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Is a clade an entity or a group?

Friday, November 30, 2012 22:30

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A clade is defined as “an ancestor and all its descendants” (in a dichotomously branching process). It means that a clade is a group consisting of one ancestor and all its descendants.

Now, since all ancestors except the first also are descendants, and since every descendant except the last also are ancestors, ancestors obviously can’t equal descendants. There are at least one ancestor and several descendants that can’t be clades.

Of these, the first ancestor appears to both be and not be a clade at the same time: it is a clade in that it has descendants, but it is not a clade in that it isn’t “an ancestor and all its descendants”. It is a clade in that it has descendants, but not in that it isn’t a group consisting of itself and all its descendants”. It is thus a clade in being a member of a group, but not in being a group.

There is thus a difference between being a member of a group and being a group that the concept “clade” confuses. In traditional logic, this confusion is called a class that is a member of itself. The particular case of “clade” is the generic case of this state expressed as “the class of all classes”. A specific case of this state is ”the list of all lists”. Typical for all such cases is that they don’t have a consistent solution, since every class of all classes, or list of all lists, but instead continuously require an additional class of all classes, making them infinite recursions. This property has been described by Bertrand Russell about a century ago called Russell’s paradox (although in the subjective approach to this inconsistency).

The fundamental question for biological systematics is thus whether a clade is an entity or a group.