Which statement best defines a monophyletic group?

Which statement best defines a monophyletic group?

• A. An ancestor and all of its descendants.
• B. An ancestor and only some of its descendants.
• C. A group that shares similar traits but lacks a recent common ancestor.
• D. A group consisting of organisms from different ancestors with similar environments.

Answer: (A)

A monophyletic group is a single ancestral lineage and all of its descendants, forming a complete clade. This means the group includes the common ancestor and every lineage that descends from it, with no offspring left out. That’s why the best statement is the one that says an ancestor and all of its descendants. Think of mammals: all living mammals share a common ancestral lineage, and all descendants of that ancestor are included in the group, so mammals form a monophyletic clade. In contrast, a group that includes the ancestor but only some of its descendants (paraphyletic) isn’t monophyletic, and a group that mixes descendants from different ancestors (polyphyletic) also isn’t monophyletic. Grouping by similar environments without regard to ancestry would capture species based on convergent traits, not shared lineage, which also doesn’t define a monophyletic group.

A monophyletic group is a single ancestral lineage and all of its descendants, forming a complete clade. This means the group includes the common ancestor and every lineage that descends from it, with no offspring left out. That’s why the best statement is the one that says an ancestor and all of its descendants.

Think of mammals: all living mammals share a common ancestral lineage, and all descendants of that ancestor are included in the group, so mammals form a monophyletic clade. In contrast, a group that includes the ancestor but only some of its descendants (paraphyletic) isn’t monophyletic, and a group that mixes descendants from different ancestors (polyphyletic) also isn’t monophyletic. Grouping by similar environments without regard to ancestry would capture species based on convergent traits, not shared lineage, which also doesn’t define a monophyletic group.