Mendel's research on hybrids in evolution|
Posted December 6, 1997
Gregor Mendel's 1866 paper on plant hybridization formed the basis for the modern study of genetics, which was used in the 1940s in support of Darwin's theory of evolution. Mendel himself was interested in the question of evolution, but ironically his experiments were done in support of the theory of special creation. He worked in the tradition of Kölreuter and Gärtner, studying Linnaeus's theory that hybrids played a role in evolution. Specifically, his experiments were designed to expose an essential difference between hybrids and species.
Gregor Mendel was born as Johann Mendel in 1822 to peasant parents in Heinzendorf, in the Czech Republic. He took the name Gregor in 1843 upon joining the Augustinian monastery at Brünn, the capital of the province of Moravia. There he became a high school supply teacher, and in 1851 he was sent to study natural science at the University of Vienna. He was ordained as a priest in 1847 and was ultimately elected to the position of Abbott. He is remembered for his research on inheritance in Pisum hybrids. First presented to the Natural History Society of Brünn in 1865 and published in 1866, his famous paper stated the laws of dominance, segregation, and independent assortment which are still used in the study of genetics.
Despite his occupation as a priest, Mendel was scientific in his approach to the question of evolution. It would be surprising for a "zealous defender of the faith" in 1866 to consider seriously ideas of evolution and in particular Darwinism (Bishop 1996), but Mendel's environment was uncommonly liberal (Voipio 1990). F. C. Napp, Mendel's predecessor as Abbott, was a member of many scientific societies and shared Mendel's interest in breeding (Orel 1996). Other members of the monastery included F. M. Klacel, who shared Mendel's interest in evolution. Klacel had been prevented from teaching by the time Mendel arrived as a result of his Czech nationalism and Hegelian philosophy (Orel 1996). Mendel himself had a scientific education at the University of Vienna, and wrote about geology and organic evolution on his 1850 teaching examination. Although Mendel was cautious, particularly in not reporting his hybrid experiments with white and grey mice (Iltis 1924), his surroundings were conducive to scientific inquiry.
Mendel had a long-standing interest in breeding and crossing. As a child, Mendel spent time in the orchard with his father, who worked with fruit trees (Voipio 1990). His high school teacher also grew fruit trees, and the curriculum included fruit growing and beekeeping (Moore 1954, cited by Voipio 1990). The Moravian agricultural community generated much interest in questions of sheep and plant breeding, and Abbott Napp's interest in plant hybridization had a noticeable influence in Brünn (Orel 1996). Much of Mendel's research concerned hybridism and its role in evolution. He transplanted unusual wild varieties of plants to his garden, and when they failed to converge with the known domestic forms he concluded that environmental influence, as in Lamarckian evolution, could not account for the modification of species (Iltis 1924).
Mendel's idea that some species might begin as hybrids was introduced by Carolus Linnaeus in the eighteenth century. In 1737 he held the special creationist view that all species had been created by God and could not deviate from "the limits of their proper kinds" (Callender 1988). He later updated his theory to account for natural hybrids. Although he did not perform any careful experiments, he was confident that they existed (Olby 1966). First he classified them as "at least permanent varieties," and by 1759 he found it "impossible to doubt that there are new species produced by hybrid generation" (Callender 1988). He proposed that God had initially created one plant in each Order, which then crossed to form Genera and Species (Callender 1988).
In the eighteenth and early nineteenth centuries, hybrids were still widely considered to be violations of nature (Callender 1988); for many, Linnaeus' theory was an assault on the idea of a natural order. Several botanists attempted to reconcile their existence with their belief in the fixity of species by demonstrating an essential difference between hybrids and true species (Olby 1966). Joseph Gottlieb Kölreuter and Carl von Gärtner, both of whom Mendel repeatedly mentioned in his papers, disputed the modified theory of special creation by qualifying the reproductive stability of hybrids (Callender 1988).
Kölreuter performed careful experiments with hybrids, and identified reversion and transformation as the processes by which their proliferation was checked. Reversion is the phenomenon of successive generations of self-fertilized hybrids returning to the form of the original hybrid's mother plant. Transformation is the convergence of hybrid generations to one parent form when they are repeatedly crossed with that parent. Together, reversion and transformation were seen by Kölreuter as the "just and certain means" by which hybrids were prevented from propagating indefinitely (Olby 1966). He described these processes in terms of male and female "seed material" combining in reproduction to form a "compound material": as they mature, plants "aim at liberating little by little, the one compound material out of which they are formed, and dividing it into the two original ground materials..." (Olby 1966). A hybrid's "compound material" would be divided before being passed on to its offspring, which would therefore resemble the original hybrids' parents more than their own.
Gärtner performed experiments which showed that although some hybrids "propagate with no change of type, like pure species," they cannot become new species because their fertility invariably declines in later generations (Callender 1988). He concurred with Kölreuter that hybrids could not be responsible for the introduction of new species (Callender 1988).
Mendel knew of Kölreuter's and Gärtners' opinions, but he was dissatisfied with the experiments they used to justify them. In an 1866 letter to Carl Nageli, he wrote of Gärtner:
"It is very regrettable that this worthy man did not publish a detailed description of his individual experiments, and that he did not diagnose his hybrid types sufficiently, especially those resulting from like fertilizations. Statements like: `Some individuals showed closer resemblance to the maternal, others to the paternal type,' or `the progeny had reverted to the type of the original maternal ancestor,' etc., are too general, too vague, to furnish a basis for sound judgement." (Mendel 1866b, p. 57)Kölreuter's language also reflected a lack of clarity and modern understanding of plant biology (Iltis 1924). Other experimenters in hybridization seemed to have been "working haphazard," and none seemed to have thought that they were working toward a solution to any problem (Iltis 1924). Mendel did think that his work was important, and he began his experiments with the intent to supplement others' inadequate data and to provide a precise account of the special phenomena of hybrids.
Mendel's experimental design shows careful attention to the work of Kölreuter and Gärtner. He specifically took measures to avoid their mistakes. In particular he went to great lengths to overcome Kölreuter's difficulties in making precise measurements. He knew that he would be counting plants, so he used the largest numbers within his reach. Only with large sample sizes would he be able to see any mathematical relations in his data. He also paid special attention to the differences between the varieties which he crossed, making sure that they could be reliably measured. He concentrated only on the differing traits which were inherited all-or-nothing, because "blending" traits would be difficult to measure consistently. These features of his experiments he hoped would mark a significant improvement over Kölreuter's and others'.
Bishop (1996) points to Mendel's "population approach" as an influence of a more modern evolutionary view, but Mendel's emphasis on populations was very different from Darwin's. Darwin saw a large population, interacting with every aspect of the environment, as the necessary focus of evolutionary study. Mendel used large numbers not because he thought it was necessary to observe the very mechanism but for the practical purpose of arriving at numerical laws which could provide insight into the mechanism. He wrote of the experiments of his predecessors:
"Thus with a relatively small number of experimental plants the result could be only approximately correct and occasionally could deviate not inconsiderably." (Mendel 1866, p. 40)He believed that the mechanisms relevant to evolution lay in the organisms themselves, but they could not be directly observed so it was necessary to study the average behaviour of a large group and treat the data statistically.
Mendel aimed to provide some of the detail that his predecessors had failed to investigate. To this end, he began his discussion of the evolutionary significance of hybrids by making a clear distinction between "variable" and "constant" hybrids (Callender 1988). The offspring of variable hybrids would display Reversion, while constant hybrids would breed true. Mendel's object was to determine which circumstances could give rise to which types, and whether constant hybrids could be produced in a reproducible experiment.
He performed the Reversion experiment on Pisum hybrids by self-fertilizing hybrids of different varieties. When he examined the statistics for hybrids whose parents differed in a single attribute, he found that one trait (the `A' trait) appeared in three quarters of the hybrids' offspring and the other (`a') was passed on to the remaining quarter. At first sight this appeared to confirm the theory that one of the parent plants had made a bigger contribution than the other, but the traits inherited by subsequent generations told a different story. Two thirds of the `A' hybrids produced seeds of both `a' and `A' types. The `a' trait had been hidden but preserved in some of the second-generation `A' hybrids, and could be passed on to their progeny. Analysis of subsequent generations revealed a clear pattern. Crossing plants with `A' and `a' traits resulted in progeny of three types: `A' hybrids, which displayed the `A' trait and passed it on to all of their offspring; `a' hybrids, which displayed the `a' trait and passed it on to all of their offspring; and `Aa' hybrids, which displayed the `A' trait but produced offspring in the same ratios as in the first self-fertilization. They appeared in the proportion 1:1:2, indicating that the combinations `AA', `aa', `Aa', and `aA' were equally represented. The fact that `Aa' and `aA' plants were identical showed that the contribution of each parent was equal; that the "A" trait is apparent in both `Aa' and `aA' plants is the phenomenon of dominance.
The proportion of each of the three types - constant `A', constant `a', and hybrid `Aa' - in successive generations of self-fertilized plants each producing four seeds, was given as follows:
Ratio Generation A Aa a A : Aa : a ---------------------------------------------------- 1 1 2 1 1 : 2 : 1 2 6 4 6 3 : 2 : 3 3 28 8 28 7 : 2 : 7 4 120 16 120 15 : 2 : 15 5 496 32 496 31 : 2 : 31 . .......... ........ n n n 2 - 1 : 2 : 2 - 1Every generation contains some hybrid forms, but their proportion diminishes exponentially. Other factors being equal, the intermediate forms will quickly be outnumbered by the constant forms. In the limit, this result agreed with the existing theory of Reversion: the intermediate `Aa' breed did not survive many generations.
Further experiments on hybrids with more than one pair of opposing traits revealed the same pattern for each individual pair. Using this result, Mendel formed a theory of inheritance based on Kölreuter's "compound material" idea. He proposed that hybrids' germ cells contained independent pairs of opposing traits from which one could be omitted in the hybrid's offspring:
"...in those hybrids whose offspring are variable a compromise takes place between the differing elements of the germinal and the pollen cell great enough to permit the formation of a cell that becomes the basis of the hybrid, but that this balance between the antagonistic elements is only temporary and does not extend beyond the lifetime of the hybrid plant. ...in Pisum the behaviour of a pair of differing traits in hybrid union is independent of any other differences between the two parental plants..." (1866; p. 42-43)Where Kölreuter had seen a compromise, Mendel saw a series of compromises. They agreed that there was a temporary compromise made in the hybrids' heritable properties, and that eventually they would sway toward one or the other of the parent attributes. Kölreuter saw this compromise in all hybrids but those which had fully reverted to one parental form. Mendel's analysis showed that each pair of traits followed the same law of reversion; most importantly, that meant that in any plant in any generation there were some traits which were in a temporary compromise and some traits which were constant. Only plants containing no compromised trait pairs could breed true. This was the distinction between constant and variable hybrids that Mendel sought; he assumed that his "laws valid for Pisum" were equally applicable to those genera which were known to contain constant hybrids (Hartl & Orel 1992).
Having identified the difference between constant and variable hybrids, Mendel had to explain Kölreuter's and Gärtner's results. He emphasized the importance of using large numbers of plants and small numbers of differing traits. The hybrid series for plants with many differing traits contains too many different forms to allow generalizations from any practical sample size. He also pointed out that the phenomenon of dominance can make it impossible to distinguish hybrid `Aa' and constant `A' forms by their outward appearance. In short, Kölreuter and Gärtner had failed to find constant hybrids mostly because their sample sizes were too small.
Mendel started his Pisum experiments in 1857, before Darwin published The Origin of Species. He read Darwin's book before presenting his paper and he drew upon several of Darwin's ideas, but no evidence of a belief in natural selection crept into his work. He shared Darwin's Lyellian approach to biology:
"Whether variable hybrids of other plant species show complete agreement in behavior also remains to be decided experimentally; one might assume, however, that no basic difference could exist in important matters since unity in the plan of development of organic life is beyond doubt." (Mendel 1866, p. 43)Darwin had also argued that the distinction between species and varieties was arbitrary. Mendel accordingly argued that his work with Pisum variety hybrids was relevant to species hybrids as well:
"The hybrids of varieties behave like species hybrids, but possess a still greater inconstancy and a more pronounced tendency to revert to the original forms." (Mendel 1866, p. 38)According to Professor Gustav von Niessl, a staff member of the school where Mendel taught, Mendel thought Darwin's theory was inadequate and "hoped that his own researches would fill this gap in the Darwinian system." (Iltis 1924).
Callender (1988) discusses an often misinterpreted paragraph of Mendel's, concerning Gärtner's Transformation experiments.
"The success of transformation experiments led Gärtner to disagree with those scientists who contest the stability of plant species and assume continuous evolution of plant forms. In the complete transformation of one species into another he finds unequivocal proof that a species has fixed limits beyond which it cannot change. Although this opinion cannot be adjudged unconditionally valid, considerable confirmation of the earlier expressed conjecture on the variability of cultivated plants is to be found in the experiments performed by Gärtner." (Mendel 1866, p. 47)Callender cites a popular interpretation, that Mendel was dissociating himself from Gärtner's position. He argues that Mendel clearly meant the opposite: he gave conditional acceptance to Gärtner's view. Both interpretations ignore the context in which the paragraph appeared. The "earlier expressed conjecture" presumably refers to the following paragraph:
"If one may assume that the development of forms proceeded in these experiments in a manner similar to that in Pisum, then the entire process of transformation would have a rather simple explanation. The hybrid produces as many kinds of germinal cells as there are constant combinations made possible by the traits associated within the hybrid, and one of these is always just like the fertilizing pollen cells." (Mendel 1866, p. 44)Mendel was arguing that the laws of variability he developed for Pisum could be applied to Gärtner's experiments to explain his results. Without committing himself to one view or the other, he proposed that his laws of variability were in accordance with Gärtner's observations. He referred to his result as the "law valid for Pisum" but he clearly intended it to be generally applicable.
The significance Mendel attached to constant hybrids amounted to a partial acceptance of Linnaeus's modified theory of special creation.
"If the compromise be considered complete, in the sense that the hybrid embryo is made up of cells of like kind in which the differences are entirely and permanently mediated, then a further consequence would be that the hybrid would remain as constant in its progeny as any other stable plant variety. (Mendel 1866, p. 42)He had established that constant hybrids did exist, but his application of that result to the question of the source of actual forms was only tentative. In his 1869 paper on Hieracium hybrids, he discussed in more detail the significance of hybridization to the question of evolution. He cited reduced fertility and unlikelihood of self-fertilization as possible means by which constant hybrids may be prevented from becoming new species. To determine the extent of those effects he proposed to study the genus Hieracium, which "possesses such an extraordinary profusion of distinct forms that no other genus can compare with it." (Mendel 1869, p. 50). It was a natural candidate for studying the possible implications of hybridization; the classification of its many varieties was notoriously difficult. Mendel published his intermediate findings but he was unable to make any conclusions. He did say that his Hieracium hybrids did not produce the same series of intermediate forms as his Pisum hybrids; they reproduced themselves "like pure species."
Mendel never reached a point where he could make a definite conclusion about the role of hybrids in the origin of species. In his publications he was elusive about his personal views, sometimes to the point of confusing his readers, but his entire research program reveals a commitment to a pre-Darwinian view of evolution. He carefully isolated his experiments from the mechanisms of Darwinian evolution, both before and after reading Darwin's theory, in order to study what Darwin called "one of the greatest obstacles to the general acceptance and progress of the great principle of evolution."
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Callender, L. A. (1988). "Gregor Mendel: an opponent of descent with modification," History of Science 26: 41-75.
Hartl, D. L., & Orel, V. (1992). "What did Mendel think he discovered?" Genetics 131: 245-253.
Iltis, H. (1924). Life of Mendel. London: George Allen & Unwin Ltd.
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Olby, R. C. (1997). "Mendel, Mendelism and Genetics." On-line document: http://www.netspace.org/MendelWeb/MWolby.html
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Voipio, P. (1990). "When and how did Mendel become convinced of the idea of general, successive evolution?" Hereditas 113: 179-181