17.3 Molecular Clocks

KEY CONCEPT Molecular clocks provide clues to evolutionary history.

17.3 Molecular Clocks

Molecular clocks use mutations to estimate evolutionary time.

• Mutations add up at a constant rate in related species.– This rate is the ticking of the molecular clock.– As more time passes, there will be more mutations.

DNA sequence from ahypothetical ancestor

The DNA sequences from twodescendant species show mutationsthat have accumulated (black).

The mutation rate of thissequence equals one mutationper ten million years.

Mutations add up at a fairlyconstant rate in the DNA of species that evolved from a common ancestor.

Ten million years later—one mutation in each lineage

Another ten million years later—one more mutation in each lineage

17.3 Molecular Clocks

• Scientists estimate mutation rates by linking molecular data and real time.

– an event known to separate species– the first appearance of a species in fossil record

17.3 Molecular Clocks

• Different molecules have different mutation rates.– higher rate, better for studying closely related species– lower rate, better for studying distantly related species

Mitochondrial DNA and ribosomal RNA provide two types of molecular clocks.

17.3 Molecular Clocks

• Mitochondrial DNA is used to study closely related species.

grandparents

parents

child

Nuclear DNA is inherited from bothparents, making it more difficult totrace back through generations.

Mitochondrial DNA ispassed down only from the mother of each generation,so it is not subject to recombination.

mitochondrialDNA

nuclear DNA

– mutation rate ten times faster than nuclear DNA– passed down unshuffled from mother to offspring

17.3 Molecular Clocks

• Ribosomal RNA is used to study distantly related species.

– many conservative regions– lower mutation rate than most DNA