Gregor Mendel

Mendel in Darwin's Shadow (p. 2)

Biography

BY DAVID ALLEN


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Constant hybrids

It has sometimes been claimed that Mendel was opposed to the very concept of constant hybrids as it conflicted with his Pisum laws; for example, Heimans wrote that we 'may assume' that Mendel only accepted the existence of constant hybrids 'with a feeling of reluctance or at least disappointment' (1970, p. 17). Why exactly we should assume this is not clear. In fact, the phenomenon of constant hybrids was very significant for Mendel.

The thrust of Callender's case is that Mendel wished to verify that constant hybrids could be produced in order to confirm the Linnaean hypothesis of descent through hybridisation. This seems reasonable; even if Mendel never stated this as his position, his work can be reconciled with it. But the simple fact that he was interested in constant hybrids does not make him a Linnaean form of ‘special creationist’.

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While the Linnaean concept of 'descent through hybridisation' caused some debate in his lifetime, it was—as Mayr points out—'largely forgotten in the ensuing period’ (1982, p. 260). In fact, in the species debate, it was Linnaeus's earlier, original doctrine that survived: Species tot sunt diversae… (etc.). For Hermann Hoffmann (writing in 1869), Linnaeus embodied the old idea of species fixity, as compared with the 'new' idea that species are 'limitlessly variable' (1869, p. 23). Similarly, in his paper Entstehung und Begriff der naturhistorischen Art, (1865), Nägeli had to explain to his readers, in a footnote, that Linnaeus was perhaps not as absolute in his doctrine of ‘species fixity’ as it might seem—as shown by his theory that species may have evolved from a few original generic types (p.6).

If, then, Mendel had wanted his audience in 1865 to understand that he believed his experiments pointed to the Linnaean concept of descent through hybridisation, he would have had to make this explicit. They would not necessarily have inferred it from what he said. In fact, in the context of the time, an argument for the existence of constant hybrids would have been read as an argument against species fixity—by implication, an anti-creationist position; and not as an argument for a form of 'special creation'.15

Hoffmann drew up a list of views on either side of the species debate, 'Thesis' vs. 'Antithesis': for example, Thesis—'species vary infinitely'; Antithesis—'species do not vary beyond the basic type'. One of his points of comparison is this:

Thesis

9. Lasting, fruitful and constant forms can also develop through the hybridisation of two species.

Antithesis

9. Not sufficiently proven in any case (cf. Dianthus)

In point 10, the 'thesis' is: 'Many current forms have originated in this way (Salix). These hybrid forms are new species'; the 'antithesis' is that hybrids revert to the parental type (Hoffmann 1869, p. 30). We can clearly position Mendel's views in the 'left-hand' column; for example, he specifically cited Wichura's Salix experiments as an example of how new 'constant' forms may be produced through hybridisation.

Mendel followed his work on peas with a series of experiments in other plants, particularly those of the hawkweed genus Hieracium. The significance of these experiments in Mendel's oeuvre is generally undervalued; his work with Hieracium is usually dismissed as a mistake, an unfortunate dead-end for his research. Thus, Sturtevant writes that it

must have been a great disappointment to Mendel. He obtained several hybrids by dint of much hard work, and all of them bred true. It is now known that this occurs because the seeds are usually produced by apomixis, that is, they are purely maternal in origin and arise without the intervention of meiosis or fertilization (Raukiär 1903, Ostenfeld 1904). In other words, this was the worst possible choice of material for the study of segregation and recombination—for reasons which could not be guessed at the time. (Sturtevant 2001, p. 11.)

Similarly, Gian Nogler argues Mendel hoped that his Hieracium experiments would verify 'the laws of inheritance that he had discovered while working on Pisum', but to his consternation, the results actually seemed to contradict them (Nogler 2006, p. 1). But if his purpose was to confirm the Pisum laws, it seems unlikely he would persist so doggedly in his experiments with Hieracium for some five or six years (from 1866 to 1871 or 1872). Some of the other plants he worked with, in fact—e.g. Matthiola incana (var. annua) and glabra, Zea and Mirabilis—did show a similar pattern of inheritance to Pisum (OG, p. 93); but he continued to devote most of his energies to his hawkweeds.

Mendel Geum
Geum rivale

Mendel is now usually remembered only as the 'father of genetics', the man who discovered the laws of particulate inheritance; the Hieracium experiments have been dismissed, or marginalised, because they do not fit in with this simplified scenario. We now know that apomixis is common among Hieracium species—and this would, indeed, have made them an unsuitable subject to test the Pisum laws, if that was what he was after; but he wasn't. Callendar is right, in fact, to argue that Mendel actually turned to Hieracium to find different laws: those that govern the evolution of constant hybrids. He selected species for further experiment that he thought would produce constant hybrids: for example, the hybrid Geum urbanum × rivale warranted special attention as one of the hybrids that Gärtner had said produced 'invariable progeny as long as they remain self-pollinated'; and he anticipated that some Hieracium species, if crossed, would produce progeny that 'would behave in a fashion similar to Geum' (OG pp. 58-59).

This is how he introduced his Hieracium experiments, in the paper he presented in 1869 (when his experiments were still unfinished):

The question of the origin of numerous and constant intermediate forms has recently acquired no small interest since a famous Hieracium specialist has, in the spirit of the Darwinian teaching, defended the view that these forms are to be regarded as [arising] from the transmutation of lost or still existing species. (OG, p. 51)

The famous specialist in question was von Nägeli.

History has been harsh to Nägeli. He is now seen as the fool who failed to recognise Mendel's genius. (Take, for example, Eric Weisstein's bizarre claim that Nägeli 'did more harm to biology than good, especially in his contemptuous dismissal of Mendel's work on pea plants'—1996.) This may explain why Mendelian scholarship in general has given surprisingly little attention to the question of Nägeli's influence on Mendel.

Mendel corresponded with Nägeli over a period of seven years, and it seems likely that he was at least sympathetic to Nägeli's views. Certainly the respectful tone in the letters suggest a supporter, rather than opponent: for example, he welcomed Nägeli's 'thorough revision of the theory of hybrids according to contemporary science' (OG, p. 60). Nevertheless, as will become clear, there were certain differences between them on the issue of hybridization and its role in evolution.

Nägeli commended Mendel's interest in producing hybrid Hieracium, 'since within a short time this might be the species which will be best recognised with reference to intermediate forms'; and suggested he might find 'substantially different results' from the Pisum experiments (cited Hoppe 1971, p. 136). We need, then, to examine Nägeli's ideas, especially with respect to Hieracium.

In 1865, Nägeli declared his support for Darwinism. He wrote:

The great scientific importance and the great success of the Darwinian theory of utilitarianism [i.e. natural selection] is based in the fact that it gives a natural explanation, following the principle of causality, for a great category of events, which could formerly only be explained in a teleological way. (Cited Junker 1989, p. 171)

Natural selection, he insisted, could only be denied through 'an ignorance of the facts or from an incorrect interpretation of the same' (ibid., p. 175). Nägeli's support for Darwin needs to be stressed, because he is sometimes misrepresented as anti-Darwin. (See, for example, Bowler 2003, p. 248.) Nevertheless, he identified what he saw as gaps in Darwin's theory, specifically in the mechanism of heredity.16

As we have seen, Nägeli (like Darwin and Mendel) saw no natural boundary-line between variety and species. Since, he argued, no other explanation is known of the relationship between different species in a genus than descent with variation,

we are entitled to consider the species of a genus as varieties of a common ancestral form which have developed further and become constant,—the original form having possibly actually disappeared or being no longer recognisable as such. (Cited Sachs 1882, p. 927.)

Under Darwinian theory, the intermediate forms—the ‘missing links’ between living and ancestral forms, have usually disappeared; and so the different species in a genus appear completely separated from one another. Nägeli seesm to have largely accepted this view. The Hieracium genus was polymorphic in form; and he claimed that there was no plant as instructive in determining whether species are constant or variable; whether they were created, or have evolved.

Nägeli's one-time assistant Albert Peter (who wrote Die Hieracien-Mitteleuropas with him) observed that the origin of intermediate forms had long been an object of debate:

Some researchers regard all such intermediate forms as hybrids, others accept this only for some of them, while they assume an independent origin for others, resulting from variation.' (1884, p. 203.)

Nägeli belonged to the latter camp. He claimed that in genera such as Hieracium, the transitional forms could not be explained, in the majority of the cases, by hybrid formation. He doubted that hybrids could last long, and become constant and stable forms in the wild; they would always find themselves surrounded and outnumbered by plants of the parental species; and so 'will only rarely achieve self fertilization, because parental pollen makes their own ineffective' (BM, p. 310). Nevertheless, hybridisation experiments were useful because they could help to determine if a particular specimen was a constant intermediate form, or a hybrid—even though 'pure parentage' was, in his opinion, on the whole far more probable (BM, p. 314).

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Nägeli was advocating, then, a theory of descent through variation rather than hybridisation—and using Hieracium to make his case. (He wrote to Darwin in 1867, telling him that he had 'worked hard for four years on Hieracium to show causes and manner and steps of variation,—hybridism etc., etc.'—Darwin 2005, p. 211.) He believed that in Hieracium, many of the missing links between species could still be found. It was as if, in this genus, you could observe all stages of the evolutionary process. 'According to the present state of our knowledge,' he declared,

no other hypothesis is possible but that all the various species of Hieracium have sprung from the transmutation (descent with variation) of forms which have either disappeared or are still in existence; and a large number of the intermediate forms still occur which must have had their share in producing several new species by the splitting up of one original species, or which would have occurred in the transformation of a still living species into one derived from it. In the case of Hieracium the species have not become so completely separated by the suppression of the intermediate forms as is the case in most other genera. (Cited Sachs 1882, p. 927)

(Here, then, is what Mendel meant, when he referred to the idea that constant intermediate forms have arisen through 'the transmutation of lost or still existing species'.)

Did Mendel, then, believe that intermediate forms in Hieracium were the product of descent through variation, or hybridisation?

*

In his copy of Darwin's book The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, Mendel marked a passage where Darwin wrote that hybridism was

one of the greatest obstacles to the general acceptance and progress of the great principle of evolution. (Darwin 1876, p. 27)17

In the Origin of Species, Darwin had already discounted the idea that hybridization played any significant role in evolution. It evidently had no place in a theory based on the gradual accumulation of small variations. (He fell back on the idea that hybrids are usually—although not always—sterile; see Darwin 1859, pp. 276-277.) Emanuel Radl noted that, while the issue of hybridisation was very actively studied during the first half of the nineteenth century, Darwin's attitude 'diverted the whole investigation into other channels' (Radl 1930, p. 337).

In 1881, in his review of the contemporary state of research into hybrids, Die Pflanzen-mischlinge, Wilhelm Olbers Focke observed that 'eager Darwinists' now tended to see transitional forms everywhere. Looking for evidence to support the Darwinian theory, they interpreted the intermediate forms they found 'where at all possible, not as hybrids, but as transitions' (1881, p. 505). Focke may have had Nägeli in mind here; but the trend was actually begun by Ernst Haeckel. In his 1862 study of Radiolaria, Haeckel argued that intermediate forms could help to decide 'the question of the gradual development of organic beings from common ancestral forms' (cited Gliboff 2008, p. 160). He listed examples of intermediate forms among Acanthometrans and other families—groups which were, like Hieracium, highly variable in form.

In his Hieracium paper, Mendel observed that there was no consensus among botanists on the role of hybridisation in the production of intermediate forms in hawkweeds. He almost paraphrases Nägeli on the issue, when he alludes to botanists who, while granting the existence of hybrids in the wild, do not regard this as very important, because such hybrids 'are only of short duration'. He then states that the purpose of his own experiments is to test for himself 'the possible influence exercised by hybridisation over the multiplicity of intermediate forms in Hieracium’. (OG p. 52.) At the end of his paper, he offered his conclusions—stressing that they were provisional at this stage in his research; nevertheless, he surmised that the Hieracium hybrids he had produced were constant and stable:

In Pisum the hybrids, obtained from the immediate crossing of two forms, all have the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In Hieracium according to the present experiments the exactly opposite phenomenon seems to be exhibited. In describing the Pisum experiments it was remarked that there are also hybrids whose posterity do not vary, and that, for example, according to Wichura the hybrids of Salix reproduce themselves like pure species. In Hieracium we may take it we have a similar case. (OG, p. 55.)

Mendel did not explicitly rule out descent through variation in the production of intermediate forms. However, he assiduously worked through the objections most frequently raised against hybridisation—for example:

  1. The issue of hybrid sterility. (All the examples he cited from his experiments were either partially or fully fertile.)
  2. The issue of self-fertilisation. Mendel found evidence that seemed to contradict Nägeli's claim that pollen from surrounding parental plants would prevent hybrids from fertilising themselves in the wild. In the case of one fully fertile hybrid, H. echiodes × H. aurantiacum, the pollen of the parental types 'was not able to prevent self-fertilisation, though it was applied in great quantity' (OG, p. 54).18 (We now know that this was probably the result of apomixis—but Mendel's basic point remains valid, that hybrids could and did endure, despite being surrounded by parental plants.)
Mendel Hieracium
Hieracium aurantiacum
(click to enlarge)

In crosses between H. auricula and H. pratense, and between H. auricula and H. aurantiacum, Mendel observed 'wildly divergent' forms, and a range of variation which was almost continuous: some traits were nearer one parental form, some the other, and some intermediate between the two. He even found new traits (H. auricula features yellow, and H. aurantiacum orange flowers; but when he first performed the cross, he found that one hybrid form had 'different leaves and a totally different flower colour'—OG, p. 89). Moreover, we know from Nägeli's colleague Albert Peter that a number of Mendel's Hieracium hybrids exhibited characteristics that were heterotic, i.e. they exceeded the parental form. (For example: in one form from the cross H. auricula × H. aurantiacum, Peter found 3.4% of all traits were heterotic—1884, p. 461.19) Given this morphological diversity, Mendel concluded:

The supposition is forced on us that we have here only single terms in yet unknown series which may be formed by the direct action of the pollen of one species on the egg-cells of another. (OG, p. 53.)

In other words: he had found that hybridisation could, indeed, produce the 'multiplicity of intermediate forms in Hieracium'.

Peter acknowledged that, following Mendel's experiments, no-one could deny the existence of Hieracium hybrids; nevertheless, he continued to see them as only a 'temporary phenomenon' (1884, p. 241). Writing in 1921, however, Karl Zahn declared that numerous cross-breeding experiments (including Mendel's), together with observations in the wild, had put the spontaneous origin of hybrids beyond doubt; and that in Hieracium, 'a far greater role is to be attributed to hybridization, and through it the production of new forms' than Nägeli and Peter had recognised (1921, p. 24). Thus, it may be argued that Mendel's experiments had an impact in clarifying the importance of hybridisation in producing new forms in Hieracium—and in so doing, countering the Nägeli/Peter emphasis on continuous variation.

It could, then, be claimed—following Callender—that Mendel's researches effectively refuted 'the Darwinian view that the constant intermediate forms of Hieracium were the result of descent with modification' (LAC, p. 61). Does it follow that Mendel was, indeed, anti-Darwin, and anti-evolutionary?

*

In his final letter to Nägeli (written 1873), Mendel noted that it is generally accepted that 'unfavorable changes in environmental conditions may result in reduced reproduction, therefore they may cause a sexual weakening or complete sterility, wherein the male organs always suffer first'. (This was Darwin's opinion; see Origin of Species 1859, pp. 131-2.) He contended that, if this were indeed the case, then

the naturally-occurring hybridizations in Hieracium should be ascribed to temporary disturbances, which, if they were repeated often or became permanent, would finally result in the disappearances of the species involved, while one or other of the more happily organized progeny, better adapted to the prevailing telluric and cosmic conditions, might take up the struggle for existence successfully and continue it for a long stretch of time, until finally the same fate overtook it. (OG, p. 101-2.)

Evidently, what Mendel means is that, if male sterility occurs in one species, it increases the likelihood that the plant will be fertilised by pollen from another species, so producing ‘naturally-occurring hybridizations’. If sterility persists, the parental species will die out; while those new hybrid forms which are better adapted to the environment (the ‘more happily organized progeny’) will survive. (It is possible that he was making this argument as a way of countering Nägeli’s view that a hybrid would always find it difficult to become established when surrounded by the more fertile and abundant parental species.)

Callender declares: 'Here at last is Mendel's conception of the process of organic change'—his explanation of how descent through hybridisation might have occurred; and his alternative to Darwinian evolution. In an evasive manoeuvre, Callender dismisses Mendel's reference to the 'struggle for existence' [Kampf ums Dasein] on the basis that the notion goes back to 'another gentleman of the church, the Reverend Thomas Malthus'—and therefore there is no necessary connection between it and Darwinism. (LAC, p. 70.) Callender's whole reading, however, tells us more about his agenda than Mendel's.

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Mendel's account may be compared with the views of Anton Kerner. (Kerner studied with Mendel at the University of Vienna; Mendel sent him a copy of his Pisum paper, but Kerner did not read it.) Kerner proposed hybridisation as a primary source of new forms of life; and he offered a clear explanation of how natural selection among such forms may occur. A community of plants might be badly affected by environmental change, he wrote; but 'in every group at all times and in all places a reserve of new forms continually arises by crossing'. Some of the older, 'less fit forms some are extinguished, whilst young, new forms step into their places'. Not all new forms will be well suited to the prevailing conditions:

Such only are able to maintain, propagate, and establish themselves, as are from their internal organization and external form in harmony with the prevailing climatic conditions of the moment. Those so constituted that they are unable to thrive under the given external conditions linger and become extinct; they are outstripped and overgrown by such as find the environment to their liking. Hence we speak of the struggle for existence. Plants in harmony with their surroundings are the victors, and they establish themselves upon the arena of this encounter. This, briefly, is Darwin's theory of Natural Selection, a theory which marks an advance upon all other theories of the origin of new species. Though many views may be held as to the precise manner of origin and transformation of forms, there can be no difference of opinion as to the significance of the struggle for existence and of the survival in this struggle of those forms best fitted by their organization to the circumstances of the environment. (1903, pp. 599-600).

The mechanism that Kerner proposes for the ‘origin and transformation of forms’—hybridisation—is indeed different from the Darwinian emphasis on gradual, ongoing selection among individuals within a population, over long periods of time. Nevertheless, Kerner explicitly allies his theory with the Darwinian ‘struggle for existence’—Kampf ums Dasein. (See Kerner 1891, p.588.)

Wilhelm Focke similarly developed a theory of evolution through hybridisation. (He thought it best to put aside Linnaeus's 'rather fantastic hypotheses' on the subject—1881, p. 506.) He did not see intermediate forms between species as transitional (in the Darwinian sense of indicating descent through variation), but as the product of hybridisation. Nevertheless, he recognised the need to counter the view of ‘many botanists’ that hybrids would inevitably lose out in the struggle for existence (‘Kampf ums Dasein’) with the parental species (ibid., p.503)—arguing they could become established through greater vegetative strength, or better adaptation to prevailing conditions (ibid., p.506).

We cannot simply sidestep (as Callender attempts to do) Mendel's use of the Darwinian phrase, 'the struggle for existence'. Alfred Kelly reports that after the publication of Origin of Species, Kampf ums Dasein (also sometimes translated as Kampf um die Existenz) 'really caught on as the Darwinian catch-phrase in Germany' (and we might add, by extension, in the wider German-speaking scientific community) (Kelly 1981, p. 30). Mendel himself read, not only the Origin, but other Darwinian texts where the phrase was used. (For example, Descendenzlehre und Darwinismus, where Oscar Schmidt wrote that Darwin had found the key to species change in a concept that had become 'a landmark, and the common property of our time'—1873, p. 127.) Nägeli himself used the phrase in its Darwinian sense, in publications that Mendel read. (See for example, BM, p. 426.) Clearly, Mendel would have been aware of the Darwinian connotations of Kampf ums Dasein; and if he wanted to use the phrase in any other sense, he would have had to make that clear—especially in a letter to Nägeli.

It appears, then, that Mendel, like Kerner and Focke, was allying a theory of descent through hybridisation with the Darwinian notion of the struggle for existence as the mechanism for the selection and survival of species. It follows that a belief in decent through hybridisation cannot be simply equated with a position that is ‘anti-evolutionary’ or ‘anti-Darwinian’, or (as Callender terms it) ‘Linnaean’. Linnaeus, as we have seen, believed that evolution was driven by 'the laws of the Creator' (which move 'from the simple to the complex'—cited Larson 1971, p.108). There is no place in his scheme for a mechanism such as natural selection. If Mendel was a ‘Linnaean’ (or ‘special creationist’), the notion of a divine plan or divine laws would have formed part of his analysis; he would have detected the hand of God in the changes he observed, and shown how they formed part of a planned or guided evolution. But instead, what Mendel describes are materialistic processes: the undirected operation of natural selection.

*

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At the same time as he was investigating variable hybrids through Pisum, Mendel was reaching for different laws which would show how new hybrid forms could become stabilized. He left his researches incomplete; but if we only think of him in terms of the Pisum laws, we are only getting half of the picture.

Iltis described Hieracium as an unsuitable choice for Mendel's work, because reproduction in the genus is 'abnormal' (1932, p. 175). However, it is only abnormal in the sense that it falls outside the 'normal' pattern of sexual reproduction. It is certainly complex, featuring a broad range of reproductive strategies. Some Hieracium species reproduce by sexual means; but some are apomictic (i.e. they reproduce asexually, without fertilisation). In this way, seeds give rise to clones—exact copies of the mother plants.20 But the situation is more complex than this. Apomictic species may have a residual sexuality, and serve, not only as pollen donors, but even as maternal plants—i.e. they produce both unreduced egg cells, and 'normal' egg cells with reduced chromosome numbers. This is sometimes termed 'amphi-apomixis'. (In Hieracium, this was first noticed, and studied in detail, in H. aurantiacum—see Krahulcovà et al. 2006, p. 329.)

From a hybrid cross between Hieracium species, four types of hybrid may be produced: apomictic; amphi-apomictic; fully sexual; and sterile. Mendel was unaware of asexual modes of reproduction such as apomixis; he assumed, in fact, that Hieracium reproduced sexually, and so he employed elaborate methods to try to prevent self-fertilisation in his specimens. In some of his first experiments, he crossed H. pilosella with pratense, praealtum and auricula; while he obtained viable seeds, he feared that he had not been able to prevent self-fertilisation. But in fact, H. pilosella is apomictic—the plants had simply produced clones of themselves. Mendel's first successful cross was H. praealtum × H. stoloniflorum. When he raised four plants from the seeds, he found that three resembled H. praealtum, but one was a hybrid form. He concluded that he had only prevented self-fertilisation in one out of four cases; but H. praealtum is facultatively apomictic; so in this case, the results may have been due to a combination of sexual and apomictic reproduction.

As we have seen, F1 hybrids which are apomictic give rise immediately to uniform progeny (or clones). In this way, apomixis leads to the rapid creation of new forms—to the point where they may be classified as 'stabilized hybrid taxa'. (Fehrer et al. suggest that the most successful of such lineages are finally classified as 'well-recognized apomictic "basic species" [Hauptarten]'—2007, p. 367.)

Ostenfeld observed that crosses in Hieracium can give rise to new forms or species that reach constancy at once through apomixis (1910, p. 275). However, new forms produced in this way have been labelled 'false hybrids'—i.e., they only simulate true-breeding species. Nogler, for example, describes them as false because they 'either do not segregate at all … or segregate according to other rules' (2006, p. 5). But this antithesis between 'true' and 'false', 'segregating' and 'non-segregating', is itself dubious.

Apomixis, once dismissed as a 'blind alley' in evolutionary terms, is increasingly seen simply as an important alternative mechanism to sexual reproduction. Many organisms capable of apomictic reproduction can also reproduce sexually. In Hieracium, the complex of different reproductive pathways cannot be reduced to a simple question of either/or—i.e. either sexual or asexual. As we have seen, amphi-apomictic species may take part in hybridization, not only as pollen donors, but also as maternal plants. Mendel himself obtained hybrids between the sexual H. umbellatum and two different apomicts, H. vulgatum and H. barbatum. Fehrer et al. recently examined a range of Hieracium species in the Pilosella subgenus, and were surprised to discover how often 'the apomict served as seed parent rather than the sexual taxon. It shows that the role of residual sexuality is evolutionarily important and might be greater than expected' (2005, p. 197). Thus, amphi-apomictic species in Hieracium, far from being an evolutionary dead-end, retain the potential to produce new forms.

*

There is a tendency in Mendelian criticism to downplay or marginalise his work on constant hybrids in Hieracium and other genera—almost as if this were an embarrassing aberration from his 'real' work as the discoverer of the basic laws of particulate genetic variation and heredity. Jacobus Heimans even characterises Mendel as a man who spent his life combating 'the elusive phantom of the allegedly constant, intermediate hybrid'. Somewhat absurdly, Heimans paints a picture of Mendel, 'lonely and steadfast', always relying on the 'fixed fundamental laws' he had discovered in Pisum; temporarily checked by his Hieracium results, it is true, but nevertheless continuing to fight for his cause. The tragedy was that he 'did not live to witness the vindication of his views' (1970, pp. 22-3).

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Mendel's investigations into constant hybrids suggest, in fact, that he saw hybridisation as a possible factor in the origin and evolution of new forms. Arguably, critics like Heimans are operating within the dominant neo-Darwinian paradigm, which does not accord a significant role for hybridization in evolution. (Wagner, for example, called hybridization 'a kind of 'evolutionary noise' which only distracts from the overall Darwinian pattern of descent through modification—cited Arnold 2006, p. 15.) Mendel's work on 'variability' fits within the neo-Darwinian paradigm; his work on constant hybrids does not. It behoves critics such as Heimans, then, to reconstruct Mendel as a good Darwinist; to stress only those aspects of his work which are consistent with descent through modification.

It may well be—as Olby suggests—that we have been 'unconsciously allying Mendel with our own perspective because we want him on our side'. But that does not mean that latter-day creationists and ID-ers are justified in claiming Mendel for their side.

Wolf-Ekkehard Lönnig argues that Mendel's laws were 'laws of constant elements for a great but finite variation'. He points out that, in his short Pisum paper, the word 'constant' appears sixty-seven times ('constant characters', 'constant combinations', 'constant forms', etc.) (Lönnig 1998, 2000, 2001). He takes this as evidence of a belief in the constancy of species. But, as we have seen, Mendel was looking for constancy in hybrids—which would actually prove the mutability of species.

Another ID critic, Giuseppe Sermonti, claims that Mendel's work proved the genetic make-up of populations is invariable. Character elements (e.g. wrinkled peas) may lie dormant in one generation, but re-emerge in subsequent generations. They remain constant; 'they do not change, either in type or frequency'. For Sermonti, this contradicts the Darwinian theory of evolution, 'a theory whose fundamental postulate was that the genetic composition of populations varied from generation to generation' (Sermonti 2005, p. 46).

We find similar views expressed by Lynn Margulis: Mendel, she claims, made it very clear that while traits reassort, 'they don't change over time' (Margulis 1996). For her, this makes Mendel's work 'anti-evolutionary'; the neo-Darwinian synthesis was, in fact, an attempt to reconcile two irreconcilable views. (We should note that Margulis, while not an ID-er, believes history will one day judge Neo-Darwinism simply as a 'minor twentieth-century religious sect'—cited Behe 2006, p. 26.) Thus, Margulis, and ID-ers like Sermonti, turn Mendel into an advocate of a form of essentialism: i.e. there is a species essence that does not change; any variation is superficial only.

The notion that microevolution may occur, but only within certain strict limits, is one of the cornerstones of ID theory. Natural selection does occur, Sermonti states, but it mainly has the effect of eliminating chance abnormalities or deviations: 'It is ever adjusting populations, but it does so in each case by bringing them back to the norm' (2005, p. 51). This is simply a recycling of the old fallacy of species stability. Mendel, as we have seen, set little store by the notion of 'species.' His emphasis on traits suggests a mosaic concept of plant form; there is no fixity or essence of type. In the pea experiments, traits remain constant; the plant form does not. Moreover, for he, there are different laws for variable and for constant hybrids. In constant hybrids—however they may be formed—the recombination of elements is permanent; there can be no return to a species 'norm.'

As we have seen, Darwinists have marginalised or misrepresented Mendel's research into constant hybrids, as it does not fit in with the dominant evolutionist paradigm. Creationists and ID-ers have long been adept at finding and exploiting weaknesses in the Darwinian case. Callender seizes on a particle of truth—Mendel was interested in the creation of new forms through hybridisation, as a mechanism for evolution—and uses it to drive a wedge into the neo-Darwinian synthesis. What is dangerous is the way that his claim that Mendel was a ‘Linnaean’ (i.e. ‘special creationist’) has been accepted by some as if it is a fact, rather than seen for what it is: a blatant propagandist ploy by the ID/creationist camp.

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The ongoing battle over Darwinism has cast its long shadow over Mendel. One side stresses constancy over variation; the other side, variation over constancy. Both sides simplify and distort the evidence, and mangle and misrepresent Mendel’s position on the role of hybridisation in the formation of new forms of life, as they fight to claim him as one of their own.

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