Speciation, Extinction, & Origins

The biological species concept defines a species as a group whose members can interbreed and produce viable, fertile offspring — and who don't interbreed with members of other species in nature. This works well for sexually reproducing animals but breaks down for bacteria (which mostly reproduce asexually) and extinct organisms.

Allopatric speciation happens when a physical barrier (a new river, a mountain range, a sea level change) divides a population. Each side evolves independently — different mutations, different selection pressures — until they can no longer interbreed, even if reunited. Grand Canyon squirrels (Kaibab on the north rim, Abert's on the south) are a classic example.

Sympatric speciation happens in the same geographic area. In plants, polyploidy (extra chromosome sets, usually from errors in meiosis) instantly produces individuals that can't interbreed with the parent type. In animals, habitat differentiation (cichlids in African lakes) and sexual selection can drive sympatric speciation.

Gradualism: slow, steady change over millions of years. Punctuated equilibrium: long periods of stasis interrupted by short bursts of rapid change (e.g., after a mass extinction). Both occur in the fossil record.

Mass extinctions (Permian-Triassic ~96% marine species lost; Cretaceous-Paleogene ~75% lost, ending the dinosaurs) clear ecological space and are followed by adaptive radiations — bursts of speciation as survivors diversify into newly empty niches.

Early Earth had no oxygen, plenty of CO₂, methane, ammonia, and water, plus energy from lightning and UV. The Miller-Urey experiment (1953) showed that under these conditions, simple inorganic molecules spontaneously formed amino acids and other organic monomers.

RNA world hypothesis: RNA, not DNA, came first, because RNA can both store information and catalyze reactions (ribozymes). Protocells — droplets of lipids enclosing nucleic acids — were the bridge to true cells. Eukaryotes arose later from endosymbiosis (an archaeal host engulfed an aerobic bacterium → mitochondria; later a cyanobacterium → chloroplasts).