SPECIES. – A biological entity whose physiological isolation enables it, on the one hand, to transmit to its descendants the complex of characters inherited from its ancestors, characters common to the various phenotypes or races composing it, and, on the other, prevents it from mixing with other related complexes. According to this description, one species does not mix with another, thereby differing from the races composing it, which can mix with one another through crossing. The species is a reality perceived by man. The author of Genesis states that God, in creating plants and animals, commanded the earth to bring forth the former and the waters to produce the latter, each according to its species.
When the zoologist studies lower organisms, although he is not yet able to grasp the intimate differences of each protoplasm, which are undoubtedly chemical in nature, he cannot doubt their existence when he sees organic and even inorganic products being deposited in each specific form, such as salts of differing constitution, and observes that the diversity is not limited to the elaboration of particular chemical substances, but that each of these assumes a physical appearance differing from species to species and from one group of species to another. In bacteria, the species manifests itself in biological behavior; even the filterable viruses which, given 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, whether animal or plant, 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 uniformly, but in specifically different ways, from the beginning. The theory of the specific differentiation of protoplasm subsequent to the emergence of life on earth does not seem tenable in the face of an objective examination of the facts.
It is not enough for the notion of species to be based on more or less perceptible differences among organisms; these differences must separate groups of similar individuals from one another, individuals descended from a single individual through apogamy or parthenogenesis, or from a pair of individuals of different sexes through amphigony, and capable of transmitting their characters to their descendants. For it to be possible to apply the designation “species” to a group of organisms displaying common characters, the hereditary transmission of the differences is therefore a necessary, though not sufficient, condition. A precursor of Linnaeus, John Ray, who lived in the second half of the sixteenth century, states in his Historia plantarum that the characters of a species perpetually preserve their nature through parents or seeds and cannot be produced from the seeds of another species. Linnaeus, as early as 1738, confirmed the perpetuity of the species, asserting that this entity had been created by God: species tot sunt quot diversas formas ab initio produxit infinitum Ens. He thereby established the principle that he himself later mitigated in the 12th ed. of his Systema Naturae, where he acknowledged the existence of varieties of plants derived from accidental causes, such as climate, soil, temperature, winds, etc., and restricted the original creation to a few forms which, by interbreeding, would subsequently have produced the species existing today.
From Linnaeus to the present day, the concept of species has become increasingly precise, especially with regard to 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 a species.
A first difficulty arises from the contradiction inherent in the fact that specific characters are essentially hereditary and yet may undergo modifications. The species is transmitted from generation to generation, but it is equally well established that intrinsic and extrinsic causes may determine in descendants modifications that are sometimes ephemeral and sometimes stable.
In the evaluation of these, or even simply in that of the specific characters—which is to say, in the criteria of specific distinction—a conventional and subjective element comes into play, varying from one scholar to another in relation to the nature of the organisms studied. It has already been stated that species react differently to the action of the environment: now, the relations between the environment and plants are so close as to make it impossible to assimilate them to those existing between animals and the environment. The plant is, so to speak, bound to the soil and cannot escape the physicochemical action of the ground or that of the climate, whereas the animal is entirely independent of the soil and of its physicochemical constitution and can escape the action of the climate through migration and hibernation.
The assessment of the specific characters of animals cannot be conducted according to uniform criteria for all groups, and it is easy to understand that those adopted in evaluating the differences among Insects cannot be the same as those applicable to distinguishing the Coelenterates or one or another class of Vertebrates. For obvious reasons of necessity, species are based for the most part on morphological characters, sometimes internal and sometimes external—characters that nevertheless vary not only from group to group, but also from author to author.
From a more general standpoint, from Buffon onward, the physiological criterion of interbreeding has been adopted as a means of distinguishing specific characters, in the sense that all forms giving rise to fertile offspring, even if morphologically different, belong to the same species, whereas those that are not fertile with one another or that produce sterile hybrids constitute distinct species. This physiological criterion is very rarely applicable, because it requires the performance of a highly difficult experimental test, especially in the case of animals. Moreover, between complete interbreeding and sterility there exists a series of intermediate conditions: 1) two interfertile forms in an experimental environment are not so in the natural environment: a) because of the complete separation of the areas they inhabit; b) because of a lack of simultaneity in the maturation of the germ cells; c) because of a lack of mutual attraction or even because of repulsion; d) because of mechanical obstacles to mating or to the penetration of the spermatozoon into the egg. 2) Two forms that mate naturally or artificially produce fertile hybrids in the homozygous sex and sterile hybrids in the other, heterozygous, sex. In the latter, the following occur: a) germ cells incapable of fertilizing or of being fertilized are formed (in birds, small or even normal eggs, but ones incapable of being fertilized); 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 that sex disappears in the second backcross; e) the sterility of that sex disappears in the third backcross. 3) The conditions mentioned above may be accompanied in either sex by parasterility, that is, by a statistically reduced production of mature germ cells and, consequently, by the production of only a few individuals that reach the adult stage. 4) The hybrids are sterile in both sexes. 5) The viable hybrids belong only to the homozygous sex and are likewise sterile.
All these conditions demonstrate that the definition of species based on the fertility and sterility of hybrids is not, by itself, as absolute as might be believed at first sight, and it is therefore advisable to moderate it by taking subordinate account of some non-physiological element. It must nevertheless be affirmed that all systematics has in practice been founded on morphological assessment.
Indeed, whereas it is easy to distinguish a species from a race when dealing with 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 in only a few cases. It follows that we must admit that, alongside the few species that really exist as such and are known to us, there is a myriad that we regard as species in systematic classification because, on the basis of differences in form or behavior, the naturalist presumes them to be real species, whereas they may not be so. These species may be designated as systematic species, in the sense that they are regarded as such within the system, without any certainty that they are truly distinct in reality. They are provisional species, in the words of the botanist De Vries, pending future experience confirming or denying their nature as species.
Having established that knowledge of the real species—that is, of the species actually existing—represents, in almost all cases, an aspiration yet to be achieved, it may be stated that in various cases there is no coincidence at all between physiological affinity and morphological affinity.
Linneo regarded the species as the real biological entity and made it the basis of his Systema Naturae and of his binomial nomenclature. According to the latter, each species is designated by two names, inseparable in specific terms, since the second name, which refers to the species, has no value when dissociated from the first, which corresponds to the genus.
It should nevertheless be noted that, whereas the species refers to an entity presumed to exist in reality, the genus is a subjective aggregate expressing the affinities among various species, without any precise fact enabling us to define its true nature and extent.
At present the 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. Even today the genus is generally an entity founded chiefly on morphological affinities, assessed 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 other related species by means of incomplete interbreeding or parasterility; the subspecies or race, designated by the third name, is a substantially genotypic form associated with a specific geographical or ecological territory.
With the advent of experimental genetics, however, the concept of species acquired an even more precise meaning. Specific characters are the expression of genes arranged along the chromosomes, which influence one another and are themselves influenced by one or more external factors, hence the appearance of phenotypes, which are likewise the result of interactions between environmental and genetic factors. It is useful to recall that many plants possess several phenotypes with respect to the color of their flowers, depending on whether they flower at medium, high, or low temperatures; and that butterflies having two generations annually display two seasonal color phenotypes, one spring and the other summer-autumn, in relation to high or low temperatures or, in tropical countries, to the dry season or the rainy season.
Modern genetics has furthermore demonstrated that many phenotypes to which a specific character is usually attributed are the result of crossings and that, according as these are homozygous (pure) or heterozygous (impure), they are stable or unstable. In the first case the species may be considered real, because, under uniform environmental conditions, they ensure phenotypic continuity—that is, continuity of their outward appearance—in their descendants. Mendel’s laws, and especially the principle of the segregation of characters in hybrids and that of the independence of genes, explain, as experience and statistical investigation have demonstrated, 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 can neither arise nor be maintained where the possibility remains of further crossings with other related phenotypes. This is the case with plants cultivated separately and in isolation and with animals raised under individual housing conditions. In nature, the most characteristic isolation is that provided by islands, especially when they belong to the same archipelago (Hawai, Galapagos, Antilles, etc.). Examples of the effect of geographical isolation on the differentiation of forms are also provided by mountains and, respectively, isolated valleys, desert oases, etc. etc.
There is also ecological isolation, caused by a different manner in which the organism reacts to distinct climatic environments, such as the desert, the forest, and the savanna, and to the different altitudes above sea level at which such geological and plant formations occur.
These phenomena are particularly evident when the forms under consideration are interfertile and may therefore be regarded as races of the same species. We are in the presence of a race whenever experience demonstrates that a population of animal or plant phenotypes consists of individuals homozygous for characters that oppose the corresponding (allelic) characters of another population of phenotypes more or less related to the one previously considered. “A race may be defined as a population that preserves its phenotype, reproducing in the homozygous state in a uniform environment.” The term “race” is essentially synonymous with subspecies, or elementary species or giordanone, after the French botanist Jordan, who regards as species groups of individuals that differ from one another in only slightly pronounced characters. Races too may be geographical or ecological, according as they are genotypes preserved in the homozygous state as a result of geographical isolation—and are then also called local races—or forms due to particular hereditary patterns of reaction to the nature of the soil or 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 either case their stabilization depends on the intervention of isolating and selecting agents, and chiefly on inbreeding. To understand what a mutation consists of in relation to the formation of a new race or subspecies and, possibly, of a species, we must always bear in mind what has already been said, namely, that the phenotype—that is, the complex of somatic characters identifying a homogeneous group of organisms—is due to a complex of elementary particles: the genes, arranged along the individual chromosomes. Experience has demonstrated that during the maturation of germ cells, external agents, such as changes in temperature and humidity, chemical agents, and radiation, may cause the loss or displacement of one or more specific genes in a single chromosome, or changes in the structure of one or more chromosomes, and finally changes in the general arrangement of the chromosome complement. These genic, chromosomal, and, respectively, genomic mutations are responsible for stable changes in the genotype and consequently in the phenotype. Usually, however, these are small species, those that we have designated as elementary or Jordanian species, arising within the sphere of a single collective or Linnaean species. By this expression we mean the species established by Linneo, large collective species that usually comprise
a sometimes very large number of small elementary species.
Biologists have also sought to determine experimentally whether other criteria, besides those we have discussed, might be useful 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 their number is often the same in very different groups of animals, or is shared by all the species of a given family (grasshoppers); in other cases, races of the same species have twice, three times, or four times the number found in the primitive race. Chemical and serological criteria, too, serve rather to distinguish races and strains of the same race than species from one another. Notable serological affinities have been found between certain apes 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 the species is the one that best corresponds to our knowledge of the subject; however, the difficulties of realizing or establishing experimentally what concerns interbreeding or amixia make the foregoing definition more theoretical than practical.
Given the enormous number of known species and the need to classify them systematically, zoologists are obliged not to confine themselves to difficult and often impossible physiological experimentation, but to take into account other morphological, geographical, ecological, and behavioral characteristics.
Alessandro Ghigi
SPECIE (philosophy): V. PREDICABILI.