PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Vol. 48, No. 6, pp. 131-140 December 21, 1993 FIFTY YEARS OF PROGRESS IN RESEARCH ON SPECIES AND SPECIATIONanne Biological Labo *oods Hole OceiograSc Stion Ub By «7. Ernst Mayr Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 Woods Hole, MA 02543 Adapted from a lecture delivered at the Golden Jubilee Celebration of the publication of Systematics and the Origin of Species at the California Academy of Sciences on October 16, 1992. Received December 31, 1992. Accepted February 11, 1993. Historians of science have taught us how much the nature of the genetic material, resulting in one can learn from studying the history of a field the theory of particulate inheritance. However, of science. This is excellently illustrated by the they drew from this the wrong conclusion as far history of evolutionary biology as a whole, and as evolution is concerned, claiming that new spe- by our growing understanding of species and spe- cies were produced by new mutations in a single ciation, in particular. saltation, completely rejecting Darwin's theory After 1859, two of Darwin's theories were ac- of gradualism. Their opponents were the biom- cepted almost at once. First, evolution as such, etricians, such as Pearson and Weldon, who cor- and secondly, the branching theory of common rectly insisted on the gradualness of evolution descent. Natural selection was with almost equal but incorrectly claimed that inheritance was unanimity rejected, being accepted only by a small equally gradual, that is, blending. As far as ge- group of naturalists. This was not too surprising netics is concerned, the Mendelians were right; since at that time no one understood variation as far as evolution is concerned, the biometri- and its origin. Finally, an active, almost violent, cians were right. There was no genuine popula- controversy developed over two other major tion thinking in either camp and the biometri- Darwinian theories, that of speciation and that cians and other opponents of Mendelism adopted of evolutionary gradualism. As a matter of fact, Lamarckian inheritance in order to account for Darwin himself was vacillating with respect to the gradualness of evolution. these two theories. In the ensuing years, the gap between the two Let us now proceed to the year 1900 and the camps narrowed appreciably as a result of the rediscovery of Mendel's laws. At that time, two new findings of genetics and systematics. Even- camps became established in evolutionary bi- tually the biometricians disappeared from the ology. One consisted of the Mendelians repre- scene and were replaced by a group of evolu- sented by Bateson, DeVries, and Johannsen. They tionists I shall call the naturalists. At the same were strict typologists who saw discontinuity ev- time the Mendelians were replaced by the pop- erywhere in nature and applied this correctly to ulation geneticists. U3 i] 132 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES, Vol. 48, No. 6 The major difference between these two new the causes of evolutionary change and speciation constellations of evolutionists was their sphere is well-documented by the widely read work of of interest. To document this it is necessary to Robson and Richards (1936). define "evolution." The geneticists adopted a re- At that time there appeared on the scene a ductionist definition that, as we now see it, was young beetle systematist who had grown up in quite misleading. They defined evolution as the the thinking of the Russian tradition with its change of gene frequencies in populations. This emphasis on organic diversity but who also had definition emphasized the wrong level of activ- had the advantage of nearly 10 years of work in ity. Evolution is the story of adaptation, of the (and stimulation by) an American laboratory in develoment of new kinds of animals and plants, population genetics. I am, of course, referring to of the origin of modes of reproduction, and of Theodosius Dobzhansky, who integrated the two all aspects of the history of organisms. Evolu- great traditions I have just described and who tionary biology deals not merely with genes but produced what could almost be called the Bible with two major sets of problems: (1) the acqui- of the evolutionary synthesis, his magnificent sition and maintenance of adaptedness, and (2) Genetics and the Origin ofSpecies ( 1 937). In spite the origin and nature of organic diversity. ofsome omissions and even a few outright errors, The geneticists dealt only with the problem of this work contained the gist of the new paradigm adaptedness. Their approach was reductionist, of the evolutionary synthesis. Within a few years concerned with the genetic changes within a pop- the synthesis was completed in zoology, as doc- ulation. It dealt only with the time dimension, umented by the publications of J. Huxley ( 1 942), with what one might call "vertical evolution." E. Mayr (1942), G. G. Simpson (1944), and B. The naturalists concentrated on the other as- Rensch (1947). In 1950 G. Ledyard Stebbins pect of evolution, the nature of organic diversity. brought in botany, showing in his monumental Their interest was in populations, species, and Variation and Evolution in Plants that the prin- macroevolution, with particular emphasis on the ciples developed in the synthesis were equally process of speciation and the geographical com- applicable to plants, contrary to the claims of ponents of evolution, what one might call the some other botanists. This is also true even for "horizontal dimension." This difference in basic the numerous special phenomena and processes concerns was, in my opinion, more important encountered in plants (Mayr and Provine 1980). than their differences in the genetic interpreta- The synthesis of the 1930s-40s was the end of tion. old arguments, in particular the final refutation There were also conspicuous national differ- of the various non-Darwinian theories of evo- ences. Evolutionary genetics flourished in the lution, but it was also the beginning of a new set Anglophone countries as indicated by the names of controversies. My limitation of time permits T. H. Morgan, H. J. Muller, S. Wright, R. A. me only to deal with those concerning species Fisher, and J. B. S. Haldane, while the study of and speciation. Even under that limitation one organic diversity in an evolutionary manner has to cover scores of books and thousands of flourished in Russia, Scandinavia, and Germany, smaller publications. This forces me to present but was poorly represented in English-speaking my findings as the final conclusions of long drawn- countries, in the United States by F. B. Sumner, out arguments and perhaps seemingly in a rather L. R. Dice, D. S. Jordan, and J. Grinnell, and in dogmatic manner. England by E. Poulton and E. B. Ford. I am often asked, what in particular had been We thus had two rather different traditions in my own contributions to the Evolutionary Syn- the 1920s and early 1930s: an Anglophone ge- thesis? They can be recorded under the two head- netic tradition studying the vertical component ings: species and speciation. ofevolution, that is, adaptive genetic change, and an essentially continental European tradition in systematics studying the horizontal component, Species that is the geographical changes of populations leading to speciation and macroevolution. As late As far as species are concerned, I demonstrated as the early 1930s (up to 1935 and 1936), several the weakness, if not invalidity, of the previously authors declared that the gap between the two most popular species criteria, particularly as stat- camps was unbridgeable. The confusion about ed in the morphological and genetical species MAYR: SPECIES AND SPECIATION 133 definitions. Instead, I promoted acceptance of definition: "an evolutionary species is a lineage the biological species concept with its emphasis (an ancestral-descendant sequence of popula- on populations and on reproductive isolation: tions) evolving separately from others and with "A species is a group of interbreeding natural its own unitary role and tendencies." The re- populations that is reproductively isolated from placement of the clear-cut criterion of the bio- other such groups." I was not the first to adopt logical species (reproductive isolation) by such the biological species concept but there is little undefined vague terms as "evolutionary tenden- doubt that it was the support I gave it in my 1 942 cies" and "evolving separately" does not permit book that led to its rapid subsequent adoption. discrimination between good species and iso- In particular, I pointed out the weakness of the lates. It is not applicable to polytypic species that morphological definition because it provided no contain geographical isolates. Nor does it even criterion by which to determine the status of permit the delimitation of an "evolutionary spe- highly distinct intraspecific variants. Another cies" within a phyletic lineage. (For a more de- weakness of the morphological definition was its tailed discussion see Mayr 1988a:323.) Simpson inability to cope with a phenomenon for which was not raised as a naturalist and, in spite of his I introduced the term "sibling species," that is, biometric work on samples of fossils, his mate- morphologically virtually identical populations rial did not allow a study of geographic specia- that were nevertheless reproductively isolated. I tion. expanded my treatment of sibling species in 1 948 Another group of opponents of the biological and 1963, and their extreme frequency is now species concept consisted of certain museum and generally acknowledged. Previously most ofthem, herbarium taxonomists. They had to assign spec- if recognized at all, had been listed as biological imens to species, particularly such from widely races. After 25 years of argument, I finally per- distant geographic locations, and were puzzled suaded even Tracy Sonneborn to recognize the what criteria to use in order to infer whether or so-called varieties of Paramecium as sibling spe- not these isolates were reproductively isolated.