Introduction to a Research Program in Evolutionary Systematics
“If a discipline belongs to modern biology, depends not on the time that has passed since its
foundation, but whether it is theoretically and methodologically in keeping with the times.”
(Rolf Löther, 1972)
Definition
The term “Evolutionary Systematics” used here and which is also included in the title [of this introduction], was borrowed from Ernst Mayr (1982: 559 ff; German 1984: 449 ff) who used it in a survey on the history of biology, to the best of my knowledge, for the first time. The term has not been used since except in a brief sketch of a corresponding research program in Glaubrecht (2007: 84–99). Mayr entitled a chapter “Advances in Evolutionary Systematics” which – admittedly from the perspective of someone personally involved – describes the beginnings of the elucidation of mechanisms that contribute to the evolution of organismic diversity and its horizontal (i.e. geographical) component. The central aspect for him was the integration of concepts and methods from systematics with population genetics, at that time a relatively new branch of biological science, against the background of a newly developing modern synthetic theory of evolution. However, Mayr has not explained the term Evolutionary Systematics in detail or even attempted a formal definition; this is done here instead as a rationale for a respective research program.
Evolutionary Systematics is the study of organismic diversity on the basis of phylogenetic systematics, especially taking into account taxonomic diversity, morphological disparity as well as genetic variability and including the study of underlying causes of evolutionary change and the origin of new species and species groups.
Evolutionary Systematics is therefore a branch of biology that does not deal with taxonomy (i.e. collecting, describing, naming and classifying) nor with phylogenetics or cladistics alone, but also with the underlying evolutionary processes. The main focus in this context is on microsystematic studies investigating biodiversity in general, and more specifically on the taxonomy and systematics on the level of species and groups of species, supplemented by studies on the geographical distribution [of species] and its causes (especially historical biogeography and phylogeography) as well as on causal mechanisms, i.e. on the causes for speciation (cladogenesis) and the transformation of species (anagenesis). Evolutionary Systematics therefore deals with uncovering what may be called the six “Darwinian mysteries”. Emphasis,however, is on questions relating to species numbers, species concept and speciation (vgl. Glaubrecht 2004, 2009, 2011).
Starting from the conviction that phylogenesis is a sequence of processes acting exclusively on species (Hennig 1957: 62), Evolutionary Systematics includes those research fields that are generally subsumed under the term micoevolution. It should be noted here that cladogenetic and anagenetic factors acting during the formation and transformation of species are to be regarded as sufficient for the explanation of the formation of new, character-rich and complex structures, i.e. also for the formation of new body plans of larger taxonomic units as for example families, orders or phyla because a so called macroevolution with its own, intrinsic factors could not be convincingly demonstrated yet.
On no account, as it may seem at first glance, the word construction Evolutionary Systematics ought to be regarded as a pleonasm, i.e. a superfluous double determination or verbal redundancy (as for example in “cold ice” or according to common opinion in [the German] “weißer Schimmel”; and even if, then only in the sense of an epithetical rhetorical figure, in which the pleonasm serves as a means of intensification, clarification or particular accentuation). Systematics is, as evidenced by the 250-year old history of the discipline, neither implicitly nor explicitly bound to the awareness of or even consideration of evolution, albeit certain implications like an actually “natural system” or the existence of a unique phylogeny that reflects the true genealogy and a resulting translation into a classification are non-coercively explained by evolutionary processes.
Starting from the basal units of evolutionary change, i.e. from species as natural entities (see below), Evolutionary Systematics is a branch of science that deals with the processes of speciation and radiation, the relationships of species (phylogeny) and their occurrence and distribution (biogeography). Aside from genealogical relationships among species (their temporal component) which are principally determined by phylogenetic research, geographical relationships (spatial component) play an important role, which are specifically determined by biogeographical research. Ultimately, Evolutionary Systematics attempts a systematization along with causal explanations for diversity that uses phylogenetic hypotheses for the reconstruction of sequential evolutionary processes and biogeography.
Studies on different research questions need to be based on sound nomenclatorial and taxonomical conclusions, i.e. on the adequately determined species composition of the species group in question in relation to the hierarchical level. An example shall illustrate this differential approach: Insofar as a question in Evolutionary Systematics – be it out of phylogenetic or biogeographical interest – is concerned with a species group occurring on both sides of an Ocean and currently treated taxonomically as two distinct families, the sampling of the most important constituent species as representatives may seem sufficient. Even though a complete taxonomical sampling of all constituent species or all actually existing species that remain after synonymization or subsuming of all named morphospecies within a systematization on the basis of a comparison of type material may also be desirable in this case, but may in the end be dispensable for answering questions on trans-Oceanic evolution in both species groups. In another case, for example in studies on speciation and radiation of a group of closely related species, a complete taxonomic sampling is an indispensable prerequisite. How else could the mechanisms of speciation (in allopatry or in sympatry, e.g. as a consequence of ecological specialization) be studied and properly understood, if not all involved taxa and resulting members of a species flock were to be considered. A plethora of examples could easily be thought of, in which the necessary amount of taxonomical-systematical research needs to be properly assessed in a differential way, in order to meaningfully contribute to the understanding of the underlying evolutionary processes.
With a research program relating primarily to questions on the origin of particular species and species groups, Evolutionary Systematics aims beyond traditional systematics with its indispensable sub-disciplines nomenclature and taxonomy as well as beyond phylogenetics as genealogical research on relationships, by not only asking questions on underlying patterns, but questions on the processes responsible behind these patterns, i.e. on questions in relation to the underlying evolutionary processes that can be investigated on the basis of an organismic approach. The uncovering of systematic relationships of a group of species of interest is always the first step, i.e. the use of taxonomical methods right through phylogenetic analyses on the basis of morphological characters and /or, ideally, combined molecular analyses. However, the attention of an Evolutionary Systematician readily shifts towards answering question in relation to evolutionary ecology and biogeography. Particular species have not only to be recognized, named, described and evaluated on the basis of their phylogenetic relationships, but their individual natural histories should be reconstructed as well: What are the genealogical relationships of the species of interest, where do they come from, why are they there, where are they now, and how did they get there; which evolutionary processes – from speciation factors to the role of morphological key features – have contributed to their evolution? Focused on individual species (groups), systematics, evolutionary biology and historical biogeography thus find their practical synergistic application and contribute to multidirectional elucidations.
For this reason, Evolutionary Systematics declaredly exceeds the scope of systematics as the discipline of studying all living and extinct species alone with the aim of unraveling the relationships between them. Sometimes a distinction was made between traditional systematics that mainly deals with the determination and naming of species (taxonomy) and modern systematics that also tries to reconstruct the phylogeny of organisms, see, e.g., Glaubrecht (2007) and references therein for a history, the definition and delimitation of biosystematics. The new quality of an explicitly extended synthesis of Evolutionary Systematics is the incorporation of research on evolutionary processes that ultimately lead to the diversity and disparity of organisms. By prepending evolutionary to systematics the more complex issue of consciously integrating evolutionary research into systematics is emphasized. It should also be noted that more than a mere systematization is pursued. The definition of Evolutionary Systematics as a discipline thus allows for a clearer correlation of the approaches of systematics and evolutionary biology briefly sketched here.
Analogously, this has been done much earlier for example in ecology, which – comparatively early introduced (but having only slowly gained acceptance) as a term to scientific literature by Ernst Haeckel (1866) – was conceptually extended progressively by adding classificatory subsystems from autecology to synecology and later on to demecology and finally to landscape ecology and ecosystem research. Thus, modern ecology considers individuals, populations and species, biocenoses and landscapes (see e.g., Nyhart 2009: 293–322; Schurig 2009). While in the case of the subsystems in the ecological discipline the research perspective was expanded through subsequently considering broader taxonomical groups, the research focus broadens in Evolutionary Systematics not by considering different taxa depending on the ranking in the hierarchical-encaptic system of the animal kingdom (as is implied by the erroneous and unsustainable distinction between micro- and macroevolution). Rather the focus is explicitly set on evolutionary biological research on the basis of – zoological in this case – systematics. Evolutionary Systematics therefore aims at an integration of Systematics, phylogenetic research and evolutionary biology.
Since taxonomists and systematicians up till now often have woefully ignored evolutionary biological contexts relevant to their work, emphasis on an explicit research program in Evolutionary Systematics seems more than justified and necessary to me. Ernst Mayr has repeatedly complained that, not least because of the fundamental importance of species [in biology], the biology of species or the “Science of Species” is not regarded as a field of knowledge in its own right like, for example, Cell Biology, Histology, Anatomy, etc. (see Sudhaus 2005: 145). After all, the “problem of evolution will only be understood by studying individual species, their origin and their extinction” (Mayr 1984: 323). However, it seems to me that a well-founded and justified science of Evolutionary Systematics is more important today than systematics focused on individual species.
- Evolutionssystematik limnischer Gastropoden (Habilitation treatise Matthias Glaubrecht 2010) [PDF]
Recently, the new concept of evolutionary systematics was explicitly applied also to other zoological taxa, e.g. freshwater crustaceans from Australia.
- Evolutionary systematics of the Australian Cyzicidae (Crustacea, Branchiopoda, Spinicaudata) with the description of a new genus (Schwentner, M., Just, F. & Richter, S. 2015. Zoological Journal of the Linnean Society, 173: 173: 271–295.) [PDF]
Excerpt (p. 272: “In the present study, we focus on the Australian representatives of the … Cyzicidae. ... The goal is to assess the diversity of the Australia Cyzicidae, their phylogenetic systematics, and their phylogeographical history within an Evolutionary Systematics framework (Glaubrecht, 2007, 2010). Therein, taxonomic diversity, disparity, and genetic variability, as well as the underlying evolutionary causes of speciation, are studied on the basis of phylogenetic systematics".