The Gall Wasp Genus Cynips: A Study in the Origin of Species/The Isolation of Species
THE ISOLATION OF SPECIES
It now remains to show how such mutant races as we have noted in the preceding sections of this study may be transformed into such large and relatively uniform populations as satisfy our concepts of species.
A mutant individual is still far from constituting a species. Its survival depends in the first place upon the condition that the new, mutant characters shall not interfere with the health of the organism. To the lethal characters which the geneticists find linked with so many mutating genes in the laboratory there are added many other controlling conditions which would kill out a large proportion of our laboratory mutations if they were exposed to the rigors of existence in nature. This negative application of the theory of natural selection would seem axiomatic—tho I must reassert, along with Crampton (1928) and others, that this is the chief aspect of the Darwinian hypothesis which seems necessary to explain species as we have met them.
But allowing that a mutant is capable of existence, its greatest handicap is the fact that it usually develops in the midst of a population so similar to itself that it is capable of interbreeding and will interbreed with this parental stock. If the mutant characters involve only a single pair of genes and if they are dominant, they will gradually disturb the conspectus of the parental species which, in the course of considerable time, should thus become a new species in the territory formerly occupied by the parental stock. But, on the other hand, mutant characters in nature are probably recessive as often as they have proved in the laboratory (Morgan 1928:59-71), and there is increasing evidence that many characters involve multiple factors in heredity. In these events, the mutant has only a remote mathematic possibility of modifying the general aspect of the parental species, and in my judgment there is every probability that it will be submerged in the parental population. It becomes apparent that the transformation of a mutant race into a species must ordinarily depend upon some sort of isolating factor which will prevent its interbreeding with closely related stocks.
Now, this isolation of species, which we may postulate, is precisely the condition which we find among the most closely HOST—GEOGRAPHIC ISOLATION OF CYNIPS
| Subgeneric names: C = Cynips, An = Antron, B = Besbicus, P = Philonix, At = Atrusca, Ac = Acraspis | ||||||||||||||||
| Q. ilex | Q. robur var. | Q. garryana | Q. Douglasii | Q. dumosa Q. durata et al. |
Q. chrysolepsis | Q. lobata | Q. Prinus Q. Michauxii, etc. |
Q. alba, etc. | Q. lyrata Q. Mühlenbergii |
Q. macrocarpa Q. bicolor |
Q. stellata, etc. | Q. minima, etc. | Q. Gambelii, etc. | Q. grisea Q. arizonica, etc. |
Q. undulata, etc. | |
| Northern Europe | 3C | (1 var. each, C. folii, C. longiventris, C. divisa) | ||||||||||||||
| Central Europe | 5C | (1 var. each, C. folii, C. longiventris, C. divisa, C. agama, C. disticha) | ||||||||||||||
| Mediterranean | 1C | 2C | ||||||||||||||
| Northern Pacific Coast | 1B | |||||||||||||||
| Northern California | 1B, 1An | 1B, 1An | 1B, 2An | (1 var. each Antron echinus, A. teres) | ||||||||||||
| Central California | 1B, 1An | 1B | 1An (1 var. each Antron echinus, A. teres) | |||||||||||||
| Southern California | 1An | 1An | ||||||||||||||
| San Bernardino | 1An | |||||||||||||||
| San Jacinto | 1An | |||||||||||||||
| Canadian, Eastern U.S. | 1Ac | 1P, 1Ac | 1Ac | |||||||||||||
| Northern Central U.S. | (1 var. each Acraspis hirta, A. gemmula) 2Ac | 1P, 1Ac | 2Ac (1 var. each Acraspis hirta, A. villosa) | |||||||||||||
| Coastal Plain | 1Ac | 1Ac | 1Ac, 1At | |||||||||||||
| Southern Central | 1Ac | 2Ac | 1Ac, 1At | |||||||||||||
| Southern Highlands | 1Ac | 1Ac | ||||||||||||||
| Florida | 1Ac | 1P | 1Ac | 1Ac | ||||||||||||
| Ozark Area | 1P, 1Ac | 1P | 1P, 1Ac | 1Ac, 1At | ||||||||||||
| Kansas-Oklahoma | 1Ac | |||||||||||||||
| East Texas | 1P | 1Ac | 1Ac, 1At | |||||||||||||
| Central Texas | 1Ac, 1At | |||||||||||||||
| West Texas | (1 var. each, Acraspis arida, A. villosa, Atrusca dugèsi, A. bella) 2Ac, 2At | |||||||||||||||
| Southern Rockies | (1 var. each, Acraspis pezomachoides, A. hirta, A. villosa) 3Ac, 3At | 1Ac | 1Ac | |||||||||||||
| Southern Arizona-New Mexico | (1 var. each, Acraspis nubila, A. villosa, Atrusca dugèsi, A. bella)2Ac,2At,1P | |||||||||||||||
| Apache Trail | (1 var. each, Acraspis villosa, A. conica) 2Ac, 1At | |||||||||||||||
| Utah | (1 var. each, A. hirta, A. villosa) 2Ac | |||||||||||||||
| Central Mexico | 1Ac, 1At | |||||||||||||||
Further discussion of this question must be limited in this place to a presentation of the data on the host and geographic distribution of our 93 species of Cynips. The detailed records and maps for each of these species are given in the systematic portion of the study. These data seem to lead to the same conclusions which we framed (Kinsey 1923) for 63 species of the gall wasp genus Neuroterus. In each geographic area there is but a single species of any phylogenetic stock on any given host. The wide applicability of the rule should be evident from the summary of the host and distribution data which /ire given in the accompanying table, and the same data are more vividly portrayed on the phylogenetic maps which constitute figures 8 to 13.
With one possible exception (C. bifurca) every one of the 93 species of Cynips is in a host-geographic area distinct from that occupied by any other derivative of the same specific stock. In 59 of the cases the isolation extends back to the subgeneric stock. This is remarkable. If our conclusions on the phylogenetic history of this genus are correct (see pp. 61 to 77) these subgenera have been separate since the Miocene, a matter of possibly ten to twenty million years. The subgenera originated in the Southwest, where the first subdivisions must have occurred. The extensions of these stocks out of the Southwest must have occurred before the Great Basin to the west or the Great Plains to the east became too arid to support an oak flora, which would suggest that it has been at least a million years since some of these stocks first came into their present host-geographic areas. The ranges of these species cover continuous instead of discontinuous regions, indicating that in all of that time no new species has succeeded in developing inside the range of any of these old species. This does not mean that the species have remained unchanged, for mutants developing in the old range and hybridizing with the parental types would remake the old species into new, hybrid species. But as for the multiplication of species, new populations can have developed only near the edge of an older range where they could spread into unoccupied territory away from the handicaps afforded by close relatives.
Another source of evidence that new species develop on the frontier of the parental range is afforded by the distribution of the short-winged species in Cynips. These insects are without doubt phylogenetically more recent than their closest long-winged relatives. Every one of the nine species which occur in southern Arizona and New Mexico, where these stocks first developed, is a long-winged species, just as the parental stocks undoubtedly were. The 42 short-winged species all lie more remote from the center of origin of the genus. Wherever long-winged and short-winged species exist in the same specific stock, the long-winged species are nearer and the short-winged species more remote from the Southwest. Thus, to cite specific instances, Cynips dugèsi has its only long-winged representative in southern Arizona and New Mexico, and shorter-winged representatives further north in the Rockies, eastward in West Texas and (C. cava) Central Texas, and southward in Central Mexico. Cynips villosa has its most southwestern varieties long-winged, with shorter-winged varieties occurring northward in Arizona, New Mexico, Colorado, and Utah, and eastward thru the Middle West. Cynips mellea, with ten varieties, spreads from the southwest across to the Atlantic seaboard, retaining its long wings everywhere except in the most remote corner of its range, in the very southeastern corner of the United States, where it is now developing short-winged species.
There are a few further aspects of the distribution of Cynips which have some bearing on the problem of species.
1. The maps of the several species show that ranges vary from relatively small (e.g. the San Bernardino area of California and the Apache Trail area of Arizona) to very large (e.g. the whole of Central Europe or the northeastern quarter of the United States), but they more often involve areas of considerable size. This means that species are relatively stable entities which are not easily disturbed by mutations or immigrations of new stocks. If such new stocks are not submerged, interbreeding must proceed fast enough over these areas to maintain the uniformity of the species. It is difficult to conceive how this can occur in the largest of the areas.
2. The ranges of these species are more often large in the regions which are topographically most uniform. The relatively rugged Central European area is an exception to this rule. Nevertheless, the general condition indicates that the isolation afforded by topographically diverse areas may be a considerable factor in favoring the multiplication of species.
3. There are, on the other hand, areas of relatively uniform topography within which distinct species have been isolated. This is true of the Canadian, Coastal Plain, northern Middle West, southern Middle West, Ozark, east Texas, and Central Texas areas in the United States. None of these areas appears to be bounded in such a way as to furnish enough geographic factors to isolate species. Distance must be an isolating factor in the origin and propagation of species. This item is never listed among factors of distribution, but I am inclined to believe that in an area like the eastern United States it is one of the most important factors. Distance would act as a barrier by preventing an interchange of genes thruout a large population, thus favoring the development of local races.
4. Another fact showing the importance of isolation is the common occurrence of hybrid individuals in transition areas between species. These hybrid populations are so extensive in the more uniform, eastern two-thirds of the United States that they are in part responsible for the widespread opinion that there are no species but highly variable complexes ranging over this part of this country. This opinion is not justified by our study of Cynipidae. In series of related species, east of the Rockies (figs. 37, 50, 59, 63, and 70), we do not find continuous gradations from one to the other end of the group, but first an area with a pure population, then an area of hybrid individuals, then another pure population, another transition population, and so on across the country. The ornithologists have ruled that the term species should be restricted to populations between which there are no intergradent individuals, and the term subspecies to populations between which such intergrades do exist. They imply that the presence or absence of hybrid individuals in the transition zones is a matter of phylogenetic significance. With this I cannot agree. To follow this rule, the ultimate phylogenetic unit (the species concept of biologists in general, the result of the most recent mutation which has been sufficiently isolated to give rise to pure populations) would usually rank as a species in the Far West. In the East populations originating in precisely the same way and representing the very same stage in phylogeny would be called subspecies.
5. The range of each species of Cynips coincides to a large degree with the range of every other species of Cynips of that part of the country. The maps thruout this paper will show the location of such concommitant ranges. These areas bear some resemblance to the life zones of current repute, but the ranges of no two species are precisely the same, and there are outstanding discrepancies (e.g. the range of Cynips pezomachoides pezomachoides vs. the range of Cynips mellea Carolina). These generalized areas certainly bear no relation to the life zones hypothesized by C. Hart Merriam (1898) and since then propagated by the U.S. Biological Survey.
6. The data given in a later section of this paper on the phylogenetic history of Cynips, and summarized in our phylogenetic maps (figures 8-13) suggest that the location and shape of these generalized areas of distribution are in part a result of the place of origin and path of migration of each subgenus. If this is so, the picture may be different for each group of organisms, and it becomes doubtful how far these approximations to life zones in Cynips may be extended to other groups of organisms.