Natural selection and evolution explained

1. Variation: individuals in a population differ, partly because of their genes (caused by mutations and sexual reproduction).
2. Competition: there aren't enough resources (food, mates, space) for all of them, and there are predators and disease.
3. Survival of the fittest: individuals with characteristics better suited to the environment are more likely to survive.
4. Reproduction: survivors breed and pass on the alleles for those characteristics.
5. Over many generations, those alleles become more common in the population.

In a population of bacteria, a mutation may make a few cells resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria die, but the resistant ones survive and multiply. Soon most of the population is resistant: this is how resistant strains such as MRSA develop. It's why doctors avoid prescribing antibiotics unnecessarily and patients are told to finish the course.

During the Industrial Revolution in Britain, soot darkened tree bark. Dark-coloured peppered moths were better camouflaged from birds than pale ones, so more survived to breed and the dark form became common in industrial areas. When pollution fell, pale moths became common again.

• Fossils show how organisms have changed over time.
• Antibiotic resistance shows evolution happening quickly.
• DNA comparisons show how closely species are related.

When populations become so different that they can no longer interbreed to produce fertile offspring, a new species has formed.

In selective breeding, humans choose which organisms reproduce, for traits we want (high-yield crops, gentle dogs). In natural selection, the environment does the choosing. Both change a population's alleles over generations.

• Saying organisms change because they need to: individuals don't evolve; populations do, through survival and reproduction.
• Saying bacteria "become immune": they're resistant because of a mutation that was already there.
• Forgetting that the advantage must be inherited to be passed on.

Learn where variation comes from in DNA and genes.

When the environment changes faster than a species can adapt, it may become extinct. Causes include new predators, new diseases, competition from other species, habitat destruction, and changes such as climate change. Fossils show that most species that have ever lived are now extinct.

1. Arctic hares are white in winter. Explain how natural selection could have led to this.
2. Why does finishing a course of antibiotics help prevent resistance?

Model answers:

1. Hares varied in coat colour. Those with paler coats were better camouflaged against snow, so fewer were caught by predators. They were more likely to survive and reproduce, passing on the alleles for pale coats, which became more common over many generations.
2. Finishing the course kills as many bacteria as possible, so fewer survive to reproduce and pass on any resistance.

• Variation: differences between individuals.
• Adaptation: a feature that helps an organism survive in its environment.
• Mutation: a change in DNA that can create new variation.
• Allele: a version of a gene.
• Species: organisms that can breed to produce fertile offspring.
• Evolution: a change in the inherited characteristics of a population over time.