AQA GCSE Biology: Variation and Evolution

The individuals in a population differ from one another, and over long periods those differences drive evolution. This guide explains what causes variation, how natural selection leads to evolution, and how new species arise. The key idea is that selection acts on variation that already exists.

Variation

Variation is the differences in characteristics within a population. It has three kinds of cause.

• Genetic factors (from mutations, meiosis and sexual reproduction), for example blood group and eye colour.
• Environmental factors (conditions during development), for example scars and the language you speak.
• A combination of both, for example height, weight and skin colour.

The third group is the largest. Height has a strong genetic component, but a child who is badly fed will not reach the height their genes would allow, so the characteristic you end up with is the result of both.

All genetic variation originally comes from mutations, random changes in the DNA base sequence that happen continuously during DNA replication. Most mutations have no effect on the phenotype. The phenotype is the result of the genome interacting with the environment.

Why do most mutations do nothing? Many fall in non-coding DNA, so no protein changes. Many others change a base without changing the amino acid, or change one amino acid in a protein of hundreds without altering its shape. A few do change the shape of a protein, and of those a very small number happen to make the organism better suited to where it lives. That small number is the raw material for everything that follows.

Evolution by natural selection

Evolution is the change in the inherited characteristics of a population over time, through natural selection, and it may lead to new species. Natural selection works in a clear sequence.

• Variation exists in a population, caused by mutations.
• Some individuals are better suited to the environment.
• These individuals are more likely to survive and reproduce.
• They pass on the advantageous alleles to their offspring.
• Over many generations, the helpful characteristic becomes more common.

The single most common mistake in this topic is to describe an individual changing. Individuals do not evolve. A rabbit born with slightly paler fur in a snowy place does not become paler because of the snow; it was born that way, and it lives longer than its darker siblings because predators see it less easily, so more of its offspring carry the pale allele. One generation later the proportion of pale rabbits is a little higher. The population has changed, not any rabbit in it.

Two phrases to keep out of an answer: organisms do not change "in order to" survive, and they do not "need" a characteristic and then develop it. The mutation happens first, blindly, and the environment then decides which versions get passed on.

Writing out the sequence in full, in that order, is usually what a four or six mark question is marking.

New species

A species is a group of organisms that can interbreed to produce fertile offspring. Two populations become separate species when they can no longer interbreed to produce fertile offspring.

The word fertile is doing real work. A horse and a donkey can breed, and the result is a mule, but a mule cannot itself have offspring, so horses and donkeys are two species and not one.

How two populations get to that point is covered in the theory of evolution and speciation guide.

How this fits together

Natural selection is evolution driven by the environment. Humans can drive a similar process deliberately, by choosing which organisms breed, which, along with directly changing genes, is the subject of the selective breeding and genetic engineering guide.