The Gall Wasp Genus Cynips: A Study in the Origin of Species/The Species Concept
THE SPECIES CONCEPT
The earliest work of the systematists emphasized the similarities of individuals within species and the sharp distinctions between individuals of diverse species. This view was an inheritance from man's primitive knowledge of plants and animals, and one which, bolstered by a widespread misinterpretation of the doctrine of the uniformity of nature, is still widely held outside of scientific circles today.
As the facilities for the more careful examination of individuals were developed early in the last century, scientific emphasis was shifted to the fact that no two individuals are exactly alike, and the species problem resolved itself into a search for the factors of evolution. As a direct result of those investigations there has developed a growing conviction that individuals are the only realities in nature, and an agreement that species are only convenient concepts originating and ending in the minds of scientists. This is the basis of recent demands that we return to what is, curiously, called the Linnean species, it being argued that that was as near reality as any present-day concept, and an article far more ready for the use of those who are not taxonomic specialists. Taxonomists have contributed little to the resolution of this confusion, for many of them are bewildered at the array of geographic variants and transition zone hybrids which have recently become available particularly from our own continent, and species are frankly defined in the codes as concepts that may be standardized and established by quasi-legal verbiage.
One wonders to what extent this confusion as to the nature of species has delayed our understanding of the origin of species. Perhaps it is this confusion which leaves us without a convincing reply for the fundamentalists who insist that data from the evolution of domesticated plants and animals and from laboratory genetics may explain the origin of varieties but not of species. There is a peculiarly hollow ring to our statement that varieties are incipient species and that the evolution of species is too slow a process for human observation. Moreover, the illogical sequence in most of our texts, where the evolution materials are followed by the data of heredity, is some evidence that the geneticists do not perceive the application of their laboratory findings to species in nature; and such students as Bateson and Morgan have suggested that that application must be made by the taxonomists if (it is implied) the taxonomists ever become convinced that there are realities in nature that deserve to be called species.
Now, with these imputations of the unreality of species, I find myself no longer in accord. In the data that follow, evidence is presented that species are realities in nature more nearly satisfying the geneticist's concepts than the conventions of current taxonomy, and that the origins of such species are more satisfactorily explained on hereditary bases than by philosophic theories that may be invoked in extenuation of the fact and the factors of organic evolution.
We may begin our analysis of species by an examination of a few individuals taken in the field. We then become impressed with the truth of the assertion that no two individuals are exactly alike. And if we extend our investigations to several dozen individuals, we shall be confused by the varying characters that enter into any population which ordinarily passes as a species.
But if, on the other hand, we extend our examination to several hundred such individuals, we shall become impressed with another opinion, namely that there are many more points of uniformity than of variation among individuals taken from a given locality and habitat. Perhaps a half-dozen characters will show appreciable variation, while the hundreds of other characters that go to make up an organism are remarkably constant. One may believe that if larger series were more often utilized in taxonomic work the current bewilderment over variation would give way to a renewed respect for a certain uniformity that exists thruout such groups of individuals.
Again, the variation that may be observed in characters that do vary occurs within narrow limits. Thus, the antennal count in species of Cynips often varies by one, but among many thousands of individuals none has shown two segments more or less than any other individual in that species. The length of the wings in relation to the length of the body of any cynipid is not altogether an invariable item, but among the many measurements I have made this wing-body ratio is always within three or four per cent of the mean for the species. In one species which constitutes an exception to this generalization, mutations are involved, and these will be treated in a later section of this paper.
Finally, the limits of variation of any character prove to be strikingly uniform thruout the great populations which we propose to call species. Whenever we have taken a reasonably large sample from any point over the usually considerable range of a species, the biometric data have not proved fundamentally different from data for any other fair sample from any other point in the range. The case of Cynips erinacei will serve to illustrate our experience. Erinacei is not only the most variable Cynips but one of the most variable cynipids I have examined. Reference to the descriptions in the systematic portion of this study will show that every one of the few characters which distinguish these insects from the most closely related species vary between limits approached by some one or another of the related species. The galls present more apparent variation, showing every gradation from smooth, naked, spherical, monothalamous structures (fig. 312) to densely spiny, ellipsoidal, polythalamous galls (fig. 315) which may have as many as eight larval cells. An initial experience with these extreme types of galls would lead one to believe they represented distinct species, and so they have always been classified heretofore. Increased material, however, has shown that every extreme and every one of the intermediate characters occurs thruout the range of erinacei.
The range of this species extends about 1,300 miles east and west and 450 miles north and south. We have samples of erinacei from nearly a hundred localities fairly well distributed over this tremendous area of possibly 500,000 square miles. In every large sample, erinacei is as variable as we have described it, and yet, after all, it is everywhere uniform—uniform in its constancy of variation. Even erinacei is, then, the sort of population which we would call a species.
Erinacei may present an extreme case, but it is not fundamentally different from the thing which one finds everywhere in nature. It is moreover, the picture of species to which our knowledge of mutation and Mendelian hybridization would lead us, and our definition of species must become genetic if we take into account the similarities and the differences which we find within a species. The essential uniformity of most of the characters of individuals depends, of course, on their possession of common genes, originating from the accessibility thru interbreeding to common stocks of genes. The uniformity of the limits of variation within a species may be the result of similarly mutating genes, or of genes derived from hybridization with some exotic influence. Such hereditary property may become fairly well distributed in the course of time thruout any interbreeding population, and if the genes belong to groups of multiple factors controlling single characters in the organism, we have an explanation for the occurrence of every degree of variation between definite limits within a population. The other possible explanation of these common limits of variability is that similar genes may have not only the same potentialities but the same lack of potentialities, reaching similar limits to their capacities for directing the growth of the individual organism or to withstand the effects of environmental factors. As Bateson put it, the degree of variation of an organism may be inherited as much as its degree of uniformity.
We may, then, allow for all individual variation while defining species as populations with common heredity. The older definition of a species as a group of similar (implying nearly identical) individuals fails because of the amount of variability actually found in nature. Definitions of species as groups of individuals distinguished by a definite number of diagnostic characters, or by certain degrees of difference from other species lead to artificial concepts that take no adequate account of individual variation, Mendelian inheritance, hybridization, or mutation. The definition of species on the basis of their fertility or infertility does not delimit phylogenetic units, for while it is true that the individuals within a species must be mostly fertile inter se if they are to maintain any sort of hereditary relationship, the failure of distinct species to interbreed may be due to geographic or seasonal occurrence or to other factors not directly concerned with reproductive physiology. But if species are defined as populations with common heredity, we obtain a concept which seems genetically sound and which, we shall try to show, is a reality in nature.
An appreciation of the fact that species are great populations distinct from Mendelian races, local colonies, or the preposterous Jordanon of botanical literature, may best be acquired by field experience with a group of related organisms. For illustration we may again utilize Cynips erinacei and some of its close relatives as they occur in the eastern portion of the United States.
We have already described erinacei as a highly variable species occurring on the leaves of the white oak, Quercus alba, over a tremendous area chiefly in the northern Middle West. On almost all of the infested trees at any locality in this area one may find a mixture of smooth and spiny galls of every extreme and intermediate type. Often all types of galls are crowded onto a single leaf, but occasionally a particular tree will have a preponderance of one type, and on several occasions we have found isolated trees well covered, as far as we could discover, with galls of only a single extreme type. These peculiar colonies, however, have always been on isolated trees or groups of trees, and they would appear to represent Mendelian races in which homozygosity in regard to particular characters had been affected by the isolation of the colony. Subsequent examinations of insects from such galls have failed to indicate that these local populations are homozygous in regard to any of the other characters that might vary within the species, and such colonies are passed by the taxonomist as ephemeral entities not satisfying the species concept.
If one will extend his collections in the first locality to a number of trees scattered over any appreciable distance—several hundred yards or a mile or two—he will leave the region with a sample that is as variable but as uniform as we have described it. If one travels to a second locality, five miles or fifty miles or a hundred and fifty miles from the first, the first collections may be duplicated. If one continues this procedure day after day, over the thousands of square miles which are the range of erinacei, one must become impressed with the fact that this is a population of inconceivably great numbers of individuals that are in certain aspects different and yet in an essential way similar over this tremendous territory. Of course, we can take only scant samples of the population, and we are reduced to glancing at most of the trees with their thousands of galls which we have no time to gather; or we fall to wondering how many inconceivable millions of individuals of erinacei there are on all the trees in all the fields and woodlands and mountain forests thru which we journey at many miles an hour, for hour after hour, yesterday, today, and tomorrow. Nevertheless, our samples seem typical, for they are surprisingly uniform; and after such field experience, one comes to feel there is a reality summed up in the word “species” which is more than a few cabinet specimens or a bottle full of experimental material or a Latin binomial in a textbook. It is an existent, tremendous population of living individuals whose identities and dissimilarities, whose divergences from all other populations, whose origin in some remote past and extension thru actual generations and years of time, whose position on these particular trees in these particular valleys and everywhere over these miles of actual country—it is this reality which, to us, constitutes the species problem.
But we have met erinacei everywhere across the miles of Indiana and across Ohio. This morning we found it in the stream valleys and over the hillsides of West Virginia. At noon we still found it in more rugged country in the heart of the mountains, and now, near the end of the day, our road-signs read Maryland and we know we are near the crest of the Alleghanies. We get out of the car and climb the hillside. It is thick woodland and we find only stray galls now and again. They are smooth and naked specimens, for aught we can tell like the smooth galls of erinacei. A mile down the road we find an open meadow where two isolated trees offer promise of richer collections. The farm boy helps, and we collect more smooth galls while we wonder about the varied mixture which spreads so many miles back of us. It is drizzling now, and sheets of fine snow come whirling off the mountain, but we espy a tree in the next open, and in the gathering dusk find—many naked but only two spiny galls for our collections! We return to the car, wondering what is the matter with the sample.
Before we sleep that night we shall have worked our way down into the valley of the Potomac. On the next day we shall collect across the valley of the Shenandoah, and in the days that follow out onto the sand coast of Virginia and southward along the shores of the Carolinas. Within a few months we shall breed the insects from the smooth galls we gather, and then we shall know that from the Maryland line to the coast we were dealing with pezomachoides, which is another species. At first the population was a mixture of insects of erinacei and pezomachoides and hybrids between, and it was only near the ocean that we found pure colonies of this new species, altho the typically smooth gall of pezomachoides was dominant to the western limits of the influence of the insect. But pezomachoides, altho very similar to erinacei, varies within limits that are different from those of erinacei. We shall find pezomachoides wherever we collect, until some day we turn in from the coast toward the heart of Georgia and meet a larger form of naked gall which will be derivatus; and then when we cross the southern front of the Blue Ridge and go into the Cumberlands we shall find the insect called advena, and in Kentucky it will be ozark, and in southern Indiana it will be erinacei again. In each area we shall find a population obviously related to every other, but each will constitute a population whose limits of variation are different from those of any other. Sometimes these limits overlap, sometimes they are wholly within, sometimes they are wholly without the limits for each other species. Sometimes the populations are wholly segregated by mountain crests and divides, sometimes the populations hybridize and intergrade over broad zones of transition; but always each population is distinct in a great area which is the heart of the range of each species. This is the picture which has gradually developed over the 32,000 miles, thru the 12 years during which we have pursued species.
And now, one confusion needs explanation before we are finished with this part of our discussion. It must be pointed out that there is a biologic concept called species and a taxonomic category called species, and that the two are not always synonymous. The concept we have developed is the biologic concept to which all except the taxonomists must refer whenever they consider the problem of species. This is the sense in which even taxonomists, including ourselves, intend the word when it is used in most biologic connections. But this biologic species is, unfortunately, the first of the taxonomic categories, the fundamental unit with taxonomic significance. As a category taxonomists may label this a geographic variant, a variety, a subspecies, or a species. There is no uniformity in current practice, and the only attempts at uniformity have been based on purely artificial distinctions. The resolution of the terminology must take into account as a question of convenience that no category higher than the genus may be written into the nomenclature, and as a matter of fact that there are often three or four degrees of phylogenetic affinities which may be recognized below what seems to be best called a genus.
The ichthyologists believe that they have the solution in calling this lowest category a species and the second category a genus, thereby making their order the equivalent of many a genus among insects. I interpret the mammalogists to mean that they call a Mendelian race a variety, and the fundamental taxonomic unit a subspecies, which they imply is an incipient “species” (their next category). The botanists, as nearly as I can perceive, call their lowest unit either a variety or a species, depending upon its remoteness from the native heath of the botanist and his field of experience.
I am at a loss for a solution of this difficulty. It seems unreasonable to expect that this first category will ever be called anything but a species by biologists who are not systematists, and in that sense I shall use the word in general discussion in this study. It will be impossible to adopt this meaning in our system of classification without inventing a new name for a category between this and the genus, and I have not the temerity to propose such a name while taxonomists are as far removed from biologic realities as the codes of nomenclature and much current systematics would indicate. Consequently, in the systematic portion of this paper I have adopted the term variety for the category which, after all, fulfills the species concept. I can only plead that I am conscious of the inevitable confusion this involves, and desirous of making amends as soon as some one proposes a solution—but I shall look for a solution that will coördinate biologic concepts of species with questions of convenience in systematic botany and zoölogy.