Variation & Evolution

No two individuals of a species are exactly alike — this is variation. Some is genetic (inherited, arising from mutation and the shuffling of genes in sexual reproduction) and some is environmental (caused by surroundings, like a plant grown in the shade). Only genetic variation can be inherited, and it is the raw material on which evolution acts.

Variation can be continuous (a range of values, like height) or discontinuous (distinct categories, like blood group).

Charles Darwin explained how species change through natural selection. Organisms produce more offspring than can survive, so there is a struggle for survival. Because of variation, some individuals have features better suited to their environment — they are more likely to survive and reproduce ("survival of the fittest"), passing on the helpful alleles. Over many generations these alleles become more common, and the population becomes better adapted.

Natural selection is not just a theory of the past — we can watch it happen. Antibiotic resistance in bacteria is a clear example: a few bacteria carry a resistance mutation, the antibiotic kills the rest, and the survivors reproduce to give a resistant population. The same logic explains pesticide resistance in insects. Over very long times, the build-up of such changes, together with the separation of populations, leads to speciation — the formation of new species.

The main evidence comes from fossils — the preserved remains of organisms in rock layers — which show how life forms have changed over millions of years and link modern species to ancestors. Further evidence comes from similarities in anatomy and, most powerfully, in DNA between related species.

Long before Darwin, humans were shaping species by selective breeding (artificial selection): choosing the individuals with the most desirable features to breed, generation after generation, so those features become exaggerated. It gave us high-yield crops, cows that produce more milk, and every breed of dog from a wolf ancestor. The mechanism is the same as natural selection — but here humans, not the environment, decide which individuals reproduce.

Speciation is the formation of a new species, and it usually needs two things: isolation and selection. If a population is split — say by a mountain range, a sea, or a river — the two groups can no longer interbreed. Over many generations, different mutations and different selection pressures make them more and more different, until eventually they can no longer breed together to produce fertile offspring. At that point they are separate species.