What is a clade?
Cladists define a clade as “an ancestor and all its descendants”. This definition is, however, extremely difficult to apply.
The first problem is: what is an ancestor? According to Wikipedia an ancestor is defined (clearly by a cladist) as “a parent”. So, what, then, is a parent? According to Wikipedia a parent is defined (clearly not by a cladist) as “a caretaker of the offspring in their own species”. Wikipedia thus defines an ancestor as “a caretaker of the offspring in their own species”, which clearly is not applicable to ancestors of clades. Ancestors of clades are instead ”predecessors” to ”postdecessors”, where postdecessors are descendants.
It means that ancestors and descendants of clades are time-relative concepts, whereof the former is defined as preceeding the latter. The members of a clade are, however, not exclusively either an ancestor or a descendant, but instead every member is both an ancestor and a descendant, except the first, which is only an ancestor, and the last, which are only descendants. This fact means that a clade can’t be consistently partitioned into subclades without splitting at least one internal line (ie, line between two nodes) into two consecutive lines, because every internal line is thus ambiguous between ancestor and descendant in the wrong direction (ie, the descendant preceeding the ancestor).
This understanding of the problem with the definition is, however, backwards. The problem is not that such a group (ie, a clade) can’t be consistently partitioned into such subgroups (ie, clades) without tearing at least one descendant-ancestor apart into a descendant and an ancestor, but that such a group (ie, a clade) is inconsistent, ie, paradoxically contradictory between ancestor and descendant, in the first place. The problem is thus not that such a group is inconsistent, but that it is inconsistent to distinguish such a group, the former is merely a consequence of the latter.
It is thus inconsistent to distinguish clades, ie, the class clade. And, the fact that this inconsistency moreover is a paradoxical contradiction means that the class clade actually can’t be defined consistently. There simply isn’t any consistent definition for a paradoxically contradictory class to be found.
The question: what is a clade? will thus remain forever. (And, cladists will thus also try to formulate a definition for clade on Wikipedia forever, just as they will search for “the true tree of life” forever. Their fundamental problem is that clade, like God, is paradoxically contradictory. It is possible to believe, but it is not possible to make belief come true.)
Source: http://menvall.wordpress.com/2013/11/29/what-is-a-clade-2/
