One origin? (viral evolution)
tdibenedetto at OCEANCONSERVANCY.ORG
Mon Jul 23 14:55:28 CDT 2001
From: Thomas Lammers [mailto:lammers at VAXA.CIS.UWOSH.EDU]
A clade is a group containing all descendents from a given common
ancestor. The tetrapods are a clade as they include all descendents from
whoever it was that first crawled up out of the water in the devonian.
A grade is a group that omits one or more descendents of a given cvommon
ancestor. Reptiles are a grade because they do not include the Birds,
which are descended from the same ancestor.
I would modify the last sentence to read "Reptiles are a grade IF you
concieve of them as not including birds".
Otherwise I agree with this description of what clades and grades look like.
I think a more basic definition however would be this:
The concept of clades is defined relative to the historical lineage system
(the "tree of life"), and refers to any complete part of that system
(including the whole part). Thus clades extend from the point at which a
branch separates from the rest of the system, and includes all sub-branches.
The concept of grades is defined relative to a model of evolutionary
progress, as orginally conceived in such concepts as the "ladder of life".
Grades are defined by circumscribing species that share what one might judge
to be a similar level of evolutionary development. Thus reptiles without
birds might be seen to have generally similar body forms and means of
existence, whereas birds are seen to have taken flight and soared to higher
levels of evolutionary accomplishment. Grades have historically been popular
amongst ornithologists and mammalogists.
More information about the Taxacom