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This set contains the content understandings, applications, skills and nature of science syllabus statements for IB Biology topic 5.4: Cladistics.
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Mutations in the nucleotide base sequences of DNA, and therefore differences in the amino acid sequence of proteins, accumulate gradually over time.
Therefore, the more differences there are in biological sequences, the more time has passed for the differences to accumulate. The more time that has passed since organisms have shared a common ancestor, the less evolutionarily related the organism are to each other.
Visa versa: the less differences there are in biological sequences, the less time has passed for the differences to accumulate. The less time that has passed since organisms have shared a common ancestor, the more evolutionarily related the organism are to each other.
Therefore, the more differences there are in biological sequences, the more time has passed for the differences to accumulate. The more time that has passed since organisms have shared a common ancestor, the less evolutionarily related the organism are to each other.
Visa versa: the less differences there are in biological sequences, the less time has passed for the differences to accumulate. The less time that has passed since organisms have shared a common ancestor, the more evolutionarily related the organism are to each other.

The DNA base sequences of the same gene in different species is compared. Because mutations (differences) in the DNA sequence will accumulate over time, the species with more similarities in the base sequence are likely more closely related (meaning they have a more recent common ancestor; have diverged from their common ancestor more recently) than species with more differences in the base sequence. The percent similarity between species can be used as the basis for creating a cladogram of hypothesized evolutionary relationships.

The amino acid sequences of the same protein in different species is compared. Differences in the amino acid sequences are due to mutations of the DNA. Because DNA mutations (and in turn amino acid sequences) will accumulate over time, the species with more similarities in the amino acid sequence are likely more closely related (meaning they have a more recent common ancestor; have diverged from their common ancestor more recently) than species with more differences in the amino acid sequence. The percent similarity between species can be used as the basis for creating a cladogram of hypothesized evolutionary relationships.
