Nonmonophyletic

Nonmonophyletic is a term used in cladistics and phylogenetics to describe a group of organisms that does not include all of the descendants of a common ancestor. In other words, a nonmonophyletic group is a collection of taxa that does not form a complete evolutionary lineage. This can occur in two main ways: a paraphyletic group, which excludes some descendants of the common ancestor, or a polyphyletic group, which includes taxa that do not share a recent common ancestor with each other, but rather have been grouped together based on convergent traits.

The concept of monophyly is central to modern evolutionary classification, as it aims to group organisms based on their shared evolutionary history. Groups that are strictly monophyletic are considered natural or evolutionary groups. Nonmonophyletic groups, on the other hand, are often considered artificial or invalid in phylogenetic analyses because they do not accurately reflect evolutionary relationships.

Identifying nonmonophyletic groups is a crucial step in constructing accurate phylogenetic trees. When a group is found to be nonmonophyletic, it often indicates that the classification needs to be revised to better represent the evolutionary history of the organisms in question. For example, a traditional classification might group certain reptiles together, but modern phylogenetic analysis may reveal that birds evolved from within that reptilian group, making the original "reptile" grouping paraphyletic. Understanding nonmonophyly helps scientists refine our understanding of the tree of life.