The concept of species and practical taxonomy: what is in-between? The concept of species and practical taxonomy: what is in-between? Rimantas Rakauskas The ecological approach considers the biological species as an ecological unit. This does not exclude recognition of biological species also as a Vilnius University, genetic and morphological (sensu lato) unit. The article deals with the M. K. Èiurlionio 21/27, theoretical and practical aspects of such a treatment of the concept of LT-2009 Vilnius, Lithuania. biological species on the examples of aphid (Hemiptera, Sternorrhyncha: E-mail: [email protected] Aphididae) taxonomy. Key words: species concepts, taxonomy, aphids INTRODUCTION performed by the species in nature. The following features seem to be characteristic of the biological Since the species are the units of evolution, and since species therefore. they are the base units of biological systematics, a 1. A species is an ecological unit. A species is a proper understanding of species is crucial for the suc- system of related populations that are similarly adap- cess of evolutionary biology and biological systema- ted (Shaposhnikov, 1981; Blackman, 1995). All popu- tics (Ereshefski, 1992). Such or similar statements lations of the species have specific adaptations for sur- seem to be generally accepted among biologists. That viving in the ecological niche of that species, and every is not the case when considering what the species re- population may have populational adaptations for sur- ally is. Numerous concepts and definitions of species viving in its particular ecological subniche. Therefore are available, and none of them is widely accepted the species is to be treated as an ecological unit. Spe- (Blackman, 1995; Grant, 1963; Klimaszewski et al., cies-specific adaptations are responsible for the speci- 1995; Mayr 1963; 1976; 1982; Matile et al., 1993; fic role performed by a particular species in nature. Çàâàäñêèé, 1968). Several papers appeared, propaga- 2. A species is a genetic unit. Species-specific adap- ting the understanding of biological species as an eco- tations must be fixed and inherited genetically. Thus, logical unit, particularly addressing the aphid (Hemi- ecological specificity of the species is underlain by the ptera, Sternorrhyncha: Aphididae) taxonomy (Shaposh- genetic specificity. Forces responsible for the specifi- nikov, 1981; 1987; Rakauskas, 1998). The aim of the city and unity of the gene pool of the species are present paper is to discuss such understanding of spe- interpopulational gene flows (Dobzhansky, 1970; Ma- cies from the point of view of practical taxonomic yr, 1982), specific mate recognition systems (Paterson, procedures, based on the examples from aphid biosys- 1993), common selective regimes (Ehrlich et al., 1992). tematic studies. Selection might be treated as the most general force of maintaining or disturbing the unity of the gene pool of BIOLOGICAL SPECIES AS AN ECOLOGICAL UNIT a particular species. 3. A species is a morphological (sensu lato) unit. Ecological methodology in the species-level taxonomy Species-specific adaptations are provided due to spe- is based on the following ideas (Rakauskas, 1997; cies-specific morphological (sensu stricto), anatomical, 1998; 2001). When describing the species as a general physiological, cytogenetic, biochemical, immunologi- biological phenomenon, a more general biological ap- cal, etc. features, the inheritance of which is underlain proach is needed. It is often quoted that in biology by the specificity of the gene pool of the species. Spe- nothing makes sense except in the light of evolution cies-specific adaptations need species-specific charac- (Dobzhansky, 1970: 6). On the other hand, biological ters. An important practical conclusion that follows evolution is the process of adaptation of living things from understanding this fact is that real species can be (Dobzhansky, 1970; Grant, 1963; Ridley, 1993; recognized by their morphological characters. The so- Ñåâåðöîâ, 1987). Following this, one may consider called morphological species of early taxonomists are that the main characteristic of the species is the role still valid in many cases therefore. ISSN 02357224. E k o l o g i j a (Vilnius). 2003. Nr. 1 3 Rimantas Rakauskas 4. Treating species as a system of populations is ques (e. g., allozymes or DNA analysis) seem to be not useful for understanding the evolutionary processes. necessary. Cytogenetic and molecular methods are ne- The idea of the complex nature of a species consisting eded when ecological and morphological ones are in- of subspecific units having a potential to diverge (or, sufficient on their own. Anyway, purely morphologi- in other expression, forego its own evolutionary (ten- cal, cytogenetic or molecular descriptions of aphid spe- dencies) was present in Darwins theory, and it is still cies, without any ecological data on new taxa, should actual in population-genetic modern synthesis (Âàâè- be avoided. Information on the life cycles and host ëîâ, 1931; Dobzhansky, 1970; Mayr, 1982). There is specificity is necessary in all cases. Let us see how this no need to multiply species numbers by giving specific methodology works in practice. names to every smallest diagnosable cluster of indi- vidual organisms (Cracraft, 1992). It is population EXAMPLES FROM APHID TAXONOMY but not the species that is an elementary unit of evo- lution, the smallest group of individuals having a po- Aphid species of the genus Aphis L. living on currants tential of independent evolution. and gooseberries (Ribes spp.) in the Palaearctics If any name for the above-mentioned understan- ding of species is needed, such a concept of species is Four species of the genus Aphis are reported as inha- to be called biological species, because understanding biting currants and gooseberries (Ribes spp.) in the species as a coadapted gene pool inhabiting a certain Palaearctics: A. grossulariae Kaltenbach, 1843, A. adaptive peak of adaptive landscape was already schneideri (Börner, 1940), A. triglochinis Theobald, 1926 present in the concept of biological species (Dob- and A. popovi Mordvilko, 1932. Numerous data con- zhansky, 1970: 26). cerning host specificity, life cycles and morphological Having recognized the above-mentioned features of features are available for the first three European spe- the biological species, what are the consequences for cies (for a wider review, see Rakauskas, 1998a). The defining the species in taxonomy? When understanding fourth one is a Siberian species, closely related morpho- the biological evolution (including speciation) as the logically to the European A. schneideri; its entire life history of adaptation of living things one might con- cycle is not known (Rakauskas, 1996). When treating sider that the main feature of species is the role per- a species as an ecological unit, one can justify the formed by it in nature. That is, the main characteristic existence of three good species inhabiting currants of the species is its ecological niche. When solving the and gooseberries in Europe: the first is holocyclic mo- problem, is it one or two species, it is necessary to noecious on Ribes spp. (A. schneideri), the second is estimate the ecological specificity of the organisms holocyclic facultatively heteroecious between Ribes spp. studied. and Onagraceae (A. grossulariae), and the third one Generally, all structural, physiological, biochemi- obligatorily alternates between Ribes spp. and various cal, etc. characters of living things serve one purpose Brassicaceae, Boraginaceae, Asteraceae hosts (A. triglo- successful existence in certain habitats. Morphology, chinis). Nevertheless, the ecological niche of aphid spe- anatomy, physiology, cytogenetics, biochemistry, etc. cies is not easy to define precisely, its boundaries re- can provide valuable information when deciding whe- main uncertain. E. g., monoecious holocyclic clones of ther the organisms under analysis are adapted to simi- A. grossulariae have been documented (Ãóñûíèíà, lar ecological niches (they belong to the same species 1963), making this species similar to A. schneideri. Re- in such case) or to different niches (the case of diffe- suming the available information on host specificity, rent species). Excellent understanding of the charac- life cycles, phenology and parasitoid-predatory species ters studied and successful intuition may result in a composition, only A. triglochinis seems to be a good proper description of the species based on one or few species, whilst A. grossulariae and A. schneideri share criteria only. Morphospecies of early taxonomists are many similar features (Rakauskas, 2000). Furthermo- still valid in many cases therefore. Nevertheless, the re, successful interspecific crossing experiments were probability of mistakes in such cases is much greater performed on these species, raising the question of the than when exploiting more criteria. possibility of natural hybridisation (Rakauskas, 1999). Concerning aphids, information on the life cycle Intermediate hybrid morphotypes appeared to be not and host specificity is supposed to be absolutely neces- so rare in natural conditions (Rakauskas, 2002). Ad- sary for understanding their ecological specificity. Mor- ditional ecological and morphological studies are ne- phological analysis is necessary for a proper diagnosis eded to clarify the taxonomic status of A. popovi it of aphid species and constructing practical identifica- might be a sibling vicariant species to