Learn & Understand

The Engine of Evolution: Mutation and the Molecular Clock

Every generation, a handful of new genetic mistakes — mutations — appear in each offspring, tiny copying errors in the DNA passed from parent to child. These errors are usually harmless or bad, occasionally good, and always essential: without mutation there would be no new variation, and evolution itself would grind to a halt.

Copying Errors

DNA is copied with astonishing fidelity, but not perfectly. Rare mistakes slip through — a base changed, added, or deleted — and some of these errors end up in the eggs and sperm passed to the next generation. Each of us carries a few dozen new mutations our parents did not have, small changes introduced fresh into the human lineage with every birth.

The Raw Material of Change

Mutation is the ultimate source of all genetic variation. Every difference between organisms, every trait that natural selection can act upon, began as a mutation somewhere in the past. Without this constant trickle of new variants, populations would have nothing new to select among, and life could not adapt or diversify. Mutation is evolution's raw material.

Mostly Neutral

Contrary to intuition, most mutations are neither harmful nor helpful but neutral — landing in DNA where they change nothing meaningful. A smaller share are harmful and tend to be weeded out by selection, and a rare few are beneficial and may spread. The vast majority simply accumulate quietly, neither helping nor hurting the organisms that carry them.

A Clock in the Genome

Because neutral mutations accumulate at a roughly steady rate over time, they act as a molecular clock. By counting the genetic differences between two species and knowing the mutation rate, biologists can estimate how long ago they shared a common ancestor. The steady tick of copying errors, useless to the organism, becomes a timekeeper for the history of life.

Estimating Mutations

To estimate mutations per genome per generation, use the Mutation Rate Calculator. Explore population divergence with the Genetic Distance Calculator, and diversity with the Heterozygosity Calculator.