by Peter Bowler
Mendelism and the Mutation Theory
The origins of the modern genetic theory of natural selection are centered on a number of curious incidents. Mendel’s work on heredity lay un-noticed for thirty-five years and brought its author only posthumous fame. When the new emphasis on the study of heredity led to its rediscovery in 1900, the potentialities of Mendel’s approach for the selection mechanism were not appreciated. Far from being seen as the salvation of Darwinism, Mendelism was presented as yet another alternative by those who felt that its emphasis on discontinuous variation contradicted the ideas of biometry
[ which concentrated on continuous variation ]. When the concept of "mutation” was introduced to allow for the appearance of new genetic characteristics, it was assumed that this would render selection superfluous. Only in the 1920s was it realized that the new approach to heredity could be interpreted to solve the problems inherent in the original form of the selection theory.
Interpretation of Mendel’s own contribution is complicated by the fact that his followers seem to have read their own ideas into his writings . It has recently been argued that far from being a pioneer, he actually was trying to extend a tradition that dates back to Linnaeus. This was the belief that new species might be produced by hybridizing two existing ones, which
Mendel apparently saw as a possible alternative to Darwinism . His famous laws of heredity were of interest to him only insofar as they allowed him to postulate the constancy of hybrids in founding new species. Nor is it clear that he thought in terms of paired hereditary particles—the alleles of modern genetics. The mathematical rigor of Mendel’s work was new, and that was exactly what his re-discoverers were seeking. Their ideas, however, were shaped by the post-Darwinian controversies over continuous versus discontinuous heredity, and they automatically interpreted Mendel’s papers in this light . The following account of Mendel’s work, for the sake of convenience, presents it as it was understood in 1900.
Gregor Johann Mendel was an unsuccessful student who received only a limited scientific education. Eventually he entered the monastery at Brünn in Austrian Moravia where he eventually rose to become abbot. His work on the hybridization of plants was performed in the monastery garden and published in the journal of the local natural history society in 1865. He received no support from the scientific community and was positively discouraged by the church.
He died in 1884, worn out by efforts to prevent his monastery from being taxed by the Austrian government.
Mendel concentrated on discontinuous variation, characteristics inherited on an all-ornothing basis, which he felt would simplify the problem of identification after hybridization . A considerable literature on hybridization existed, and the phenomenon of

discrete inheritance had been noted from time to time, but no one had focused on the point to see if regular mathematical laws might apply. It is clear that Mendel knew exactly what he was looking for and had picked out a suitable experimental subject—the garden pea . The suggestion has even been made that his results are a little too good, perhaps “improved” by an assistant who knew what Mendel was looking for.
Mendel picked out seven characteristics in the garden pea . . . These included the size of the plant, the color of the flower, whether the seeds are wrinkled or not, and so forth . In the case of height, our example, the pea exhibits two distinct forms, tall and short. The experiment began with pure strains for each characteristic, that is, strains known to have bred true over many generations. The first step was to cross-fertilize plants with the opposite form of each characteristic, pure tall crossed with pure short, and so on. When the resulting seeds were grown,
Mendel could see how the characteristics had been inherited by the hybrid. In every case there was no blending; the hybrid showed only one form of the characteristic that had been crossed.
The hybrids from tall-short cross were all tall—there was no blending to give a medium stature, and all trace of the short characteristic seemed to have disappeared . The hybrids were then self-fertilized and the seeds grown to maturity. In the second hybrid generation,
Mendel found his famous 3:1 ratio. There was still no blending, but the characteristic that had apparently vanished now reappeared in one quarter of the plants in this generation.
On the average, there were three tall plants for every short one .
To Mendel’s later followers, it seemed obvious that in such cases there must by a particulate mechanism for heredity ; a single particle or unit in the germ plasm had to be responsible for transmitting the characteristic from one generation to the next. It was equally obvious that this unit must exist in two distinct forms, one for each state of the characteristic (tall or short). The unit is what we now call the “ gene
” and the different forms its “ alleles
.” Two crucial assumptions were required to complete the theory. First, each organism must carry two units for each characteristic. This is required by the nature of sexual reproduction: if the offspring is to derive a part of its germ plasm from each parent, it must get one unit for each characteristic from the male and one from the female parent. When it, in turn, reproduces, it will pass on just one of the units; that unit will combine with one from the other parent to give a pair for their offspring. The second critical assumption is that when the two opposite forms of the unit for a certain characteristic are combined in the same individual, they do not blend their effects. Instead, one is “ dominant ,” the other “ recessive ”—one takes complete control of producing that characteristic in the new organism, while the other lies dormant in the germ

plasm. Only when both units correspond to the recessive characteristic will it actually be produced in the mature organism . . .
Surprise is sometimes expressed that Mendel’s work went unnoticed for thirty-five years . Although the journal of the Brünn natural history society was not the most prestigious, copies were circulated even in
England, and there were occasional references to
Mendel’s paper in later literature. Mendel was in contact with one leading biologist, Carl von Nägeli, who told him his results were of little significance and put him onto other subjects whose complexities defied analysis by any technique available at the time. The fact is that the scientific community was not able to make use of the work in the 1860s . The experiment was probably seen as a contribution to the now outdated debate on the production of new species by hybridization. Discontinuous variation within a species was considered a rare exception to the normal rule of blending . Most cases of heredity are, in fact, far more complex than the simple example Mendel so carefully picked out . The same rules apply, but they cannot be seen to apply unless the case is examined with techniques far more sophisticated than those available to Mendel. What was needed was a situation in which scientists would become convinced that something was wrong with the traditional view of heredity, and hence readier to seek out a new approach.
By the end of the century, developments in the study of heredity had created a new environment in which the modern interpretation of Mendel’s results could be promoted . . .