What is Molecular Evolution?
Molecular evolution studies how DNA, RNA and protein sequences change over time. It combines molecular biology and evolutionary theory to trace relationships between species and estimate when they diverged from a common ancestor.
Molecular evolution is the accumulation of changes in DNA, RNA and protein sequences over generations; comparing these sequences between species, using the molecular clock, lets scientists estimate divergence times and build phylogenetic trees.
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Step-by-step worked examples
Two species' cytochrome c proteins differ by 12 amino acids, and fossil evidence puts their divergence at 60 million years ago. Estimate the substitution rate.
K = 12 differences, T = 60 Myr r = K / (2T) = 12 / (2 × 60) r = 12 / 120 = 0.1 differences per million years
A gene's molecular clock rate is known to be 0.5 substitutions per million years. Two living species differ by 5 substitutions in that gene. Estimate when they diverged.
Rearrange: T = K / (2r) T = 5 / (2 × 0.5) = 5 / 1 T = 5 million years ago
Human and chimpanzee beta-globin genes differ by about 5 substitutions, with a fossil-calibrated divergence time of roughly 6.5 million years. Estimate the substitution rate.
K = 5, T = 6.5 Myr r = K / (2T) = 5 / (2 × 6.5) r = 5 / 13 ≈ 0.38 differences per million years
Flashcards
Quick quiz
Q1.Two sequences differ by 20 substitutions and diverged 5 million years ago. What is the substitution rate?
Q2.The neutral theory of molecular evolution states that most mutations are:
Q3.Orthologous genes arise through:
Q4.The molecular clock is mainly used to:
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Common mistakes
The molecular clock ticks at exactly the same rate in every gene and species. — Correct: Substitution rates vary by gene, lineage and selective pressure — the clock is approximate, not exact.
All molecular differences are caused by natural selection. — Correct: The neutral theory shows most molecular substitutions are neutral and fixed by genetic drift, not selection.
Orthologs and paralogs mean the same thing. — Correct: Orthologs arise from speciation; paralogs arise from gene duplication within a lineage.
More sequence differences always mean more time has passed. — Correct: Rate differences between lineages can distort this relationship, so molecular clocks need fossil calibration.
FAQ
What is molecular evolution?
It is the study of how DNA, RNA and protein sequences accumulate changes over generations, used to reconstruct evolutionary relationships.
What is the molecular clock formula?
r = K / (2T), where K is the number of sequence differences and T is the time since divergence; it estimates the substitution rate per lineage.
How do you calculate divergence time from molecular data?
Rearrange the molecular clock formula to T = K / (2r), using a known substitution rate r and the observed sequence difference K.
What are examples of molecular evolution?
Comparing cytochrome c, hemoglobin, or beta-globin sequences across species to estimate when they last shared a common ancestor.