Temperature, evolution and speciation — Edexcel International A Level Biology
Temperature and enzyme activity (Q₁₀), the development core practical, evolution by mutation and selection, and allopatric and sympatric speciation.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Temperature affects the rate of enzyme-controlled reactions: rate increases as kinetic energy rises, up to an optimum, and then falls as enzymes denature. The temperature coefficient Q₁₀ is the factor by which the rate increases for a 10 °C rise. Many organisms, especially ectotherms, are affected strongly by environmental temperature.
Core practical 12 investigates the effect of temperature on the development of organisms, for example the growth rate of seedlings or the hatching of brine shrimp eggs. Temperature is the independent variable, set with water baths or incubators, while all other variables are controlled. The investigation must take account of the ethical use of living organisms.
Evolution is a change in allele frequency in a population over many generations. Gene mutation produces new alleles and variation. Natural selection then acts on this variation: individuals with advantageous alleles are more likely to survive, reproduce and pass those alleles on, so the frequency of advantageous alleles in the gene pool increases.
A species is a group of organisms that can interbreed to produce fertile offspring. Isolation reduces gene flow between populations, so different mutations and selection pressures cause their allele frequencies to diverge. If they can no longer interbreed, new species form: allopatric speciation after geographical isolation, or sympatric speciation within the same area.
Exam questions
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Quick check questions
Write the formula for Q₁₀.
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Rate at (T + 10) °C ÷ rate at T °C.
Why does the rate of an enzyme-controlled reaction fall above the optimum temperature?
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The enzyme denatures: bonds break and the active site changes shape.
Define evolution.
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A change in allele frequency in a population over generations.
What is the difference between allopatric and sympatric speciation?
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Allopatric involves geographical isolation. Sympatric occurs in the same area.
Frequently asked questions
What does Q10 mean in biology?
Q₁₀ is the temperature coefficient: the factor by which the rate of a reaction increases for a 10 °C rise. It is found by dividing the rate at the higher temperature by the rate at the lower temperature. For enzyme-controlled reactions it is often about 2 below the optimum.
Why do enzymes denature at high temperatures?
High temperature gives enzyme molecules so much kinetic energy that hydrogen and other bonds holding the tertiary structure break. The active site changes shape, and the substrate can no longer fit. Denaturation is usually irreversible, so the rate of reaction falls.
How does natural selection lead to evolution?
Mutations create new alleles. If an allele gives an advantage, its carriers are more likely to survive and reproduce, passing the allele on. Over generations the frequency of the allele increases in the population, and this change in allele frequency is evolution.
What is the difference between allopatric and sympatric speciation?
Allopatric speciation occurs when populations are separated by a geographical barrier, which stops gene flow. Sympatric speciation occurs within the same area, when reproductive isolation arises, for example through different breeding times, habitats or polyploidy. Both lead to divergence of allele frequencies.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026