Variation and natural selection

Types of variation

Genetic variation: differences caused by different alleles (inherited from parents). Cannot be changed by the environment. Examples: blood group, eye colour, genetic disorders.

Environmental variation: differences caused by the environment (surroundings, lifestyle). Examples: language spoken, scars, body mass (partly), tan.

Both genetic and environmental: height and skin colour are influenced by both genes and environment (nutrition/sunlight).

Mutations

A mutation is a random change in DNA. Mutations:

• Occur spontaneously during DNA replication.
• Are increased in rate by mutagens: UV radiation, ionising radiation (X-rays, gamma rays), certain chemicals (e.g. carcinogens in cigarette smoke).
• Most are neutral (no effect on phenotype) or harmful; very occasionally beneficial.
• Are the source of all new genetic variation — the raw material for natural selection.

Natural selection — Darwin's theory

Darwin's theory of evolution by natural selection (1859) states:

1. Variation: individuals in a population show variation.
2. Overproduction: more offspring are produced than can survive (competition for resources).
3. Struggle for survival: individuals compete for food, mates, and space.
4. Selection: individuals with characteristics better suited to the environment survive longer and reproduce more (they are "naturally selected").
5. Inheritance: the favourable characteristics are inherited by offspring.
6. Over many generations: the frequency of advantageous alleles increases in the population → evolution.

Evidence for evolution

Fossil record: fossils show that organisms have changed over geological time. Sequence of fossils shows a progression from simpler to more complex organisms.

Antibiotic resistance in bacteria — the best modern evidence:

• Bacteria with a mutation giving antibiotic resistance survive treatment.
• Non-resistant bacteria die.
• Resistant bacteria reproduce; the resistance allele increases in frequency.
• After many generations, the population is largely resistant.
• This happens in real time — we observe it directly.
• Example: MRSA (methicillin-resistant Staphylococcus aureus).

Comparative anatomy: homologous structures (e.g. the pentadactyl limb — five-bone structure found in human arm, bat wing, whale flipper) suggest common ancestry.

DNA evidence: closely related species have more similar DNA sequences. Comparing DNA allows evolutionary trees (phylogenies) to be constructed.

Antibiotic resistance — implications

CCEA examiners frequently ask why antibiotic resistance is a growing problem and how to slow it:

• Complete the full course of antibiotics (so all bacteria are killed, not just the most susceptible).
• Do not overprescribe antibiotics.
• Do not use antibiotics for viral infections.
• Develop new antibiotics.