Speciation & The Tree of Life

Carl Linnaeus, the Swedish naturalist who in 1735 gave us binomial nomenclature (Homo sapiens, Felis catus), organized living things into a static hierarchy — kingdoms, classes, orders, genera, species — that he believed reflected God's creation. Charles Darwin, a century later, reinterpreted the same hierarchy as a historical record: every taxonomic group descends from a common ancestor, and the hierarchy is a tree. On the Origin of Species (1859) contains the only diagram in the book — a branching tree showing how species diverge — and Darwin had no fossil or genetic evidence for it, only the logical structure of natural selection acting on inheritable variation. Molecular sequence data has since confirmed the tree's shape with extraordinary precision: every species ever sequenced fits into a single tree traceable back ~3.5 billion years to a Last Universal Common Ancestor (LUCA).

Speciation is the process by which one ancestral population splits into two reproductively isolated populations. Reproductive isolation is the operational definition of species in Ernst Mayr's biological species concept (1942). Mechanisms run prezygotic (different mating seasons, courtship rituals, sperm-egg incompatibility) and postzygotic (the hybrid is inviable, sterile, or unfit — mules from horse × donkey, hybrid breakdown in plants). Modes are allopatric (geographic separation, the dominant mode for animals, with Darwin's finches in the Galapagos as the textbook case watched in real time over four decades by Peter and Rosemary Grant), sympatric (without geographic separation, through ecological specialization, sexual selection, or polyploidy), and parapatric (along a continuous geographic gradient), with the pace typically 10⁵ to 10⁷ years for animal speciation, faster for plants via polyploidy. Phylogenetics reconstructs evolutionary trees from molecular sequence data: monophyletic groups (clades) include all descendants of a common ancestor; paraphyletic groups include some but not all ("reptiles" excluding birds); polyphyletic groups arise from multiple independent ancestors. Carl Woese (1977) showed by ribosomal-RNA sequence that life divides into three domains — Bacteria, Archaea, and Eukarya — with Eukarya the youngest, originating from an ancient archaeon that engulfed a bacterium (the ancestor of mitochondria). Horizontal gene transfer among microbes complicates the tree, so some authors prefer a web or network of life metaphor for the deepest microbial relationships. Molecular clocks under approximately constant rates of neutral mutation give divergence times calibrated with fossils — chimpanzee-human ~6 Mya, primate-rodent ~80 Mya, vertebrate-arthropod ~600 Mya.