Evolution — Natural Selection and Adaptation

Evolution is supported by multiple independent lines of evidence: Fossil record: Fossils show the history of life — simpler organisms in older rock layers, increasingly complex and diverse organisms in younger layers. Transitional fossils document the evolution of major groups. Tiktaalik (fish-tetrapod transition), Archaeopteryx (dinosaur-bird), and numerous human ancestor fossils provide detailed evidence. Comparative anatomy: Homologous structures: same basic structure, different function, derived from a common ancestor. Human arm, bat wing, whale flipper, horse leg — all have the same bones (humerus, radius, ulna, carpals) despite very different functions. Vestigial structures: reduced or non-functional organs that were useful in ancestors. Human coccyx (tail vertebrae), arrector pili muscles (for raising hair, now only causes goosebumps), wisdom teeth. Molecular evidence: DNA sequence comparisons show evolutionary relationships. More closely related species have more similar DNA. Human and chimpanzee DNA differs by ~1.2%. The cytochrome c protein (involved in cellular respiration) is almost identical across all eukaryotes — conserved because any change disrupts function. Biogeography: Species distribution reflects evolutionary history. Similar environments on different continents have similar-looking but unrelated species (convergent evolution). Island species most closely resemble mainland species from the nearest continent. Direct observation: Evolution has been observed directly: Galapagos finch beak changes during drought years, peppered moth colour changes with industrial pollution, bacterial antibiotic resistance development, HIV evolution within a single patient.

Population genetics studies how allele frequencies change over time in populations. The Hardy-Weinberg principle: In an idealised population (no selection, no mutation, no migration, no genetic drift, random mating), allele frequencies remain constant. This provides a baseline to detect when evolution IS occurring. Hardy-Weinberg equations: If p = frequency of allele A, q = frequency of allele a, then p + q = 1. Genotype frequencies at equilibrium: p² (AA) + 2pq (Aa) + q² (aa) = 1. Application: If q² = 1/10,000 (frequency of autosomal recessive disease), then q = 1/100, p ≈ 99/100, carrier frequency (2pq) ≈ 2/100 = 1 in 50. Useful for counselling. Example: cystic fibrosis affects ~1/2,500 Europeans (q² = 1/2,500), so q = 1/50, carrier frequency = 2 × (49/50) × (1/50) ≈ 1/25. Factors that change allele frequencies (drive evolution): Natural selection: changes frequencies of alleles affecting fitness. Mutation: creates new alleles. Genetic drift: random changes in small populations. Founder effect (small group establishes new population) and bottleneck effect (population reduced by catastrophe). Can fix harmful alleles by chance. Gene flow: migration brings in or removes alleles. Sexual selection: non-random mating based on heritable traits.

Speciation is the process by which new species arise from an ancestral population. Species definition (Biological Species Concept): A species is a group of organisms that can interbreed and produce fertile offspring, and are reproductively isolated from other groups. Limitations: doesn't apply to asexual organisms, fossils, or hybridising species. Types of speciation: Allopatric speciation: geographic isolation separates a population. The two groups accumulate different genetic changes over time until they can no longer interbreed. The most common mechanism. Darwin's finches on different Galapagos islands are a classic example. Sympatric speciation: speciation without geographic isolation. Polyploidy (whole genome duplication) in plants can instantly create a new species — the polyploid can no longer interbreed with the parent species. Common in crops (wheat, cotton, potatoes are all polyploids). Reproductive isolation mechanisms: Pre-zygotic: different habitats, different mating seasons, behavioural differences, mechanical incompatibility, gamete incompatibility. Post-zygotic: hybrid inviability (hybrids don't survive) or hybrid sterility (mules — horse × donkey — are sterile). Phylogenetics: The study of evolutionary relationships. Phylogenetic trees (cladograms) show relationships based on shared derived characters. All life on Earth shares a common ancestor — the universal common ancestor (LUCA) that lived ~3.5–4 billion years ago. The three domains: Bacteria, Archaea, and Eukarya. Convergent evolution: Similar traits evolving independently in unrelated lineages due to similar selective pressures. Eyes have evolved independently ~40 times. Dolphin and shark body forms are convergent. Wings in birds, bats, and insects are convergent.

Evolutionary principles are essential in medicine: Antibiotic resistance: The most important evolutionary phenomenon in modern medicine. Bacteria with mutations conferring antibiotic resistance survive treatment and reproduce. Resistance spreads through reproduction (vertical gene transfer) and between bacteria of different species (horizontal gene transfer — via plasmids). WHO lists antimicrobial resistance as one of the top global public health threats. Prevention strategies: complete antibiotic courses, avoid unnecessary use, rotate antibiotic classes, develop new antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), and extensively drug-resistant tuberculosis (XDR-TB) are critical concerns. Viral evolution: Viruses mutate rapidly. Influenza virus undergoes antigenic drift (gradual mutation) and antigenic shift (reassortment of genome segments from different strains). This is why flu vaccines need annual updating. HIV mutates so rapidly (reverse transcriptase lacks proofreading) that resistant variants are selected within a patient during treatment — requiring combination antiretroviral therapy (HAART) to prevent resistance. Cancer as evolution: Cancer is somatic evolution. Mutations accumulate in somatic cells. Cells with growth advantages proliferate (natural selection). Tumour heterogeneity (different cells with different mutations) is a major cause of treatment failure — therapy selects for resistant subclones. Evolutionary medicine: Understanding why humans are susceptible to certain diseases. Sickle cell trait protects against malaria — in malaria-endemic regions, the heterozygous state is selected for despite the homozygous disease. This explains the high frequency of the sickle cell allele in sub-Saharan Africa. Menopause: evolution of post-reproductive lifespan may relate to the "grandmother effect" — post-menopausal women improve survival of grandchildren by providing care, increasing inclusive fitness.