Where does variation come from, and how does natural selection drive evolution?
Genetic and environmental causes of variation, mutation, evolution by natural selection, evidence from fossils and antibiotic resistance, selective breeding, genetic engineering, and the work of Darwin.
A focused answer to the OCR Gateway GCSE Combined Science A topic B5 on variation and evolution, covering genetic and environmental variation, mutation, natural selection and Darwin, evidence from fossils and antibiotic resistance, selective breeding and genetic engineering.
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What this topic is asking
OCR wants you to explain the causes of variation, describe mutation, explain evolution by natural selection with evidence, and describe selective breeding and genetic engineering, including their benefits and risks.
Variation and mutation
Most variation is caused by a combination of genes and the environment. Mutations happen continually and at random; they change the sequence of DNA, sometimes creating a new allele with a slightly different effect. Most mutations have little or no effect, some are harmful, and very occasionally one produces a useful new characteristic. Crucially, mutations are the original source of all the genetic variation that natural selection acts on, because without new alleles there would be nothing new for selection to favour.
Evolution by natural selection
Evolution is the gradual change in the inherited characteristics of a population over many generations, which can lead to the formation of new species. Charles Darwin proposed the theory of evolution by natural selection, which works as follows: there is variation within a population; individuals with characteristics best suited to the environment are more likely to survive the competition for resources (the "survival of the fittest"); these survivors reproduce and pass on the alleles for the useful characteristics; so over many generations those alleles become more common and the population changes. If two populations of a species become so different that they can no longer interbreed, a new species has formed.
Selective breeding and genetic engineering
Humans can change organisms deliberately. Selective breeding (artificial selection) means choosing the individuals with a desired characteristic (such as high milk yield, disease resistance or large fruit), breeding them together, and repeating this over many generations so the characteristic becomes more common. Its main risk is reduced genetic variation: a smaller gene pool makes the population more vulnerable to a new disease, and inbreeding can increase inherited defects. Genetic engineering transfers a gene from one organism into another, for example inserting the human insulin gene into bacteria so they make insulin, or giving crops a gene for pest resistance. It can produce useful organisms quickly, but raises concerns about long-term safety, the effect on wild populations, and ethics. OCR expects you to weigh benefits against risks rather than simply approving or rejecting these techniques.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksExplain how a population of bacteria can become resistant to an antibiotic, using the theory of natural selection.Show worked answer →
A Biology Paper 2 six-mark extended response, marked on levels. Reward: within a population of bacteria there is variation caused by random mutations, and by chance some bacteria carry a mutation that makes them resistant to the antibiotic. When the antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive. The surviving resistant bacteria reproduce (and pass on the resistance allele to their offspring), so over many generations the proportion of resistant bacteria increases until the whole population is resistant. Top answers name mutation as the source of variation, identify the antibiotic as the selection pressure, and explain survival and reproduction of the resistant individuals passing on the gene. They may add that overuse of antibiotics speeds this up.
OCR 20214 marksDescribe how selective breeding is used to produce cows that yield more milk, and give one risk of selective breeding.Show worked answer →
A B5 structured question on selective breeding. Reward a clear method: choose the cows that produce the most milk (the desired characteristic), breed them together, then from their offspring again select those that yield the most milk and breed them, repeating this over many generations so the high-yield characteristic becomes more common. One risk: selective breeding reduces genetic variation (the gene pool), so the population is more vulnerable to a new disease, and inbreeding can increase the chance of inherited defects. Markers credit selecting the best individuals, breeding them, repeating over generations, and a valid risk (reduced variation, inbreeding problems, or susceptibility to disease).
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