Evolution

Darwin's mechanism needs only five steps, and the papers test them one at a time. Individuals of every population vary. More offspring are produced than can survive. Resources such as food, water and space cannot increase at the rate at which populations grow, so there is a struggle for existence. The individuals best fitted to their surroundings survive and reproduce more. The advantage is inherited, so it accumulates generation on generation. Two of those steps carry the technical vocabulary the examiner wants. Overproduction against finite resources is the struggle for existence, which is defined as competition between members of the same species, an intraspecific struggle, not a fight for territory. And the gradual accumulation of an inherited advantage is descent with modification, the phrase the papers take as the key point of Darwinism, and the name Darwin gave to his view of the history of life as a branching tree.

What Darwin did not claim is as examinable as what he did. Asked which idea was not part of his theory, the key is variation arising by gene mutation: Darwin took variation as given and explained its sorting, not its origin. Mutation enters later with the neo-Darwinian synthesis, which is why "Neo-Darwinism" sits in this chapter's option lists as a wrong answer, and why "Darwin" itself is the name examiners reach for when they want the mechanism rather than the mutation. Nor is survival random. The statement that survival in the struggle for existence is random is keyed as the one statement that is not true of Darwin's theory, because selection is a sorting process with a direction. The other half of the definition comes in from the other side: natural selection affects heritable variation, it can improve the adaptation of a species, and it is regional in nature.

One framing point governs the whole chapter. Darwin's book is chiefly about speciation, not about the origin of life and not about adaptation alone. The ideas of Charles Lyell, James Hutton and Thomas Malthus belong to the early development of Darwinism, and Malthus appears in the papers only as a distractor option. Wallace sits in that same option list: where the papers put Wallace beside Darwin, the key is Darwin, which is also where syllabus outcome 7.3 puts natural selection.