Module 4: Evidence for Natural Selection
These free OCR A Level Biology Evidence for Natural Selection revision notes have been written for specification points 4.2.2(e) and 4.2.2(i).
Evidence for Natural Selection and Evolution
Anatomy, DNA, fossils, biomolecules and real-time observations provide evidence and examples for species evolving by natural selection.
Fossil Evidence
Fossils are the preserved remains or traces of organisms from the past.
The fossil record provides direct evidence of:
- Extinct species that once existed
- New species which resemble older ones
- Transitional fossils which show intermediate changes
Anatomical Evidence
Similarities and differences in the anatomical structures of different species offer insight into evolutionary relationships.
Homologous structures are anatomical features which are present in different species, but have adapted over time to serve different functions, such as the bones found in a human hand vs the same bones found in a bat’s wing.
Genetic Evidence for Evolution
The discovery of DNA and the development of techniques to sequence and compare different genes and genomes allow for evolutionary changes and relationships to be accurately explored.
The table below outlines some of the different types of genetic material that can be investigated:
| Evidence | What It Shows |
|---|---|
| DNA sequence similarity | Closely related species have more similar base (DNA) sequences. |
| % of shared genes/conserved genes | Homologous (matching) genes appear in many species, indicating descent from a common ancestor and the genes’ importance in being selected for. |
| Genetic markers/mutations | Shared mutations or marker sequences indicate common ancestry. |
Biomolecular Evidence
The biomolecules synthesised in organisms are determined by their underlying genetic code. In some situations it may be easier, or more informative, to examine and compare these, particularly proteins, to determine evolutionary relationships.
The table below outlines the different biomolecules which can be investigated:
| Evidence | What It Shows |
|---|---|
| Shared amino acid sequences in proteins | Closely related species have similar or identical amino acid sequences. |
| Differences in protein primary structure | Distantly related species have more differences. |
| Shared non-coding DNA | Identical sequence insertions passed down from a common ancestor. |
| Mutated DNA repeat regions | Some mutations have harmful effects, but may still be passed on. |
Observing Evolution Today
While most evolutionary changes take place over long periods, some examples of natural selection can be seen happening within just a few generations. These examples often arise in response to strong selection pressures such as drugs, pollution, or pesticides.
They demonstrate how beneficial traits can become more common over time.
The table below outlines some important examples of modern evolution in action:
| Example | What It Shows |
|---|---|
| Antibiotic resistance | Random mutations made some bacteria resistant, and these resistant strains became more common. |
| Peppered moths | Changes in pollution affected camouflage success, so darker or lighter morphs were selected. |
| Mosquitoes and DDT | Insecticide use favoured resistant mosquitoes, so the population became resistant over time. |