Orthophyletic? was Re: Tree numbers

[Jason at]

But how do you define a "major cladogenetic event"?


>From: Gary Rosenberg <rosenberg at ACNATSCI.ORG>
>Reply-To: Gary Rosenberg <rosenberg at ACNATSCI.ORG>
>To: TAXACOM at USOBI.ORG
>Subject: Re: [TAXACOM] Orthophyletic? was Re: Tree numbers
>Date: Wed, 4 Jun 2003 11:57:36 -0400
>
>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

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