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A Reduction of "Species" Resolves the Species Problem ------ Jody Hey, January
1997
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CONTEMPORANEOUS SPECIES AND HISTORY
One of the major dichotomies that arises in the discourse on species concepts
is between contemporaneous, or "snapshot" viewpoints and historical viewpoints
(Endler, 1989). A contemporaneous view is most likely to be useful to a
population geneticist or ecologist focusing on ongoing population processes.
The biological species concept, and the genetic species concept are
contemporaneous concepts. However, systematists take a historical view and
generally refer to species within the context of ancestor-descendant
relationships. For example, (Simpson, 1961) defined a species as an
"ancestral-descendant sequence of populations". Cracraft defines a phylogenetic
species as "an irreducible (basal) cluster of organisms, diagnosably distinct
from other such clusters, and within which there is a parental pattern of
ancestry and descent" (Cracraft, 1989). One kind of resolution of these two
different viewpoints of species (contemporaneous and historical) has been
to argue that both are real but distinct kinds of entities (Donoghue, 1985;
de Queiroz and Donoghue, 1988). In particular, de Queiroz and Donoghue (1988)
argue that there are two processes, interbreeding and common descent, that
are both valid species criteria. However, the two criteria differ fundamentally
in their incorporation of time. Interbreeding is a process that can be viewed
in a short time interval (e.g. a generation). Common descent is an explicitly
historical criterion - whether or not a group of organisms have descent in
common depends on what ancestors they had. The processes of interbreeding
and common descent are distinct merely because one (i.e. common descent)
includes the passage of time. If there exists a kind of species that can
be defined by historical relationships, then these historical relationships
must have arisen because of processes that occurred over time. For example,
Simpson's definition of an evolutionary species explicitly refers to the
existence of a population that exists at a particular point in time. Cracraft's
definition of a phylogenetic species refers explicitly to a "parental pattern
of ancestry and descent". Though not defined precisely, this definition clearly
implies some contemporaneous process by which groups of parents leave offspring.
In both of these cases, Simpson's and Cracraft's, the definition of a historical
species concept supposes the existence of some kind of cohesive group of
organisms that exists for each slice of time in the history of the historical
species.
The gap between historical and "snapshot" concepts can be bridged by considering
the process that creates a contemporaneous species, and then extending that
process through time to generate a picture of a historical species. Indeed
much of the literature that forms the debate on the phylogenetic species
concepts includes this very exercise for the case of contemporaneous populations
of interbreeding organisms (Hennig, 1979, p73; de Queiroz and Donoghue, 1988;
Nixon and Wheeler, 1990; Davis and Nixon, 1992; Baum and Shaw, 1995). This
can also be done for the genetic species concept. The historical extensions
of contemporaneous situations, with and without sex, can be considered.
1) For organisms that do not have sex, the common ancestors of a genetic
species become fewer, the further one goes back into the past, and this reaches
a limit of a single individual. For two genetic species that have recently
diverged, there may be many shared ancestral organisms, and these may or
may not have existed as a single genetic species. Thus whether or not a
historical viewpoint includes ancestral and descendant species depends on
how much time is being considered and the history of genetic drift. For an
ancestor and descendant that are far apart in time, the ancestor is literally
a single organism and could not be a species. Furthermore, this is true whether
or not the descendant organisms are a contemporaneous genetic species.
2) For a group of sexual organisms, different portions of the genome will
have different gene tree histories. The nodes of these gene trees are ancestral
portions of the genome, and the spatial and temporal distribution of the
organisms that carried these ancestral DNAs can be considered as a function
of the historical pattern of genetic drift. From models of the variance of
the coalescent process in the presence of population structure, it is clear
that the variance of the time and the geographic location of gene tree nodes
can become large, arbitrarily large, depending on the degree to which populations
of ancestral organisms depart from panmixia (Slatkin, 1987; Hey, 1991). It
is possible for a group of organisms to exist as a contemporaneous genetic
species, and yet have a history of ancestors that did not occur in genetic
species. This type of history would cause different gene tree estimates,
for different portions of the genome, to have a very large variance for the
pattern of node spacing (see EMPIRICAL
CONSIDERATIONS). It is also possible for a group of organisms that do
not exist as a contemporaneous genetic species to have a history of ancestors
that did occur as genetic species.
In summary, when the genetic species concept is extended to a historical
context, the exercise does not reveal the emergence of a distinct historical
entity. There is no epiphenomenon that could be called a historical species
when the history has not included the persistence of contemporaneous genetic
species. In the literature on phylogenetic systematics, one graphical tool
is to depict historical species as a tube, with time as an axis that runs
the length of the tube. In these diagrams, a view of a species or a population
at a point in time can be represented as a cross section of a tube (e.g.
Hennig, 1979, p59). The point made here is that the histories of organisms
need not have a shape that can be represented in this way. In the absence
of sex, the more recent ancestors of a group of organisms may not have shared
genetic drift. For times longer in the past, the ancestor of a group of asexual
organisms, regardless of whether they occured as part of a genetic species,
is a single individual. In the presence of sex, the ancestors of a
contemporaneous genetic species may not have been a genetic species. The
ancestors may have been spread across a wide expanse of geography, in an
isolation by distance relationship, or with a complex structure of multiple
populations.
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© 1997 Jody Hey
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