What do we call groups that include an ancestral species and all of its descendants?

A group that includes an ancestral species and all of its descendants is known as a clade. Clades are fundamental concepts in the study of evolutionary biology and systematics. They are constructed based on the idea of common ancestry, representing a branch on the tree of life. By definition, a clade consists of an ancestor and all its descendants, making it a monophyletic group, which is crucial for understanding evolutionary relationships among species.

In contrast, families, tribes, and orders are higher taxonomic ranks that categorize groups of organisms but do not necessarily reflect the complete lineage from a single ancestor down through all its descendants. A family may include multiple genera, while an order encompasses multiple families, often without direct reference to a singular ancestor-descendant relationship. Thus, while these terms are important in taxonomy, they do not convey the precise meaning that a clade does in terms of evolutionary lineage.