diploid parents.
  • Chromosome Structure Changes: Species may evolve from common ancestors but exhibit different gene arrangements.
    • Example: Human chromosome 2 involved a fusion event not present in closely related primates.
    • Chromosome 3 in orangutan: inversion shown, absent in closely related primate species.

Part 2: Phylogenetic Trees

  • Phylogeny: Sequence of evolutionary events for species.
  • Phylogenetic Tree: A diagram depicting evolutionary relationships based on homologous traits.
  • Major Terms:
    • Clade: A group of organisms that includes an ancestor and all its descendants.
    • Cladistics vs. Phenetics:
    • Cladistics: Focuses on evolutionary relationships based on shared derived features.
    • Phenetics: Constructs trees based on overall similarities without consideration of evolutionary pathways.
Constructing Phylogenetic Trees

  • Cladistic Approach:

    • Character evaluation to determine ancestry of species.
    • Distinction between:
    • Ancestral characters: Traits shared with distant ancestors.
    • Shared derived characters: Traits shared by a specific group but not with distant ancestors (e.g., flippers in cetaceans).

  • Phenetic Approach:

    • Considers overall similarities between gene sequences.
    • Example: UPGMA method assumes similar rates of neutral mutation across species.

Tree Thinking Practice

  • Relationships illustrated in the phylogenetic tree help identify evolutionary connections.
  • Examples involve understanding relationships between species like green algae, moss, seals, and horses, noting sister species and their common ancestors.

Modern Evolutionary Models

  • Recent models of evolution incorporate horizontal gene transfer.

  • Illustration of implications seen in Bacteria, Archaea, and Eukaryotes revealing genetic community dynamics.

  • The root of the eukaryotic tree remains uncertain; divisions exist among:

    • Alveolata
    • Stramenopila
    • Rhizaria
    • Amoebozoa
    • Opisthokonta
Final Notes
  • Understanding evolutionary genetics and mechanisms of speciation is critical for comprehending biodiversity and the evolutionary history of life on Earth. The interconnectedness of species through genetic lineage emphasizes the importance of molecular genetics in evolutionary studies.