Shared molecular systems and sequence comparisons reveal common ancestry, while accumulated DNA, RNA, and protein differences indicate divergence and relatedness.
Coverage 2018–2024 · Updated 15 Jul 2026
Similarity in DNA or protein sequences can provide evidence that organisms share a common ancestor.
Homologous genes retain inherited sequence features, while substitutions and insertions accumulate after lineages split. The strength of an inference depends on sequence choice, alignment, sampling and the possibility of gene transfer.
Use sequence evidence carefully:
A conserved gene with fewer differences between species A and B than between A and C supports a closer relationship between A and B.
Sequence similarity supports ancestry; it does not prove that one modern species descended directly from another.
This objective is assessed through essay response, commonly using Explain / Discuss / Identify.
Explain / Discuss / Identify / Describe
Build the answer around this relationship: The universality of DNA and the genetic code supports common ancestry among living organisms.
Equating sequence similarity with identical whole organisms rather than using it as evidence of recent common ancestry.
Representative question
Discuss how variations in proteins can indicate phylogeny and be used as an evolutionary clock.
The core A4.1 answer moves from evidence to mechanism. Define evolution as heritable population change across generations. Use molecular universals and sequence differences, selective breeding, homology, and convergence as evidence for common ancestry and divergence. Then explain speciation as reduced gene flow followed by divergence of isolated gene pools through selection, mutation, and drift.
