Final answer:
The horizontal axis of a phylogenetic tree represents time, with the most recent common ancestor on the left and extant taxa on the right. Vertical spacing does not indicate time unless specified, and branches show evolutionary order, not time duration.
Step-by-step explanation:
If a phylogenetic tree depicts extant taxa on the right and the point of most recent common ancestry on the left, then the horizontal axis represents time. This means that as you move from left to right across the tree, you are moving forward in time from the most recent common ancestor to the present day. An important aspect to understand is that unless the phylogenetic tree specifies otherwise, the vertical spacing between taxa does not indicate the amount of time since their divergence from the common ancestor. Rather, a phylogenetic tree is like a map of evolutionary history, showing how species are related through shared common ancestors. It is important to note that the branches of a phylogenetic tree do not necessarily reflect the length of time, but they do show the order in which evolutionary events took place.