By definition in cladistics, a clade is a group of organisms that is:

Introduction / Context:
This question is from evolutionary biology and systematics. It asks for the correct definition of a clade within cladistics, the method of classifying organisms based on common ancestry. Knowing what a clade is helps you interpret phylogenetic trees and understand how biologists group species according to evolutionary relationships rather than just superficial similarities.

Given Data / Assumptions:

• The term to be defined is clade.
• Options include monophyletic, polyphyletic, parsimonious, and paraphyletic groups.
• Basic familiarity with phylogenetic terminology is assumed.

Concept / Approach:
In cladistics, a clade is defined as a monophyletic group. Monophyletic means that the group contains a single common ancestor and all of its descendants, and no species that fall outside that lineage. In contrast, a paraphyletic group includes a common ancestor and some, but not all, of its descendants, leaving out one or more lineages. A polyphyletic group combines organisms from different ancestors based on similar traits that evolved independently, making it evolutionarily misleading. Parsimony is a principle used to choose the simplest evolutionary tree that explains the data, but it does not define what a clade is. Therefore, the correct definition of a clade is that it is a monophyletic group.

Step-by-Step Solution:

Verification / Alternative check:
Systematics and evolution textbooks define a clade as a monophyletic group and show diagrams where a clade can be visualised by taking a branch on a phylogenetic tree and including all organisms descending from that branch. They contrast this with examples of paraphyletic and polyphyletic groupings that are not considered true clades in modern classification. For instance, birds and crocodiles share a common ancestor with other reptiles, and a clade would include all of them, whereas excluding birds from “reptiles” produces a paraphyletic group. These explanations reinforce that the accurate definition of a clade is a monophyletic group.

Why Other Options Are Wrong:

Polyphyletic, combining unrelated organisms with similar traits, is wrong because such groups do not reflect shared ancestry and are not considered clades.

Parsimonious, containing the smallest possible number of species, is incorrect because parsimony is a principle about choosing the simplest tree, not about defining groups.

Paraphyletic, excluding some descendants of the common ancestor, is wrong because this type of group leaves out some lineages and does not meet the requirement of including all descendants.

Common Pitfalls:
Students sometimes confuse monophyletic and paraphyletic, especially when both groups share a common ancestor. Another pitfall is to think that any “natural” or traditional taxonomic group is a clade, even if it excludes some descendants. To avoid confusion, remember that a true clade can be cut from a phylogenetic tree with a single snip that includes a branch and all of its twigs. In formal terms, a clade is a monophyletic group that includes a common ancestor and all of its descendants.