Final answer:
A branch point in a phylogenetic tree indicates where one lineage splits into two or more lineages. If more than two lineages emerge from a branch point, it is called a polytomy, suggesting a need for more data to clarify relationships. Sister taxa stem from the same branch point and share an ancestor, but do not evolve from each other.
Step-by-step explanation:
In a phylogenetic tree, the point where a split occurs, called a branch point, represents where a single lineage evolved into distinct new ones.
When two lineages stem from the same branch point, they are called sister taxa. However, when a branch point has more than two lineages emerging, this is known as a polytomy, which often indicates that more data is needed to determine the evolutionary relationships among these lineages. A lineage that evolved early and remains unbranched is known as a basal taxon. It is crucial to understand that sister taxa and those involved in polytomies share a common ancestor, but this does not imply that one taxon directly evolved from the other.
For instance, in a phylogenetic tree examining primate relationships, humans did not evolve from chimpanzees nor did chimpanzees evolve from humans, though they are closely related as sister taxa with a common ancestor. Similarly, a polytomy involving several gorilla subspecies signifies that their precise evolutionary relationships are yet to be fully resolved. Such complexities in a phylogenetic tree help to identify areas where additional scientific research and data are required.