Questions: Molecular Clock Calibration and Fossil Dating

Question 1

Researchers find the oldest known bat fossil, dated to 52 million years ago by radiometric methods. They use this fossil to calibrate the node separating bats from their sister group. What conclusion is justified?

• A. Bats and their sister group diverged exactly 52 million years ago
• B. Bats and their sister group diverged approximately 52 million years ago, within dating error margins
• C. Bats and their sister group diverged at least 52 million years ago — the fossil sets a minimum age for this node
• D. Bats and their sister group diverged at most 52 million years ago — the fossil sets a maximum age for this node

Answer: (C)

A fossil sets a minimum age for its node, not the actual divergence time. The bat fossil demonstrates that the bat lineage already existed 52 mya — so the divergence from its sister group must have occurred before that. The actual divergence could be substantially older: the absence of earlier fossils in the record may reflect incomplete sampling, not actual absence. Option A (exact date) is wrong because early fossilization is rare and the earliest fossil is unlikely to be the first individual of the lineage. Option D reverses the logic entirely.

Question 2

Why do modern molecular dating analyses use relaxed clock models rather than strict molecular clock models?

• A. Strict clock models require fossil calibration points, which are too unreliable to use
• B. Substitution rates vary among lineages, genes, and time periods, violating the strict clock assumption of rate constancy
• C. Relaxed clock models are computationally simpler and produce narrower confidence intervals
• D. The strict molecular clock always overestimates divergence times for ancient splits

Answer: (B)

The strict molecular clock assumes a single, constant substitution rate across all branches of the tree. This assumption is often violated: rodents evolve faster than elephants, mitochondrial DNA evolves faster than most nuclear genes, and rates can shift over time within a lineage. Relaxed clock models allow rates to vary branch by branch according to a statistical distribution (e.g., lognormal or uncorrelated), estimated jointly with divergence times from sequence data and fossil constraints. Option A is wrong — both strict and relaxed models use fossil calibrations. Option C is wrong — relaxed models are computationally more demanding and typically produce wider uncertainty intervals.

Question 3

A single misidentified or misplaced fossil calibration point can systematically distort divergence time estimates across the entire phylogenetic tree, not just at the calibrated node.

• T. True
• F. False

Answer: True

Because calibration points anchor the entire time scale, an error at one node propagates through rate estimates that affect all other nodes. If a fossil is placed on the wrong branch (misidentified) or assigned to the wrong geological stratum, the substitution rate inferred for that region of the tree will be wrong, and that rate estimate is then used (directly or indirectly) to date other nodes. This is why paleontological judgment about calibration quality is as important as molecular sophistication, and why using multiple independent calibration points distributed across the tree helps — errors at one node are partially corrected by the others.

Question 4

A molecular clock can convert sequence divergence into absolute divergence times without any external calibration information, as long as enough sequence data are available.

• T. True
• F. False

Answer: False

Without calibration, a molecular clock provides only relative time — it tells you that lineage A and lineage B have accumulated twice as many substitutions as lineage C and lineage D, but not what that difference corresponds to in years. Converting to absolute time requires anchoring the rate scale to an external reference with a known age, which is what fossil calibration provides. No amount of sequence data alone can supply an absolute time scale; the data determines relative rates and topologies, but the translation to calendar years requires fossil or geological constraints.

Question 5

Why does a fossil constrain only the minimum age for a divergence node rather than the actual divergence time, and what types of evidence can suggest a soft maximum age for the same node?

Model answer: A fossil constrains the minimum age because it demonstrates that the lineage already existed at the fossil's geological age — the actual divergence must have occurred at or before that time. Fossilization is a rare event and the fossil record is highly incomplete, so the oldest known fossil is almost certainly not the first individual of that lineage. Soft maximum age constraints come from the absence of fossils in well-sampled older deposits: if intensive sampling of rock strata older than a certain age consistently fails to find the lineage, this is probabilistic (not certain) evidence that the lineage had not yet originated. Biogeographic events, mass extinctions, or vicariance events (continental rifting) with known geological ages can also provide maximum age constraints when the biology and geology are tightly linked.

The asymmetry between hard minimum and soft maximum calibrations reflects the logic of the fossil record: a fossil's presence is positive evidence of existence, but absence is hard to interpret — it could mean absence from the record (sampling bias) rather than absence from the biota. This is why molecular dating analyses specify calibrations as probability distributions rather than hard bounds, with the minimum age as a hard constraint and the maximum as a soft upper tail that allows some probability above the bound.