Sequence Differences Act as a Molecular ClockDefinitionMolecular ClockThe use of the gradual accumulation of mutations to infer when species diverged from a common ancestor.Scientists estimate evolutionary timelines by measuring changes in DNA sequences.This technique is called the molecular clock.Molecular clockAnalogyIt is like estimating the years between two old family photos from the visible signs of ageing.Why Sequence Differences MatterDNA encodes proteins, so changes in DNA often change amino acid sequences.The more differences between two species' homologous sequences, the longer since they diverged.These differences arise through mutations: base substitutions, insertions, or deletions.TipIn exams, state that differences accumulate gradually at a relatively constant rate.The Basis of the Molecular ClockMutations change the nucleotide base sequences of DNA.Some change amino acid sequences, while others are silent.Over time, differences accumulate between lineages that no longer interbreed.These differences can be quantified as an approximate measure of evolutionary time.TipState that the molecular clock gives estimates, not exact dates, because mutation rates are not perfectly constant.Factors Affecting the RateGeneration time: species with shorter generations accumulate mutations faster.Population size: large populations keep more variants, while small ones lose them to genetic drift.Selective pressure: harmful mutations are removed, while neutral ones accumulate.DNA repair: efficient repair enzymes mean fewer mutations accumulate.NoteRNA viruses like influenza and HIV evolve very fast because their polymerases lack proofreading.Applications of the Molecular ClockHuman and Primate EvolutionThe human–chimpanzee split is estimated at about 4.5–6 million years ago.The chimpanzee–bonobo split is estimated at about 1 million years ago.Mitochondrial DNA traces the most recent common ancestor of humans, sometimes called Mitochondrial Eve, to about 150,000 years ago.NotemtDNA is useful because it is inherited only maternally, does not recombine, and mutates at a fairly steady rate.Conserved Proteins Like Cytochrome cHighly conserved proteins like cytochrome c are compared across species.Fewer differences mean a more recent common ancestor.This has helped settle classification controversies, such as the placement of tarsiers.ExampleCytochrome oxidase subunit I (COX1) is widely used as a DNA barcode for identifying species.Limitations of the Molecular ClockMutation rates are not perfectly constant.Some DNA regions mutate faster than others.Environment, selection, and life history can speed up or slow accumulation.So the clock gives approximate estimates, not precise times.NoteTo improve accuracy, biologists compare results from multiple genes.How Reliable Is a Constant Rate?Theory of KnowledgeHow does the assumption of a constant mutation rate show the limits of scientific models?What other fields rely on simplifying assumptions that are useful but imperfect?Active recallWhat is the molecular clock, and how does it estimate divergence times?Describe two factors that affect mutation rates and how they affect the clock.Why is mitochondrial DNA useful for tracing human ancestry?
