The logic of natural selection

Natural selection follows from four observations and their conclusion:

1. Variation — individuals in a population differ in their heritable traits (ultimately from mutation and, in sexual species, recombination).
2. Overproduction — organisms produce more offspring than can survive; resources are limited.
3. Struggle for existence & differential survival — some variants are better suited to the environment and are more likely to survive and reproduce ("survival of the fittest").
4. Inheritance — those advantageous traits are passed to offspring.

Conclusion: over generations, advantageous alleles become more common and the population's characteristics shift — it adapts. Fitness in biology means reproductive success, not strength.

Mechanisms of evolutionary change

• Natural selection — non-random survival and reproduction based on traits. Can be directional, stabilising, or disruptive.
• Genetic drift — random changes in allele frequency, strong in small populations (e.g. founder effect, bottlenecks).
• Gene flow — movement of alleles between populations via migration.
• Mutation — the ultimate source of all new variation.

Speciation and evidence

Speciation occurs when populations become reproductively isolated (e.g. geographically) and diverge until they can no longer interbreed. Lines of evidence for evolution include: the fossil record (transitional forms, dated strata), comparative anatomy (homologous structures like the pentadactyl limb; vestigial organs), embryology, biogeography, and — most powerfully — molecular biology (shared genetic code; DNA/protein similarities that map cleanly onto the tree of life).

Selection is a filter, not a designer

The commonest misconception is that organisms evolve traits because they need them, or that individuals evolve. Neither is true. Individuals do not evolve — populations do. Variation arises randomly by mutation, before and independent of any need. Selection then acts as a filter: the environment "chooses" which existing variants reproduce. A bacterium does not become resistant because antibiotics are present; rather, a few bacteria already carry a resistance mutation by chance, and the antibiotic kills the rest, leaving the resistant ones to multiply. Nothing is designed; the appearance of design emerges from selection accumulating tiny advantages over vast time.

Worked example: the peppered moth and directional selection

The peppered moth (Biston betularia) is the classic documented case. Before industrialisation, most moths were pale ("typica"), camouflaged on lichen-covered trees; a rare dark form ("carbonaria") stood out and was eaten by birds. Trace what industrial pollution did.

1. Establish the starting variation. Both pale and dark alleles already existed in the population — variation was present, not created by pollution.
2. Change the environment. Industrial soot (mid-1800s) killed lichens and blackened tree bark in industrial regions.
3. Identify the selection pressure. Now the pale moths were conspicuous against dark bark and heavily predated by birds; the dark moths were camouflaged and survived.
4. Predict the allele-frequency shift. Dark survivors reproduced more, passing on the dark allele. Over generations the frequency of the dark form rose sharply in polluted areas (up to ~90% in industrial cities by 1900) — directional selection.
5. Test the reversal. After 20th-century Clean Air legislation, bark lightened again; the prediction is that pale forms should recover — and they did, a natural re-run that confirms selection, not chance, drove the change.
6. Draw the general model. A pre-existing variant, a change in environment, differential predation, heritable advantage → shift in population. This is natural selection observed within human lifetimes.

Test yourself