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1 Agamospecies: Are Sexual and Asexual Species the Same?
5.1 Agamospecies: Are Sexual and Asexual Species the Same?
the concomitant reticulation among organisms is lacking in asexuals. Asexuals do
form lineages and historical entities (individuals), but these are ontologically like
clear-cut non-reticulating supraspecific monophyla and not like sexual species:
“cladistic structure will go down to the organism level” (Mishler and Theriot
2000a, p. 51). This might warrant a dichotomous species pluralism (sexuals vs
asexuals) so that what is called species in sexuals and asexuals is actually two
biologically different kinds of entities. In line with this, many researchers do not
accept asexual species or at least emphasize that they are not directly comparable,
among them Mayr (1987, 2000a), Ghiselin (1997), Meier and Willmann (2000a),
Bock (2004) and Dobzhansky: “the species as a category which is more fixed, and
therefore less arbitrary than the rest, is lacking in asexual and obligatorily selffertilizing organisms [. . .] The binominal system of nomenclature, which is applied
universally to all living beings, has forced systematists to describe ‘species’ in the
sexual as well as in the asexual organisms. [. . .] Nevertheless, systematists themselves have come to the conclusion that sexual species and ‘asexual species’ must
be distinguished [. . .] In the opinion of the writer, all that is saved by this method is
the word ‘species.’ A realization of the fundamental difference between the two
kinds of ‘species’ can make the species concept methodologically more valuable
than it has been” (Dobzhansky 1937, p. 321). Dobzhansky again points out here that
our taxonomic nomenclature leads to the same kind of name (binomials) being
assigned to what may be very different entities, which is a reminder that T species
are not the same as E species. The arguments of those who hold that sexual and
asexual species are not the same should not easily be dismissed as they imply that
there is yet another form of homonymy of the term species involved. Certain
species concepts such as the Evolutionary, the General Lineage or the Unified
Species Concept explicitly embrace asexuals, and their adherents consider the
fact that these concepts are flexible enough to cover the whole spectrum of
reproduction an advantage. However, it could be argued that while different kinds
of lineages (such as reproductively isolated lineages, those with different ecological
niches, etc.) are rightfully subsumed under the same name (“species”), to include
also asexuals may be stretching the lineage pluralism a bit too far.
What, then, could be arguments in favour of combining sexual and asexual
organisms into the same kind of taxonomic unit (“species”)? First of all, sexual
and asexual reproduction are the extreme points in a continuum, with all sorts of
intermediate (mixed) reproductive strategies in between (Mishler and Theriot
2000a). This in itself, however, is not conclusive evidence because, as is repeatedly
argued in this book, fuzziness does not mean that boundaries do not exist. The
groups that first come to mind when thinking about asexual reproduction are
prokaryotes, but the main problem with these organisms may be a very different
one, namely, that there is so much horizontal gene flow among them that it is
doubtful that taxonomic individuation can be carried out in a way comparable to
p. 297), but this seems not to have been the case as it was denied by Mayr later (see Stamos 2003,
p. 150; Mayr 1987).
5 Species Concepts and Beyond: Selected Topics Relating to the Species Problem
eukaryotes, and then a large part of asexuals will be something else than what we
usually call species anyway (see Sect. 5.7). For eukaryotic asexuals, perhaps, the
main argument in favour of their forming usual species is to do with cohesion.
Genetic exchange via reproduction is viewed as one of the main forces of cohesion
in sexual species. Beneficial mutations can spread through a population in selective
sweeps, and the homogenizing effects of gene flow can keep populations or demes
from diverging through drift. Asexuals have none of that. Still, they often form
well-defined clusters in character space that are isolated from other such clusters:
“One particular troublesome aspect of excluding nonsexual species is that most
parthenogenetic ‘species’ display the same patterns of phenotypic cohesion within
and discontinuity between as do sexual species” (Templeton 1989, p. 8). This has
led some authors to doubt that gene flow plays a major role in cohesion (the locus
classicus is Ehrlich and Raven 1969, but see also Grant 1980 and Lande 1980). As a
consequence, Templeton’s (1989) Cohesion Species Concept combines organisms
into one species that shows phenotypic cohesion and genetic and/or demographic
exchangeability: “For asexual taxa, genetic exchangeability has no relevance, and
species status is determined exclusively by demographic exchangeability”
(Templeton 1989, p. 21). A very similar view was already expressed by R. A.
Fisher. Although he, too, makes a distinction between sexual and asexual species,
he also regards exchangeability as a key criterion: “Species, properly speaking, we
could scarcely expect to find [in asexuals], for each individual genotype would have
an equal right to be regarded as specifically distinct, and no natural groups would
exist bound together like species by a constant interchange of their germ-plasm.
The groups most nearly corresponding to species would be those adapted to fill so
similar a place in nature that any one individual could replace another, or more
explicitly that an evolutionary improvement in any one individual threatens the
existence of the descendants of all the others” (Fisher 1930, p. 121).
Many biologists will not be convinced that this suffices to combine sexuals and
asexuals into the same notion of species. After all, even if gene flow through sexual
reproduction does not play an important role in cohesion as often assumed, it still
occurs in sexuals, and it does not in asexuals. This is still a fundamental difference
between the two, or is it? In practice, this difference often simply does not exist.
Think of allopatric sexual populations—they might exchange genes through reproduction, but they don’t; they are as tokogenetically separate as asexual organisms.
In the absence of gene flow, however, what else keeps the various separate
populations of a sexual species together if not common selection pressures and a
common history, i.e. a relatively recent common ancestor which may result in
common developmental constraints, etc.? In other words, the very same processes
are responsible for cohesion and thus for the phenotypic clustering and gaps that we
find in asexual organisms! Therefore, unless we strictly classify allopatric
populations of sexual organisms as distinct species, the distinction between sexuals
and asexuals is not as clear-cut as often claimed. It is in this context that Templeton
(1989, p. 9f.) writes “At what point is isolation by distance and population subdivision sufficiently weak to bring a taxa [sic] into the logical domain of the isolation
and recognition concepts [i.e. sexual species concepts]? [. . .] there is a continuum
5.2 The Hierarchy of Species Concepts: The Evolutionary, General Lineage and. . .
from panmictic evolutionary dynamics to genetically closed evolutionary
In a nutshell, there are differences between sexual and asexual species taxa, but
just how pronounced they are is not at all clear (and may well vary from case to
case). Whether sexual and asexual taxa can and should be subsumed under the same
notion of E species (and not just T species in nomenclature) seems to be an open
question. The important dimension of this conundrum is, ultimately, in how far
biological analyses based on species taxa (their number, distribution, etc.) will be
skewed by lumping sexual and asexual species. We may never definitively know,
but this is only part of a larger problem that will be discussed in Chap. 7.
The Hierarchy of Species Concepts: The Evolutionary,
General Lineage and Unified Species Concepts
I therefore believe myself to have found, on all essential points, the final solution of the
problems. And if I am not mistaken in this belief, then the second thing in which the value of
this work consists is that it shows how little is achieved when these problems are solved.
Ludwig Wittgenstein (1922), Tractatus Logico-Philosophicus (Preface, Pears/
A basic dilemma of the species problem as perceived by many has been
formulated by David Hull. He lists three criteria that concepts in science are
expected to fulfil: universality or generality, applicability and theoretical significance. The problem with available species concepts is, according to Hull, that none
of them meet all three: “Most importantly, if a species concept is theoretically
significant, it is hard to apply, and if it is easily applicable, too often it is theoretically trivial” (Hull 1997, p. 358). Add to this the problem of universality4 (Hull
explicitly mentions as intractable problems for species definition those of asexual
reproduction and hybridization), and the prospects for a solution to the species
problem are bleak. By separating theoretical significance from practical applicability and making a virtue out of their incompatibility, as it were, something like a
solution has been found—although this solution admittedly only pertains to the
theoretical dimension of the problem. The introduction of the notion of a hierarchy
of species concepts in which a single one functions as a true ontological or primary
concept and all the others as secondary species identification criteria has arguably
been one of the major conceptual breakthroughs in recent decades.5 The primary
According to Hull, universality of species concepts does not covary with either their theoretical
significance or their applicability (Hull 1999, p. 42).
Richards (2010, p. 143) states that if there are indeed two kinds of species concepts—an
ontological one and several secondary criteria—then a framework like that of Hull where the
perfect species concept should fulfil all three of his criteria “guarantees a species problem. We
have treated [. . .] concepts as competitors, rather than as complements”. Hull (1999, pp. 38–43)
briefly comments on why he (I think) seems to agree with Mayden’s approach but justifies his own
from a more open-minded philosophical and less involved (scientific) perspective.
5 Species Concepts and Beyond: Selected Topics Relating to the Species Problem
concept (a term from Mayden 19976) is that of species as lineages, either as defined
by the Evolutionary Species Concept (see Mayden 1997, 1999, 2002; Wiley and
Mayden 2000a, b, c) or the General Lineage or Unified Species Concepts
(de Queiroz 1998, 1999, 2005a, b, 2007).7 Importantly, it also embraces the view
that all species concepts listed in Chap. 4 are based on biological realities, and that
means that they may not be applicable to all taxa or situations but that they cannot
be simply wrong.
Let us start with the Evolutionary Species Concept according to which species
are ancestor-descendant lineages that evolve separately from other such lineages
and have their own evolutionary tendencies and historical fate. This definition is
something like the consensus definition of several publications (Simpson 1951,
1961; Wiley 1978; Wiley and Mayden 2000a), and it was probably not a coincidence that it was the palaeontologist (Simpson) among the main architects of the
Modern Synthesis who came up with a notion of species as lineages through time.
The second half of the concept (“own evolutionary tendencies and historical fate”)
is important in that it precludes the assignment of species status to each and every
ephemeral offshoot of a species (e.g. small captive populations or a temporary, alloor peripatric population) and therefore holds that there should be some kind of
assessment of biological relevance involved when delimiting species in practice
(see Chap. 6), which means that species delimitation is only possible in a meaningful way in hindsight. The Evolutionary Species Concept “demands only that
speciation and evolution are natural processes involving lineages that maintain
cohesion and have unique identities”—something that probably all biologists
would agree is true—and thus has “the greatest generality” of all species concepts
(Mayden 1997, p. 416). It is conceded that it is not operational, i.e. it will not help in
a concrete case of whether a certain group of organisms form a species or not but
“[w]hile this may be viewed as a possible shortcoming, it is not so for a primary
concept” (p. 419) because it is about what a species is and not how to identify one.
Therefore, “it requires bridging concepts permitting us to recognize entities compatible with its intentions. To implement fully the ESC we must supplement it with
more operational, accessory notions of biological diversity—secondary concepts”
(p. 419). The Evolutionary Species Concept is considered the single appropriate
primary (ontological) concept because it unites all those entities that are identified
as species by the other (secondary) species concepts which function as identification criteria. Mayden (1997, p. 414, 421) draws an analogy between the hierarchy of
species concepts and phylogenetics: monophyly is defined as the property of a
group of taxa to comprise all and only the descendants of a stem species (and that
Mayden (1997, p. 418) adapted it from Mayr (1957). See also Hey (2006, p. 448, Box 1 where
Hey shows that Mayr did not follow up on this distinction) and de Queiroz (2005c) on Mayr’s early
role in the general conception of species as population or metapopulation lineages.
Naomi (2011) summarizes both Mayden’s and de Queiroz‘s approach and concludes that they are
basically equivalent. Naomi presents what he calls a revised version of this integrated framework
of species concepts, but I have to admit that to me he simply reformulates what Mayden and de
Queiroz have stated.