The Gall Wasp Genus Cynips: A Study in the Origin of Species/Hybridization
HYBRIDIZATION
Altho we have evidence that new species most often originate thru mutation and subsequent isolation, the possibility still remains that species may on occasion have hybrid origins. Mendelian hybrids in the midst of populations in which mutations have occurred have already been described in this paper (pp. 49, 53), with the suggestion that such hybrids are submerged, or that they give rise to local variations of the original population, or that they may after considerable time change the complexion of the old species and thus give rise to a new species. But the question is raised whether hybrid individuals originating from interspecific crosses may give rise to a third species without the replacement of either of the parental stocks.
Jeffrey's charge (1925-1928) that Drosophila melanogaster has had a hybrid origin pertains to the product of inter-specific hybridization; the geneticists have considered the criticism unimportant probably because they have in mind such Mendelian races or mutant individuals' as may readily be admitted to have entered into the constitution of probably every species. The real issue is evidently that which Lotsy (1916), Jeffrey, and many others have had in mind, but the solution must depend upon a knowledge of hybrids and species in nature, as well as upon the more cryptic means which Jeffrey would employ.
Everyone who has studied a large group of closely related species in the field knows that hybrid individuals of apparently inter-specific origin are not uncommon in the transition areas that usually occur between related faunas and floras, especially in the relatively uniform eastern two-thirds of the United States. The recognition of these individuals as hybrids depends, of course, upon the recognition of combinations of characters typical of Mendelian heredity, and of such graded series as we would be led to expect from crosses in which many of the characters were controlled by multiple factors in heredity. Further than that, we may expect that hybrids will occur within or between the areas occupied by the two hybridizing species.
On the above bases, hybrid populations are recognizable among the Cynipidae of the northern half of the Lower Peninsula of Michigan; thruout most of New England south of the Androscoggin River; in the southern third of Indiana and the adjacent areas of southern Illinois; in the Southeast wherever the northern faunas of the Appalachians come into contact with southern species at lower elevations, particularly in the eastern two-thirds of Kentucky and in the Cumberlands and the lower hill country of central Tennessee; and farther west in more limited areas that lie between practically every one of the cynipid faunas all the way to the Pacific Coast. I have similar hybrid series in my European collections of Cynips from more northern Denmark, the southernmost portion of Finland, from Bohemia, and from the upper Danube valley.
In some of these localities, as for instance in the neighborhood of our own laboratory at Bloomington, Indiana, the hybrid individuals may constitute 30 to 50 per cent of each collection. In places in the Cumberlands of Tennessee the hybrids may amount to 80 per cent or more of the cynipid populations. Whether the areas of transition among the Cynipidae are the same as those among other organisms must be determined by studies on these other groups. Nevertheless, if Jordan's Law holds as often as it would appear, species usually have close relatives in adjacent areas, whatever group of plants or animals they represent, and such close relatives are usually fertile inter se and should give rise to inter-specific hybrid individuals as often as we have found them among the Cynipidae.
But do such inter-specific hybrids ever give rise to populations that deserve to be called species?
It must be remembered that transition zone populations grade in every direction into the pure populations between which they are hybrid. Any portion of such a hybrid population is different biometrically from any other portion of that population. The genes available at one point in the transition zone are not equally available at every other point in the zone. There is no common heredity within the population. It does not satisfy our concept of a species (p. 20), no matter how extensive the area over which it occurs.
But if such a population, of hybrid origin, should in some way become isolated, then it might in the course of time become a fairly uniform population. Relieved from the continual introduction of genes from the parental stocks, the hybrids might finally achieve a thoro exchange of genes thruout the whole of the population. Then a sample taken from any part of the range would vary within the same wide but uniform limits typical of every other sample of the population. It would then satisfy our concept of a species, for it would have a common heredity.
It is this problem, the attainment of homogeneity out of a hybrid population, that has become the immigration problem of the American people in the last half-century. Whether homogeneity is a biologic virtue or not, the statesmen have shown themselves good taxonomists in their insistence that we cannot become a true species until barriers are erected to protect us from continued contributions of parental stock. Whether in peoples or insects, the melting pot cannot blend diverse materials that pour in too rapidly.
Now it becomes obvious why hybrid populations cannot often give rise to new species. Such inter-specific hybrids must usually arise in limited areas between the parental species. As long as the area is limited and the parents are close at hand, the hybrids will continue to be hybrids of every shade and extreme and intermediate combination of parental characters. On theoretic grounds, it would appear that hybrids may become species only when geographically removed or in some other way isolated from the parental stock. These circumstances would seem so rare that we cannot believe that hybrids account for the origin of many of the species with which we are acquainted in the field today.
And yet, there is one group of species among the Cynipidae which would seem to have had hybrid origin. These species occupy that very portion of the northeastern United States in which so many of our biologic studies have been pursued. Cynips erinacei, to which repeated reference has already been made, is the most certain of these cases in the genus Cynips.
Erinacei, it will be recalled, is a highly variable species occupying about 500,000 square miles of the area which we have just defined. It is unique among Cynips in the extent of its individual variation. The extreme individuals of the group have previously been considered representatives of two distinct species, but the specific unity of erinacei is affirmed by the existence of every type of intergrade between these extreme individuals, and by the occurrence of all these variations in every large series which we have from nearly a hundred localities well spread over the range.
The interpretation of this population has become possible thru the discovery of its closest relatives, Cynips wheeleri to the north and Cynips derivatus to the south of the area occupied by erinacei (see map, fig. 63). Erinacei occurs exactly where we might expect a hybrid of wheeleri x derivatus origin.
Wheeleri is a uniformly small insect; derivatus is large; erinacei shows every gradation between the extremes. Wheeleri is largely black; derivatus is prominently rufous over most of the body; erinacei shows every sort of combination and recombination of these characters. The mesonotum of wheeleri is largely smooth and naked; the mesonotum of derivatus is rough and more hairy than in any of the related species; the mesonotum of erinacei varies from smooth and more naked to rough and hairy, again showing every gradation and combination between the supposed parents. The galls of wheeleri are ellipsoidal, polythalamous, and uniformly spiny; those of derivatus are spherical, one- or two-celled, and uniformly naked; the galls of erinacei (figs. 312-315) show these extremes and a remarkable series of every conceivable intermediate between and combination of these extremes. Detailed descriptions of the insects are given in the systematic portion of this study. Remembering that many of these characters are probably controlled by multiple factors in heredity, erinacei appears as just that variable combination of characters which we might expect from a wheeleri x derivatus cross.
This interpretation finds confirmation in such series as the 107 insects which we have from Meadville, in the northwestern corner of Pennsylvania. In this series 41 per cent of the individuals show clear evidence of wheeleri affinities, 37 per cent are practically identical with our Alabama and Georgia material of derivatus, and 21 per cent show gradations between wheeleri and derivatus that would pass as good erinacei. It is certain that erinacei is not of present-day origin, for wheeleri and derivatus are separated thruout most of their ranges by several hundreds of miles. This distance is too great to allow any present-day hybridization of pure stocks of wheeleri and derivatus. The occurrence of the three types of insects at Meadville is, however, the picture of segregation from a hybrid population which continues to proclaim its parentage. Similar series of erinacei are in our collections from many other localities.
There is, of course, a ready explanation of a past contact of wheeleri and derivatus. If wheeleri was in existence during the Pleistocene glaciation, its range must have been pushed southward at least as far as southern Indiana and the Ohio River, and still further south in the eastern mountains. If the range of derivatus at that time was comparable to the range of the present-day species, wheeleri first hybridized with derivatus in the Ohio Valley and in the valleys adjacent to the southern Appalachians. But as the glaciers retreated to the north, wheeleri retreated with them, leaving derivatus far to the south, and a tremendous area between where the hybrid wheeleri x derivatus found its opportunity to breed and interbreed until it had acquired the uniformity which warrants its present recognition as a species.
The Pleistocene origin of erinacei finds confirmation in three other hybrid species of Cynips in the same Northeastern area of the United States. These species are Cynips fulvicollis, C. gemmula, and C. macrescens. The detailed data are presented in the systematic portion of this study. They parallel the case of erinacei. These four cases account for all of the stocks of Cynips which are known to have penetrated far enough into the Northeast to have developed sub-Canadian varieties which would have been affected by the Pleistocene glaciation. These four cases are the only ones among the 93 species of Cynips which we now have reason for believing of hybrid origin, except for C. advena, of the Cumberland Highlands, which we shall show in a moment to date also from the Pleistocene. Of all the areas occupied by Cynipidae in the United States, this Northeastern area, the Eastern mountain country, and areas immediately adjacent to these are the only ones which had a Pleistocene history that would have provided the opportunity for the multiplication of hybrid individuals and which would have offered a subsequent isolation sufficient for the origin of new species. The apparent restriction of the hybrid Cynips to those areas seems confirmation of our explanation.
The fifth case C. advena, which we have mentioned, involves a wheeleri x pezomachoides cross in the Cumberland Highlands and the Appalachian areas of central and eastern Tennessee, North Carolina, northern Georgia, and the borders of adjacent states in the South. Individuals that are certainly hybrids between wheeleri and pezomachoides are common from northern New England to Georgia wherever wheeleri still comes into contact with pezomachoides. Such hybrid individuals may be interpreted with much certainty and they confirm our explanation of advena in the Southern Highlands. The unusual amount of variation, the occurrence of segregates that appear as pure wheeleri or pure pezomachoides, and the geographic position of the hybrid between the supposed parents is, as with erinacei, the basis for recognizing the origin of advena. Segregates of wheeleri are more common in advena than in erinacei, probably because advena is not yet free from current contributions from the nearly pure populations of wheeleri which occur in the southern mountains. The galls of advena are interesting because they run largely to the smooth form typical of pezomachoides, indicating some dominance of pezomachoides characters; but advena galls are very finely bristly, and large series do include a few that are as strictly spiny as those of wheeleri.
We have then, out of the 93 species in the genus, the following which we would recognize as of hybrid origin:
| C. fulvicollis (= C. canadensis x major) |
| C. gemmula (= C. suspecta x fuscata?) |
| C. erinacei (= C. wheeleri x derivatus) |
| C. advena (= C. wheeleri x pezomachoides) |
| C. macrescens (= C . scelesta x opima) |