Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 26.5 Molecular Clocks Help Track Evolutionary Time
Molecular clocks use the rate of genetic change to estimate the timing of evolutionary events. By assuming some genes mutate at a relatively constant rate, scientists can trace lineage divergence even beyond the fossil record.
The Concept of a Molecular Clock
- A molecular clock estimates absolute time of evolutionary change by measuring the number of genetic mutations.
- Based on the assumption that nucleotide substitutions in orthologous genes occur at a relatively steady rate over time.
- In paralogous genes, substitutions reflect the time since gene duplication.
- Example: Researchers estimated the origin of Hawaiian silversword plants at ~5 million years ago using a molecular clock.
Calibrating a Molecular Clock
- Clocks are calibrated by plotting the number of genetic differences against known divergence times from the fossil record.
- Once calibrated, molecular clocks can date events lacking direct fossil evidence.
- Not perfect:
- Mutation rates vary.
- Natural selection can influence some regions of DNA.
- Some genes evolve in bursts rather than at a constant pace.
Improving Clock Accuracy
- Use multiple genes to average out rate fluctuations.
- Use different genes for different evolutionary scales:
- Slow-evolving genes for deep divergences (e.g., vertebrates).
- Fast-evolving genes for recent events (e.g., HIV).
- Example: A study of 658 nuclear genes in vertebrates produced estimates closely matching fossil data, despite covering 600 million years.
Applying Molecular Clocks: The Origin of HIV
- HIV likely originated in humans from viruses infecting chimpanzees and other primates.
- HIV-1 M, the most widespread strain, was traced back using viral gene sequences from multiple time points.
- Molecular clock estimates suggest HIV-1 M entered humans around 1930, possibly as early as 1910.
Differences in Clock Speed
- Genes evolve at different rates depending on:
- The importance of their protein product.
- Whether mutations are neutral (no effect on fitness) or selected against.
- Essential genes evolve slowly; mutations tend to be harmful.
- Less critical genes evolve faster due to higher tolerance for neutral mutations.
Potential Problems and Solutions
- Molecular clocks can be thrown off by:
- Selection (non-neutral mutations).
- Rate changes across lineages.
- Limited fossil calibration points.
- Solutions include:
- Using multiple genes.
- Calibrating rates with known divergences.
- Focusing on genes with mostly neutral mutations.
In a Nutshell
Molecular clocks estimate divergence times based on mutation rates in genes. Although not perfectly precise, they are powerful tools for dating evolutionary events, especially where fossils fall short. Careful calibration and use of multiple genes help improve their accuracy.