Darwin-L Message Log 4:27 (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.

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<4:27>From DARWIN@iris.uncg.edu  Wed Dec  8 22:30:58 1993

Date: Wed, 08 Dec 1993 23:37:21 -0400 (EDT)
From: DARWIN@iris.uncg.edu
Subject: Extinction and pseudoextinction
To: darwin-l@ukanaix.cc.ukans.edu
Organization: University of NC at Greensboro

Diane Nelson's original query about the different meanings of "extinct"
is an interesting one, as witnessed by the many replies it has generated.

Greg Mayer is correct in saying that in evolutionary biology we usually
distinguish between genuine extinction and "pseudoextinction" (taxonomic or
morphological extinction: no organisms exist today that _look_ like the
"extinct" taxon, but modified descendants of it do exist).  This point
recently came up in one of my classes in the context of Darwin's description
of his tree diagram in the _Origin of Species_.  This diagram has a row of
capital letters across the bottom, standing for several species in a genus,
and then a range of branches leading up from some of them to the top of the
diagram where the end-points are given small letters and numbers.  Part of
the description of the diagram reads:

  "If then our diagram be assumed to represent a considerable amount of
  modification, species (A) and all the earlier varieties will have become
  extinct, having been replaced by eight new species (a14 to m14); and (I)
  will have been replaced by six (n14 to z14) new species."  (p. 122)

Species (A) and (I) are in this case not genuinely extinct, but rather
pseudoextinct; that is, there is nothing around today that looks like (A),
but there are descendants of (A).  (Species (A) is thus like Latin.)  Darwin
contrasts this sort of extinction with that of some of the other species at
the bottom of his diagram, such as (E), which are "extinct, and have left no
descendants."  (p. 123)

Greg also says that it is often neither important nor practical for
paleontologists to determine whether a particular lineage has undergone
extinction or pseudoextinction.  I certainly agree about the practical part
(it may be an exccedingly difficult or impossible question to answer), but
depending upon what one wants to do with the information it may be very
important to distinguish between genuine and pseudoextinction.  There is a
large genre of literature in paleontology, most of it from the last 20 years
or so, that attempts to tablulate the number of species, genera, or families
from different geological periods, and to use these data to say something
about rates of extinction and origination of taxa.  The data on which these
studies are based almost certainly contaminated with pseudoextinctions, and so
their results must be regarded critically.  One valuable paper that challenged
these studies on this ground is:

  Smith, Andrew B., & Colin Patterson.  1988.  The influence of taxonomic
  method on the perception of patterns of evolution.  Evolutionary Biology,
  23:127-216.

Smith and Patterson suggest that as many as one third of the extinction events
recorded in paleobiological compilations, and used in statistical calculations
of rates of extinction, are in fact pseudoextinctions.  It's as though the
historical linguists had tens or hundreds of thousands of languages to study,
and they wanted to figure out how frequently languages "die out", but had
listed both Latin and Tasmanian as having "died out", when in fact one of them
(Latin) not only didn't die out, but flourished and diversified.

To bring the issue of pseudoextinction home, and also the contrast between
morphologically vs. genealogically defined taxa, one has only to consider
dinosaurs, the archetypal "extinct" taxon.  In point of fact, of course,
the pseudoextinct Dinosauria are alive and well; we just call them birds.

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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