Module 4: Evolution and Speciation
These free OCR A Level Biology Evolution and Speciation revision notes have been written for specification points 4.2.2(h) and 6.1.2(g).
Speciation
Speciation is when an isolated population of a species evolves to become different enough that it can no longer reproduce with the original species’ population to produce fertile offspring.
Populations become isolated – either geographically or reproductively – and stop interbreeding. Once isolated, the populations experience different selection pressures and, over time, accumulate genetic differences.
Types Of Speciation
There are two main types of speciation:
- Allopatric: Occurs when populations are geographically separated.
- Sympatric: Occurs when populations are reproductively isolated.
Allopatric and sympatric speciation do have their own technical terms, which are not required for OCR A level Biology, and are not explored here.
Examples of Allopatric Speciation
The table below outlines some situations that result in geographical isolation:
| Description | Example |
|---|---|
| Continental or sea-level changes separate populations | Ostriches and emus are evolving separately |
| Ice sheets isolate species during glaciation | Sticklebacks in isolated post-glacial lakes |
| Separated populations adapt to contrasting local conditions | Lizards on either side of a canyon |
Examples of Sympatric Speciation
The table below outlines some processes that result in reproductive isolation:
| Process | Description | Example |
|---|---|---|
| Habitat Differentiation | Populations evolve to be adapted to different resources in the same area | Apple vs hawthorn maggot flies |
| Sexual Selection | Preferences for certain traits isolate groups (mate choice divergence) | African cichlid fish |
| Temporal Isolation | Groups reproduce at different times of year or day | 13-year vs 17-year cicadas |
Convergent Evolution
Convergent evolution is when unrelated species independently evolve similar traits or adaptations in response to similar selection pressures, even though they are not closely related.
The key features of convergent evolution is that it:
- Occurs in unrelated species living in similar environments
- Driven by similar selection pressures, not shared ancestry
- Results in analogous structures (same function, different origin)
The table below outlines some common examples of convergent evolution:
| Species 1 | Species 2 | Shared Adaptation(s) | Reason for Convergence |
|---|---|---|---|
| Marsupial mole | Placental mole |
– Streamlined body – Strong forelimbs – Reduced eyes | Adaptation for life underground (burrowing) |
| Shark | Dolphin |
– Fins – Streamlined body – Powerful tail | Fast swimming in aquatic environments |
| Cactus | Euphorbia |
– Thick – Fleshy stems – Spines | Water storage and herbivore protection in desert environments |
The image below compares the typical anatomical profile of sharks (fish), ichthyosaurs (reptiles) and dolphins (mammals), which became similar through convergent evolution:
