DNA differences alone don't give you a date
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two species differ at 20 sites in a gene, they split about 20 million years ago.”
More differences means more time apart, so the count looks like a clock. Clocks need a known tick rate, and genes tick at different speeds.
The short version
The mistake: If two species differ at 20 sites in a gene, they split about 20 million years ago.
What's actually true: Differences turn into years only after a split dated another way, often by fossils, tells you how fast that gene changes.
- A molecular clock compares DNA or protein sequences between species. A count of differences has no time unit of its own.
- To calibrate it, find a split dated another way, often by fossils. That pair's differences divided by its age give the gene's rate.
- If 10 differences built up in 25 million years, the rate is 0.4 per million years. Then 20 differences means about 50 million years.
- The clock assumes a fairly steady rate. Genes where most changes harm the protein change slowly, so each gene needs its own calibration.
On the exam: Differences aren't years. Get the rate from a dated split, then divide the new count by it.