Evolutionary Anthropology 23:5–7 (2014)
ISSUES
Recognizing Species, Present and Past
IAN TATTERSALL
Nobody disputes that nature is meaningfully “packaged” in some way. But
debate persists over exactly how (and even whether) the boundaries dividing taxa
should (can) be drawn. At one end of the scale, some zealots abstrusely deny real
existence to higher taxa.1 At the other, laborers at the taxonomic rock-face confront genuine challenges in recognizing and delineating the species that systematists agree constitute the most fundamental unit of taxonomic analysis.2, 3
These difficulties exist at both conceptual and operational levels. In
theory, they are independent, but
they are liberally confused because
operational
considerations
have
often been allowed to feed back into
species definitions. In other words,
defining what species are in principle has too often been a matter of
taxonomists’ convenience. This is
one major reason why Coyne and
Orr4 were recently able to list well
over 30 definitions of the species in
active current use. Nobody benefits
from the consequent muddle, which
is best sidestepped by adopting Ghiselins5 elegant characterization (not
definition) of species as individuals.
Surely we can all agree that species
are, most fundamentally, the smallest historically and genomically individuated lineages of organisms.
Yet the individuation of populations is often hard to confirm or
deny. In large part, this is because
Ian Tattersall is a curator emeritus at the
American Museum of Natural History. He
has worked on lemur systematics and
ecology as well as in paleoanthropology, where his special interest is in
hominid diversity and cognitive evolution.
iant@amnh.org
Key words: speciation; species recognition;
fossils; diversity; systematics
C 2014 Wiley Periodicals, Inc.
V
DOI: 10.1002/evan.21385
Published online in Wiley Online Library
(wileyonlinelibrary.com).
species are the products of speciation, and speciation is far from being
a unitary process. Rather, it is something we recognize in retrospect:
the effective cessation of genetic
continuity among sister populations
– their individuation – may be mediated by numerous different mechanisms that act at all levels, from the
genomic to the anatomical and
behavioral. As a result, there are
many ways in which individuation
may or may not express itself. This
diversity of causation can make life
difficult for systematists of all kinds,
whether their subjects are living or
extinct.
For paleontologists, there is the
supreme operational difficulty that
speciation is not simply a passive
consequence
of
morphological
change.6 This discordance is particularly problematic because neither
geological age nor geographical origin is a reliable key to the affinity of
any individual fossil.7 At best, systematists can add these attributes to
a probability assessment perforce
based on morphology. Also, because
of the disconnect between morphology and speciation in living primates, which furnish our only verifiable
yardstick for judging the past, provide us with examples of both geographically widespread species with
well-differentiated local populations
and sympatric individuated species
that don’t appear very different to
the human eye.
Individuals, of course, possess their
anatomical attributes by virtue of
belonging to a particular species, so
that a fossil’s hard-tissue anatomy
should, in theory, be a robust indicator of its identity. But while this is
true in principle, anatomical variation
itself also varies widely among species,6 confounding simple statistical
assessment of differentiation. Combined with the loose linkage between
morphology and speciation, this
inconvenient fact might seem to
deprive morphology of yet more of its
systematic
sheen.
Complicating
things still further is the observation
that, among most primates, morphological variation among species of the
same genus tends to be expressed in
external soft-tissue characteristics
that do not impress themselves significantly on the durable bony tissues
beneath. This leads to the odd result
that, while it is undoubtedly species
that split to generate new lineages,
the most robust hard-tissue gestalt
category is the genus.
There is, then, no morphological
silver bullet out there for recognizing
species. Nonetheless, such awkward
complicating factors as sexual
dimorphism aside, it remains true
that the consistent presence of a distinctive morphology in a subset of a
fossil assemblage argues strongly in
favor of independent species status
for that subset.
The implications for the primate
paleontologist are clear. If closely
related species tend to differ rather
little in bony and dental morphology,
there will be a tendency to underestimate the actual variety of species represented in fossil samples. From the
systematist’s perspective, this bias is
actually preferable to the alternative.
6 Tattersall
For while a low estimate of species
diversity will undeniably oversimplify
the historical picture, unlike an overestimate, it will not actively distort
perceived evolutionary patterns within
the larger clade involved.
In the hothouse of paleoprimatology, other significant factors are also
at work. The boom-and-bust pattern,
whereby species and genera accumulate within a clade and are then dramatically reduced before beginning
to proliferate once more (Miocene
apes are the classic example), exemplifies a Kuhnian phenomenon that
is not only independent of the actual
fossil evidence, but sometimes also
independent of any species concepts
that might have crept into the picture. The same may be said of the
powerful historical biases that have,
from the very start, promoted hyperlumping in paleoanthropology.8
For systematists trying to understand the structure of the living primate fauna, the operational – and
philosophical – difficulties are very
different from those in paleontology,
although they are no less intractable
for that. Speciation is a complex matter that may take considerable time to
unfold completely. This makes it hard
to discern sharp genetic boundaries
between closely related populations
that might occasionally interbreed if
given the chance, but that are nonetheless already effectively individuated. On its own, then, cross-mating
may have zero relevance for species
status, while Mallet9 has even proposed that introgression between differentiated populations early in their
existences can be an important influence on their subsequent evolution.
For most students of the primates
that live today in the world’s tropical
forests and woodlands, internal
hard-tissue morphology of the kind
paleontologists depend on is at best
a secondary systematic consideration. It is certainly not by virtue of
the post-choanal pits in its cranial
base, which are almost the only
striking cranial autapomorphy to be
found among all the species of Eulemur, that we recognize E. rubriventer
as an individuated species. Just as
important as that they look different
to us is that, out there in the rainfor-
ISSUES
est, red-bellied lemurs behave as a
genetically independent entity.
However, things are not always
that simple. In the latest edition of
the lemur Field Guide10 all of the former subspecies of Eulemur fulvus
are recognized as species in their
own right, for no better reason than
that they differ among themselves in
external characters that are readily
visible to the observer. Yet they also
have largely allopatric distributions
and freely interbreed when given the
opportunity. There is scant evidence
that any of them has yet departed on
its own individuated evolutionary
. . .while a low estimate
of species diversity will
undeniably oversimplify
the historical picture,
unlike an overestimate it
will not actively distort
perceived evolutionary
patterns within the larger
clade involved.
trajectory, as E. rubriventer so clearly
has done.
The rationale for splitting the fulvus-group in this way is furnished by
the Phylogenetic Species Concept
(PSC), which sees species as
“irreducible cluster[s]. . . that [are]
diagnosably distinct.”11 The PSC has
also been used to justify the recognition of numerous more cryptic nocturnal lemur species (for example, of
Microcebus and Lepilemur10) almost
uniquely on the basis of mtDNA distances. Yet, while it is magnificently
easy to apply, the PSC ignores the
vital fact that the engine of new biodiversity is the emergence of distinct
varieties within species. Without subspecies, there is no place for new
species to start; and individuation
will emerge independently, under
other controls.
Because of the dubious inflation of
lemur species numbers engendered
by the PSC, the use of which typically increases species numbers by
50% compared to other species concepts,12 I have energetically advocated2,3 the use of a much broader
range of criteria than simple diagnosability in recognizing living primate
species.
When
inferring
individuation, systematists should
consider all available information,
much as juries do in civil court
(although, given the complexities of
nature, the “beyond all reasonable
doubt” criterion used in criminal
court is probably asking too much).
Such lines of evidence include morphology in its broadest sense,
embracing superficial characters and
olfactory signaling systems as well as
internal anatomy; social behaviors;
vocal and visual communication;
DNA markers; geographical and ecological distributions; environmental
preferences; and interactions with
sympatric populations, including
putative gene flow.
Having tried over many decades to
apply my experience of lemur systematics to understanding species
diversity in the hominid fossil
record, I find myself in a curious
position. Among many students of
the lemurs, I am known as a reactionary who is reluctant to acknowledge all the species diversity out
there in the forests of Madagascar.
Yet in paleoanthropology I am
simultaneously seen as a crazy splitter who sees a new species under
every rock. And then, of course,
there is what I like to think of as the
real me, trying to eschew both ideology and received wisdom and simply
to be as judicious as I can in the face
of the manifold difficulties that any
alpha-taxonomist has to grapple
with.
Paleontology and neozoology both
pose particular systematic challenges, some of which look insuperable in the near future. But one thing
is clear: the effort to recognize species realistically has to be made.
Basic systematics is far from being a
humble clerical business that we
should brush under the rug as fast
as possible so we can get to the good
stuff. Instead, it is what we must
imperatively get right before we can
say anything reliable at all, either
Recognizing Species, Present and Past 7
ISSUES
about our ancestry or our current
place in nature.
ACKNOWLEDGMENT
My appreciation goes to John Fleagle
for inviting me to contribute to this special issue of Evolutionary Anthropology.
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