4.1. Historical Overview on the development of variability ideas.
The paleontologist Wilhelm Waagen applied the term «mutation» to morphological discontinuities in a temporal series of fossil ammonites. Hugo de Vries in Holland and William Bateson in England described mutations as discontinuous hereditary variations causing major, easily recognizable changes. De Vries argued that the building blocks of evolutionary change are sudden mutational changes rather than the gradual «individual variability..... [that] cannot lead to a real overstepping of the species limits even with the most intense steady selection». De Vries based his mutation theory of evolution on the results of segregation in progenies of crosses of the evening primrose, Oenothera.
Actually, the «mutants» described by de Vries were due to a variety of hereditary changes, particularly chromosomal aberrations. Richard Goldschmidt and others argued that there are two kinds of mutations: some produce variation among individuals, modifying their adaptation to the environments, while others produce new species, genera, families, etc.
The mutations allegedly responsible for drastic changes were called «systemic mutations», «macromutations», and the like. The modern concept of mutation is due to Thomas Hunt Morgan and his associates who worked with the fruit fly, Drosophila melanogaster. Mutations are defined as changes in single genes with effects ranging from barely detectable to very drastic. Some mutations cause mor¬phological variations, but others result in behavioral changes or modify the viabil¬ity, fertility, or rate of development of their carriers.
Mutations are said to be «spontaneous», i.e., due to unknown naturally occurring agencies. H.J. Muller showed that mutations arise with definite regular¬ity, at the rates that can be measured. He also discovered that the frequency of muta¬tion is increased in the progenies of flies treated with X-rays. Other high-energy radiations were later shown to be also mutagenic. Charlotte Auerbach was the first to demonstrate unambiguously that a chemical, namely mustard gas, had mutagenic properties. It was later shown that exposure to a variety of chemical agents, or to higher temperatures, increases the frequency of mutations.
Since the 1940-s, mutations have been extensively studied in the mold Neu¬rospora, in yeast and other unicellular organisms, in bacteria such as Escherichia coli, and in viruses. Morphological mutations modifying the appearance of colo¬nies were discovered in these organisms, but major contributions to genetic knowledge and to the understanding of the mutation processes were made through the study of so-called auxotroph mutations. In contrast to prototrophic (wild-type) organisms, auxotrophic organisms require special nutritional supple¬ments for growth and reproduction. Other types of mutations commonly studied in haploid organisms are those causing resistance or sensitivity to certain drugs, such as penicillin or streptomycin, or to infection by specific viruses. In the vi¬ruses themselves, mutations were discovered that modify the viruses’ ability to infect their bacterial hosts or to multiply in them.
The discovery of the double-helical structure of DNA, and later of the genetic code, opened the way to an understanding of the process of mutation and its causes in physicochemical terms.