How do mutation, variation and natural selection lead to the evolution of new species?
Mutation as the source of new alleles, variation and natural selection, and speciation through isolation by geographical, ecological or behavioural barriers followed by independent mutation and selection.
An SQA National 5 Biology answer on the evolution of species, covering mutation as the source of new alleles, variation and natural selection, and speciation through isolation by geographical, ecological or behavioural barriers followed by independent mutation and selection.
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What this dot point is asking
The SQA wants you to explain mutation as the source of new alleles, describe natural selection acting on variation, and describe speciation: how isolation by a barrier, followed by independent mutation and natural selection, leads to two populations becoming separate species.
Mutation: the source of new alleles
Mutations happen by chance, but their rate is increased by mutagenic agents such as radiation and some chemicals. New alleles from mutation are what make evolution possible, because without new alleles a population could not change.
Variation and natural selection
Within any population there is variation, arising from mutation and from sexual reproduction (which shuffles alleles). Natural selection acts on this variation.
This is how a population becomes better adapted to its environment over time. Those without the advantageous alleles are less likely to survive and reproduce, so their alleles become rarer.
Speciation
It happens in stages:
The isolation barrier can be:
- Geographical: a physical barrier such as a mountain range, river or sea.
- Ecological: a difference in conditions such as pH, salinity or humidity in different microhabitats.
- Behavioural: a difference in behaviour such as breeding season or courtship display.
Examples in context
Example 1. Antibiotic resistance. When an antibiotic is used, the few bacteria that already carry a resistance allele (from a chance mutation) survive while the rest die. They reproduce, passing on resistance, so the population becomes resistant over generations. This is natural selection happening fast enough to observe in hospitals.
Example 2. Darwin's finches. On the Galapagos islands, finches isolated on different islands evolved different beak shapes suited to different foods. Geographical isolation, different mutations and independent natural selection produced several distinct species from one ancestor, a classic case of speciation.
Try this
Q1. State the only source of new alleles in a population. [1 mark]
- Cue. Mutation.
Q2. Name the three types of isolation barrier. [1 mark]
- Cue. Geographical, ecological and behavioural.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksExplain how natural selection can lead to a population of bacteria becoming resistant to an antibiotic.Show worked answer →
A 4-mark answer should run through variation, advantage, survival and inheritance.
Within the bacterial population there is variation, caused by random mutation. By chance, a few bacteria have an allele that makes them resistant to the antibiotic.
When the antibiotic is used, it kills the non-resistant bacteria. The resistant ones have an advantage and are more likely to survive.
The surviving resistant bacteria reproduce and pass on the resistance allele to their offspring.
Over many generations, the frequency of the resistance allele increases, so most of the population becomes resistant.
Markers reward (1) variation from mutation, (2) resistant bacteria having an advantage and surviving, (3) survivors reproducing and passing on the allele, and (4) the allele increasing in frequency over generations.
SQA N5 style4 marksDescribe the stages of speciation, starting from one population.Show worked answer →
A 4-mark answer should give isolation, mutation, selection and the inability to interbreed.
First, an isolation barrier (geographical, ecological or behavioural) splits one population into two groups that can no longer interbreed.
Different mutations then occur in each isolated group, giving different new alleles.
Natural selection acts independently in each group, because their environments may differ, so each becomes adapted in its own way.
Over many generations the two groups become so genetically different that they can no longer interbreed to produce fertile offspring, so they are now separate species.
Markers reward (1) isolation by a barrier, (2) different mutations in each group, (3) natural selection acting independently, and (4) the groups no longer able to interbreed.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)