Questions: Molecular Clocks and Phylogenetic Dating

Question 1

Two insect lineages are compared by molecular phylogenetics, and their sequences differ by a known number of substitutions. Without any fossil calibration points, what can researchers confidently conclude?

• A. The exact divergence time in years, by applying a universal substitution rate derived from well-studied vertebrate lineages
• B. That lineage A diverged twice as long ago as lineage B if its sequence divergence from the outgroup is twice as large
• C. The relative divergence of the two lineages compared to each other, but not their absolute ages in years
• D. Nothing interpretable, because molecular clocks are only valid when fossil evidence is already available

Answer: (C)

Sequence differences alone provide relative divergence — you can say lineage A and B are more or less divergent than lineage C and D, but not how many million years have passed. Converting sequence differences into absolute time requires knowing the substitution rate, and calibrating that rate requires an independent time anchor (typically a fossil). Without calibration, you have a relative timeline with no scale bar. Applying substitution rates from unrelated groups (e.g., vertebrates applied to insects) is unreliable because rates vary substantially across lineages.

Question 2

A phylogenetic study uses a strict molecular clock and finds that its divergence date estimates for rodents are systematically younger than paleontological evidence suggests, while whale estimates are too old. The most likely explanation is:

• A. The fossil calibrations were mistakenly set as maximum ages rather than minimum age constraints
• B. Violation of the strict clock assumption: rodents evolve faster than whales, and forcing a single rate across both lineages produces biased estimates for both
• C. Rodent genomes have fewer neutral sites available for substitution, reducing their apparent evolutionary rate
• D. The strict clock model applies only to mitochondrial DNA; nuclear genes require a relaxed clock

Answer: (B)

A strict clock assumes a single substitution rate across all lineages. Rodents are known to have faster molecular evolution than whales (shorter generation time, higher metabolic rate). If a single rate is forced onto the data, it will be a compromise — overestimating the time needed for slowly-evolving lineages (making whale dates too old) and underestimating the time for fast-evolving lineages (making rodent dates too young). Relaxed clock models solve this by allowing rates to vary across branches, producing more accurate estimates when rate heterogeneity is present.

Question 3

Fossil calibration points used in molecular dating are typically treated as minimum age constraints for the node they calibrate, because the true divergence event must have predated the first appearance of fossils from that lineage.

• T. True
• F. False

Answer: True

Correct. Fossilization is a rare, improbable event, and the oldest known fossil of a group is almost certainly younger than the actual divergence. A lineage must exist before it can leave fossils, and fossil preservation depends on many taphonomic factors. Therefore, a fossil dated to 65 million years ago tells us the divergence occurred at least 65 million years ago — but possibly earlier. In Bayesian molecular dating, this is encoded as a minimum age prior on the relevant node, with a probability distribution that allows for older dates.

Question 4

When molecular dating estimates disagree with paleontological divergence dates, the molecular clock estimate should generally be trusted over the fossil record, because molecules provide more direct evidence of evolutionary time.

• T. True
• F. False

Answer: False

Neither source of evidence automatically trumps the other. Molecular clock estimates can be wrong due to rate variation across lineages, poor calibration, model misspecification, or violation of the strict clock assumption. Fossil estimates can be wrong due to preservation gaps, misidentification, or incorrect stratigraphic dating. Disagreement is a signal that something needs investigation — it could be an undetected rate acceleration in one lineage, a significant gap in the fossil record, or a calibration error in either direction. The most productive response is to examine both sources critically and seek additional evidence.

Question 5

Why can molecular dating estimate divergence times for groups with no fossil record at all, and what is required for this to work?

Model answer: Molecular dating can work for groups without fossils by borrowing calibration information from other parts of the same phylogenetic tree. If a well-calibrated node (with fossil evidence) exists elsewhere in the tree, the substitution rate estimated at that node can be applied — through the relaxed clock model — to poorly-calibrated branches. The result is a complete time-calibrated tree, with dates estimated even for lineages that left no fossil trace.

This is one of the most powerful applications of molecular clocks: recovering the temporal history of groups like fungi, most marine invertebrates, and viruses that have minimal or no fossil records. The key requirements are: (1) at least some calibration nodes elsewhere in the tree from groups with fossils, (2) a good molecular phylogeny showing how the fossil-free group relates to calibrated groups, and (3) appropriate rate models (typically relaxed clocks) that allow rates to vary among lineages rather than forcing an inaccurate single rate onto the whole tree.