Species

SPECIES. - A biological entity which, through physiological isolation, on the one hand allows the complex of characters inherited from its ancestors to be transmitted to descendants — characters common to the various phenotypes or races that compose it — and on the other hand prevents it from intermingling with other related complexes. According to this description, a species does not intermingle with another, thereby differing from the races that compose it, which can intermingle with one another by crossing. The species is a reality perceived by man. The author of Genesis affirms that God, in creating plants and animals, ordered the earth to germinate the former and the waters to produce the latter, each according to its kind.

The zoologist, when studying lower organisms, although still unable to grasp the intimate differences of each protoplasm, which are undoubtedly of a chemical nature, cannot doubt their existence when he sees organic and even inorganic products being deposited in each specific form, such as salts of different constitution, and sees that the diversity is not limited to the elaboration of determinate chemical substances, but that each of these assumes a physical aspect different from species to species, from one group of species to another. In bacteria the species manifests itself in biological behavior; the same filterable viruses, which in the present state of our knowledge require the presence of living cells in order to multiply, demonstrate their specificity through the varied reactions they provoke in the organism, animal or vegetable, that hosts them. These facts seem to prove that there is no life without species and that, presumably, species arose from the very origin of life, in the sense that protoplasm or living substance was not organized in a uniform manner, but in a specifically different manner, from the very beginning. The theory of the specific differentiation of protoplasm, subsequent to the establishment of life on earth, does not seem able to hold up before an objective examination of the facts.

It is not enough that the notion of species be founded on more or less perceptible differences among organisms; these differences must separate from one another groups of similar individuals, descended from a single individual by apogamy or parthenogenesis, or from a pair of individuals of different sex, by amphigony, capable of transmitting their characters to descendants. For it to be possible to attribute the denomination "species" to a group of organisms offering common characters, the hereditary transmission of differences thus constitutes a necessary condition, but not however a sufficient one. A precursor of Linnaeus, John Ray, who lived in the second half of the 16th century, affirms in his Historia plantarum that the characters of a species preserve their nature in perpetuity through the parents or seeds and cannot be produced by seeds of another species. Linnaeus, as early as 1738, confirmed the perpetuity of the species, affirming that this entity was created by God: species tot sunt quot diversas formas ab initio produxit infinitum Ens. He thus established the principle which he himself attenuated in the 12th ed. of his Systema Naturae, where he recognized the existence of varieties of plants derived from accidental causes, such as climate, soil, temperature, winds, etc., and limited the primitive creation to a few forms which, intermingling, would then have produced the species existing at present.

From Linnaeus to our own day the concept of species has become ever more precise, especially in what concerns the conditions recognized as necessary for a group of organisms, similar to one another, to be distinguished from other more or less related groups and to be considered as a species.

A first difficulty is that which derives from the contradiction inherent in the fact that specific characters are essentially hereditary, and yet can undergo modifications. The species is transmitted from generation to generation, but it is equally proven that intrinsic and extrinsic causes can determine in descendants modifications now ephemeral and now stable.

In the evaluation of these or even simply in that of specific characters — which is to say in the criteria of specific distinction — a conventional and subjective element enters into play which varies from one scholar to another in relation to the nature of the organisms studied. It has already been said that species react differently to the action of the environment: now, between the environment and plants there exist relationships so close as to make impossible their assimilation to those that exist between animals and the environment. The plant is, so to speak, bound to the soil and cannot escape the physico-chemical action of the terrain nor that of the climate, whereas the animal is entirely independent of the terrain and its physico-chemical constitution and can escape, by migration and by hibernation, the action of the climate.

The appraisal of the specific characters of animals cannot be conducted with uniform criteria for all groups, and it is easy to realize that those adopted in the evaluation of the differences among Insects cannot be the same as those that serve to distinguish the Coelenterates, or one or another class of Vertebrates. For evident reasons of necessity, species are founded for the most part on morphological characters, now internal and now external, characters which vary moreover not only from group to group, but also from author to author.

From a more general standpoint, since Buffon, the physiological criterion of interfertility has been adopted as a means of distinguishing specific characters, in the sense that all forms which give rise to fertile products, even if morphologically different, belong to the same species, whereas those which are not fertile with one another or which produce sterile hybrids constitute distinct species. This physiological criterion is of very rare application, because it requires the realization of a very difficult experimental test, especially when it is a question of animals. Moreover, between complete interfertility and sterility there exists a series of intermediate conditions: 1) two forms interfertile in an experimental environment are not so in the natural one: a) because of complete separation of the areas inhabited by them; b) because of lack of contemporaneity in the maturation of the germ cells; c) because of lack of reciprocal attraction or indeed of repulsion; d) because of mechanical obstacles to mating or to the penetration of the sperm into the egg. 2) Two forms which mate naturally or artificially produce hybrids fertile in the homozygous sex and sterile in the other, heterozygous, sex. In the latter are formed: a) germ cells incapable of fertilizing or of being fertilized (in birds, small or even normal eggs, but not fertilizable); b) germ cells are not formed (in birds, complete infertility of the hybrid female); c) the sterility of the heterozygous sex disappears in the first backcross; d) the sterility of the same disappears in the second backcross; e) the sterility of the same disappears in the third backcross. 3) The aforesaid conditions can be accompanied in each of the two sexes by parasibility, that is to say by a statistically reduced production of mature germ cells and consequently of few specimens that reach the adult state. 4) The hybrids are sterile in both sexes. 5) The viable hybrids belong only to the homozygous sex and are equally sterile.

All these conditions demonstrate that the definition of species, founded on the fertility and sterility of hybrids, is not, by itself, as absolute as one might believe at first sight, and it is therefore appropriate to attenuate it, taking into account subordinately some non-physiological element. It must moreover be affirmed that all systematics has been practically founded on morphological evaluation.

Indeed, while it is easy to distinguish a species from a race in the case of domestic animals and cultivated plants, whose number is extremely limited, it is equally difficult to distinguish the species found in nature, because experience has been able to establish their identity only in a few cases. Hence the necessity of admitting that alongside a few species really existing as such and known to us, there exists a myriad that we regard as species in systematics, because on the basis of differences in form or behavior the naturalist presumes them to be real species, whereas they may not be. These species may be designated by the name of systematic species, in the sense that they are considered as such within the system and without any certainty that they are truly distinct in reality. They are provisional species, in the expression of the botanist De Vries, awaiting a future experience that will confirm or deny their nature as species.

Granted that knowledge of the real species, that is, the one really existing, represents in almost all cases an aspiration to be attained, it may be affirmed that in various cases there is no coincidence at all between physiological and morphological affinity.

The species was considered by Linnaeus as the real biological entity and was placed by him at the basis of his Systema Naturae and of his binomial nomenclature. According to this, every species is designated by two names, indissoluble from the specific standpoint, since the second name, which refers to the species, has no value when dissociated from the first, which corresponds to the genus.

It should be noted, however, that while the species refers to an entity presumed to exist really, the genus is a subjective aggregate, which expresses the affinity of various species among themselves, without any precise fact giving us the possibility of defining its true nature and extent.

At present nomenclature is trinomial: the first name is always that of the genus; the second therefore refers to the species and the third to the subspecies or race. The genus is usually even today that entity founded especially on morphological affinities, evaluated according to the subjective and personal criteria of each author, and generally corresponds to the Linnaean species; the species is the real entity, physiologically more or less separated from the other related ones by means of incomplete inter-fertility or parasterility; the subspecies or race, designated by the third name, is a substantially genotypic form, tied to a determined geographical or ecological territory.

With the advent of experimental genetics, however, the concept of species has acquired an even more precise meaning. Specific characters are the expression of genes aligned in the chromosomes and which influence one another and in turn undergo the influence of one or more external factors, whence the appearance of phenotypes, which are likewise the result of interactions of environmental factors and genetic factors. It is worth recalling that many plants possess several phenotypes for flower color, according to whether they bloom at medium, high or low temperatures, that butterflies which have two annual generations offer two seasonal phenotypes of color, one vernal and another summer-autumnal, in relation to high or low temperature or, in tropical countries, in relation to the dry season or the rainy one.

Modern genetics has furthermore demonstrated that many phenotypes, to which a specific character is customarily attributed, are the result of crosses and that these, according to whether they are in a homozygous (pure) or heterozygous (impure) state, are stable or unstable. In the first case the species may be considered real, because they ensure to their descendants, under uniform environmental conditions, phenotypic continuity, that is, of their external appearance. Mendel's laws, and especially the principle of the segregation of characters in hybrids and that of the independence of genes, explain, as is demonstrated by experience and statistical investigation, the mechanism of this phenomenon, which is moreover conditioned by territorial and geographical isolation. It is evident that the homozygous state of a phenotype resulting from hybridization cannot be determined or preserved where the possibility of further crosses with other related phenotypes persists. Such is the case of plants cultivated separately and in isolation and of animals raised under cellular regime. In nature the most characteristic isolation is that offered by islands, especially if they belong to the same archipelago (Hawaii, Galapagos, Antilles, etc.). Examples of the effect of geographical isolation on the differentiation of forms are also given by mountains and respectively by isolated valleys, by desert oases, etc., etc.

There also exists an ecological isolation, due to a different manner of reaction of the organism to distinct climatic environments, such as the desert, the forest, the savanna, and to the different altitudes above sea level of such geological and vegetal formations.

These phenomena are particularly evident when the forms considered are inter-fertile and may therefore be considered as races of the same species. We are in the presence of a race every time experience demonstrates that a population of phenotypes, animal or vegetal, is composed of individuals homozygous for characters that are opposed to the corresponding ones (alleles) of another population of phenotypes more or less related to that previously considered. "A race may be defined as a population that preserves its own phenotype, reproducing in the homozygous state in a uniform environment." The expression "race" is substantially synonymous with that of subspecies, or elementary species or Jordanon, from the French botanist Jordan, who considers as species groups of individuals that differ from one another by slightly accentuated characters. Races too may be geographical or ecological, according to whether they are genotypics that are preserved in the homozygous state by effect of geographical isolation and which are also called local races; or else they are forms due to particular hereditary norms of reactions to the nature of the soil or of the climate. The origin of such races, both geographical and ecological, must be sought, at least in part, in new conditions determined by crossing, but for the most part in mutations: in the one and the other case their stabilization is subordinated to the intervention of isolating and selecting agents and principally to consanguineous reproduction. In order to understand what a mutation consists of for the purposes of the formation of a new race or subspecies and, possibly, of a species, one must always bear in mind what we have already said, namely that the phenotype, that is, the complex of somatic characters that individuate a homogeneous group of organisms, is due to a complex of elementary particles: the genes, aligned in the individual chromosomes. Experience has demonstrated that during the maturation of the germ cells, external agents, such as changes of temperature and humidity, chemical agents, radiations, may provoke the falling out or the displacement of one or more determined genes in a single chromosome, or changes in the structure of one or more chromosomes, and finally changes in the general arrangement of the chromosomal complement. These gene mutations and respectively chromosomal and genomic ones are responsible for stable changes of the genotype and consequently of the phenotype. Usually, however, these are small species, of those that we have indicated as elementary or Jordanian species, arisen within the circle of a single collective or Linnaean species, intending by this expression to allude to the species instituted by Linnaeus, large collective species, which usually comprise a sometimes very great number of small elementary species.

Biologists have also sought to test whether criteria other than those we have discussed can be valid for identifying species, but the results have been negative. Cytological criteria, especially those concerning the number and form of chromosomes, have not yielded positive results, since often their number is the same in very different groups of animals, or belongs to all the species of a single family (grasshoppers); in other cases races of the same species have double, triple or quadruple the number existing in the primitive race. Chemical and serological criteria too serve rather to distinguish races and strains of the same race than species from species. Notable serological affinities have been found between certain monkeys and man, but no one thinks that these forms belong to the same species.

From what has been set forth it follows that the definition we have given of species is the one that best corresponds to our knowledge on the subject, but the difficulties of achieving or experimentally ascertaining what concerns interfecondity or amixia make the aforesaid definition more theoretical than practical.

Given the enormous number of known species and the necessity of classifying them systematically, zoologists are compelled not to limit themselves to difficult and often impossible physiological experimentation and to take account of other morphological, geographical, ecological and behavioral characters.

Alessandro Ghigi

SPECIE (philosophy): v. PREDICABILI.