Evolution by natural selection

Natural selection follows from four observations. Organisms produce more offspring than can survive. Individuals within a population vary. Some of that variation is heritable. Variants that improve survival and reproduction in the current environment become more common over generations.

Nothing in that list requires intention. No organism chooses to adapt, and no individual evolves. Populations evolve, because the proportions of variants within them shift. The environment does not create the variation, it filters variation that already existed.

Mutation is the original source, introducing new alleles through copying errors, radiation or chemical damage. Most mutations are neutral or harmful; occasionally one is useful in a given environment.

Sexual reproduction then shuffles existing variation through crossing over and independent assortment in meiosis, plus the random combination of two parents' genomes. This is why sexually reproducing populations generate variety far faster than mutation alone would allow.

• Fossils show sequences of intermediate forms and appear in a consistent order in rock layers.
• Comparative anatomy reveals homologous structures, such as the same bone arrangement in a human arm, a bat wing and a whale flipper.
• Vestigial structures, like the pelvis of a whale, make sense only as leftovers from ancestors that used them.
• Embryos of very different vertebrates pass through strikingly similar stages.
• Molecular biology shows a shared genetic code, and species that look closely related also have closely matching DNA sequences.
• Direct observation: antibiotic resistance in bacteria and pesticide resistance in insects evolve on human timescales.

Speciation usually begins with reproductive isolation. In allopatric speciation a physical barrier such as a river or a mountain range splits a population, and the separated groups accumulate different changes until they can no longer interbreed. In sympatric speciation, isolation arises without a physical barrier, through differences in timing, behaviour or chromosome number.

Related mechanisms include genetic drift, where allele frequencies shift by chance in small populations, and gene flow, where migration mixes populations and slows divergence.

• Evolution is not a ladder toward complexity. Many lineages have become simpler.
• Individuals do not evolve during their lifetime. Acquired traits are not inherited.
• Fitness means reproductive success in a particular environment, not strength or health.
• Theory in science means a well-supported explanatory framework, not a guess.
• Organisms do not develop traits because they need them. Variation arises first and is then filtered.