5.3.9Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Isolation and Speciation — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 36 Unit quiz
Section 1 of 5The Idea of Speciation
A species is a group of organisms that can interbreed to produce fertile offspring.
Definition
- Speciation is the formation of new species from existing species over time.
Isolation needed
- Populations must be separated so that gene flow between them stops.
Different selection
- Isolated populations may face different pressures, so different alleles become advantageous in each one.
Genetic drift
- Random changes in allele frequency, called genetic drift, can also cause the populations to diverge.
Result
- Differences accumulate in physiology, behaviour and structure until individuals from the two groups cannot produce fertile offspring.
Section 2 of 5Allopatric Speciation
1
A geographical barrier such as a mountain range, river or motorway divides one population into two.
2
There is now no gene flow between the two populations.
3
The environments differ, so different alleles are favoured by natural selection in each population.
4
Allele frequencies in the two gene pools change differently, and the phenotypes begin to differ.
5
Eventually the populations cannot interbreed to produce fertile offspring, and they are separate species.
Section 3 of 5Sympatric Speciation
Sympatric Speciation
No barrier
- The populations live in the same area, but random mutations stop some individuals from breeding with the others.
Seasonal
- Individuals may flower or mate at a different time of year, so they no longer meet reproductively.
Mechanical
- Changes to reproductive structures may prevent mating from being successful.
Behavioural
- Changes in courtship behaviour mean that individuals no longer attract mates from the other group.
Section 4 of 5Comparing the Two Types
Allopatric
- Caused by a physical barrier and regarded as the more common type of speciation.
Sympatric
- Caused by a genetic or behavioural change that prevents interbreeding in the same habitat.
Shared stages
- Both need variation, reduced gene flow, different allele frequencies and finally inability to produce fertile offspring.
Evidence
- Fruit flies that come to prefer different foods are an example of sympatric divergence.
Mountain example
- A new mountain range dividing a population of trees can, over thousands of years, give two species that no longer interbreed.
| Feature | Allopatric | Sympatric |
|---|---|---|
| Barrier | Geographical | None, reproductive isolation instead |
| Gene flow stopped by | Physical separation | Genetic, timing or behaviour change |
| Habitat | Separate | Same |
Section 5 of 5Exam Checklist & Tips
5.3.9 – Key Points for the Exam
- Speciation The formation of a new species from an existing one.
- Gene flow The exchange of alleles between populations through interbreeding.
- Genetic drift Random change in allele frequency, most important in small populations.
- Reproductive isolation A situation in which two groups cannot interbreed to produce fertile offspring.
Common Mistakes
- Saying a barrier alone makes a new species; the populations must also diverge until they cannot interbreed.
- Forgetting to state that there is no gene flow between the isolated populations.
- Describing sympatric speciation as involving a geographical barrier.
- Using the term species for populations that can still produce fertile offspring together.
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