Geographic Isolation and Speciation
The other major advance has been the gradual realization that in the general case (with the exception of the origin of new species in some plants by polyploidy), speciation, that is to say, the splitting of one species into two separate species, has always been preceded by some form of spatial. generally geographic, isolation, such as is entailed in the formation of geographic races. Some of Darwin's earliest thoughts on evolution arose from observation that “the several islands of the Galapagos Archipelago are tenanted… in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world”99. As early as 1844 he concluded100 “that those areas in which species are most numerous have oftenest been divided and isolated from other areas, united and again divided…” and “that isolation is the chief concomitant or cause of the appearance of new forms”. Yet in the “Origin” he allows the possibility of what we would now call “sympatric speciation”, and declares101 “I believe that many perfectly defined species have been formed on
[Footnote] 99 Origin: 339.
[Footnote] 100 Life and Letters, II: 28.
[Footnote] 101 Origin: 150.
strictly continuous areas” so that he was left with the major difficulty why organisms do not all intergrade—the problem J. S. Huxley102 has termed “how discontinuity of groups is introduced into the biological continuum”. The subject was later touched on by A. R. Wallace103, who asked Darwin, “How do species ever arise except when a variety is isolated?” but the question was rhetorical. The same year Moritz Wagner, in Germany, insisted on isolation by geographic barriers, and in 1876 Darwin admitted to Wagner, “I do not believe that one species will give birth to two or more species so long as they are mingled together in the same district104”. Two years later105 he coined the word “specification” (“speciation” of modern terminology) for the process contrasted with “phyletic evolution”. Hutton, in New Zealand, clearly saw that this emphasis on isolation was the foundation of the new Darwinism, so that he wrote106: “In addition to selection, isolation is shown to be necessary for organic evolution; and this, in my opinion, is the only real advance since Darwin's death”.
The flowering of genetics at the beginning of the century led to long neglect of the factor of geographic isolation until it was revived by Rensch, and more strongly, by Ernest Mayr in 1940, in a paper107 which I personally rank as a milestone in my thinking, expanded in book form in 1942108. In the same year J. S. Huxley produced his “Evolution, the Modern Synthesis”, in which he still failed to give geographic isolation its due place in speciation; the triumph to Mayr's ideas came in 1943 when Huxley accepted and incorporated them in the American edition of his text book.
The process of geographic speciation can be seen in progress wherever local populations, more or less isolated, have diverged in response to local differences in selective pressure. Geographic races, now often designated by trinomial names as Darwin predicted, occur in every group of organism and differ in physiology and ecology as well as morphology109. A simple case of speciation occurs when the
[Footnote] 102 Huxley, J. S. 1940. The New Systematics, Clarendon Press: 2
[Footnote] 103 More Letters, 1: 294.
[Footnote] 104 Life and Letters, III: 159.
[Footnote] 105 Life and Letters, III: 160.
[Footnote] 106 Hutton, op. cit.: 5, 9.
[Footnote] 107 Mayr, E., 1940. Amer. Naturalist, 74: 249–78.
[Footnote] 108 Mayr, E., 1942. Systematics and the Origin of Species, Columbia Univ. Press.
[Footnote] 109 Mayr, E., 1947. Evolution, 1 (4): 263–88.
same stock has invaded an isolated area on two or more occasions; divergence of the early invaders has led to the development of some form of ecological, behavioural, physiological, or seasonal isolation so that the new arrivals do not blend. Two notably cases at our own back door are the two species of white-eye formerly on Noifolk Island (a third final invasion having arrived about 50 years ago) and at Lord Howe Island, and the two parakeets of New Zealand derivation occupying Antipodes Island110. A further much publicised example is the formation of a ring of subspecies in the large gulls of the northern circum-polar shores—close relatives of our black-backed gull. Each of the forms grades into its neighbour, from Europe eastward round the Arctic Ocean to North America Western Europe has been occupied by races from opposite ends of this chain, which there behave as almost totally different species, the herring and lesser blackbacked gulls These two, though connected by intergrading forms (or subspecies) are themselves isolated, not only in plumage, but in voice, breeding season and migration habits, and very rarely hybridise. Since “there is no geographic speciation that is not at the same time ecological and genetic speciation”111 owing to the differences of selection pressure in different environments, there are good chances that two geographic races will diverge during their separation to an extent that when they meet through “double invasion” or “back invasion” or breakdown of barriers, they will prove better adapted to separate ecological niches in their common habitat112. When two daughter stocks meet again after geographic isolation they commonly show such ecological differences, which are subsequently accentuated by selection operating on the principle that no two species can survive if they occupy an identical ecological niche (Gause's Law). So we find the two white-eyes of Lord Howe and the two parakeets of Antipodes Island differ in size and, I suspect, in food requuements, the two British gulls in habits and the 14 species of Darwin's finches of the Galapagos, the result of multiple repetition of this process, have diverged to occupy a great variety of ecological positions in the Galapagos Archipelago113. Such “adaptive radiation” is the process whereby, by analogy, Hutton supposed the moas became diversified after an early archipelagic stage in the history of New Zealand, so that they are now classed by Oliver in 7 genera and 27 species114.
[Footnote] 110 Fleming, C. A., 1952. N.Z. Sci. Rev., 10: 86.
[Footnote] 111 Mayr, op. cit., 1947: 285.
[Footnote] 112 Lack, D., 1947. Darwin's Finches, Cambridge: Univ. Press: 137–40.
[Footnote] 113 Lack, op. cit.: 102.
[Footnote] 114 Oliver, W. R. B., 1949 Dom. Mus. Bull., 15.