An Undiscoverable Essence of Species?

At the turn of the 5th to 6th centuries, the Neoplatonist Boethius expressed his concern about the foundations of the logical generic-species scheme by declaring that “if we do not know what species is, nothing would secure us from delusion” (cited after [Boethius 1906]). This became a kind of testament for many subsequent generations of thinkers: first were scholastics, then natural philosophers, and finally biologists. The main question that determined the content of the species puzzle then was this: if we designate something as species, is there any real essence hidden behind it in Nature, or is it only a nominal category? Over time, this puzzle preserved its fundamental status, though its content changed in one way or another.

In scholasticism, a nominal treatment of species seemed to dominate. Among the naturalists of the 17th and 18th centuries, a belief in the reality of species began to take hold: for example, J. Ray and after him C. Linnaeus believed that species appeared in the days of the Divine creation (see Section 2.3.2). Buffon, agreeing with them, wrote in one of his treatises that “Species are the only creatures of Nature, eternal and unchanging like itself’ (cited after [Buffon 1843: 52]). At that time, the monistic concept of the overall System of Nature dominated, with one of its basic elements being just the species. And since the System of Nature was imagined to be unified, both the species unit and category were also considered real and unified for all Nature; this implied that a true understanding of species must also be unified. This general position is well reflected in the unified taxonomic species of systematics.

Starting in the second half of the 19th century, Darwinian evolutionary theory has significantly shaken (for some time, even subverted) belief in an exclusive status of the species category: the latter began to be understood as just a final stage in the gradual differentiation of local geographical races. And it was Charles Darwin who expressed an idea inserted into the title of this section: he believed that biological science had to become free “from the vain search for the undiscovered and undiscoverable essence of the term species” [Darwin 1859: 485]. Darwin’s “species nihilism” gave rise to a “species crisis” [Mayr 1965; Skvortsov 1967; Zavadsky 1968] and was taken up by biosystematics of the first half of the 20th century [Turesson 1922; Lotsy 1931; Mayr 1942]. With this, the species question emerged, which was basically about how to distinguish between species and intraspecific categories [Britton 1908; Turrill 1925]. In the synthetic theory of evolution that emerged, speciation was recognized as a nodal point in the evolutionary process; this led to the restoration of the priority status of the species category in those branches of biology that were in one way or another oriented to that theory, including several research programs of systematics [Mayr 1965]. A kind of apotheosis of this return became several proposals to establish a special “science of the species,” called hexonomy [Skvortsov 1967], eidology [Zavadsky 1968], or eidonomy [Dubois 2011].

Most recently, interest in species increased with the emergence of the concept of biodiversity as a natural resource that requires special study and conservation [Wilson 1988]. The reason is that, in the currently dominating noticeably simplified version of biodiversity, the species is usually given the same key place as in “Linnaean” systematics: it is considered the main “counting” unit of the diversity of organisms [Claridge et al. 1997; Sigwart 2018].

A gradual return of interest in species led to the fact that, throughout most of the 20th century, there was an active search for its “undiscoverable essence.” By the end of the 1930s, it became clear that, in different groups of organisms, a structural unit commonly called species manifests itself in quite different manners. These differences were primarily associated with various mechanisms (ecological, behavioral, epigenetic, etc.) that ensured species integrity. Interpretation of species as a singameon [Lotsy 1931] led to the fundamentally important conclusion that different breeding systems might presume different kinds of species [Faegri 1937; Turrill 1938; Cain 1954]. Depending on particular mechanisms, different species concepts were formulated based on the general notion of species. These concepts, each with its own rationale, appeared to be scarcely generalized by any single “superconcept”: that was how the species problem in its original version emerged [Robson 1928; Faegri 1937; Clark 1956].3 Its further development led to the fact that a general intuitive understanding of the “natural species” (in the sense of [Kunz 2012]) was supplanted by its conceptualization and related formalizations [McOuat 2001; Pavlinov 2013a, 2018; Bartlett 2015]. Thus, species pluralism appeared to come into conflict with the classical species monism. As this problem was developing, the situation did not become clear but worsened because the species concepts got multiplied: theorists

Actually, this word combination appeared earlier in the sense of the “species question” (e.g., [Turesson 1922]).

“established a minor industry devoted to the production of new definitions for the term species” [de Queiroz 1988: 57]. Hundreds of articles and more than a dozen books devoted to this problem and its possible solutions appeared; among the books the most significant seem to be: [Ghiselin 1997; Wilson 1999; Wheeler and Meier 2000; Hey 2001; Stamos 2003; Wilkins 2009; Richards 2010; Pavlinov 2013d; Slater 2013;Zachos 2016].

Thus, the main reason for the ascendancy and persistence of the species problem in its traditional sense is due to the existence of different species concepts. However, it seems to be only part of this problem. The entire cognitive situation construed around the general species concept is problematic, not just because of the diversity of particular concepts as such, but rather due to a contradiction between aspiration and inability to reduce this diversity to a single species concept, be it the most general (“omnispective”) or a particular “most principal” one [Pavlinov 2009b, 2013a, 2018]. So, in more general terms, this problem is caused by a contradiction between species monism and species pluralism: as can be assumed, it is this contradiction that accounts for the current content of the species puzzle.

Part of the latter is an “erosion” of the species category, which is reflected in the specific terminology denoting different degrees of species differentiation: hypospecies, semispecies, “almost species,” allospecies, ex-conspecies, superspecies [Mayr 1965, 1969; Graybeal 1995; Dubois 2006; Mallet 2013]. As a result, the species category turned out to be vague, and it was suggested that the pool of taxonomic studies addressed to it should be called “around-species systematics” [Mikhailov 2003].

Currently, there are about two dozen particular species concepts; they differ in what is considered the key parameter in the definition of the species unit [Mayden 1997; Hey 2001; Mallet 2001; Wilkins 2009; Zachos 2016; Pavlinov 2018]. These concepts can be classified into various categories; of the “ontically motivated,” two seem to be the most general—structural (static, synchronous) and processual (dynamic, diachronic) [Dobzhansky 1935; Lee and Wolsan 2002; Stamos 2003]. Besides, it is important to distinguish between theoretical and operational concepts. The following are lower-level categories to which most of the species concepts can be referred:

  • • species as a unit defined by similarity!, including the following concepts: phenetic (similarity of phenotypes), genetic (similarity of genotypes), typological, as well as all concepts based on operational definitions of species units
  • • species as a least reproductively isolated unit, according to the biological concept, the self-recognition concept
  • • species as a historical unit, according to the classical generative concept (“like gives birth to like”), phylogenetic, and genealogical (including lineage) concepts
  • • species as an ecological unit, including the ecospecies concept
  • • species as an ontological unit, including understanding of species as a natural kind.

There is a number of complex concepts trying to combine several criteria, of which the evolutionary species concept is worth mentioning here [Simpson 1961; Wiley and Mayden 1981; Mayden 1997].

Looking at the species problem from a “non-classical” point of view does not allow us to consider the diversity of species concepts as “seditious.” Each concept fixes a certain particular manifestation of a particular structural unit of taxonomic reality, which is usually called species (or “around-species”). This position legitimizes species pluralism and pledges not so much to fight against it but rather to look for its natural causes. A general prerequisite for such a search is provided by the structure of the cognitive situation in which the species problem is considered [Pavlinov 2013a, 2018].

Taking into account the hierarchical (“pyramidal”) structure of the conceptual space shaping this situation (see Section 3.2.3), let us first assume that any particular species concept of a certain level of generality can be rationally defined within the context of a higher, more inclusive level of the respective conceptual pyramid. In this regard, a question inevitably arises as to how to develop some “most general” species concept that would serve as a kind of “umbrella” to integrate (“shelter”) particular concepts [Reydon 2005; Yuichi 2017]; in a way it is similar to the question posed by David Hull about an “ideal” species concept [Hull 1997]. It is evident that, for such a general concept to be properly developed, it is necessary to raise the respective pyramid’s highest level of generality so that it permits consideration of the general species concept within the context of a certain more general biological conception/ theory. In more formal terms, it means the need to denote such a “logical genus,” by which division it would become possible to get a “logical species” containing a general definition of the natural species phenomenon biology deals with. In order to avoid the logical “genus-species” tautology, it is necessary to define, in the same, more general context (“logical genus”), some other biological units (“logical species”) of the same level of generality as the species phenomenon proper, but which are not species. Such an approach is aimed at understanding what the natural species phenomenon is and how it differs from any “non-species” phenomena (such as life forms, syntaxa, guilds, etc.); otherwise, it seems to be impossible to decide positively why we think of a phenomenon as of species and not anything else.

Taking into consideration that natural “species” and “non-species” units are different elements of biota, it seems reasonable to fix the latter’s certain conception at the “top” of the entire conceptual pyramid. In this respect, treating biota as a developing non-equilibrium system seems quite attractive; it allows the main emphasis to be placed on those natural causes (factors) that operate at the level of biota and structure it, as it develops and functions, thus generating and individualizing its various structural units. The joint co-action of these causes yields dynamic stability of both biota and any of its units, including species, as one of their most fundamental properties to be comprehended [Brooks and Wiley 1986].

So, to get a metaphysically consistent understanding (and eventually definition) of the general concept of species as a natural phenomenon, the entire conceptual pyramid should be construed as a descending cascade of those causes that structure biota and provide dynamic stability of species as a natural phenomenon. There are several of them (external, internal, historical, etc.) which interact in a complex manner with each other, so no one of them, taken separately, can explain the species phenomenon exhaustively. If this is so, then the desired definition (or at least understanding) of the natural “species in general” should not be reductionist but as exhaustive (omnispective) as possible to incorporate all causes ensuring its existence [Sluys 1991; Pavlinov 2013a], On the one hand, this would provide a certain comprehension of what makes the natural “species in general” just species differing from other units of the structure of developing and functioning biota. On the other hand, this would allow clearer recognition of the particular causes responsible for the particular manifestations of the natural species reflected in the respective particular concepts [Ereshefsky 1992; Pavlinov 2018].

According to the considerations given above, attempts to find a metaphysically sound general understanding (and eventually definition) of the natural species phenomenon seems to plunge the species problem into the context of “new' essentialism” [Ellis 2001; Rieppel 2010b; LaPorte 2017; Maxwell et al. 2020]. This means acknowledging that, if a species is not reducible to just a sum of its constituent organisms, it must be endowed with some kind of emergent essential property [Sober 1984, 2000]. The latter may be informally designated as a specieshood of whatever content, distinguishing it from any “non-species” with their owm essences or “-hoods” [Pavlinov 1992. 2009b, 2013a, 2018; Griffiths 1999; Wilkins 2007; LaPorte 2017]. A part of this metaphysics is the assumption that specieshood, however conceptualized, is an integrated part of the overall natural history of organisms. It incorporates, in a certain unobvious way, particular mechanisms responsible for the dynamic stability of the particular species—their self-reproduction and mutual isolation, their place in the niche structure of ecosystems, their persistence as genealogical successions, etc. Such an understanding seemingly presumes the dependence, to a greater or lesser degree, of specieshood on other aspects of an integrated natural history of organisms. This inevitably makes it group-specific: even if we suppose that some natural species considered in general may be endowed with certain emergent properties common to all (or to the vast majority of) living beings, it may have different manifestations depending on the specific biological properties of particular groups of organisms. One of the outcomes of such group-specific specieshood manifestations is the above-mentioned existence of different “kinds of species” associated with different breeding systems.

From this, the assumption follow's that specieshood manifestations change together with other biological properties of organisms in the course of the evolutionary development of the functional and structural organization of biota in general and its various elements in particular. So w'e have here something like an “evolving essence” w'hich might be pertinent to so-called “historical essentialism” [Griffiths 1999; Pedroso 2012; Maxwell et al. 2020]. More particularly, this means the supposition that biological mechanisms responsible for the dynamic stability of the species and constituting their respective specieshoods were quite loose (fuzzy) at the beginning of evolution, which resulted in quasispecies or pseudospecies in viruses, prokaryotes, and lower eukaryots [Dobzhansky 1970; Eigen 1983; Nowak 1992; Cohan 2001, 2002; Domingo 2002; Stamos 2003; Hanage et al. 2005; Wilkins 2006; Pavlinov 2013a; Andino and Domingo 2015]. Finally, those mechanisms shaping specieshood became more perfect in more advanced organisms, thus making their species more cohesive and discrete, so they became euspecies (“real species”) described by the reproductive (or self-recognition) species concept [Dobzhansky 1970; Eldredge 1985; Pavlinov 2013a]. Thus, it might be an important task of comparative eidology/ eidonomy to study the distribution of various kinds of species, with their specific specieshoods, over groups of organisms with different natural histories to reveal the causal intercorrelations of these “-hoods”and “histories.”

Were it to appear possible, contrary to Darwin, to discover a certain essence of species (“specieshood”), the probable theoretical verdict regarding the species problem might be as follows: the natural (biological in its most general sense) species as a natural phenomenon manifested itself in a universal unit of the structure of biota of a certain level of generality most likely exists, but it is implemented in different ways in different groups of organisms with different natural histories. This will serve as justification for the use of a unified taxonomic species concept in systematics and, at the same time, a certain motivation for a more thorough elaboration of different species concepts. Otherwise, it will be necessary to acknowledge that there is no such universal natural unit in its general meaning; instead, there are actually different natural phenomena (units), each with its own essence, inherent in different groups of organisms. And systematics will most possibly be obliged to respond appropriately to such a conclusion by inventing local classification units to replace the currently used universal one (as a legacy of scholastics) in order to correspond better to what occurs "in reality” [Dubois 2011; Nathan 2017].

The latter position corresponds to the proposal to exclude the general species concept from the biological thesaurus, since it is both outdated and cannot be rigorously and unambiguously defined [Burma 1954; Michener 1962; Sokal and Sneath 1963; Riddle and Hafner 1999; Hendry et al. 2000; Kober 2008]. However, when considering such a far-reaching suggestion, one should take into consideration that both the general species concept and notion are sturdily embedded in the thesaurus of many fundamental and applied biological disciplines. So, realization of this suggestion may lead to a substantial reorganization of a large part of the conceptual space of all of biology. The reason is quite obvious; such a rejection would necessarily entail rejection or serious correction of other concepts and notions that are associated, in one way or another, with the notion of species.

For instance, in ecology, this notion is not essential for describing the structure of the local ecosystems, as far as such descriptions are based on biomorphs [Krivolutsky 1971; Chernov 1991]. However, in comparative analyses of different ecosystems, there is the evident need for a certain unified basis of comparison (the above “denominator”) that would allow the biomorphs recognized within each of the local ecosystems to be linked to each other. It happens that currently it is the species that fulfills such a function; as a matter of fact, for the evolutionary ecologists, biomorphs exist in local ecosystems not by themselves but as manifestations of local populations of widespread species [Schwarz 1980].

This particular aspect of the species puzzle is not only a consequence of conservatism of the conceptual apparatus of biology but also reflects one of the universally valid epistemological principles. According to the latter, in order to explore any differences between objects, one must have some more or less solid unified basis of comparison making these objects components of a certain unity (elements of the same class, tokens of the same natural kind, etc.) by possessing some fundamental feature(s) in common. For many research tasks in biology, such a currently acknowledged basis refers to conspecificity, i.e., to organisms, differing from each other in some details, belonging to the same species as a real existing natural unit. Such reference function of species faces the serious problem of the substantive incommensurability of the species units recognized in biologically different groups of organisms, with their “fuzzy” treatment providing a rather weak solution to this problem. And yet, it is clear from this perspective that, in order to get rid of the notion of species in biology, it would be necessary to introduce some other no less general notion as a solid unified basis of comparison, substantiating such a replacement by reference to some biologically meaningful and sufficiently comprehensive theory. With this, it should be kept in mind that any new concept replacing that of species will certainly face the same ontic and epistemic limitations and pitfalls outlined in the philosophical chapter of the book: ontic reduction, incompleteness, fuzziness, etc. So the species problem will not actually be “closed,” but will simply turn into another, no less challenging, problem.

 
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