Question

what is the purpose of an evolutionary tree?
to show the lineage of species and when they branched off from their common ancestor
to show the age of a species and the fluctuations in its population
to show a common structure between living organisms

Explanation:

An evolutionary tree, also known as a phylogenetic tree, is a diagrammatic representation. It uses branches to depict the evolutionary relationships among various biological species or other entities. The key aspect is that it shows the lineage of species. Each branch - point (node) represents a common ancestor. The branching off from a common ancestor at different points in time is what the tree illustrates. It is not about showing population fluctuations (which would be more related to population dynamics studies) or just a common structure (homology can be part of the analysis but not the main purpose).

Answer:

to show the lineage of species and when they branched off from their common ancestor