Variation and Evolution

Variation

Variation is the differences between individuals. It has two causes:

• Genetic variation – differences in the alleles inherited from parents (e.g. eye colour, blood group).
• Environmental variation – differences caused by surroundings/lifestyle (e.g. a scar, a language spoken, a plant grown in the shade).
• Most variation is a combination of both (e.g. height depends on genes and diet).

Mutations are random changes to DNA. They are the source of all new alleles. Most have no effect, some are harmful, and occasionally one gives an advantage — providing the raw material for evolution.

Natural selection and evolution

Evolution is the gradual change in the inherited characteristics of a species over many generations, through natural selection. Charles Darwin proposed the theory. The steps:

1. Variation – individuals in a species show genetic variation.

2. Competition/selection pressure – organisms compete for limited resources (food, mates, space); predators and disease act as pressures.

3. Survival of the fittest – individuals with characteristics best suited to the environment are more likely to survive and reproduce.

4. Inheritance – survivors pass on the advantageous alleles to their offspring.

5. Over many generations, these beneficial characteristics become more common in the population — the species evolves.

Evidence for evolution

• Fossils – preserved remains in rock layers show how organisms have changed over time.
• Antibiotic-resistant bacteria – a fast, observable example of natural selection: bacteria with a resistance mutation survive antibiotics and reproduce, so the resistant strain spreads (e.g. MRSA).

Selective breeding (artificial selection)

Humans choose organisms with desired characteristics to breed together, repeated over generations — e.g. cows with high milk yield, disease-resistant crops, dogs with gentle temperaments. Risk: reduced variation can cause inbreeding problems.

Genetic engineering

Transferring a gene from one organism to another to give a desired characteristic — e.g. crops resistant to pests, or bacteria that produce human insulin. Benefits (higher yields, medicines) are weighed against ethical and environmental concerns.

Worked example

Explain how a population of bacteria becomes antibiotic-resistant.

1. Random mutation makes a few bacteria resistant.

2. When antibiotics are used, non-resistant bacteria die, but resistant ones survive.

3. Survivors reproduce, passing on the resistance allele.

4. Over time the whole population becomes resistant. ✓

Common mistakes

• Saying organisms "choose" or "try" to adapt — variation is random; the environment selects.
• Confusing natural selection (nature selects) with selective breeding (humans select).
• Forgetting that mutations are the origin of new alleles.

Exam tips

• Learn the natural selection steps in order: variation → competition → survival of the fittest → inheritance → over generations.
• Use antibiotic resistance as a strong example of natural selection.
• Distinguish selective breeding and genetic engineering (both human-driven, but different methods).

Key facts to remember

• Variation = genetic + environmental; mutations create new alleles.
• Natural selection: those best suited survive, reproduce and pass on advantageous alleles → evolution over many generations (Darwin).
• Evidence: fossils and antibiotic-resistant bacteria; humans also drive change via selective breeding and genetic engineering.