The essentials
Evolution means change in the heritable characteristics of populations over many generations. Before genetics was understood, two major explanations were offered.
Lamarckism (Jean-Baptiste Lamarck, early 19th century) proposed that organs develop through use and weaken through disuse, and that such acquired characters are passed to offspring. The classic example is the giraffe stretching its neck to reach leaves and passing a longer neck to its young.
Darwinism (Charles Darwin, "On the Origin of Species", 1859) proposed evolution by natural selection. Organisms produce more offspring than can survive, individuals vary, there is a struggle for existence, and those with useful variations survive and reproduce more ("survival of the fittest", a phrase popularised by Herbert Spencer). Alfred Russel Wallace reached a similar idea independently.
Key points
- Neo-Lamarckism: later attempts to revive inheritance of acquired characters, stressing the direct role of environment in producing heritable change.
- Neo-Darwinism (modern synthetic theory): combines natural selection with Mendelian genetics and population genetics. Variation comes from mutation and genetic recombination; selection, drift, gene flow and isolation shape populations.
- August Weismann's germ plasm theory argued that changes in body (somatic) cells are not passed on; he cut the tails of mice for many generations and the offspring still had tails.
- Darwin's weakness: he could not explain the source and inheritance of variation; genetics later filled this gap.
Exam tips
- Link each theory to its key phrase: "use and disuse" (Lamarck), "natural selection" (Darwin), "mutation plus selection in populations" (Neo-Darwinism).
- Weismann's experiment is the standard evidence against Lamarckism.
Common mistakes
- Thinking Darwin knew about genes or DNA; he did not.
- Treating "fittest" as strongest; in evolution it means best at surviving and reproducing in a given environment.