Orthophyletic? was Re: Tree numbers

[Gary Rosenberg]

Orthophyletic corresponds to the latter concept of stem group, "the taxa in a clade that precedes the major cladogenesis event." 

Gary

>>> "Stephen C. Carlson" <scarlson at mindspring.com> 06/04/03 09:55AM >>>
At 05:18 PM 5/30/03 -0400, Gary Rosenberg wrote:
>I hadn't realized that the term "orthophyletic" was not used outside 
>malacology. It appears on page three of  www.cabi-publishing.org/Bookshop/ 
>ReadingRoom/0851993184/0851993184Ch1.pdf. I think it was coined by 
>Gerhard Haszprunar in the 1980s. An orthophyletic group is a stem group, 
>i.e., a group that is paraphyletic because a single clade (the crown 
>group), has been excluded.

Thank you for this explanation.  There seems to be two different and
somewhat conflicting definitions of stem and crown group out there,
and, since I am not a malacologist, I'm wondering which definition is
meant in your field.  One approach is based on living (or Recent)
species in that a "crown group" is a clade consisting of the most
recent common ancestor of a set of living members and all its
descendents, while a "stem group" includes extinct members more
closely related to the crown group than to another but more basal
than the MRCA of the crown group.  Thus, do you mean that an
orthophyletic group consists only of fossil members?

Another approach is make a crown group consist of all the taxa
descended from a major cladogenesis event, while the stem group
comprises the taxa in a clade that precedes the major cladogenesis
event.  Putting aside what makes an event major, this approach
would restrict orthophyletic groups to singly paraphyletic.

>The point of the calculation is that the number of monophyletic groups is 
>proportional to n, whereas the number of paraphyletic groups is proportional
>to n squared. This is basically an argument against recognizing 
>paraphyletic groups. Say we had a tree of life for a million species (the 
>true tree). Ignoring infraspecific groups, the maximum number of nameable 
>monophyletic groups would be about 2 million (including the million 
>species) whereas allowing paraphyletic groups increases the number of 
>nameable groups to about a trillion.

I see the point, thanks.

Stephen Carlson