Evolution, Speciation, and Phylogeny Notes

Origin of Species

• Source of Biological Diversity

  • Microevolution: Change in the gene pool of a population over time.
  • Speciation: Evolutionary process where one species splits into two or more new species.
  • Biological Species Concept:
  • Defines species as groups of populations whose members can interbreed and produce fertile offspring with each other, but not with members of other species.
  • Emphasizes reproductive isolation.

• Key Questions:

  • How does microevolution differ from speciation?
  • Which species concepts apply to both asexual and sexual species?

Species Concepts

• Morphological Species Concept: Defines species based on observable physical traits (e.g., shape, color, structure).

  • Examples:
    • Prokaryotes based on cell wall, cilia, habitat, gene expression.
    • Insects based on legs, body segments, mouthparts.
    • Plants based on leaf shape and flower traits.
    • Primates based on opposable thumbs, brain size.

• Ecological Species Concept: Defines species by their ecological role or niche; focuses on unique adaptations to habitat.

  • Example Differences:
    • What species eat, where they live, their interactions in the ecosystem.

• Phylogenetic Species Concept: Defines species as the smallest group of individuals that share a common ancestor.

Reproductive Barriers

• Types of Reproductive Barriers:
  • Pre-zygotic Barriers: Prevent mating or fertilization between species.
  1. Habitat Isolation: Species live in different habitats.
  2. Temporal Isolation: Species breed at different times.
  3. Behavioral Isolation: Differences in courtship rituals prevent species from mating.
  4. Mechanical Isolation: Incompatible reproductive structures.
  5. Gametic Isolation: Incompatible gametes prevent fertilization.
  • Post-zygotic Barriers: Operate after hybrid zygotes form.
  1. Reduced Hybrid Viability: Hybrids fail to develop or reach sexual maturity.
  2. Reduced Hybrid Fertility: Hybrids are sterile (e.g., mules).
  3. Hybrid Breakdown: Next-generation hybrids are inviable or sterile.

Modes of Speciation

• Concept 24.2: Speciation can occur with or without geographic separation:
  • Allopatric Speciation: Populations are geographically isolated.
  • Geographic isolation restricts gene flow.
  • Drives genetic changes leading to reproductive barriers.
  • Sympatric Speciation: Populations are not geographically isolated.
  • Can occur through mechanisms like polyploidy, sexual selection, or habitat differentiation.

Hybrid Zones and Outcomes

• Hybrid Zones: Regions where different species interbreed and produce hybrid offspring due to incomplete reproductive barriers.
• Possible Outcomes:
  1. Reinforcement: Strengthening of reproductive barriers when hybrids are less fit.
  2. Fusion: Weakening of reproductive barriers if hybrids are as fit as parents, leading to species fusion.
  3. Stability: Continued formation of hybrid individuals due to ongoing gene flow.

Fossil Record and Evolutionary Patterns

• Punctuated Equilibria: Sudden changes in species followed by long periods of stasis.
• Macroevolution: Accumulation of changes over many speciation events, affecting the diversity and complexity of life.

Mechanisms of Evolution

• Natural Selection: Acts on individuals, favoring traits that enhance survival and reproduction.
• Genetic Drift: Random changes in allele frequencies, more pronounced in small populations.
  • Founder Effect: New population established by a small group has different allele frequencies.
  • Bottleneck Effect: Sudden population size reduction can change gene pool.
• Gene Flow: Movement of alleles among populations reduces genetic differences.

Hardy-Weinberg Equilibrium

• Describes a non-evolving population, where genotype and allele frequencies remain constant under certain conditions:
  • No mutations
  • Random mating
  • No natural selection
  • Large population size (no genetic drift)
  • No gene flow

Natural Selection Types

• Directional Selection: Favors individuals at one end of the phenotypic spectrum.
• Disruptive Selection: Favors individuals at both extremes.
• Stabilizing Selection: Favors intermediate variants, acting against extremes.

Sexual Selection

• Intrasexual Selection: Competition among one sex (often male) for mates.
• Intersexual Selection: One sex chooses mates based on certain traits.
  • The “good genes” hypothesis suggests female choice is influenced by male traits indicative of genetic quality.

Taxonomy and Phylogeny

• Taxonomy: Classification of living organisms, reflecting evolutionary relationships.
• Biological Classification: Organisms are grouped by:
  • Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Phylogenetic Trees: Illustrate evolutionary relationships among species.

Domains of Life

• Three domains:
  1. Bacteria
  2. Archaea
  3. Eukarya
• Reflects fundamental differences in cellular organization, genetics, and evolutionary history.

Key Characteristics of Domains**