A lineage is a sequence of populations connected through ancestry. When one ancestral lineage splits into descendants that then evolve independently, their histories diverge.

The phylogeny of a group is its evolutionary history of ancestry and lineage splitting.

If species B and C descended from a population that split more recently than either did from species A, then B and C share a more recent common ancestor with each other. This statement concerns ancestry, not necessarily present-day similarity.

Closely related species can look very different if they experience different selection pressures. Conversely, distantly related species can evolve similar forms independently.

The branching view is therefore fundamental:

        ┌─ B
    ┌───┤
────┤   └─ C
    └───── A

B and C are not descendants of one another. Both descend from their shared ancestral lineage.

Likewise, living species should not usually be described as 'more evolved' or as ancestors of other living species. Every extant lineage has been evolving since its own ancestors lived.

A phylogeny is a historical hypothesis inferred from evidence such as morphology, fossils and molecular sequences. The aim is to recover patterns of common ancestry and divergence, not to arrange organisms from primitive to advanced.