6.14 Evolution by natural selection
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What is meant by a species?
Mark scheme
a group of organisms that can (inter)breed to produce fertile offspring
Model answer
A species is a group of organisms that can interbreed to produce fertile offspring.
Explain how the dark form of the peppered moth became common in industrial areas.
Mark scheme
- variation: some moths dark due to mutation (1)
- trees blackened by soot (1)
- pale moths more visible / eaten by birds (predators) (1)
- dark moths camouflaged so more likely to survive and breed (1)
- pass on the allele for dark colour (1)
- over many generations dark moths became more common (1)
Maximum 5 marks
Model answer
A mutation produced some dark moths. Soot blackened the tree trunks, so pale moths stood out and were eaten by birds, while dark moths were camouflaged. More dark moths survived and bred, passing on the allele for dark colour. Over many generations, the dark form became more common.
Where does the variation that natural selection acts on come from?
Mark scheme
mutations / differences in genes / alleles
Model answer
From mutations, which produce different alleles.
When are two populations said to have become different species?
Mark scheme
- they can no longer interbreed (1)
- to produce fertile offspring (1)
Model answer
When they can no longer interbreed to produce fertile offspring.
Some bacteria can reproduce every 20 minutes. Suggest why natural selection can be seen happening quickly in bacteria.
Mark scheme
- many generations in a short time (1)
- so advantageous alleles / resistance spread quickly through the population (1)
Model answer
Bacteria reproduce very quickly, so there are many generations in a short time, and a useful allele, such as resistance, can spread quickly.
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