Variation, Evolution and Selection
Variation describes the differences between individuals of the same species, caused by genetic factors such as different alleles, by environmental factors, or by a combination of both. Evolution is the change in the inherited characteristics of a population over many generations, and Charles Darwin's theory of evolution by natural selection explains how individuals best suited to their environment are more likely to survive, reproduce and pass on those characteristics.
Method
- Learn where new variation comes from: mutation, a random change in an organism's DNA. Most mutations have no effect on the phenotype, a few influence it, and very rarely one determines it completely.
- Structure natural-selection answers as a clear chain: variation exists in a population, due to different alleles; a change in the environment, or a new predator, competitor or disease, creates a selection pressure; individuals with an advantageous characteristic are more likely to survive and reproduce; their alleles become more common in the next generation; over many generations the whole population changes.
- Define speciation precisely: populations of the same species become so different, usually because they are geographically separated and experience different selection pressures, that they can no longer interbreed to produce fertile offspring, so they have become two separate species.
- Compare selective breeding and genetic engineering using the same three headings each time: what happens, a use of it, and a downside or concern.
- Learn selective breeding as a repeated cycle: from an existing population, select the individuals that show the desired characteristic most strongly, breed them together, then from their offspring again select individuals with the desired characteristic, repeating this over many generations.
- Learn genetic engineering at GCSE depth: a gene for a desired characteristic is identified and cut out, or copied, from the DNA of one organism, and inserted into the DNA of another organism at an early stage of its development, so it develops with the new characteristic.
- For evidence questions, know the limits as well as the support: fossils show how much organisms have changed, but because so many organisms that have ever lived have not left fossil remains, scientists cannot be certain exactly how life on Earth began; antibiotic resistance in bacteria is directly observable evidence of natural selection happening today.
Worked example
A patient in hospital is treated with an antibiotic for a bacterial infection. A few resistant bacteria survive the treatment because of a random mutation, and these multiply. Explain, step by step, how this can lead to a population of bacteria, such as MRSA, that this antibiotic can no longer treat.
- Identify the source of variation: within the large bacterial population, a random mutation has already given a small number of bacteria an allele that makes them resistant to the antibiotic.
- Identify the selection pressure: treating the infection with the antibiotic kills the non-resistant bacteria, but the resistant bacteria survive because the antibiotic cannot kill them.
- Explain the effect on reproduction: with the non-resistant competitors killed off, the surviving resistant bacteria have more resources and space to reproduce rapidly.
- Explain the effect on the population: because bacteria reproduce asexually, passing on identical copies of their DNA, all the offspring of resistant bacteria also carry the resistance allele, so the proportion of resistant bacteria in the population increases quickly.
- State the outcome: after enough generations, the bacterial population is dominated by resistant bacteria, such as MRSA (meticillin-resistant Staphylococcus aureus), so the original antibiotic can no longer treat this infection and a different one is needed.
Practice questions
Try each question, then tap to reveal the answer.
Q1Define mutation.Show answer
Answer: A random change to an organism's DNA, or to the sequence of bases in a gene.
Q2Give one example of variation that is caused purely by the environment rather than by genes.Show answer
Answer: Any valid example that is not genetic, for example a scar from an injury, or an accent picked up from where a person grew up.
Q3State what is meant by a selection pressure.Show answer
Answer: An environmental factor, such as a predator, a disease, competition for resources, or a change in climate, that affects the chance of an organism surviving and reproducing.
Q4Define speciation.Show answer
Answer: The process by which populations of the same species change so much, usually due to different selection pressures after geographic separation, that they can no longer interbreed to produce fertile offspring, forming two separate species.
Q5State one advantage and one disadvantage of selective breeding in farm animals.Show answer
Answer: Advantage: for example it can increase meat or milk yield, or disease resistance. Disadvantage: for example it reduces the gene pool, making the population more vulnerable to a new disease, or increases the risk of inherited health problems from inbreeding.
Q6Explain why genetic engineering can introduce a new characteristic in a single generation, while selective breeding cannot.Show answer
Answer: Genetic engineering inserts the specific gene for the desired characteristic directly into the organism's genome, so the first generation already has the new characteristic; selective breeding relies on a characteristic that already exists in the population being reinforced by breeding chosen parents together over many generations.
Q7Name the scientist who proposed the three-domain system of classification, and name the three domains.Show answer
Answer: Carl Woese; Archaea, Bacteria and Eukarya.
Q8Explain one reason it took many years for Darwin's theory of evolution by natural selection to become widely accepted.Show answer
Answer: Any valid reason, for example there was not enough supporting evidence available at the time to convince other scientists, or the theory contradicted the widely held religious belief that God had created all species in their current form.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain why antibiotic resistance in bacteria, such as MRSA, is a good example of natural selection.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
A dairy farmer wants to increase the average milk yield of her herd of cows over the next twenty years. (a) Describe how the farmer could use selective breeding to achieve this. (b) Suggest one risk of using this method.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 4 available
The peppered moth, Biston betularia, exists in a pale, speckled form and a dark form. Before the Industrial Revolution, most peppered moths in Britain were the pale form. After the Industrial Revolution, soot from factories darkened tree bark and killed pale lichen in many areas, and the dark form of the moth became far more common in those areas. Explain this change in the population using the theory of natural selection.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 6 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the variation, evolution and selection worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 16 of GCSE Biology Workbook, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on variation, evolution and selection, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.