17.3.2—DNA evidence for evolution

DNA sequence comparison provides evidence for evolutionary relationships. DNA from suitable samples can be sequenced and aligned; the more similar the nucleotide sequences of two organisms, the more closely related they are inferred to be.

• Obtain DNA from an organism or fossil sample and determine its base sequence.
• Compare homologous sequence regions between organisms.
• Shared sequence similarities support common ancestry and closer evolutionary relatedness.
• Greater sequence differences are consistent with a longer time since the groups separated, because differences can accumulate over generations.
• Multiple sequence comparisons can be used as evidence for a phylogenetic relationship or tree.

Interpret the evidence comparatively: similarity supports a closer relationship relative to a less similar comparison, while differences support greater evolutionary distance. The inference concerns relative relatedness or relative recency of separation; it does not supply an exact time or, by itself, identify every mutation, pathway or ancestor.

Similar DNA does not mean that two organisms are identical or that sequence comparison alone proves a complete evolutionary history. A sequence comparison is evidence that must be interpreted with the sampling and comparison context. This card owns DNA evidence, not the general definition of evolution or the detailed genetic-isolation pathways of speciation.