Evolution__3rd_Edition

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Box 23.1
Pseudoextinction
Species (or higher taxa) may go extinct for two reasons. One is
“real” extinction in the sense that the lineage has died out and left
no descendants. For modern species, the meaning is unambiguous,
but for fossils real extinction has to be distinguished from
pseudoextinction. Pseudoextinction means that the taxon appears
to go extinct, but only beause of an error or artifact in the evidence,
and not because the underlying lineage really ceased to exist. We
can distinguish three kinds of pseudoextinction, the first two of
which are due to taxonomic artifacts (Figure B23.1).
1. A continuously evolving lineage may change its taxonomic
name. As a lineage evolves, later forms may look sufficiently
different from earlier ones that a taxonomist may classify them
as different species, even though there is a continuous breeding
lineage (Figure B23.1a). This may be because the species are
classified phenetically (Section 13.2.3, p. 354), or it may be
because the taxonomist only has a few specimens, some from
early in the lineage and some from late in the lineage such that
the continuous lineage is undetectable. Either way, this kind of
taxonomic extinction is conceptually different from the literal
death of a reproducing lineage. The taxonomic survivorship
curves that we looked at in Sections 21.6, p. 609, and 22.7,
p. 637, contain some (usually unknown) mix of real extinction
and pseudoextinction.
2. A higher taxon may cease to have any members if it is defined
phenetically and only some divergent lineages persist. A higher
Time
t
Species 1
Species 2
Character state
Pseudoextinction
of species 1
Family A Family B
CHAPTER 23 / Extinction and Radiation 647
taxon, such as a family, can undergo pseudoextinction if
the taxon is defined phenetically (Figure B23.1b). For instance,
a paraphyletic group could go extinct even though some
descendants of that group continue to exist. In this sense, the
extinction of the dinosaurs was a pseudoextinction. Birds are
lineal descendants of one dinosaur group, and birds continue
to exist.
3. Lazarus taxa. A lineage may disappear temporarily from the
fossil record, perhaps because appropriate sediments were not
laid down for a while. Later it reappears. The first disappearance
is a pseudoextinction, and may be misrecorded as a real
extinction if the later reappearance is overlooked for some
reason. (The term “Lazarus” taxa alludes to a man in the
Christian Bible. Jesus Christ is there reported to have
miraculously raised Lazarus from the dead.)
The topic of pseudoextinction is worth keeping in mind when
considering theories to explain extinction and diversity patterns.
Most of the theories we shall look at in this chapter make sense only
for real extinctions of species lineages. For example, the theory that
mass extinctions are caused by asteroid impacts makes sense if the
mass extinctions are real but not if the evidence is largely composed
of pseudoextinctions in the sense of Figure B23.1a and c.
However, if the evidence for a mass extinction mainly consists of
pseudoextinctions of higher taxa, in the sense of Figure B23.1b,
that evidence could be compatible with an asteroid impact.
(a) (b) (c)
Figure B23.1
(a) Pseudoextinction within one lineage. If a continuous
phylogenetic lineage is taxonomically subdivided, the
earlier species will go “extinct” at the dividing line, even
though the lineage persists just as it did before. The
“extinction” of species 1 at time t is called pseudoextinction.
Time
†
†
†
†
Pseudoextinction
of family B
Time
Species 1
Pseudoextinction
of species 1
(b) Pseudoextinction of a higher taxon. Family A and
B have been defined phenetically, and family B is a
paraphyletic group (Section 16.6.2, p. 481). At time t, family
B becomes extinct, even though some lineal descendants
of the common ancestor of family B continue to exist.
(c) Lazarus taxa, with a fragmentary fossil record.