Variation and Evolution (B6.2)

Variation

Variation is differences between individuals of the same species. Causes:

• Genetic: differences in DNA (mutations, sexual reproduction). Causes continuous (e.g. height) and discontinuous (e.g. blood group) variation.
• Environmental: diet, sunlight, exercise (e.g. weight, skin tan). Cannot be inherited.
• Both: most traits (intelligence, height) are influenced by genes AND environment.

Mutations

A mutation is a random change in DNA sequence. Mutations:

• Occur spontaneously during DNA replication
• Can be caused by mutagens: ionising radiation, UV light, certain chemicals
• Most mutations are neutral (in non-coding DNA) or harmful; rarely beneficial
• If they affect gametes, they can be inherited

Natural selection (Darwin)

Theory of evolution by natural selection:

1. Individuals in a population show variation.
2. More offspring are produced than can survive (struggle for existence).
3. Those with favourable variations are more likely to survive and reproduce (survival of the fittest).
4. Favourable variations are passed on to offspring via genes.
5. Over generations, allele frequencies shift — the population evolves.

Evidence: fossil record, comparative anatomy, antibiotic-resistant bacteria (observable evolution).

Selective breeding (artificial selection)

Humans select individuals with desirable traits and breed them together, over many generations, to enhance those traits. Examples:

• Wheat: high yield, disease resistance
• Cows: high milk production
• Dogs: specific temperament or size

Benefits: improved crop yields, disease resistance in animals.
Drawbacks: reduced genetic diversity → population vulnerable to new diseases; inbreeding leading to genetic disorders.

Genetic engineering

Desired gene is cut out using restriction enzymes, inserted into a vector (often a plasmid or virus), and introduced into a host organism. The host then expresses the gene.

Examples:

• GM insulin: human insulin gene inserted into bacteria → bacteria produce human insulin at scale.
• GM crops: herbicide-resistant crops (Roundup Ready); Bt crops produce insecticide.
• Golden Rice: engineered to produce β-carotene (vitamin A precursor).

Benefits: more precise than selective breeding; produces novel traits rapidly.
Concerns: unknown ecological effects; ethical objections; corporate control of food supply; potential allergens.

Common exam errors

1. Confusing selective breeding with genetic engineering — selective breeding uses natural reproduction; genetic engineering transfers specific genes.
2. Saying natural selection "chooses" individuals — the environment does the selecting; the organism has no choice.
3. Forgetting that mutations must be in gametes to be heritable.