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Re: Re: Linnean Classification and Creationism
I have heard it this way: At any one point in time, a species is (more
or less) a reproductively mixed population of creatures (in the sense
that mating leads to reproductively successful offspring), which is
reproductively isolated from all other such reproductively mixed
populations (that is, other species). The "more or less" has to do
with the problem that "reproductive isolation" is only crudely a
yes/no issue, about which one could say lots.
There is an immediate problem with talking about the "same" species at
different points in time, for there are no time machines in nature;
time provides excellent reproductive isolation of populations, and it
is hard to tell who could mate with whom if a time machine were
available from a dating service. I will get back to this in a moment.
Now imagine the grand canonical phylogenetic evolutionary tree of
reproductively isolated populations, evolving upward through time.
Slice through the whole tree at any plane of constant time. Each
separate branch, limb or twig that you cut through is one of the
separate species that existed at that time. If you happen to cut
through a knot where a branch is splitting into two or more, you
have to look closely at the details of what is happening, and the
simple concept of "species" may require elaboration.
Now, do another experiment on the tree. Make a single saw cut through
just one branch, limb, or whatever. Reach out and grab the hunk of
tree that is about to fall to the ground thereby, with all its
attached smaller branches, limbs, twigs, and leaves. What you hold in
your hand is called a clade (definition thereof). Whether you name
it, how you name it, and what kind of name you give it, is up to you.
Confusion between named clades and named taxa in other taxonomies
sometimes results from the convention, in other taxonomies, of naming
groups of separated twigs or leaves without the name also applying to
all of the limbs and branches from which they originated. Cladists
don't do this; that is a convention of that taxonomic system.
Because of the problem of talking about the "same" species at
different points in time, some new words are useful. A "lineage"
(definition) is sometimes said to be a maximal unbranching section of
limb, branch or whatever -- that is, a whole piece of tree from where
it starts off from a fork somewhere up through time to where it itself
first forks. That is, a lineage comprises all the variation in a
species between speciation events. But it's just a definition -- if
it isn't useful, don't use it.
-- Jay Freeman