Paraphyly: the continuing saga

[Thomas Pape]

The discussion on *reality* of paraphyletic groups seems to be partly semantic:

Ken Kinman wrote that: "we can separate and delimit the traditional Class Amphibia, which originates at one synapomorphy (tetrapody or stegocephaly) and ends where the synapomorphy of amniotic eggs appears".

Traditional Amphibia is therefore a grade rather than a clade, yet that we can use "Amphibia" as a communicative tool does not make paraphyletic groups *real*.

Another issue is how cladogenesis works. It is important to remember that processes (like speciation) are difficult to describe in discrete terms. Species do not replicate, DNA does, but we traditionally use species as 'units' of evolution for convenience (e.g., ease of observation and character coding) and because the DNA replicators in general flow along those lineages we call species. And along the road there will be small changes - the "descent with modification".

As systematists we try to map those changes, building the Tree of Life, and cladistic analysis is currently our most powerful tool. Yes, some 'mother species' may be 'budding off' daughter species so fast that few or no changes occur in those replicators that by chance follow the lineage we call the 'mother species'. As systematists we have practical problems with that in that we cannot get our cladograms fully resolved. 

Some daughter species may even bud off practically simultaneously, which means that the non-resolved part of the cladogram may be a reasonable description of what happened, yet we cannot distinguish that from pure lack of knowledge. And it is certainly no argument for reality of paraphyletic groups.

Thomas Pape
Naturhistoriska riksmuseet - Stockholm