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A Reduction of "Species" Resolves the Species Problem ----- Jody Hey, January
1997
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PROPOSALS FOR POPULATION BIOLOGISTS
Biologists often apply "species" without a clear meaning of the word or,
if a specific meaning is articulated, with uncertainty over whether a group
of organisms actually fit the meaning. For cases when "species" is used to
convey some degree of individuality on the part of a group of organisms,
meaning some degree of spatio/temporal integrity, a two step convention is
proposed. First, the specific meaning that should be used is that of genetic
species. However, for most groups of organisms, including many that might
be called species under other concepts, the population structure will not
resemble panmixia. Thus the second component of species identification is
the inclusion of a description of the pattern of genetic drift.
Having a meaning of "species" makes it possible for population biologists
to avoid the word and associated uncertainties. If genetic drift underlies
mechanistic species concepts, it follows that an assessment of the history
of genetic drift for some group of organisms will obviate the need for species
identification of those organisms. One or more species names may be applied,
but will convey no additional information.
GENETIC SPECIES AND SYSTEMATICS
It has been proposed that some organisms do exist in groups that are real,
and that the spatiotemporal distinctness of these groups arises when organisms
share in a process of genetic drift. The genetic species is also a distinct
kind of individual, different from the individual status that might be considered
for larger groups of organisms. For example, it has been argued that species
are not different from genera or other taxa, and that a taxon at any level
can be thought of as an evolutionary unit (Nelson, 1989). However, the criteria
of genetic species is shared genetic drift among organisms. It is possible
that some organisms occur in a pattern of population structure with nested
levels of genetic drift among organisms (see GENETIC
DRIFT AND POPULATION STRUCTURE) so that genetic species may occur within
larger genetic species. However this pattern is still caused by genetic drift
among organisms, which is in turn caused by gene flow and
demographic exchangeability. These causes of individuality at the species
level need not be the same as processes that occur among species. While it
may be argued that there are processes of species turnover that are analogous
to genetic drift, these processes will not be identical to gene flow and
demographic exchangeability. In short, the genetic species is a distinct
kind of individual that includes multiple organisms.
The finding that an organism may not be part of a species, either in a
contemporaneous sense or a historical sense, has implications for the study
of the historical relationships among organisms and for classifying organisms.
These implications may be roughly categorized as theoretical and practical.
The primary theoretical implication is that a systematic theory that assumes
that all organisms occur in nature as parts of species, may
be incorrect. For example, investigators in the field of phylogenetic systematics
strive to use historical relationships among organisms as a guide for the
classification of organisms (Hennig, 1979, p73). It is implicit, and sometimes
explicit, within this perspective that organisms truly do occur in phyla
(e.g. species or higher taxa). For instance Hennig stated that species and
higher taxa "are all segments of the temporal stream of successive `interbreeding
populations'" (1979, p81). If interbreeding populations are not, in fact,
a necessary occurrence for organisms, then the theory of phylogenetic systematics
has an error. However, as severe as the theoretical implications may be,
the practical implications are largely unknown. It is not known how many
organisms do not occur in genetic species, and it is not known to what degree
the ancestors of present day organisms occurred in genetic species. Also,
the estimation of gene trees as a way to estimate phylogenetic relationships
will appear to be insensitive to whether or not ancestral organisms were
in species. For instance, in the absence of sex there will still exist a
bifurcating gene tree history for extant DNAs, regardless of the species
status of ancestors (see CONTEMPORANEOUS SPECIES
AND HISTORY). Such a tree may be misidentified as a phylogeny, when it
actually does not represent phyla.
If the theoretical implications of the genetic species concept were to be
included in a theory of systematics, then at least two possible courses can
be considered. First, a theory of systematics could focus solely on the
historical relationships of individual organisms (Vrana and Wheeler, 1992).
This method would have the advantage that the identification of individual
organism is vastly easier than for individual species. However, such a system
would also face the difficulty that for any point in time, a sexual organism
will likely have many ancestral organisms. This means that a bifurcating
hierarchical tree will often not be a good historical model. An alternative
proposal is that a systematic investigation could begin with a process of
identifying candidate species and estimates of historical relationships among
candidate species. For example, candidate species could be identified by
relatively practical criteria (e.g. morphological similarity). This general
approach is commonly used with other species concepts. In addition systematics
would include a process of examining the genetic species status of candidate
species. This kind of systematic protocol, with a theoretical component that
includes the genetic species concept, would also need to address the issue
of the classification of organisms that are not in species.
Both of these proposals, using organisms as the individuals of systematics
and including the genetic species concept within systematic theory, address
the theoretical basis of systematics. A third alternative would be to forego
biological theory, and develop a taxonomic system based on criteria identified
by observers. For instance, organisms could be clustered on the basis of
similarity (Sneath and Sokal, 1973) or on the basis of shared characters
(Nelson, 1989) without assumptions of underlying biological processes. A
taxonomic species identified in this way would be a class of organisms, and
would have no necessary relationship to genetic species that occur as individuals
in nature.
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© 1997 Jody Hey
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