- Using molecular clock, it was estimated that two species A and B must have diverged from their common ancestor about 9 x 106 years ago. If the rate of divergence per base pair is estimated to be 0.0015 per million years, what is the proportion of base pairs that differ between the two species now?
(1) 0.0270 (2) 0.0135
(3) 0.00017 (4) 0.0035The proportion of base pairs that differ between species A and B is 0.0270, corresponding to option (1).
Molecular Clock Basics
The molecular clock hypothesis assumes DNA mutations accumulate at a roughly constant rate, allowing estimation of divergence times between species. For two species diverging from a common ancestor, the total genetic divergence D (observed proportion of differing base pairs) equals D=2×r×t, where r is the divergence rate per base pair per unit time, and t is time since divergence. The factor of 2 accounts for independent mutations accumulating along each lineage post-split.
Problem Data Breakdown
Species A and B diverged 9 × 10⁶ years (9 million years) ago, so t=9 million years. The given rate is r=0.0015 per base pair per million years. This rate represents substitutions fixed per site per million years in a lineage.
Step-by-Step Calculation
Convert time to consistent units: t=9 MYA (already aligned).
Compute lineage contribution: r×t=0.0015×9=0.0135
Total divergence: D=2×0.0135=0.0270
Verification: 0.0015×9×2=0.027 , matching option (1).Why This Formula Works
Post-divergence, each species evolves independently, doubling differences relative to the ancestor. Real rates vary by gene (e.g., cytochrome b ~0.01 per MYA total), but here the problem specifies r=0.0015. Option (2) omits the ×2 factor; others mismatch math.
Applications in Evolution
Molecular clocks calibrate phylogenies using fossils or known events, aiding biodiversity timelines. Limitations include rate variation, addressed by models like RelTime
