A Step Forward: Evolutionary Revolution

Historically (and maybe conceptually), the primary basis of the world picture of mankind is fixism, asserting the immutability of forms after their appearance (creation, etc.). Such is, for example, the biblical concept of the Divine creation, after which the System of Nature remains unchangeable. This natural philosophy forms an ontic core of those taxonomic theories (primarily scholasticism, stationary typology, etc.), in which “genera” and “species” are constant and unchanging universals or essences.

The fixist standpoint is opposed by the idea of development, or transformism, which means transformation (transmutation) of some forms into others. Transformism has two main versions, atemporal and temporal [Grushin 1961]. According to the first version, development does not imply its actual duration in time: this is an “ideal” transformation exemplified by “evolution” of the prototype giving rise to the Ladder of Perfection, metamorphoses of Goethe’s archetype, etc. According to the second version, development is a material process that has both temporal extension and sequence, and within its route some forms actually turn into others; transformations of parts of an organism during ontogenesis provide a clear example. This second version is part of the materialistic natural philosophy: it is centered around the idea of the self-developing Nature, with cosmogonic conceptions of Descartes, Leibniz, and Kant playing a key role in its formation in the epoch under consideration [Gaydenko 1987; Hoque 2008]. On this basis, Kant (with reference to Buffon) differentiates between the fixist “descriptions of nature” (Naturbeschreibung) and the transformist “history of nature” (Naturgeschichte).

In the 19th century, the notion of “evolution” begins to be used to denote real historical transformations, thus emphasizing a certain analogy between historical and individual development [Chambers I860]; this is what Haeckel means and emphasizes by coining the new terms "phylogeny” and “ontogeny” [Haeckel 1866]. In the middle of the 19th century, in the works of philosopher Herbert Spencer, an iconic notion of the “theory of evolution” appears and then becomes widely acknowledged [Bowler 1975; Richards 1992]. In biology, the general idea of transformism is embodied in a more particular idea of the origin of some species from others, so the transformist (evolutionary) concept, as applied to living matter, is often called the theory’ of origin [Haeckel 1866; Cope 1887].

The first historically interpreted transformist ideas relevant to systematics appear at the beginning of the 19th century based on different natural philosophies: J.-B. Lamarck thus interprets the Ladder of Nature, while Gottfried Treviranus and Friedrich Tiedemann infer an evolutionary worldview from organismism [Raikov 1969; Richards 1992]. Their adoption leads to the “historicization” of systematics, making it evolutionary interpreted [Lenoir 1988; O’Hara 1988; Pavlinov 2009a, 2018; Pavlinov and Lyubarsky 2011]. This gives a significantly new understanding of the Natural System, in which the unity of organisms is based on the unity of their historical (evolutionary) origin and blood (genealogical) kinship relations. This idea forces taxonomists to reconsider the major old problems of systematics in a new way: how to define natural groups of organisms, whether they are real or nominal, how they can be recognized, etc. The main illustrative representation of the Natural System in its historical understanding becomes the genealogical tree, which refers to the timing of the origins of the respective groups, so it is interpreted as the tree of history’ [Barsanti 1992; O'Hara 1992, 1996]. All this makes up the content of the next scientific revolution in systematics—the evolutionary revolution, which begins in the 1860s.

First Ideas

Lamarck appears the first taxonomist to integrate his evolutionary concept directly into the respective taxonomic theory and classification [Lamarck 1809]; his natural method becomes the first to be evolutionarily interpreted, and it can be designated as classification Lamarckism [Pavlinov 2009a, 2018; Pavlinov and Lyubarskiy 2011]. Its main ideas are briefly as follows: the historical development of organisms is represented as a continuous (without breaks) time-extended chain of live beings; any gaps separating its fragments are due to extinct or unknown intermediate forms; the taxa corresponding to these fragments are arbitrary, and are introduced as discrete classification units to indicate the main stages of the general unidirectional trend of evolution from lower to higher organisms; this trend determines an arrangement of taxa in the classification according to the above-mentioned rule of progression. As a “ladderist,” Lamarck clearly distinguishes between the natural order (or arrangement), which reflects the evolutionarily interpreted Ladder of Nature, and artificial classifications developed according to “systemic” principles.

Early attempts to master the evolutionary idea by systematics during the first half of the 19th century have no success due to their probable prematurity because of the domination of the theories of natural systematics (narrowly understood) and typology. In a full-fledged manner, evolutionary (in the contemporary sense) natural philosophy penetrates systematics in the second half of the 19th century thanks mainly to the works of Charles Darwin and Ernest Haeckel. They both are “systemists,” so one of their tasks is to justify evolutionarily the tree-like representation of the Natural System [Winsor 2009]; they are unanimous about a fundamentally new understanding of the Natural System as genealogical. These two persons become the authors of two interrelated, yet different, evolutionary concepts, which lay the foundations for two respective research programs in systematics. Darwin is mainly interested in the origin of species, so his theory refers (in modern terms) to microsystematics; Haeckel develops a theory of phylogeny of the animal kingdom relating to macrosystematics.

Charles Darwin’s contribution to the development of the evolutionary interpreted systematics is fundamentally important: as a matter of fact, he appears the first to express clearly the general idea that the Natural System is the genealogical one. His taxonomic conception, which is sometimes called Darwin’s “central discovery,” at least as far as systematics is concerned [Reif 2006, 2007], can be outlined as follow's. According to him, “propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications” [Darwin 1859: 413-414]. Therefore, “the arrangement of the groups within each class, in due subordination and relation to the other groups, must be strictly genealogical in order to be natural” [Darwin 1859: 302], the latter statement can be formalized as the principle of genealogical arrangement. With this, Darwin provides a very important justification for the selection of characters for the reconstruction of genealogies, rejecting their essentialist interpretation. He emphasizes that “the characters [...] showing true affinity between any two or more species, are those which have been inherited from a common parent” [Darwin 1859: 420], therefore, “the less any part of the organization is concerned with special habits, the more important it becomes for classification”

[Darwin 1859: 414]; this statement is referred to as Darwin’s principle [Mayr 1965]. This principle in its expanded form includes several important evolutionary criteria for assessing taxonomic significance (weighting) of the characters. One of them goes back to the ideas of Jussieu and Cuvier about character ranking based on their constancy; it can be designated as the criterion of commonality; the second resembles partly Adanson’s method based on the principle of congruence; the third repeats in many respects the principles of parallelism of Oken, although in Darwin’s interpretation it is the principle of evolutionary’ parallelism; finally, according to Darwin, “amount or value of the differences between organic beings all related to each other in the same degree in blood” [Darwin 1859: 421] should not be taken into consideration if organisms are to be grouped according to their genealogical relationships; this is the principle of irrelevance of differences. Most of these principles (except parallelism) constitute the foundation of the modern cladistics, so the latter’s adherents are better to call themselves Darwinists rather than Hennigians; the principle of parallelism is especially significant in the evolutionary taxonomy of Simpson (on these schools of taxonomic thought, see Section 5.7).

An important part of Darwin’s natural method is a denial of the particular importance of the species category; the latter should be emphasized—not species as a natural unit (as is often misbelieved), but just a certain fixed category [Komarov 1940; Zavadsky 1968; Stamos 1996, 2007a; Ereshefsky 2011; Pavlinov 2013a], This position is substantiated by reference to the continuity of the microevolutionary process, which is a gradual transformation of local races into different species, so a distinction between categories of race and species is not qualitative but quantitative. Indeed, based on his observations supporting this evolutionary model, Darwin becomes convinced how “entirely vague and arbitrary is the distinction between species and varieties” [Darwin 1859: 48]. So he iterates that “species [are] only strongly-marked and well-defined varieties” [Darwin 1859: 55], while “a well-marked variety may be justly called an incipient species” [Darwin 1859: 52]. All this urges him to call against “a vain search for the undiscovered and undiscoverable essence of the term species” [Darwin 1859: 485]. Referring to this thesis, supporters of the extraordinary importance of species in both evolution and systematics accuse Darwin of “eliminating the species as a concrete natural unit” [Mayr 1963: 14], although this is hardly true. At any rate, it is this view of the gradual transition between species and infraspecies units that constitutes an ideological core of what was called above classification Darwinism.

The background of Ernst Haeckel’s consideration of biological evolution, as noted above, is clearly natural-philosophical: in one of his lectures he refers to Kant as the person who was the first to give mankind a comprehensive transformist doctrine [Haeckel 1868]. For Haeckel, the history of living matter, i.e., phylogeny, is the development of a “genealogical individual,” by its fundamental properties being similar to the development of particular individuals, i.e., ontogeny [Haeckel 1868]. These general ideas go back to the natural philosophy of organismism of the beginning of the 19th century, in particular, to the transformist ideas of Treviranus and Tiedemann [Richards 1992; Rieppel 2016]. In this regard, it is pertinent to note that Haeckel is a great admirer of Goethe: quotations from him are placed as epigraphs to all chapters of his fundamental “General Morphology” [Haeckel 1866].

The macroevolutionary character of Haeckel’s transformism is clearly seen from his genealogies. In contrast to the well-known Darwinian schemes, in which races and species are represented, Haeckel’s scale of consideration of the history of organisms is set mainly by the level of classes and types. With this, Haeckel understands phylogeny as genealogical history reconstructed on the basis of the study of fossils: for him, “phylogeny includes paleontology and genealogy” [Haeckel 1868].

Haeckel in all his works follows Darwin in insisting that “the tree-like form of the Natural System can only become understandable when we acknowledge it as a real genealogical tree of organisms.” However, generally speaking, he is more a phylogenetic morphologist rather than a systematician; at the center of his attention are the problems and law's of historical changes in animal body plans, which determine the groups of different levels of generality. Accordingly, he designates his approach as systematic phylogeny, as is evident from the title of his three-volume book thus entitled [Haeckel 1894-1896]; it will only turn out to be phylogenetic systematics in the works of his adherents. Studying this systematic phylogeny, he simply “applies” genealogical interpretations to ready-made classifications, mainly borrowed from Cuvier and Baer, turning them into phylogenetic schemes [Remane 1956; Williams and Ebach 2008; Rieppel 2016].

For Haeckel, the ascending genealogical tree, with its main trunk and lateral branches, is a standard representation of phylogeny as a “genealogical history.” The predominating orientation of the trunk, as in the Ladder of Nature, is based on Aristotle’s principle of perfection, from lower organisms at its base to higher ones at its top; each of its branches is “an aggregate of all those organisms, the common origin of w'hich from one ancestor we cannot doubt” [Haeckel 1868]. Haeckel calls such an “aggregate” monophyletic and refers to it as phylon; if a group combines descendants of different ancestral forms, it is polyphyletic. The main task of systematic phylogeny is to identify monophyletic groups of organisms; together they constitute the Natural System, while polyphyletic groups cannot be considered elements of the Natural System. This general provision is known as the principle of monophyly; it is implemented by the principle of phylo-taxonomic correspondence, according to which a certain (in this case, non-strict) isomorphism should be established between a genealogical tree and a classification reflecting it.

Another phylogenetic concept is developed in the same years by the zoologist paleontologist Edward Cope: unlike Haeckel’s, it is based on an assumption of unidirectional parallel macroevolution, in which dominate certain general trends (scales) of transformations of organisms not tied by close genealogical relationship [Cope 1887]. Each trend is manifested in many parallel phyletic lines: they originate from different “roots,” and in all of them evolutionary transformations obey the same trend in both the general direction and successive passing through the same phases. So, not a tree but a phylogenetic “lawn” serves as an adequate representation of this phylogenetic model. According to Cope, the natural system is a hierarchy of polyphyletic groups, in w'hich each “class is a scale of orders, the order of tribes [...] the family composed of one or more scales of genera” [Cope 1887: 45]. Each of these scales forms a homologous series; each successive phase passed by organisms in parallel scales forms a heterologous series (both terms borrowed from organic chemistry).

The groups encompassing these scales are distinguished by characters that are ranked into species, generic, familial, etc., though the criteria for their ranking are not specified. Characters of different ranks are mutually independent in their evolution: “the process of development of specific and generic characters does not proceed pari passu [...] therefore, species may be transferred from one genus to another without losing their specific characters, and genera from order to order without losing their generic characters” [Cope 1887: 123]. The last passage paradoxically repeats the idea of Theophrastus that plants can “pass” from one species to another because of their becoming modified under varying environmental conditions (see Section 2.2.2).

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