Darwin-L Message Log 4:8 (December 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

<4:8>From DARWIN@iris.uncg.edu  Sat Dec  4 23:29:15 1993

Date: Sun, 05 Dec 1993 00:35:44 -0400 (EDT)
From: DARWIN@iris.uncg.edu
Subject: Ancestral and derived character states in systematics
To: darwin-l@ukanaix.cc.ukans.edu
Organization: University of NC at Greensboro

This is a belated second installment in answer to some questions Lynn
Hanninen asked several days ago about systematics.  I tried in the first
reply to distinguish between phylogenetic (historical) inference and
classification, and here will try to describe the difference between
ancestral and derived character states.  Although I will describe these
concepts as they are used in systematics, I think the general ideas will be
very clear to philologists as well.  As always, this is a brief summary;
many things said here could be qualified and expanded upon.

Systematists reconstruct phylogeny on the basis of _character_ data.  What
is a character?  A character is _an observed difference from which we infer
an evolutionary event_.  The two "sides" of the difference are the character
_states_.  (Characters may have more than two states, but for purposes of
exposition I will consider only two-state characters.)  For example, if we
were to examine the 300 or so species of woodpeckers, we would notice many
differences among them, such as the fact that some of the species are
four-toed while others are three-toed.  "Number of toes" in this case would
be a character, and its states would be "four" and "three".  If woodpeckers
are a clade (a whole branch of the evolutionary tree) then it is likely that
the ancestral woodpecker was either four-toed or three-toed, and that at
some point during the history of the woodpecker clade one branch changed
from one state to the other (an evolutionary event).  The original state for
the clade as a whole is the _ancestral state_, and the innovation that now
occurs in a portion of the entire clade is the _derived state_.  In this
particular case we believe that "four-toed" is the ancestral state of the
character "toe number" in woodpeckers, so the transformation went like this:
four-toed --> three-toed.  This does not mean that there are no more four-
toed woodpeckers; there are.  What it means is that when we look at the
species we see today, any particular pair of four-toed species are not
necessarily more closely related to one another than either is to the
three-toed branch, because the ancestral four-toed state may _retained_
anywhere in the tree.  Thus _similarity_ is not the criterion of historical
relationship; it is similarity in _derived states_ that is evidence of
relationship (the similarity among the three-toed species, in this example).

A couple more examples: In the case of mammals, laying a shelled egg is the
ancestral state, and live birth is the derived state, of a character we
might call "mode of reproduction".  The monotremes (Spiny Anteater,
Duck-billed Platypus) retain the ancestral state, while most other mammals
exhibit the derived state (live birth).  Notice from this example that the
designation of a character state as ancestral or derived is always dependent
upon the level in the tree we are talking about (mammals, in this case), and
is not some absolute attribute of the state itself.  Laying a shelled
egg is a _derived_ state for the tetrapods (the four-legged vertebrates),
because for the tetrapods as a whole the ancestral state is usually
considered to be the unshelled "amphibian" egg (like a frog or salamander
egg); but the shelled egg is an _ancestral_ state of the mammalian clade.

A number of synonyms for these terms are in common use: "primitive" or
"plesiomorphic" are sometimes used in place of "ancestral"; "apomorphic" is
sometimes used in place of "derived".  Thus "a plesiomorphy" is an ancestral
character state, and "an apomorphy" is a derived character state.  We could
speak of the three-toed condition as being an apomorphy of one sub-clade of
woodpeckers, or of the woodpecker clade as a whole being plesiomorphically
(ancestrally) four-toed.

I hope that is of some help, but I know that these ideas can be very
difficult to absorb from prose.  A few minutes with a piece of paper and
some tree diagrams can bring it all into focus, however.  One of the most
important things anyone can do when studying systematics is to get in the
habit of "tree thinking", because all the terms and concepts of modern
systematics are defined in the context of evolutionary trees.  When you are
confronted with a systematic concept or term you always want to ask yourself
"How does this relate to the tree?"  When told about any organismal
attribute you always want to ask yourself "How is it distributed on the
tree?"  (It might require a major research project to answer that question,
of course, but that is the question that is of systematic interest.)

The question of how, practically, one distinguishes the ancestral from the
derived states of a character is another question entirely, and one that
I won't address here.  I will however follow this message with a collection
of some stray quotations and references that touch on the notion of ancestral
and derived character states in systematics and philology, in the hope that
they may be of interest to some people.

Bob O'Hara, Darwin-L list owner

Robert J. O'Hara (darwin@iris.uncg.edu)
Center for Critical Inquiry and Department of Biology
100 Foust Building, University of North Carolina at Greensboro
Greensboro, North Carolina 27412 U.S.A.

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