Final answer:
A branch point in a phylogenetic tree represents where two lineages diverged from a single lineage into distinct new ones, indicating a common ancestor.
Correct option is b) A common ancestor
Step-by-step explanation:
The term branch point in a phylogenetic tree best describes b) A common ancestor. A branch point indicates where two lineages diverge, representing a split from a single lineage into distinct new ones. This point serves as evidence of an evolutionary relationship between species. It is important to note that sister taxa, which stem from the same branch point, do not imply that one taxon evolved from the other; rather, they share a common ancestor that no longer exists.
For example, in a phylogenetic tree, humans and chimpanzees are considered sister taxa since they share a common ancestor, which is represented by the branch point from which both lineages diverge.
Therefore, a phylogenetic tree may also have a basal taxon, which is an unbranched lineage evolved early from the root, and polytomy, which is a branch with more than two lineages that indicates unresolved evolutionary relationships.