Introduction to Evolution and Evolutionary Genetics

Foundations of Evolutionary Thought

• Pre-Darwinian Perspectives:
  • Natural Theology: Formulated by William Paley (1802), viewing adaptation as evidence of divine creation.
  • Inheritance of Acquired Characters: Proposed by Jean-Baptiste de Lamarck (1744–1829) as a mechanism for evolutionary change.
  • Uniformitarianism: Proposed by Charles Lyell (1797–1875), postulating that current geological processes explain Earth's history.
• Darwin-Wallace Theory of Evolution by Natural Selection:
  • Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently established natural selection after reading Thomas Malthus's An Essay on the Principle of Population (1798).
  • Postulated two core concepts: evolution as a pattern of descent with modification, and natural selection as its primary driving mechanism.
• Prerequisites for Natural Selection:
  • Variation: Individuals in a population differ in their physical or physiological traits.
  • Heredity: Offspring resemble their parents more than unrelated individuals.
  • Differential Fitness: Specific trait variations enhance survival and reproduction success in a given environment.

Key Concepts of Biological Evolution

• Core Definition: Biological evolution is the change in inherited attributes of populations across generations.
• Scales of Evolutionary Study:
  • Microevolution: Evolutionary patterns and mechanisms occurring within species.
  • Macroevolution: Evolutionary patterns and processes occurring among species.
• Fundamental Principles:
  • Evolution acts on populations, not individuals.
  • Variation is non-directed and not induced by environmental demand.
  • Adaptation: Traits that enhance survival and reproduction in an environment (noun), or the evolutionary process leading to them (verb).
  • Biodiversity: The total variety, number, and composition of living organisms in a specified area.

Lines of Evidence for Evolution

• Geology and Fossils:
  • Fossil records reveal ancient lineages and transitional forms linking distinct taxa (e.g., Glyptodon fossils leading to modern armadillos; transitional whale series; Tiktaalik roseae from 375 million years ago illustrating the fish-to-tetrapod transition).
• Homology and Vestigial Features:
  • Structural Homology: Similarity in anatomical structures resulting from shared ancestry (e.g., pentadactyl forelimb architecture shared across humans, moles, horses, porpoises, and bats).
  • Vestigial Structures: Reduced, non-functional traits derived from functional ancestral structures (e.g., flightless cormorant wings; human tailbone, appendix, ear muscles, and goosebumps).
  • Molecular Homology: Approximately 500 core genes conserved across all domain life forms for basic cellular functions.
• Biogeography and Convergence:
  • Island endemicity (e.g., Galápagos tortoises and finches) illustrates speciation following geographic isolation.
  • Convergent Evolution: Independent evolution of similar phenotypic adaptations in unrelated taxa under comparable environmental pressures (e.g., Australian marsupial potoroo vs. European placental rabbit; convergent Anolis lizard ecomorphs in the Caribbean).
• Domestication and Direct Observation:
  • Artificial selection in domestic pigeons, maize (from teosinte), and dogs (from wolves) demonstrates rapid evolutionary change driven by selective breeding.
  • Real-time evolutionary responses documented in nature (Peter and Rosemary Grant's 50-year study of Galápagos finch beak size) and laboratory settings (Aneil Agrawal's experimental evolution in Drosophila).

Sources and Inheritance of Genetic Variation

• Origins of Genetic Variation:
  • Mutation: Stable, non-directed changes in DNA sequence (point mutations, indels, repeat number shifts, inversions).
    • Human mutation rate: Approximately $16 \times 10^{-9}$ per base pair per generation (~96 new mutations per zygote).
  • Independent Assortment: Shuffling of parental chromosomes into $2^n$ possible gametes ($2^{23} \approx 8.4 \times 10^6$ in humans; $2^{510} \approx 3.4 \times 10^{153}$ in Ophioglossum petiolatum).
  • Recombination: Meiotic crossing over generating novel haplotype combinations.
• Inheritance Models:
  • Blending Inheritance: Outdated hypothesis claiming parental traits blend irreversibly, which would eliminate phenotypic variation necessary for natural selection.
  • Particulate Inheritance: Formulated by Gregor Mendel (1822–1884), demonstrating that discrete hereditary factors (genes/alleles) segregate and assort independently without blending.
  • Quantitative Traits: Continuous phenotypic traits (e.g., human height) governed by complex polygenic inheritance at multiple loci interacting with environmental factors.
  • The Modern Synthesis: Synthesis by R.A. Fisher, J.B.S. Haldane, and Sewall Wright uniting Mendelian genetics with Darwinian natural selection.

Population Genetics and Mechanisms of Evolutionary Change

• Metrics of Genetic Diversity:
  • Polymorphism ($P$): Proportion of gene loci with two or more alleles in a population.
  • Heterozygosity ($H$): Average proportion of heterozygous individuals across examined loci.
• Evolutionary Forces:
  • Mutation: Increases diversity by introducing new alleles ($+$).
  • Recombination: Increases haplotype diversity ($+$).
  • Genetic Drift: Random fluctuations in allele frequencies, causing loss of genetic variation ($-$); pronounced in small populations.
  • Natural Selection:
    • Purifying/Negative Selection: Eliminates deleterious mutations ($-$).
    • Positive/Directional Selection: Fixes advantageous alleles ($-$).
    • Balancing Selection: Maintains genetic diversity via mechanisms like heterozygote advantage ($+$).
  • Migration (Gene Flow): Exchanges genetic material between populations, increasing within-population diversity ($+$) and reducing divergence between populations.

Models of Genetic Variation in Populations

• Classical School (T. H. Morgan, H. J. Muller): Predicted low $H$ and $P$, arguing mutation-selection balance dominates and purges rare, deleterious variants.
• Balance School (E. B. Ford, T. Dobzhansky): Predicted high $H$ and $P$, arguing balancing selection maintains abundant genetic variation.
• Electrophoresis Revolution (R. C. Lewontin): Allozyme gel electrophoresis revealed high levels of genetic polymorphism and heterozygosity across species.
• Neutral Theory of Molecular Evolution (Motoo Kimura): Proposed that the majority of molecular genetic variation and evolutionary change is selectively neutral, driven by mutation and random genetic drift rather than natural selection.