Continuous and discontinuous variation Notes

Variation between members of a species can be genetic (different alleles), environmental (diet, sunlight) or both, like human height.

Continuous variation is controlled by many genes and is strongly affected by the environment. Discontinuous variation is controlled by one or few genes and gives clear categories.

Natural selection acts on existing variation. A selection pressure such as predators, disease or food supply means that some individuals survive to breed more than others.

For example, if predators hunt by sight, darker moths on dark bark survive, so dark alleles become more common over generations.

The fossil record shows change over time and transitional forms such as Archaeopteryx. Bacteria evolving antibiotic resistance is rapid, observable natural selection.

Selective breeding shows variation can change a population, and homologous structures such as the human arm, bat wing and whale flipper suggest common ancestry.

Darwin's theory suggested species share common ancestors and change by natural selection. It challenged religious views of creation and suggested humans evolved from other animals.

He could not explain how variation arose or was inherited. Mendel's work, recognised later, helped fill this gap.

Selective breeding takes many generations. Genetic engineering uses restriction enzymes to cut a gene, a vector to carry it and DNA ligase to seal it, then transfers it into host cells. Cloning by embryo splitting or nuclear transfer (Dolly the sheep) gives identical organisms.

Benefits include pest-resistant crops and insulin; risks include unknown long-term effects and loss of variation.

Geographic isolation stops gene flow. Each population faces different selection pressures and accumulates different alleles. After enough divergence, they cannot produce fertile offspring even if reunited: they are separate species.

Darwin's finches on different islands evolved different beaks for different seeds.