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Science · Genetics & Evolution · ages 19-20

Name ______________________ Date ____________

Two lineages differ at 84 neutral sites. Their calibrated pairwise difference rate is 3 sites per million years. What is the estimated split age in millions of years?
Answer: ______________
Why do neutral differences tick like a clock?
- Chance and mutation-linked fixation
- Selection speeds every neutral change in the sequence
- Neutral changes are never fixed in populations
What job does a dated fossil do in a clock study?
- It removes all uncertainty from every date
- It links sequence gaps to an estimated date
- It slows all mutations down in every lineage
Why can a lineage with a shorter generation time accumulate more copying changes per century?
- It may complete more DNA-copying rounds per century
- It must have a larger body and a much faster metabolism
- It always has slower metabolism in every generation
How do tightly constrained genes tick compared with less constrained stretches?
- They tick faster
- They tick the same
- They tick slower
Why average over long stretches instead of one short gene?
- Single genes are always exact and need no averaging
- Short regions give noisy estimates
- Long regions cost less to sequence
A friend says one universal rate can date every gene in every lineage. Is that right?
- Yes, every gene and lineage has exactly the same rate
- Yes, a dated calibration is optional because every molecular clock ticks alike
- No, rates differ, so use a suitable gene and lineage calibration
A clock rate calibrated between two slow-changing lineages is used to date a split between two lineages that gain pairwise differences faster each year. What happens to the estimate?
- The split date comes out too old
- The split date comes out too young
- The split date is unaffected

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading time in silent typos W1-mt_yIidVpa703-s1

28 · The calibrated pairwise rate is 3 differences per million years. Divide 84 differences by 3 to estimate 28 million years.
Chance and mutation-linked fixation · Under the neutral model, neutral substitutions are not favored or removed by selection, so chance and mutation-linked fixation can produce an approximately steady rate.
It links sequence gaps to an estimated date · A dated fossil or other event supplies a calibration point that links sequence difference to an estimated time. It does not slow mutations or remove uncertainty.
It may complete more DNA-copying rounds per century · More generations per century can mean more germline copying rounds, although species-specific mutation and repair also affect the rate.
They tick slower · In a constrained gene, fewer of the possible changes are neutral and selection removes the harmful ones, so fewer substitutions accumulate and the clock ticks slower.
Short regions give noisy estimates · Averaging over more sites smooths out chance differences and gives a steadier estimate of the underlying rate.
No, rates differ, so use a suitable gene and lineage calibration · Mutation, generation time, functional constraint, and lineage history can change the rate. A study must use a suitable model and calibration for the comparison.
The split date comes out too old · The faster pair of lineages gathers more pairwise differences per million years than the borrowed rate assumes. Dividing that larger count by the slower rate gives more years than really passed, so the split date comes out too old.