Parasites. At a symposium on the co-evolution of parasites and their vertebrate hosts in Neuchatel, Switzerland in 1957, many cases of "host transfer" were discussed which prevent a complete parallelism between the classifications of parasites and vertebrate hosts. Also, the rate of morphological change in different lines of parasites is unequal (Mayr 1958b).
Evolutionary novelties. At the Darwin celebration organized by the University of Chicago Mayr (1959d) discussed the emergence of evolutionary novelties which neither Rensch nor Simpson in their recent books dealing with macroevolution had treated in detail. Mayr stressed the importance of a change in function and the fact that evolutionary novelties are in most cases acquired gradually. During the new wave of saltationism in the 1970s, this paper which established gradual evolution so well was almost completely ignored.
Chance and necessity. At another Darwin Centenary Symposium in Australia, Mayr chose as the subject of his contribution the eternal problem of chance or necessity in evolution, or as he called it, "Accident and Design, the Paradox of Evolution" (1962b). He again emphasized here that natural selection is a two-step process, the first step consisting of the production of genetic variability and dominated by chance (mutation and recombination), and the second step, the differential survival and reproduction of the new generation, determined by natural selection. Other aspects of adaptation and selection were also dealt with. His final conclusion was: "The solution of Darwin's paradox is that natural selection itself turns accident into design" (p. 14). He was aware, of course, that natural selection is always constrained by the potential of the existing genotype, i.e., by the preexisting general morphological construction of an animal or plant.
During the course of evolution the overall amount of variation produced varied and "periods of high phenotypic variation (as in the Pre-Cambrian, Cambrian) have alternated at other geological periods with periods of relative stasis. As natural selection is active at all periods, it would seem that evolutionary change depends more on the availability of variation than that of selection. [...] What environmental factors favor high variability? What others favor stability?" are challenging questions, as Mayr (2005) emphasized in his last publication, a Foreword to a book, Variation. A central concept in biology.
Adaptation. Thetopic of adaptation is treatedinmanyofMayr'spublications from the 1950s onward and has been occasionally the subject of separate articles. At the International Ornithological Congress in Helsinki 1958 he introduced a symposium on adaptive evolution stressing "that not every aspect of the phenotype is necessarily adaptive. What is being selected is the total phenotype and through it the total genotype [.] The phenotype is almost always a compromise between several opposing selection pressures" (1960e, pp. 496-497). The word adaptation refers to the condition of being adapted. To counter claims in the literature that a major part of evolution is "Non-Darwinian," effected by neutral genes and neutral characters, Mayr (1982f) discussed several concepts of evolutionary biology, particularly adaptation and selection, and concluded that none of the new insights necessitate any essential correction of the basic neo-Darwinian theory. Since the individual as a whole is the target of selection, many neutral or even slightly deleterious genes may be carried in a population as "hitch hikers" of favorable gene combinations. Selection never succeeds in achieving perfection because of chance factors and several constraints of which Mayr (l.c.) listed no less than seven. In his defense of the "adaptationist program" (which attempts to determine what selective advantages have contributed to the shaping of the phenotype), Mayr (1983a) concluded that "its heuristic power justifies its continued adoption under appropriate safeguards."
Sexual selection. Darwin's most useful thesis of sexual selection was hardly ever mentioned during the first two-thirds of the 20th century, the age of genetics, and none of the architects of the evolutionary synthesis discussed the presumed significance of sexual selection as an evolutionary factor. Some reasons for this neglect are: (1) The assumption that the gene rather than the individual is the target of selection virtually wiped out the distinction between the two kinds of selection, (2) during the evolutionary synthesis the emphasis was on speciation and those aspects ofbehavior that served as isolating mechanisms. This drew attention away from the behavioral interactions of conspecifics which is where sexual selection takes place (Mayr 1992f). Mayr (1963b: 199-201) called attention to this neglect and Ghiselin (1969) dealt with sexual selection in more detail. Revival of general interest in sexual selection occurred during the centenary of the publication of Darwin's Descent of Man in 1971 and led, in 1972, to the publication of a book edited by B. Campbell, to which Mayr (1972g) contributed an essay on sexual and natural selection. Here he discussed female choice, females' sense for "beauty," struggle among males and features which do not contribute to the fitness of the species but merely to the reproductive success of the possessor of these characters.
Speciational evolution. The theory of "punctuated equilibria" (Eldredge and Gould 1972) was based on Mayr's views of peripatric speciation in founder populations (1954c, 1982l), but claimed that all species enter pronounced stasis after completion of the speciation process. In other words, evolutionary change occurs exclusively during speciation. These claims of the prevalence of total stasis and of the impossibility of evolutionary change without speciation, are clearly invalid. Mayr (1982p, 1989i, 1999g) clarified the false claims and misunderstandings associated with "punctuationism" and, like other authors, admitted its stimulating impact on evolutionary discussions and the conduct of fruitful empirical research. Speciational evolution is fully consistent with Darwinism. The discussion of punctuationism "finally brought general recognition to the insight of those who had come from taxonomy (Poulton, Rensch, Mayr) and had consistently stressed that the lavish production of diversity is the most important component of evolution" (1989i, p. 156).
Haeckel's "biogenetic law." Based on the observations of earlier authors Haeckel, during the 1860s, became enthusiastic about the phylogenetic information content of the embryological development of animals and formulated the slogan: "Ontogeny is the short and rapid recapitulation of phylogeny." However, he failed to provide a convincing causal explanation assuming that phylogeny worked like a physiological process. Mayr (1994l) reviewed the history of the "biogenetic law" and showed how often proximate (physiological or genetical) and ultimate or evolutionary causations were confounded in attempts at explaining recapitulation. He emphasized that this phenomenon must be explained not only physiologically but also in terms of ultimate causations, i.e., recapitulated features must have some function to be preserved by natural selection. It became obvious in recent decades that recapitulated ancestral features were necessary stages in the developmental process; each stage has a causative influence on the next following stage. The inducing capacities of the surrounding embryonic tissues serve as "organizers" and form a somatic program. The gill arch system of the human embryo was preserved by natural selection because it was needed. It serves as the somatic program for the further development of the neck region in the embryo. This is the basis of an ultimate explanation for recapitulation, which also permits an explanation of why recapitulation is so irregular. Some characters are recapitulated, others are not, because some ontogenetic developments require a somatic program, others do not. The reason for this difference, however, is not yet clear.
Ontogeny recapitulates—with minor or major deviations—the ontogenetic (but not the adult) stages of the ancestors.
Human blood groups and schizophrenia. While still in New York Mayr argued repeatedly with Dobzhansky as to the meaning of blood group polymorphism in man. The latter was convinced of the neutrality of the differences, while Mayr seeing how many cases of polymorphism had been discovered in recent years to be balanced polymorphism, was convinced that blood group frequencies were controlled by selective compromises. Dr. Louis K. Diamond made all the blood group data of the Children's' Hospital in Boston available. However, the frequencies of the A, B, O blood groups of the so-called "blue babies" who had gone there for heart surgery, were almost exactly those of the Boston population. Mayr's former associate on the Whitney South Sea Expedition, Dr. Hannibal Hamlin, who had become a neurosurgeon, made the blood group data of the Neurology Division of the Massachusetts General Hospital available. Most of the hard work of this research was done by Mayr's wife Gretel, who copied all the blood group data from thousands of patients' files in several Boston and New York hospitals. There was one malignancy adenoma of the pituitary gland which strikingly deviated from the Boston population and from other brain tumor (a considerable excess of blood group 0 was found). Other medical studies in the meantime also clearly established a correlation between certain blood group frequencies and certain pathologies. The idea that the blood groups were a neutral polymorphism was thereby refuted.
Mayr has also been interested for a long time in the genetic basis of schizophrenia and, with J. Huxley and two medical colleagues, developed a model of balanced polymorphism for it (1964o). Schizophrenia probably depends primarily on a major dominant gene, Sc, with about 25 percent penetrance, that is, is manifested in only 25 percent of the carriers of this gene. The authors concluded that the Sc gene may be in morphic balance conferring certain selective advantages to compensate for its obvious disadvantages.
"For many years I have been trying to get the psychiatric community to take a greater interest in the problem, but without success. The arguments of course are always the same, it is a syndrome that is difficult to diagnose, it is highly variable and no one knows whether it is one or several diseases, it has a large environmental component, etc. To me these defeatist arguments do not make any sense. I agree with Popper that one cannot advance in science unless one proposes models that can be falsified. Sclater's genetic model for the inheritance of schizophrenia, as well as the Mayr-Huxley model of morphism, are models that can be disproven. This is entirely independent of the question whether or not schizophrenics can be chemically identified and whether or not such chemical components are cause or effect. [...] If the paper in Nature stimulates further research I will be happy even if this research should lead to a falsification of the proposed model" (letter dated February 1,1965).
In a later status report Mayr (1972b) concluded "that there can no longer be any argument about the existence of genetic factors in schizophrenia" (p. 530).
This encouraged the search for new approaches in the treatment of this disease. Mayr emphasized the need for an adequate consideration of multiple causation, "a process particularly important in the shaping of the phenotype," for which both genotypic and environmental factors are important. The purely environmental theory of schizophrenia has been abandoned.
Explanations of concepts and definitions of terms. The clarity of discussions of theoretical concepts depends on clear definitions of terms. Therefore Mayr included in most of his books detailed glossaries and published an article on the "Origin and history of some terms in systematics and evolutionary biology" (1978c), many of which he had proposed or introduced from foreign languages into the English literature. Examples are: allopatric, cladogram, founder principle, genetic revolution, nondimensional species, phenetics, population thinking, semispecies, sibling species, superspecies, teleonomic and others. He discussed and analyzed repeatedly theoretical concepts like "Darwinism," "adaptation" and "selection." As late as 1997(e), Mayr devoted a discussion to the subject of selection coining the new term "selecton" for the target or object of selection (the gamete, the individual organism, and certain social groups of cooperating animals or early humans). However, there is no "species selection," because the species as an entity does not answer to selection. When one species replaces another one due to competition, then this "species turnover" is due to individual selection discriminating against the individuals of the losing species. The expression "selection for" is used for any aspect of the phenotype (or the phenotype as a whole) that favors survival or reproductive success and which, therefore, will be favored by selection.
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