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Faculty Publications from the Harold W. Manter Laboratory of Parasitology Parasitology, Harold W. Manter Laboratory of

1989

Tribune: and : Concepts and Natural Groupings

Armand R. Maggenti University of California - Davis

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Maggenti, Armand R., "Tribune: Genus and Family: Concepts and Natural Groupings" (1989). Faculty Publications from the Harold W. Manter Laboratory of Parasitology. 105. https://digitalcommons.unl.edu/parasitologyfacpubs/105

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GENUS AND FAMILY : CONCEPTS AND NATURAL GROUPINGS

Armand R. MAGGENTI University of California Davis, Department of Nenzatology, Davis, CA 95616, USA.

A little over two hundred and fiftyyears ago Linnaeus to take the step beyond seeing whatothers had seen to (= Linne) began to maneuver his concepts of the of thinking what others had not thought. As arrangement into Aristotle’s logic of classes. Twenty- a result Darwin’s thesis on the origin of has three years elapsed between the publication of his first influenced for over onehundred years and and tenth editions of (( D. The tenth Wallace is remembered for presenting a similar hypoth- edition (1758) is theacknowledged starting point of esis that his theology would not allow him to expand. zoological nomenclature.Often forgotten but highly Linnaeus provided the grist upon which Darwin and significant is the fact that he spent those intervening others worked. However, Darwin and others were lead- twenty years orchestrating the then known into ers in initiating the shiftfrom theconcept of a uti- the world of philosophy. litarian classification based on the procedures of logic Linnaeus’genius was conceivingthat science and that primarily functioned as an instrument for identifi- philosophy can blend and that theology, though highly cation,to themuch broader interpretation that the influential to his concepts, was not the al1 controlling diversity of resulted from evolutionarydiver- unit. He was influenced by Aristotle’s logic of classes, gence (Mayr, 1969). Thus, there was a major shift from ’s “ essence ” and Thomistictheology. This may be identification to classification and the process of reason- best explained bysimplistic definiti0ns:science at- ing became inductive rather than deductive. Recently, temptsto delimitwhat is true;philosophy seeks to in nematology, there has been an inclination to revert to define truth; theology, which Linnaeus was unable to deductive reasoning in the formulationof classifications completely escape, intervenesin as much as the de- (Andrassy, 1976; Fotedar & Handoo,1978; Siddiqi, mands of faith supersede delimitation and definition. 1986). In these classifications the categories above the Understand, Linnaeus was polarized by the sixteenth species are basedon ana prioriarrangement of - century observation : philosophia theologiae(phi- logic characters ratherthan on natural groupings as losophy is the maid-servant of theology). interpreted through known biology. To a large extent, and unfortunately so, science has Linnaen classificationhas not been without oppo- come to limit the genius of Linnaeus to G creating )) the sition and criticism. Attacks on the system generally arise binomial system of nomenclature. Binomial nomencla- from an inability to comprehend a rigid hierarchy with ture was extant hundreds of years before Linnaeus; his an inherent need forarbitrary ranking. The assignment proposa1 established the binomialas unambiguous. The of intractable values to ranks withinthe hierarchy would names and groups he offered are not important. The paralyze the system into a desinence that would preclude introduction of an unambiguous binomial nomencla- improvement. Perception of the intrinsic subjectivity of ture and thelogic ofclasses and its application tobiology the system allows for the incompleteness ofOur know- are important because for the first time thediversity of ledge of relationships and presents us with the oppor- living organisms was organized in a manner that rev- tunityto testalternate models of relationship, thus olutionized thought. However, Linnaeus’ philo- maximizinginformation. Systematists should accept sophy remained Aristotelian and washe dedicated to the and welcome the fact that the systemwill forever remain application of Aristotle’s system of logic to classification provisional. (Tuxen, 1973). Later generations up to and including There are seven basic categories that prevail in the Darwinsucceeded in excising Linnaean philosophy modern interpretationsof Linnaeus’ hierarchies. Al1 are from Linnaeus’ proposa1 while retaining the essence of not those proposed by Linnaeus (classis, ordo, genus, a rigid hierarchy of categories andan unambiguous species, and varietas) butthose that newknowledge . demanded. After more than two hundredyears only fïve Darwin’s contributionsencouraged us to use Our (genus, family, , and ) permit US tO abilities to see what everyone else has seen andyet think place a species with any degreeof accuracy. Even these beyond this neurologic limitation.It is this that spotligh- didnot satisfy Our needto more accurately portray ted Darwin. Wallace, hindered by theology, was unable relationships;therefore, over the years moreprecise A. R. Maggenti

~~ ~ designations were imperative and the prefxes super- false faith or faith in a rigid system based on numbers and sub- were attached to the basic categories. cannot escape thereality thar taxaare based on zoologi- With no sense of the purpose of Linnaean hierarchy cal realities. Categories,no matter how depictedare some other terms have been introduced and have, not concepts; however, they are concepts based on natural unexpectedly, become the subjectof confusion because occumng units. theydo not conveyrelationship nordo they add to The most objective of al1 categories remainsthe perception of biology. The terms referred to species, for a further discussion of the objectivity and are : biotype, race and pathotype. On the other hand definition of the speciessee Maggenti (1983). The someterms that have received anotable degree of species category differs from al1 other divisions in the acceptance are : , between family and genus, and hierarchy in that itsignifies singularity, distinctnessand group between genus and .However, asuseful difference (Mayr, 1969). Categories above the species as they are, they have not warranted recognitionby the are collective conceptsinasmuch as they have the International Commission of Zoological Nomenclature function of grouping and ordering by de-emphasizing forinclusion in the rules for “ binomial(trinomial) differences between species and emphasizing affinities nomenclature ”. It is not surprising that these perceptive among groups of species. Mayr (1969) States : “ Even additionsamplify the ability to expressrelationships though an operational definition for the higher catego- within the two most informative categories in the hier- ries does not exist, nor for therank which they signify, archy of classification; i.e., the genus and family. Both they do have anobjective basis because a placed additions are designed to expandOur ability to cope with in a higher category (if correctly delimited) is natural, newknowledge of relationships without the need to consisting of descendants from a common ancestor. ” overinflate the basic categories. This simply means that Since there is no operational definition of the genus 1 genera do not have to be raised to families or families will adopt Mayr’s pragmatic definition: “ A genus is a to orders. Thus funher illustrating the genius of the taxonomiccategory containing a single species, ora Linnaean-Darwin-Mayr philosphy of classification and monophyletic groupof species, which is separated from perhaps,with an affordabledegree of magnanimity, other taxaof the same rank (other genera) by a decided Aristotle’slegacy to biology: the gft of the logic of gap. ” In general, the size of thegap is inversely classes. proportional to thesize of the taxon. Thisis a schooled This disquisition set-out to address the logical conse- observation and not a rule. quence of purporting to understand and accept that a In view of the fact that there is no operational defi- classification and ranking in a hierarchy reflect rela- nition of the genus and inasmuch as there is no dis- tionships. From the foregoing statements it should now tinctive single or group of characters thata priori make be evident that the backboneof a Sound classificationis a genus, then its circumscription must be sought else- entrenchedin Our perception of thegenus and its where. Since strict morphology cannot delineate thege- projected concept - the family. nus then the alternative to understanding the affiities Linnaeus in 1737 gave lasting advice in his dictum : among species must lie in their biology and ancestry. “ Its the genus that gives the characters and not the The latter, obviously, refers to their phylogenetic deve- characters that make the genus. ” Though it would be lopment from a common ancestor, and in this context highlydesirable to have charactersor even a single a genus will have common features that facilitate reco- characterthat unequivocally designated categorical gnition. Therefore, oneof the featuresof a genusis that rank, we must accept the fact that such does not exist it embrace in the hierarchy a collective assemblage of (Mayr, 1969). Therefore, it also follows that taxonomic species that comprise a phylogenetic unit. Even though characters that prove generic distinctions do not exist. the genus is a phylogenetic unit there need not be a It is also truethat non-arbitrary definitionsat the single species which is immediatelyancestral to al1 categorical level of genus and above are not possible to species inthe genus. A genuscan be derived from give. several species in a genus in which theancestral species canonly florish in the realmof flexibility; were grouped. It is not required that a new genus be it is a science of concepts and realities orpragmatically, proposed for each ancestral species. definitions and descriptions. Absolute definitions would As a phylogenetic unit the genus differs from similar stifle future knowledgebecause in the world of the and related assemblages by reflecting anecological unit absolute, freedom to express new information concern- that is adapted to a particular mode of Iife; Le., a valid ing relationships could not exist. Many scientists find genus occupies a niche. Because the genusoccupies an this situation difficult to accept and even moredifficult ecological niche it makes an evolutionary statement that to work with. It is among these scientists that there is an the individual species, making up the assemblage, can- expressed dissatisfaction with the Linnaean hierarchy. not make independently. It has even been suggested that category names be re- Among parasitic groups, in Our case , it placedwith numbers (Lervtrup, 1979). Numbersare should be easierto recognize genera (where the biology , only unit expressions of mathematical concepts and a is generally well known) than it is among the so-called

4 Revue Nématol. 12 (1) :3-6 (1989) Genus and family freeliving genera (where-the biology is seldom known). gains as well as evidence of are most evident. However, this does not seem to be the case among the There aresome broad features or generalizations parasitic nematodes northe animalparasitic nema- that, thoughnot generally applied in systematics to todes. Over the last twenty to thirty years the number distinguish the family, are.applicable to the family of nominal genera in Tylenchina has increased at a concept. Unequivocally, families are older than genera logarithmic rate. The recent revision of Tylenchina and oftenworld-wide in distribution. In general, and thk (Maggenti et al., 1988) has accepted a 46 O/o reduction should be given serious consideration among nemato- in the number of genera in Tylenchoidea and a 58 O/o logists, the family should have a general facies easily reductionin the number of generaaccepted in the recognizable. The latter is easily verified by Our ability Criconematoidea. The philosophyemployed by this to recognize some families under the dissection micro- team of researchers, Luc, Maggenti, Fortuner, Raski scope; this should not be interpretedas a mechanism to andGeraert (1987),was thatoutlined above: seek recognize families. phylogenetic units, ecological units and a generic niche; It should not be surprising that as knowledgeof taxa these preceptswere extended to theirlogical conclusion increases world-wide thatfamily designations often - the family. require rethinking. World-wide families have a tendency The family being an abstract concept of a naturally to internally break into distinctive groups and itis to be occurring assemblage of taxa (genera) cannot be given expected that intermediate groups will be found and a nonarbitrary definition and therefore, the only assi- quite oftenrelic groups discovered thatCloud the issues. gned definition is nearly equivalentto thatgiven for the One only has to considerrecent finds among the genusby Mayr (1969): " A family is ataxonomic Heteroderidae to confirm this generalization. Whenthis category containing a single genus or a monophyletic occurs we have two choices: the known families can be group of genera, which is separated from otherfamilies raised to superfamilies (the simplest solution, though by a decided gap. " Once again it has been generally more often than not the least satisfactory) or we can observed that the gapis inversely proportional to the size rethinkthe available informationand expand Our of the family. concepts by combining and reducing the number of If thegenus should occupy an ecological niche it families. follows that its logical projection, the famfly, of necessity The heart of a classification isfounded on thestability must also occupy awell-defined %Che or adaotive zone. of its recognized families. Family stabilityis essential to The family owes its origin to the invasion of '$sis zone a useful classification that affords maximum commu- by one'or more founder species and to the subsequent nication and in turnoffers the greatest amount of infor- active and which follows a successful mation with maximum retrieval. Splitting, at any level adaptive shift. Becauseof this the family is the category within the hierarchy, makes information retrieval more that should provide the most information amongal1 the difficdt and in some instances impossible. The uncurb- assignedranks in the entire hierarchy. The available ed addition of taxa without regard for their effects on information on any assemblageof taxa decreases above other categories or their illogical placement within the and below the family level; therefore, it is a rank to be system, can only result in the continued repression of assigned with a great dealof caution and consideration. nematological knowledge. It is imperative that we re- Obviously, since there is no operative definition nor examine nematode classification and the philosophies single or group of morphological characters that deli- that are being applied to determine the categorical ranks neate a family, then family designation in any given within the hierarchy. phylum will be individualistic but the philosophy ap- plied should be the same. In the class Insecta there are someone million species distributedamong some REFERENCES 940 families or roughly one family for 1 060 species ANDRÀSSY,1. (1976). Evolution as a basisfor the systematization (obviously there are some smaller and some larger). In of nematodes. London, Pitman Publishing, 288 p. Nemata there aresome 200 families forsome 15 O00 nominal species. Within Nemata this implies that FOTEDAR,D. N. & HANDOO,Z. A. (1978). A revised scheme a great deal of extinction has occurred and that what of classification to order Tylenchida Thome, 1949 (Nema- remainsare clear-cut taxonomic isles. Any cursory toda). J. Sci. Univ. Kashnlir, 3 (1975) : 55-82. attempt to work with the families of nematodes quickly LINNAEUS,C. (1758). Systenza naturae per regna tria naturae, convinces one that thedifferences between families are secundum classes,.ordines, genera, species cum characteribus, often so slight as to defy recognition or identification, differentiis,synonymis, locis, Editio decima, reformata, To- when the opposite should be more often true. Careful 1. Laurentii Salvii, Holmiae, 824 p. consideration should be given to whether some of Our L0vTRUP, S. (1979). The evolutionary species: fact or fiction. superfamilies and perhaps even some suborders are not syst. Zool., 28 : 386-392. in reality families; it is often at thislevel in the classifi- Luc, M., MAGGENTI, A. R., FORTUNER, R., RASKI, D. J. & cation of Nemata that well-defined niches or adaptive GERAERT,E. (1987). A reappraisalof Tylenchina (Nemata).

Revue Nématol. 12 (1) :3-6 (1989) 5 A. R. Maggenti

1. For a new approach theto of Tylenchina. Revue MAYR,E. (1969). Principles of Systernatic . New York, Nématol., 10 : 127-134. McGraw-Hill Book Co., 428 p. MAGGENTI,A. R. (1983). Nematodekigker classification as SIDDIQI,M. R. (1986). Tylenchida parasites of plantsand injluenced by species and familyconcepts. In : STONE, A.R., . Slough,U.K., Commonwealth Agricultural Bu- l’un, H. M. & KHALIL,L. F. (Eds). Concepts in Nematode reaux, 645 p. Systematics. London & New York, Academic Press : 25-40. TUXEN,S. L. (1973). Entomology systematizes and describes : MAGGENTI, A. R., LUC,M. RASKI, D. J., FORTUNER,R. & . 1700-1815. In : SMITH,R.F., MI-ITLEK,T. E. & SMITH, GERAERT, E. (1988). A reappraisalof Tylenchina (Nemata). C. N. (Eds). History of entomology. Ann. Rev. Entont. Soc. 11. List of generic and supra-generic taxa, with their junior Amer. :95-118. synonyms. Revue Nénzatol., 11 : 177-188.

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