Where does variation come from, and how does natural selection lead to evolution?
Continuous and discontinuous variation and their genetic and environmental causes, the role of mutation, the theory of evolution by natural selection, antibiotic resistance as an example, and the evidence from fossils.
A focused CCEA GCSE Double Award Science (Biology Unit B2) answer on variation and evolution, covering continuous and discontinuous variation, genetic and environmental causes, mutation, natural selection, antibiotic resistance as an example, and fossil evidence.
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What this dot point is asking
CCEA Double Award wants the two types of variation and their causes, the role of mutation, the theory of evolution by natural selection, antibiotic resistance as a modern example, and the evidence from fossils. Natural selection is the central idea; learn it as an ordered sequence.
Types of variation
Causes of variation
Variation has two sources. Genetic causes come from the alleles inherited from parents, mixed by meiosis and fertilisation. Environmental causes come from surroundings - for example a plant grown in shade is shorter, or a person who eats well grows taller. Many characteristics are due to both. New alleles arise by mutation, a random change to the DNA, which is the ultimate source of all genetic variation.
Natural selection and evolution
The steps of natural selection are:
- Variation - individuals in a population differ, partly because of mutation.
- Competition and survival - resources are limited, so individuals compete; those best adapted are more likely to survive.
- Reproduction - the survivors reproduce and pass on the alleles for the helpful characteristics.
- Change over time - the helpful alleles become more common, so the species gradually changes.
Antibiotic resistance
Antibiotic resistance is natural selection happening now. In a population of bacteria, a chance mutation makes a few resistant. When an antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive and reproduce, passing on the resistance. Over time the whole population becomes resistant. This is why doctors avoid overusing antibiotics.
Fossil evidence
Fossils are the preserved remains or traces of organisms from long ago, usually in rock. By comparing fossils of different ages, scientists can see how organisms have changed over millions of years, providing evidence for evolution. The deeper (older) rock layers contain simpler organisms, and more complex forms appear in younger layers.
Examples in context
Example 1. Peppered moths. When pollution darkened tree bark, dark moths were better camouflaged from birds, so they survived and reproduced more than the pale moths. Over time the dark form became common. This is natural selection driven by a change in the environment.
Example 2. Why finishing a course of antibiotics matters. Stopping antibiotics early can leave the slightly more resistant bacteria alive to reproduce, speeding up resistance. Finishing the course kills more of them, reducing the chance that resistant bacteria are selected.
Try this
Q1. Give one example of continuous variation in humans. [1 mark]
- Cue. Height (or mass).
Q2. What is the ultimate source of new alleles? [1 mark]
- Cue. Mutation.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style5 marksExplain how a population of bacteria can become resistant to an antibiotic. Use the idea of natural selection.Show worked answer →
Tell the natural-selection story in order for up to five marks.
There is variation in the bacteria; a chance mutation makes a few resistant to the antibiotic.
When the antibiotic is used, it kills the non-resistant bacteria but the resistant ones survive.
The survivors reproduce and pass on the resistance allele.
Over time the whole population becomes resistant.
Markers reward variation, mutation, survival of the resistant, reproduction, and passing on the allele.
CCEA-style3 marksExplain the difference between continuous and discontinuous variation, with one example of each.Show worked answer →
Define both with examples for three marks.
Continuous variation has a range of values with no distinct groups, such as height or mass.
Discontinuous variation falls into distinct categories with no in-between, such as blood group or eye colour.
Continuous variation is usually controlled by many genes and the environment, while discontinuous is usually controlled by one or a few genes. Markers want the definitions and a valid example of each.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)