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Old January 7 2013, 06:06 PM   #7
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Location: Banana Slug Land
Re: Evolutionary biology questions...seems things have changed in 20 y

The answers to the hominin/hominid question given above are correct. Hominin refers to the tribe Hominini (a tribe is the taxonomic level below subfamily), because most apes are now included in the family Hominidae.

To answer the question about a clade...

A clade is a monophyletic group - which means that it includes a single common ancestor and ALL of its descendants. Some of the traditional taxonomic groups are not clades (are not monophyletic), but are paraphyletic "grades" (paraphyletic means that it includes a common ancestor but only SOME of the descendants. For example, Pisces is not a clade because tetrapods (reptiles, birds, mammals, etc.), which evolved from one group of fish, are not included. As a result, Pisces is no longer used as a valid group, although many individual fish groups are clades.

Clades do not supersede the Kingdom/Class/... organization (they are still used), but it is preferable that a given Class or Family or Subfamily or whatever be a clade rather than a paraphyletic grade. Clades are like those Russian nesting dolls, in that (for example) the clade Hominini (a tribe) nests within the clade Homininae (a subfamily) that nests within the clade Hominidae (a family) that nests within the clade Hominoidea (a superfamily)...

The increasing use of clades reflects the increasing importance of cladistics in reconstructing phylogeny (taxonomic relationships). Cladistics is a quantitative method that classifies taxa based on the shared occurrence of newly-evolved, or derived, characters (called synapomorphies). When Linnaeus created Pisces it was basically defined (although not explicitly) on lacking the derived characters found in tetrapods, which is now viewed as a bad way to define groups.
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