What you need to know
- 5.23Evolution as a change in allele frequencyHow common an allele is in a gene pool, as a proportion of all the alleles of that gene. through geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. mutationA change in the sequence of bases in DNA. and natural selectionIndividuals with alleles that give an advantage are more likely to survive and reproduce, so those alleles become more common over generations..
- 5.24Isolation reduces gene flowMovement of alleles between populations when individuals migrate and interbreed., leading to allopatric speciationSpeciation after populations are separated geographically, for example by a river or mountain range. or sympatric speciationSpeciation in populations living in the same area, for example through differences in behaviour or breeding times..
Hardy–Weinberg calculations remain 4C. This lesson reapplies mutation, selection and isolation to climate. It does not replace 4C 4–5 and does not claim that climate change has already speciated a named animal in your lifetime.
By the end, you should be able to…
- Define evolution as a change in allele frequency.
- Link mutation and natural selection, including climate as a selection pressure.
- Distinguish allopatric from sympatric speciation via reduced gene flow.
Build your understanding
Evolution is a gene-pool change
Specification 5.23 defines evolution as a change in allele frequencyHow common an allele is in a gene pool, as a proportion of all the alleles of that gene.. mutationA change in the sequence of bases in DNA. introduces new alleleA different version of the same gene, found at the same locus.. natural selectionIndividuals with alleles that give an advantage are more likely to survive and reproduce, so those alleles become more common over generations. changes how common those alleles become if they affect survival or reproduction. Climate is a selection pressure when heat, drought, season length or new competitors alter which phenotypes reproduce. Individuals do not evolve in this sense; populationAll the individuals of one species living in one area at the same time. do.
A heat-tolerant allele does nothing if it never arises. A useful mutation does nothing if every carrier dies for another reason. You need both mutation and selection. 4C already taught that Hardy–WeinbergEquations p + q = 1 and p² + 2pq + q² = 1 that predict allele and genotype frequencies in a large population that is not evolving. is the null model; a climate-driven change in p and q is evolution whether or not you calculate p².
Isolation cuts gene flow
gene flowMovement of alleles between populations when individuals migrate and interbreed. is the movement of alleles between populations by breeding or migration. Isolation reduces that flow. allopatric speciationSpeciation after populations are separated geographically, for example by a river or mountain range. happens when a geographic barrier — a new desert, a risen sea, a fragmented forest — splits a range so the groups no longer mix. Climate change can create those barriers as habitatThe place where an organism lives. dry, flood or shift.
sympatric speciationSpeciation in populations living in the same area, for example through differences in behaviour or breeding times. happens in the same geographic area: different breeding times, host plants, or habitats within a landscape stop gene flow without a mountain. A climate-shifted breeding season could, in principle, split a population that still shares a map reference. Isolation is the condition; speciationThe formation of a new species, when populations become reproductively isolated and their gene pools change. is the possible outcome after mutation and selection accumulate.
Do not rush the ending
Range shifts from 5C 8 are often ecological responses, not finished speciation. Many populations go extinct locally rather than evolve. Allopatric and sympatric are about how gene flow stopped, not about whether the organisms “wanted” to adapt.
If two groups can still produce fertile offspring, the biological species conceptDefining a species as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups. says they have not finished speciating. That is the same caution as 4C 5, now with climate as the isolating or selecting agent.
NOT: a species moving north with the climate has not, by that fact, become a new species.
- Evolution here means a change in allele frequency
- Individuals do not evolve in this definition; populations do
- Many populations die out locally instead of speciating
- Fertile offspring still mean one biological species
- Allopatric and sympatric name how gene flow stopped
Two traps this diagram blocks
- Do not call a northward range shift a new species.
- Do not swap allopatric and sympatric.
From the diagram alone
Question
Explain the difference between allopatric and sympatric speciation, and state the role of mutation. [4]
Show a model answer
Mutation introduces new alleles. Allopatric speciation occurs when a geographic barrier stops gene flow, so selection can change the separated populations differently. Sympatric speciation reduces gene flow in the same area, for example by different breeding times, without that geographic split.
| Idea | Genetic meaning | Climate example (teaching) |
|---|---|---|
| Mutation | New allele appears | A drought-tolerance variant in a plant population |
| Natural selection | Allele frequency changes with differential success | Dry years favour that variant |
| Allopatric | Geographic isolation cuts gene flow | A spreading arid belt splits a woodland range |
| Sympatric | Isolation without a map barrier | Heat shifts breeding time so two groups no longer mate |
Make your reasoning visible
Say allele frequency
“They got used to the heat” is not 5.23.
Name gene flow
Isolation matters because alleleA different version of the same gene, found at the same locus. stop being shared.
Key explanations
- Both are needed. A useful allele that never arises cannot spread, and a new allele that does not affect success need not spread.
Avoid this misconception
The claim: If a bird’s range moves north, a new speciesA group of organisms that can interbreed to produce fertile offspring. has formed.
The correction: That can be the same species tracking its climate envelope. speciationThe formation of a new species, when populations become reproductively isolated and their gene pools change. needs reduced gene flowMovement of alleles between populations when individuals migrate and interbreed. and accumulated genetic difference, not only a new map.
Order natural selection and speciation
Natural selection changes allele frequencies. If two groups are isolated, the changes can make them separate species. Order each chain.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Natural selection or Allopatric speciation.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
The climate changes. Put natural selection in order.
1 of 5 in the right place
Speciation usually takes many generations. Sympatric speciation (for example by behaviour or polyploidy) has no geographical barrier and is not ordered here.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Define evolution as used in 5.23.
Show suggested answer
A change in allele frequency in a population, arising through gene mutation and natural selection.
Practice question
Retrieval and explanation · suggested answer
How can a hotter climate act as a selection pressure?
Show suggested answer
Phenotypes that survive and reproduce in the heat leave more offspring, so alleles for those traits become more common.
Practice question
Retrieval and explanation · suggested answer
Distinguish allopatric from sympatric speciation.
Show suggested answer
Allopatric: gene flow stops because of geographic isolation. Sympatric: gene flow stops in the same area, for example by season or habitat.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A river dries into isolated pools. Fish in each pool experience different temperatures. Outline how 5.23 and 5.24 could both apply, and what evidence would show speciation was complete.
Show a hint
Isolation first, then allele change, then breeding.
Explore the reasoning
Drying is geographic isolation (allopatric) so gene flow falls. Mutation and temperature-based selection can then change allele frequencies independently. Speciation would be supported if the groups no longer produce fertile offspring if reunited — not merely if they look different.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the original source of new alleles on which selection can act.
Evolution by mutation and selection
Practice question
Retrieval and explanation · suggested answer
Name the original source of new alleles on which selection can act.
Lesson: 5C 9: Climate Change and Evolution
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Speciation is isolation plus time
5C · Climate Change and EvolutionScitable’s speciation article treats new species as the product of reduced gene flow plus diverging selection and mutation — allopatric when a barrier splits a range, sympatric when…Specification 5.24Read & use the scienceClose article
Why this matters
Scitable’s speciation article treats new species as the product of reduced gene flow plus diverging selection and mutation — allopatric when a barrier splits a range, sympatric when isolation is ecological or temporal. Climate can supply both the barrier and the selection. The article is enrichment; required words remain allele frequency, gene flow, allopatric and sympatric. A change in average trait is not itself proof of a new species. Researchers must ask whether mating and gene flow remain possible, then test whether divergence persists over many generations.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.24 understand how isolation reduces gene flow between populations, leading to allopatric or sympatric speciation
Use the science
Why is a single hot summer not sympatric speciation?
Explore a possible response
Gene flow has to stay reduced while different alleles accumulate. One season of heat is a selection episode, not a completed reproductive barrier.
Explore the source
- Scitable: speciation — the origin of new speciesNature Education
Finches show selection within a lifetime
5C · Climate Change and EvolutionPrinceton researchers studied Galápagos finches through droughts, measuring which birds survived and how inherited beak traits changed in the next generation. Genetic work linked a beak-size locus to…Specification 5.23 · 5.24Read & use the scienceClose article
Why this matters
Princeton researchers studied Galápagos finches through droughts, measuring which birds survived and how inherited beak traits changed in the next generation. Genetic work linked a beak-size locus to the observed shift. This is stronger than the popular cartoon in which an individual bird changes its beak because it needs a different seed. Natural selection changes the frequency of inherited variants in a population. The case does not, by itself, show completed speciation: that also requires reduced gene flow and persistent divergence. Long-term field records and DNA together reveal more than a single before-and-after photograph.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.23 understand how evolution (a change in allele frequency) can come about through gene mutation and natural selection
- 5.24 understand how isolation reduces gene flow between populations, leading to allopatric or sympatric speciation
Use the science
Why is a smaller average beak after drought evidence of selection, but not automatically of a new species?
Explore a possible response
Inherited variants may have survived at different rates, changing allele frequencies. Speciation also needs enduring reproductive isolation or greatly reduced gene flow.
Explore the source
- Gene behind evolution in action in Darwin’s finchesPrinceton University