What term describes the hypothetical last common ancestral interbreeding population of a taxon?

The term that accurately describes the hypothetical last common ancestral interbreeding population of a taxon is "common ancestor." This concept refers to the most recent individual or population from which a group of species has descended. In phylogenetics, the common ancestor represents a crucial point in evolutionary history where genetic divergence begins to separate different lineages. This concept is foundational in understanding evolutionary relationships among organisms, as it underpins the lineage through which various species have evolved.

A terminal node represents the current species or taxa in a phylogenetic diagram, but it does not embody the ancestral population from which they evolved. Phylogenetic trees and cladograms are visual representations used to illustrate the evolutionary relationships and ancestry of different species, but they are not terms specifically denoting a common ancestral population. Thus, the most accurate term in this context is "common ancestor," as it directly pertains to the ancestral lineage from which various taxa descend.