A3.2 Classification and cladistics (HL)

The molecular clock principle states that mutations accumulate at an approximately constant rate over time. Therefore, fewer amino acid differences between two species indicates a more recent divergence from a common ancestor.

P vs Q = 3 differences, P vs R = 19 differences, P vs S = 31 differences. The pair with the fewest differences (P and Q) diverged most recently; the pair with the most differences (P and S) diverged longest ago.

Since P and Q differ by only 3 amino acids while P and R differ by 19, the conclusion that P and Q shared a common ancestor more recently than P and R is directly supported by the data.

The question asks which conclusion is best supported by the amino acid difference data. We need to apply molecular clock reasoning to evaluate each option.

Terminal taxa on a cladogram are not ancestors of other terminal taxa. Having the most differences from other species does not make S an ancestor — it means S diverged earliest, but all species shown are contemporary descendants.

We are not given R vs S data directly, but P vs R = 19 and P vs S = 31. We cannot conclude R and S are closer to each other than to P without the R–S comparison. This option is not supported.

The data explicitly states P and Q differ by 3 amino acids — their sequences are similar but not identical. This option directly contradicts the data.

'Species P and Q diverged from a common ancestor more recently than Species P and R' is supported because 3 differences (P–Q) < 19 differences (P–R), indicating less time has elapsed since P and Q shared a common ancestor.

Fewer sequence differences between two species indicates more recent common ancestry. The pair with the smallest difference is most closely related; the pair with the largest difference is most distantly related.

W vs X = 5 differences — the smallest value in the table, so W and X are most closely related and form a sister pair.

Y vs Z = 36 differences — the smallest remaining value not involving W–X. This suggests Y and Z form the next sister pair. Meanwhile, Z is most distant from W (47) and X (45), confirming Z is an outgroup relative to W, X, and Y taken together.

The topology ((W, X), (Y, Z)) correctly represents W and X as sister taxa and Y and Z as sister taxa, with these two pairs diverging from a more ancient common ancestor.

The topology of a cladogram based on sequence data should group the most similar species (fewest differences) as sister taxa. We must match the data to the branching pattern.

This topology implies Y and Z are most closely related (sister taxa). But Y vs Z = 36 differences, while W vs X = only 5 — W and X are far more similar than Y and Z. This topology is incorrect.

This pairs W with Z (47 differences — the largest value in the table) and X with Y. W and Z are the most distantly related pair, so grouping them as sisters contradicts the data.

While Z is indeed the most distant species overall, this topology incorrectly suggests W, X, and Y are all equally related to each other. The data shows Y is much less similar to W and X than W and X are to each other.

((W, X), (Y, Z)) correctly reflects: W–X are closest (5 differences), Y–Z form the next pair (36 differences), and Z is most distant from W and X (47, 45 differences).