"It is not the strongest of the species that survives... but the one most responsive to change." — apocryphally attributed to Darwin (actually Megginson, 1963), but the spirit is right.
Charles Darwin's central idea was almost embarrassingly simple. Wherever there is variation, inheritance, and differential reproduction, populations must change over generations.
Individuals differ. Mutations, recombination, and developmental noise generate the raw material on which selection acts.
Offspring resemble parents more than they resemble strangers. Mendelian particles, since 1900; genes, since the 1920s.
Some variants leave more offspring than others. The world cannot support all that is born.
Given those three, evolution is mathematically inevitable.
Failed in medicine at Edinburgh, took holy orders at Cambridge instead. In 1831, at 22, sailed as gentleman naturalist on HMS Beagle's five-year survey voyage. The experience — fossils in Patagonia, finches and tortoises on the Galápagos, coral atolls in the Pacific — convinced him species are not fixed.
He read Malthus on population in 1838 and the puzzle's last piece fell into place. He sat on the theory for twenty years, terrified of the consequences. In 1858 a letter from Alfred Russel Wallace, working independently in Indonesia, forced his hand. They presented jointly at the Linnean Society. On the Origin of Species followed in November 1859 — sold out in a day.
— sketch after Darwin, 1845
Random changes to DNA. Rate ~10⁻⁸–10⁻⁹ per base per generation in mammals.
Differential survival/reproduction. Natural, sexual, kin, group. Directional, stabilizing, disruptive.
Random changes due to finite population. Dominates when Ne is small or selection is weak.
Migration mixes populations. Counteracts local adaptation.
Inbreeding, assortative mating. Changes genotype frequencies even without changing alleles.
Same bones, four jobs.
A schematic, not to scale or completeness.
All living organisms share the same genetic code, the same handedness of biological molecules, and large blocks of homologous genes. The simplest explanation: they share an ancestor. The Last Universal Common Ancestor (LUCA) lived ~3.8 Gya, perhaps near hydrothermal vents.
Geographic isolation. Most common mode. Galápagos finches; Lake Baikal seals; Hawaiian silverswords.
Divergence in same place — niche partitioning, host shift. Cichlid fish in African rift lakes (~500 species in Lake Malawi alone).
Adjacent ranges with limited gene flow. Ring species, e.g. Larus gulls around the Arctic.
Reproductive isolating mechanisms: prezygotic (mating time, place, behavior, gamete incompatibility) and postzygotic (hybrid inviability, sterility — see the mule).
Cabinet of curiosities. The 19th-century museum was the engine of comparative zoology.
1744–1829
First systematic theory of transmutation. Inheritance of acquired characters — mostly wrong, but he raised the question.
1809–82
Natural selection. Origin of Species, 1859. Descent of Man, 1871.
1823–1913
Co-discoverer of natural selection. Biogeography founder.
1822–84
Particulate inheritance. Critical to the modern synthesis.
1920s–30s
Founders of population genetics. Reconciled Darwin with Mendel.
1900–75
Modern Synthesis architect. "Nothing in biology makes sense except in the light of evolution."
1904–2005
Biological species concept. Systematics and the Origin of Species, 1942.
1936–2000
Inclusive fitness, kin selection (1964). Hamilton's rule: rB > C.
Genotype frequencies in absence of evolution. Deviation diagnoses force at work.
Altruism evolves when relatedness × benefit to relative exceeds cost to actor.
Rate of increase in mean fitness equals additive genetic variance, scaled.
Allele frequency change per generation under selection of strength s.
Random walk in finite population. Effective N ≪ census N usually.
George Price (1970): unifies all evolutionary change.
Earliest chemical signatures of life — graphite in Greenland metasediments.
Stromatolites; cyanobacteria.
Great Oxygenation Event.
Eukaryotic origin via endosymbiosis (mitochondria).
Sexual reproduction.
Ediacaran biota — first multicellular animals.
Cambrian explosion — most modern phyla appear.
Plants colonize land.
Tetrapods: Tiktaalik et al.
Amniotes — life freed from water for reproduction.
Permian–Triassic extinction. ~96% of marine species lost.
Birds branch from theropod dinosaurs.
K–Pg extinction. Mammals radiate.
Hominin–chimpanzee LCA.
Anatomically modern Homo sapiens.
Domestication of plants and animals.
Sixth. The Holocene/Anthropocene biodiversity crisis: extinction rates 100–1,000× background. ~1 in 4 mammals threatened. Defaunation of large vertebrates.
"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one..."— Charles Darwin, On the Origin of Species, 1859
Humans share ~98.7 % of nuclear DNA with chimpanzees, ~96 % with gorillas, ~93 % with orangutans. Our split from chimpanzees was ~6 Mya, in Africa. The fossil record since then is rich and tangled — at least a dozen recognized hominin species, several coexisting.
The Modern Synthesis (1930s–40s) — Fisher, Haldane, Wright, Dobzhansky, Mayr, Simpson, Stebbins — fused Darwin with Mendelian genetics. Evolution is changes in allele frequencies; speciation is the splitting of gene pools.
Since the 1990s, an Extended Evolutionary Synthesis argues for adding: developmental constraints (evo-devo), epigenetic inheritance, niche construction, multilevel selection, plasticity-led evolution. The debate is more about emphasis than mechanism.
Svante Pääbo's lab (Nobel 2022) sequenced Neanderthal & Denisovan genomes. Pleistocene mammoth DNA > 1 My old recovered (2021).
Lenski's E. coli LTEE, ~80,000 generations and counting. Citrate utilization evolved at gen ~31,500.
Mammoth, thylacine, dodo proxies under engineering. Bringing functional ecologies back, not "Jurassic Park".
Whole-genome trees resolve historic disputes; reveal incomplete lineage sorting and introgression.
Henrich; Boyd & Richerson. Dual-inheritance models for human cooperation, language change.
Maynard Smith & Szathmáry's framework: levels of selection re-organize. From RNA to cells to multicellularity to societies.
Plus David Attenborough's "Life on Earth" (1979) — still unmatched.
"Have, and are being, evolved." — closing words, Origin of Species.