Explaining Change: Variation and Evolution — AQA Combined Science: Synergy
This topic covers variation, mutations, evolution by natural selection, the evidence for evolution, classification, selective breeding and genetic engineering.
Variation
Variation within a species is caused by genes, the environment, or a combination of both. Genetic variation comes from sexual reproduction (which shuffles alleles) and from mutations.
Mutations
Mutations are random changes to DNA that occur continuously. Most have no effect on the phenotype; some have a small effect; very rarely one significantly changes a characteristic and, in a changed environment, may give a survival advantage.
Evolution through natural selection
The theory of evolution by natural selection (Charles Darwin) states that all species evolved from simple life forms over billions of years.
The process:
- There is variation within a species.
- Individuals best suited to the environment are more likely to survive and reproduce.
- They pass on the beneficial alleles.
- Over many generations, the species changes (evolves).
If two populations become so different they can no longer interbreed to produce fertile offspring, a new species has formed (speciation).
Evidence for evolution
- Fossils — preserved remains showing how organisms have changed; the record is incomplete because many early organisms were soft-bodied.
- Antibiotic resistance in bacteria — a modern, observable example of natural selection.
- Extinction — species die out when they cannot adapt to environmental change, new predators, disease or competition.
Identification and classification
Living things are classified into groups. The traditional Linnaean system uses kingdom, phylum, class, order, family, genus and species, with the binomial naming system (e.g. Homo sapiens). As biochemical and DNA evidence improved, the three-domain system (Archaea, Bacteria, Eukaryota) was proposed.
Selective breeding
In selective breeding (artificial selection), humans choose organisms with desired characteristics to breed together over generations (e.g. higher-yield crops, docile animals). A risk is reduced variation, leading to inbreeding problems.
Genetic engineering
Genetic engineering transfers a gene from one organism to another (e.g. bacteria producing human insulin, herbicide-resistant crops). Benefits include higher yields and medical uses; concerns include effects on wild populations, ethics and long-term safety.
Exam tips
- Learn the four steps of natural selection and apply them to any example.
- Give the three pieces of evidence for evolution.
- Distinguish selective breeding (choosing existing variation) from genetic engineering (transferring genes).
- Explain antibiotic resistance as natural selection in action.