Biosemiotics (2016) 9:61–71 DOI 10.1007/s12304-016-9259-2 R E V I E W A RT I C L E The Biosemiotic Concept of the Species Kalevi Kull 1 Received: 24 January 2016 / Accepted: 14 March 2016 / Published online: 22 March 2016 # Springer Science+Business Media Dordrecht 2016 Abstract Any biological species of biparental organisms necessarily includes, and is fundamentally dependent on, sign processes between individuals. In this case, the natural category of the species is based on family resemblances (in the Wittgensteinian sense), which is why a species is not a natural kind. We describe the mechanism that generates the family resemblance. An individual recognition window and biparental reproduction almost suffice as conditions to produce species naturally. This is due to assortativity of mating which is not based on certain individual traits, but on the difference between individuals. The biosemiotic model described here explains what holds a species together. It also implies that boundaries of a species are funda- mentally fuzzy, and that character displacement occurs in case of sympatry. Speciation is a special case of discretisation that is an inevitable result of any communication system in work. The biosemiotic mechanism provides the conditions and communica- tive restrictions for the origin and persistence of diversity in the realm of living (communicative and semiotic) systems. Keywords Assortative mating . Biparentality . Character displacement . Family resemblance . Recognition window . Species problem Introduction A semiotic approach in biology means the study of the organisms’ own approach, the study of distinctions that they make, what they recognise, what they intend, what they know, in a broad sense. Thus the biosemiotic concept of species has to explain how species arise and persist on the basis of organisms’ capacity of recognition and making distinctions. The species problem is not the question of defining the species, but the question of what the species actually is. * Kalevi Kull kalevi.kull@ut.ee 1 University of Tartu, Tartu, Estonia 62 K. Kull After two centuries of discussions, the problem of the species in biology with its diversity of incomplete solutions appears as an eternal inescapable jumble (see, e.g., Ghiselin 1997; Otte and Endler 1989; Pavlinov 2013; Stamos 2003; Wilkins 2009; Wilson 1999). However, this does not mean that there has been no progress in these discussions: from Ch. Darwin to E. Mayr to H. Paterson, to A. Templeton, M. Ghiselin, K. de Queiroz, we can certainly detect progress in understanding the mechanisms that create and delimit the diversity of species. Still, the earlier solutions are clearly unsatisfactory for the theoretical biologist who simultaneously is a naturalist in the wild. One of the characteristics of progress in species theory has been the growing number of conceptions or major definitions of species. Every contemporary treatise on the species problem must list several of these mutually incompatible definitions (e.g., Queiroz 2007: 880, table 1). The situation has been well characterised by Pigliucci (2003), who pointed out that different concepts of species form a category on the basis of family resemblance. Family resemblance means that there may not exist any trait that all elements of the category have in common, while each element has a trait in common with some other element of the category.1 Using this Wittgensteinian notion,2 Pigliucci even says that Wittgenstein has posthumously solved the species problem. Indeed, many different concepts of species as used in biological discourse are linked on the basis of family resemblance. However, one can notice that this is a common feature (not only of words and phonemes in common language, 3 but also) of many concepts used in the humanities — take, for instance, ‘culture’, ‘melody’, ‘language’, ‘sign’… Concepts based on family resemblance are not Aristotelian concepts which are based on formal definitions. In physicalist biology it was mostly possible to build theories exclusively on the basis of logically strict (therefore well mathematizable) universal categories, yet if we are going to apply a semiotic approach, the non-Aristotelian categories (i.e., for which the boundaries cannot be exactly defined) become unavoidable. Why is it so? In a recent book on the species problem Werner Kunz (2012: 60) observes that Bbiological species do not possess a single trait that is present in all members of the species. […] There is always some trait that individual organisms of a species lack, and in spite of this, these organisms still belong to the species. This is one of the arguments for the conclusion that the biological species as a class of organ- isms cannot be defined by essential traits and, therefore, cannot be a natural kind^. He adds: BA class like this is a polythetic class as opposed to a monothetic class, in which a single factor determines whether a particular organism belongs to this and no 1 That is, no single diagnostic trait can characterise all individuals of the category. Also, common origin is not obligatory for the category; thus, polyphyly is possible. 2 The concept of family resemblance itself was known long before Wittgenstein; for instance, it was defined precisely by Dugald Stewart already in 1818 (see a review in Mizak 2005). 3 Krzeszowski (1990: 215–216) writes: „natural categories, characterized by lack of clear-cut category boundaries (fuzziness, ability to stretch), […] permeate natural languages. […] Classical phonology was incapable of handling the well-known phenomenon of phonemic overlap, which involves situations in which many non-prototypical variants of some phonemes belong to a given category because of family resemblance […].B The Biosemiotic Concept of the Species 63 other class […]. Wittgenstein (1953) called this phenomenon Bfamily resemblance^^ (Kunz 2012: 61). Thus the important point here is that family resemblance is related not just to concepts of the species, but is a fundamental feature of the natural species themselves. We agree with Kunz on this — the model we present below describes the mecha- nism that produces categories characterized by family resemblance. However, further, Kunz finds the gene-flow conception (of de Queiroz) to be an acceptable general model of species. According to Kunz, species is a gene-flow community. This is acceptable, yet it does not explain why species exist, or what holds them together. Kunz asks ‘do species exist?’ and responds affirmatively. Why and how they exist still remains to be explained. In what follows, we argue that it will be possible to say more about the nature of species if we focus on inter-individual recognition. Describing a mechanism of species dynamics, we are going to demonstrate that what it reveals is a semiotic model of the species. The first axiom one should certainly abandon when studying the nature of the species is the Linnean idea that every organism belongs to some species. Only on this condition can we study what the species-creating mechanisms can be, and whether these concern certain features of certain groups of organisms themselves.4 A rejection of the Linnean ‘every-organism-has-a-species-name’ principle was proposed already in E. Mayr’s concept of the biological species, yet it was superficial as the common result was just adding the taxa of hybrids (between particular species) and the species of ‘non- biological-species’ (e.g., of uniparentals) that still allowed to continue with the logic of the Linnean axiom in only a slightly modified form. The Model In most cases in which we can observe the existence of clearly separate species in living nature, these organisms are biparental. Thus we hypothesise that mating is somehow responsible for the creation and preservation of species. This means that we can take mating not as a criterion for the species (as does Mayr’s conception of the biological species), but as a process that plays a crucial part in species creation. What holds a species together? This basic question in understanding species can be answered using the following model. On the level of population processes, not all individuals produce offspring. However, consider the difference between uniparental and biparental organisms (Figs. 1 and 2). If no selection is assumed, the distribution range of an uniparental population tends to increase due to stochastic changes in reproduction. The distribution range of a biparental population, however (despite the stochastic changes in reproduc- tion), will be dependent on a recognition window of individuals and will decrease if the range is initially larger than the recognition window. Biparental reproduction assumes that pair relations will be established on the basis of mutual recognition between individuals. The individuals in a sexually reproducing 4 At the same time, we still retain the assumption that every organism belongs to some genus, familia, and other upper taxa — because (differently from the species) we do not assume that these taxa have been produced and delimited by the organisms themselves. 64 K. Kull Fig. 1 Direction of population distribution change in case of uniparental organisms in the absence of selection. G — characteristic of character space axis; N — number of specimen with corresponding inheritable trait population are all slightly different from one another. The individuals that deviate too much from the rest have a lower probability to mate and reproduce because of the difficulties in finding a mate to match. This keeps the variability of the population within certain limits. The recognition between individuals is generally not determined by any particular inheritable trait, but depends on a large number of characteristics, both heritable and non- heritable. Since successful recognition depends on the availability of another sufficiently similar individual nearby, the differences in reproduction rates are, in general, not dependent on the particular alleles or the genotype that an organism carries. 5 As a consequence, the differences in reproduction rates between individuals are primarily dependent not on any individual characteristics, but on the availability of partners. To formulate this more radically — in case of biparentality, the reproduction rate is not a function of features of individuals, but of relations, of difference.6 Thus, biparental reproduction in a general sense always means assortative mating. (But this assortativity is not based on certain traits, but on the limits of the recognition window and difference.) Assortativity in mating is a filtering and purifying mechanism that leaves out the unrecognisable on any level of communication during a sexual act, including the intracellular processes during and after fertilisation. We can say that this is the main reason why sex reduces variability (Gorelick and Heng 2011). The process described here as the homogenisation effect or normalisation effect occurring in biparental populations is a very general effect in any communicative system. Communication makes the individuals more similar in the first place. In a simple, more formal version, the core model of the speciation mechanism can be described as follows (Figs. 2 and 3).7 5 This is equivalent to the absence of natural selection, the latter being defined as the differential reproduction of genotypes (see also Kull 2014). 6 Therefore this mechanism is not the one of stabilising selection. In case of stabilising selection, the lower fitness of extreme specimen is due to their particular genotypes, not due to their difference from the average. 7 Some aspects of this model were described in Kull 1988a, 1993; Lambert and Spencer 1995; Schult 1992; a computer simulation was presented in Kull 1988b. The Biosemiotic Concept of the Species 65 Fig. 2 Direction of population distribution change in case of biparental organisms in the absence of selection. Individual e which is closer to the edge of the distribution curve has fewer potential mates in the region of its recognition window we than individual f which is closer to centre and therefore has more potential mates in its recognition region wf. This difference shifts the distribution in the next generations, narrowing it until its amplitude is approximately equal to w Let there be individuals that reproduce biparentally. Let individual i be characterised by features Gi. In order for two individuals i and j to be able to cross (interbreed), their difference should not exceed a certain value, abs (Gi – Gj) ≤ w, where w is the width of the recognition window. Let the features of offspring Gk stay close to their parents, for instance, abs (Gk – 0.5 (Gi + Gj)) ≤ w. If in these conditions pairs are meeting randomly, mating is nevertheless assortative due to the limit on the difference between the individuals (w) for successful mating. As a result, the number of individuals close to the maximums of the distribution D (G) tends to increase, while the minimums decrease even more, due to the autocatalytic nature of reproduction. In other words, the distribution D (G) is unstable if it is represented by a considerable number of individuals of values of G in the range larger than w, except if these are separated by a hiatus wider than w. However, the distribution turns to be quasi-stable if the range of the individuals in the population does not exceed w (and the width of hiatuses between the close species is at least about w). In the latter case, all individuals within a species are compatible to any other as regards mating, which is the normal ideal situation for a panmictic species. Thus, the simple assortativity of mating leads to speciation, and makes a continuous distribution (i.e. one with all “intermediate forms” represented) unstable. 8 It self- regulates the range of variability. In case of increased variability, it may happen either by cutting off the extremes, or by making a hiatus in between. Since it is the range (and not the existence of particular genotypes) that is regulated by the recognition window, the genetic isolation between close species is not necessary for the stability of these species.9 This means that the existence of fertile hybrids (and inter-species gene flow 8 In many cases, however, we can observe a Bdouble assortativity^ (or multi-stage recognition), which means that the recognition of a mate has (at least) two stages — the first, which restricts the range of potential mates (a wider Bwindow^), and the second, where the more detailed comparison (fitting) of the (usually sexual) characteristics of the mates takes place (a narrower Bwindow^); this is obviously a factor that allows the coexistence of close species. 9 See also the critique of the isolationist concept of species in Paterson (1993). 66 K. Kull Fig. 3 Formation of species boundaries as a result of constraints in sexual communication (biparental reproduction without selection assumed). wa — the width of recognition window (shown for specimen a from the boundary region of species A). Two specimen can interbreed if their distance on axis G is less than w. Accordingly, a can interbreed with b and c, but not with d. Thus, species A and B become communicatively separated, except some edge specimen. Solid line presents a quasi-stable population density; dotted line presents a changing density; and the arrows show the direction of changes as a result of recognition processes via the hybrids) might not destroy the relative hiatus. As is well known, the regular low- frequency natural hybridisation between close species tends to be a rule (e.g., Grant and Grant 1997). Thus the boundaries between species can normally be fuzzy. The exis- tence of a boundary does not assume the absence of communication (including one on the genetical level). It is important to notice that the biosemiotic mechanism is relational. That is, it depends on the relation between individuals and almost not at all on the adaptation to the environment. In other terms, the existence of species is not an adaptational phenomenon. It is rather a communicational phenomenon, because it requires (a) biparental communication (mating), and (b) compatibility (recognition) between the individuals. Thus, speciation based on this mechanism can also be sympatric (as described, for instance, in Grant and Grant 1989). Empirical Evidence The communicative mechanism of species and its consequences become particularly visible in comparison with the possibilities of speciation in case of uniparentally reproducing organisms. In case of uniparental reproduction, the differences between the reproduction rates of individuals (besides being random) can depend on suitability of the environment available at the site. The organisms (of value G) that manage better can reproduce more, and if the overall number of individuals is limited, the result will be that those that fit the environment better will prevail and those who reproduce more slowly will disappear. In this case, the existence of species is related to the heterogeneity of the environment. Dobzhansky straightforwardly formulated the question: BWhy should there be species?^ Put clearly, the question was also given a very clear answer by him (Dobzhansky et al. 1977: 168): Bbecause there are many adaptive peaks^. This The Biosemiotic Concept of the Species 67 answer, in more or less clear variations, has been at the heart of understanding the problem of species in neodarwinism (and not only there) for a very long time. If the adaptive landscape, i.e., the heterogeneity of the environment, was the main cause of the existence of species, the diversity of species should represent a map of the environment. The extent to which it does so could be a measure of the role of this mechanism. This question certainly requires a more detailed analysis, yet it is evident that the regularity of differentiation into species that we can observe in almost all groups of eukaryots is far too strong in order to be predominantly caused by the environmental ‘landscape’, since the relief of the latter is largely quite irregular. While the environmental dependence of the reproduction rates of genotypes, i.e., natural selection (sensu stricto) can potentially be universal and relevant for all organisms, this does not imply that the major patterns of organisms’ diversity result from (are produced by) natural selection. In order to have natural selection (defined as the differential reproduction of genotypes), some particular genotypes should have an advantage in reproduction. In case of mating, however, what primarily counts is the relationship between the (genotypes of) mating individuals, their relative distance, not the particular genotypes themselves. Thus, in case of a biparental group, there may be no particular genotype that has an advantage in reproduction; instead, a species is a collective phenomenon in which it is the pairwise compatibility between individuals that counts. Empirical evidence can illustrate the role of this mechanism. If biparental recogni- tion is responsible for the existence of species, it can be predicted that in case of secondary disappearance of biparental reproduction, its replacing by uniparental repro- duction should lead to the dissolving of the species. Indeed, there are some natural experiments in which biparentality has been lost. This is the case, for instance, in the plant genera Hieracium, Alchemilla, and, partly, Rubus, whose seeds can be formed apomictically. For taxonomists, these happen to be Bhard taxa^, as the variability in these groups is quite irregular, which is described with the help of a large number of microspecies. In other words, loss of biparentality indeed does dissolve species. Furthermore, as the analysis of a large dataset has demonstrated, Bcontemporary hybridization among species of the same ploidal level failed to cause taxonomic problems, despite its frequent mention as the primary cause of ‘fuzzy’ species-boundaries in plants^ (Rieseberg et al. 2006: 525) — precisely as our biosemiotic model predicts. Character Displacement as a Communicative Boundary Effect A well-known phenomenon in the boundary region of close species is character displacement. The concept seems to have been first introduced by Brown and Wilson (1956), who distinguished it from the divergence of character as a result of competition as described by Darwin.10 Character displacement is an effect that appears in case of partially overlapping distribution areas of close species. Character displacement that characterises biparental species can be caused by interaction in reproduction or 10 Thus it is not the same as the Wallace effect, either. See Pfennig and Pfennig (2010); they mention that E. Mayr did not make a clear distinction between character displacement and character divergence. 68 K. Kull reproductive interference (Konuma and Chiba 2007) — that is, communicative incom- patibility. This process may include partner selection and habitat selection. If character displacement is caused by a communicative mechanism, this means that an analogous effect could appear in many communicative systems, including language, and could be found in phonology and geographical lexicology. And this, indeed, is the case (Blevins and Wedel 2009); these fields of specialty even use the same term (character displacement, Merkmalverschiebung). Character displacement demonstrates well the relationality of species — species are linked (and move or evolve together, as if entangled) in the character space. In an analogous way, just like adding a new phoneme to a dialect would shift the other (close) phonemes, shifts in interaction of the species, too, are a result of communicative processes in the first place. For instance, it has been observed how bird song can change. One of the Galápagos islands was inhabited by two species of Geospiza — G. fortis and G. scandens. Then, a third species (G. magnirostris) with a similar song was introduced to the island. As a result of the invasion, the song of the other two species changed towards being more distinctive in the next generations (Grant and Grant 2010). Some Further Corollaries of the Model The discretisation produced by biparentality (or recognition-based reproduction) does not concern only discretisation in the character space (i.e., speciation). Analogically, it may discreticise spatial distribution, modelled as a result of the Allee effect (Gyllenberg et al. 1999). Another consequence can be temporal discretisation, seen as synchronisa- tion of reproduction events in a population even in case of non-seasonal environments (Tammaru 1993b). An additional generalisation of the model points to obligatory symbiosis as a form of biparentality. This would mean that even in the case of uniparentally reproducing organisms whose life cycles use an obligatory symbiotic connection with other groups of organisms, the mutual recognition between them assumes a recognition window, which may result in a self-delimitation of variability, i.e., in a species. We will call this type of species a co-species. Reciprocal recognition between the species in case of symbiosis can also create a higher-level delimitation of variability that simultaneously covers a group of species; the category of this type can be named compound species (Kull 1993: 136–137). A Note on the History of the Biosemiotic Conception of Species The biosemiotic conception of species largely stems from the work of Hugh Paterson (Paterson 1993). His recognition concept of species is basically communicational and semiotic. For instance, this is what he says about recognition (window): “The response of one mating partner to a signal from the other is here regarded as an act of recognition […]. Recognition is thus a specific response by one partner to a specific signal from the other. This process might be compared to the recognition of a specific antigen by its specific antibody, and hybridization involves a process having something in common The Biosemiotic Concept of the Species 69 with a cross-reaction” (Paterson 1993: 146). Paterson (ibidem, 144) further writes: “This subset of adaptations, which are involved in signaling between mating partners or their cells, constitutes the specific-mate recognition system or SMRS”. However, it is possible to trace the main ideas of the biosemiotic understanding of species (at least in some of its important aspects) even further back. One of the first biologists who understood the fundamental role of mutual recogni- tion between individuals (and described its mechanisms) in the formation of species was the entomologist Wilhelm Petersen (Petersen 1903; 1905a, b; see also Tammaru 1993a).11 In particular, it was only with Petersen’s work that the morphological traits of genitalia became a major characteristic for distinguishing close species in entomology. As mentioned by J. Mallet (2013), W. Petersen was one of the first to argue, Bin contrast to Darwin and Wallace, that species should be defined physiologically, i.e. by means of their reproductive isolation^. Some views on species that can be interpreted as protobiosemiotic were expressed by George Romanes and Samuel Butler. Romanes stated that a difference in itself is a cause for speciation (concerning the rarity of intermediate forms); that species differ by non-adaptational traits. In this respect, we could point at the distinction between two major traditions in the description of species boundaries as seen by Krementsov (1994:39). According to the tradition of Moritz Wagner, the primary factor in speciation is geographical segregation, while sexual isolation is secondary. The tradition of George Romanes describes sexual isolation and physiological differentiation as primary factors in speciation that do not necessarily require geographical isolation.12 Conclusions The factor that divides potential partners into compatible and incompatible ones can be called a recognition window. As an inevitable characteristic in biparental reproduction, it creates assortativity, which leads to splitting of diverse sets into species. Speciation can be seen as a special case of a general discretisation or categorisation effect that is an inevitable result of any communication system. The resulting categories cannot be defined formally as they are based on family resemblance. 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