Author Institution: 3 Davie Circle, Chapel Hill, N. C

McAtee, W. L.

The Ohio Journal of Science

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SPECIATION WITHOUT ISOLATIONW. L. McATEE3 Davie Circle, Chapel Hill, N. C.Often we find in older discussions of evolutionary processes, statements possiblycorrect according to then existing information, but which subsequent accumulationof knowledge has rendered doubtful. In scarcely any instance has there beenmore frequent and rapid shifting of ground than in treatment of the general pro-position of isolation.Darwin recognized the importance of separation, both in artificial and naturalselection, but he did not especially emphasize it. Moreover, he did not agreewith Moritz Wagner "that migration and isolation are necessary elements forthe formation of new species" (?1912, p. 90). In fact he could not well do sowithout depreciating just so much the force of the arguments for natural selection.The latter is in fact only a form of isolation (by allegedly restricting breeding tosimilar favorable variants) as recognized by Romanes, and every admission as tothe importance in evolution of physical and other forms of isolation restricts thefield of natural selection. It is not my intention to review the literature of isolationas a factor in evolution but rather to comment briefly on various phases in whichthe idea has been presented. Geographical isolation as a necessary factor to theformation of varieties and species of organisms was an early postulate. This crudegeneralization could not be approved for exceptions to it were part of the knowl-edge of every naturalist. It is well known both that isolation is not necessary tocoexistence of related species, and that isolation does not necessarily result indifferentiation. Then in turn ecological or seasonal separation were appealed to,and these not covering every case, finally physiological and morphological isolationwere assumed as necessary to permit the development of specific differences amongforms obviously not separated in any spatial sense.It is unnecessary to cite cases typical of those that caused these changes of argu-ment on the part of adherents to the hypothesis that some form of isolation is anecessity for speciation. It will serve to consider a few instances in which allof the suggested forms of isolation except the morphological or physiological areexcluded. I may allude to the 8 species, more or less, of Antennaria that inhabitdry open places in the Northeastern States, to the same number of species of Bidens,all marsh plants of the same region. In Florida swamps there are 8 species ofTillandsia epiphytic on trees and 10 of Utricularia living in ponds and ditches.For a western example, there are 13 species, more or less, of Eriogonum in theMojave Desert. Hodges (1936) has reported the identification of 32 species(besides several varieties) of Fusarium growing on sugar beets in storage atSyracuse, N. Y.Woltereck (1933, p. 515) referred to "the fifty, about, endemic Haplochromisfishes living pell-mell in Lake Victoria, which, with one exception, are all unique, orthe three hundred gammarids in the bottom mud of Lake Baikal, or the twelve totwenty species of cyprinid fishes in Lake Lanao; we cannot deny some internalcauses of such differentiations or explosions, because they are not fifty or threehundred or twenty different localities (environments) within these lakes. And if weconsider, for instance, the hundreds of very different Radiolaria or Tintinnidae orCoccolithophorides living pelagically under identical conditions in the same regionand the depths of the Pacific Ocean, we face again the same necessity of admittingimmanent capacity and trend to change."Kofoid (1907, p. 503) stated that 50 species of Coscinodiscus occurred in Ant-THE OHIO JOURNAL OF SCIENCE 54(2): 104, March, 1954.No. 2 SPECIATION 105arctic plankton taken on one expedition between depths of 60 and 40 meters, andfurther, that in his own studies of the genus Ceratium of the dinoflagellates "in theplankton of the Pacific immediately at the surface, that is in the upper 2-3 meters,it is no unusual thing to take 25-30 species in a single 14-inch net."It can hardly be argued, with reference to these cases, that there are niches inthe necessarily very uniform environments named sufficient to accomodate such amultiplicity of species, but it can be postulated, although unreasonably so, thatthese species originated in spatially separated environments and congregatedlater. As a crucial case in which even such a far-fetched doubt cannot intrude, Iwould cite the tiny leafhoppers of the genus Erythroneura of which there are alreadyknown from the eastern United States about 50 species that live upon the leavesof grape. Several species so distinct that immature as well as mature examples canbe recognized at a glance, as well as others more difficult of separation, can be simul-taneously collected from a single plant, and individuals swarm.Now in a case of this sort where isolation, if it exists at all, must be off a physio-logical (intersterility) or morphyological type (differentiation of the genitalia),it would seem that it is not isolation that permits the species to develop but ratherthat development of physiological or morphological differences have brought aboutwhatever kind of degree of isolation that does exist. In other words, crowding in auniform environment would provide such a free opportunity for interbreeding thatspeciation would be prevented except for intervention of some internally developedmechanism for preventing free crossing.In such cases as that of the grape leafhoppers, and the example is merely ex-treme, not unique, it is clearly out of the question that specific characters could bedeveloped by a process of selection of beneficial variations, for the organismscontinuing to live together under identical conditions, there could be no advantagein merely being distinct. So far as advantage is concerned, such groups of speciescould just as well be one. If survival of the fittest ruled, there would be only onekind of grape leafhopper on the leaf at the same season, not several. Certainlyin a stable environment an organism can be fittest only by being superlativelyitself, not by diverging. Such examples are opposed to Darwin's postulate ofdivergence being promoted by natural selection as an advantage enabling occu-pation of new range, for in these instances divergence has clearly occupied withoutspread to new environment. The existing diverse species have manifestly deve-loped together in identical environment. The picture presented is of organisms inthe grip of an evolving power that brings about speciation without regard to en-vironment .The phenomenon probably may be taken as illustrative of what we shall find inmany groups when their specific differentiation is worked out according to the mostdiscriminative methods. Previously recognized specific groups will prove compo-site and we shall accordingly find numbers of species in environmental niches souniform as to rule out the influence of external factors upon speciation.Close analysis of groups has characterized modern work especially in insecttaxonomy and the use of genitalic structures for separation of species has becomeincreasingly prevalent. Peterson (1905, p. 224) has concluded from his work inthis direction (upon butterflies) that "specific divergences have their origin inmorphological differentiations in the sexual apparatus, which led to physiologicalisolation of groups of individuals and further that in the course of their isolation,indifferent and superficial characters by restricted breeding, become specific."The tenor of the argument is that in cases really testing the validity of theisolation hypothesis, the origin and maintenance of characters preventing inter-crossing has provided the degree of isolation necessary to the existence of closelyrelated species in identical environment. Hence, speciation brought on isolation,not the reverse. The process is orthogenetic evolution and it undoubtedly pre-vails even in cases where isolation is more tangible, but has been neglected as an106 W. L. McATEE Vol. LIVexplanation while other theories were having their day. Although endless discus-sion of external factors and their influence has served greatly to delay recognition ofthe fact, organic evolution clearly is in the main an innate process.LITERATURE CITEDDarwin, C. 1912? The origin of species by means of natural selection. . . . Reprint, 6th ed.,xix+501 pp. Hurst & Co., New York, N. Y.Hodges, F. A. 1936. Fungi on sugar beets. Phytopathology, 26: 550-563.Kofoid, C. A. 1907. The limitations of isolation in the origin of species. Science, N.S.,25: 500-506.Petersen, W. 1905. Urber die Bedeutung der Generationsorgane fur Entstehung der Arten.6th Congr. Internat. Zool., 213-224.Woltereck, R. 1933. Endemism and the probable primary causes of organic evolution. Philip-pine Agriculturist, 21: 513-520.

Speciation Without Isolation

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Speciation Without Isolation

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