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1 Agamospecies: Are Sexual and Asexual Species the Same?

1 Agamospecies: Are Sexual and Asexual Species the Same?

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5.1 Agamospecies: Are Sexual and Asexual Species the Same?



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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).



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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. . .



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from panmictic evolutionary dynamics to genetically closed evolutionary

dynamics”.

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.



5.2



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/

McGuiness translation)



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



4



According to Hull, universality of species concepts does not covary with either their theoretical

significance or their applicability (Hull 1999, p. 42).

5

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.



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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

6

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.

7

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.



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