Final answer:
In a phylogenetic tree, a node represents a common ancestor, and time is usually represented by the branch lengths, with the caveat that this only holds true if the tree denotes time scaling on the branches.
Step-by-step explanation:
In a phylogenetic tree, a node represents a common ancestor. Specifically, it is the point where a single lineage splits into two or more distinct lineages, representing a past evolutionary event. This branch point is crucial for understanding the evolutionary relationships among different species; when two lineages stem from the same node, they are called sister taxa.
Time in a phylogenetic tree is often represented by the branch length, where longer branches may correspond to longer periods of time since the divergence occurred from a common ancestor. This correlation between branch length and time, however, is only accurate if the tree explicitly states that branch lengths are scaled to time. In some cases, branch lengths may not represent time, and instead only show the relative order of evolutionary events.