The phylogenetic tree below shows three species X, Y and Z. The branch length from the root to the common ancestor of X and Y is $0.10$ substitutions per site. From that node to X is $0.20$ and to Y is $0.35$. The branch from the root directly to Z is $0.60$ substitutions per site. (a) Which two species are most closely related? (b) Assuming a molecular clock of $1\times10^{-8}$ substitutions per site per year, estimate the time since X and Y diverged. (c) The earliest fossil of this genus is dated at 5 million years ago. Discuss whether the molecular estimate supports the fossil evidence.

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X and Y; about 27.5 million years; the molecular estimate is much older than the 5 million‑year fossil, suggesting the fossil record may be incomplete or the clock rate overestimated.

Igcse Model Questions Vol1 2021 Biology Question 16

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