Gregor Mendel and the theory of species multiplication (2024)

Abstract

Introduction

To biologists, Gregor Mendel (1822–1884) is the founder of genetics. This Augustinian friar from Brünn (Now Brno, Czech Republic) published his famous pea-crossing experiments in 1866, but this publication was neglected until 1900. Today these experiments are still used as an introduction to genetics. According to this traditional interpretation, the investigation of the inheritance of traits was Mendel’s original goal.

However, many historians of science do not share this orthodox interpretation of Mendel’s work. According to the revisionist interpretation, Mendel followed the tradition of earlier plant hybridists such as Josef Gottlieb Kölreuter (1733–1806) and Carl Friedrich von Gärtner (1772–1850) and was interested in the question of whether new species could arise from hybridization between existing species. This revisionist interpretation was first suggested in a 1979 article by the historian of science Robert C. Olby with the remarkable title, “Mendel no Mendelian?” (Olby 1979). He was supported in this by Brannigan (1979) and L.A. Callender, in a then-unpublished thesis of which an elaborated version did not appear as an article until 1988. In Olby’s view, Mendel’s re-discoverers in 1900 had seen more, and something different, in Mendel’s work than Mendel had originally meant. Moreover, Olby argued that Mendel’s interpretation was not that of later Mendelian genetics. Olby also found it odd that the word “inheritance” did not occur in the title of the 1866 article if this was the article’s subject.

Olby’s paper had a major impact. Müller-Wille (2021) writes in the “Handbook of the Historiography of Biology” that: ‘“Mendel no Mendelian?” provided ample historical evidence for a new assessment of Mendel’s achievements that inspired a whole new generation of Mendelian scholarship.’ (p. 113). Oddly enough, few geneticists are familiar with the revisionist interpretation of Mendel’s work.

A common problem in understanding Mendel’s original intentions for his experiments is that primary historical sources are few. After he died in 1884, nearly all of his notes were destroyed, and when his work was rediscovered 16 years later, the memories of people that had known him had faded. This is in stark contrast to Charles Darwin, the other great 19th-century biologist, whose notebooks and thousands of letters have been preserved. After 1900, 10 letters from Mendel to the Munich botany professor Carl Nägeli were found, as were a few fragments of Nägeli’s replies. The lack of good historical sources has led to the fact that research into Mendel’s motives for the pea experiments is mainly based on the interpretation of the text of the 1866 article.

Mendel conducted his Pisum experiments from 1854 to 1863. After that, from 1866 to 1873, he worked on crosses in a wide range of plant genera with a focus on hawkweeds (Hieracium). For a long time, biologists had thought that the Hieracium crosses served to verify the results of the Pisum experiments and that when they failed, Mendel became frustrated and abandoned research. Historians of science have followed Olby’s reading in assuming that the Hieracium experiments were more satisfying to Mendel than the Pisum experiments. According to the transcription of Correns (1905), Mendel did indeed write in his first letter to Nägeli that he intended to use Hieracium to confirm his Pisum results. However, an examination of the written letter showed that this interpretation is not secure, and it is likely that a page is missing, producing what appears to be an unintended contradiction in the text (Van Dijk and Ellis 2016). In agreement with Olby (1979), Brannigan (1979), and Callender (1988), we concluded that Mendel had found variable hybrids in Pisum and was looking for constant hybrids in Hieracium. In variable hybrids, characters segregate in the next generation, whereas this does not happen in constant hybrids. Shortly after 1900, Hieracium was discovered to have a special and rare breeding system producing clonal seeds (apomixis). Apomixis preserves maternal heterozygosity.

In the Concluding Remarks, Mendel (1866) wrote about constant hybrids as new species:

We meet with an essential difference in those hybrids which remain constant in their progeny and propagate themselves as truly as the pure species. According to Gärtner, to this class belong the remarkably fertile hybrids Aquilegia atropurpurea-canadensis, Lavatera pseudolbia-thuringiaca, Geum urbano-rivale, and some Dianthus hybrids; and, according to Wichura, the hybrids of the willow family. For the history of the evolution of plants this circ*mstance is of special importance, since constant hybrids acquire the status of new species.

[We used the translation of Bateson (1902), which can be found on MendelWeb (http://www.mendelweb.org/) and, as recommended by an anonymous referee, a slightly different, searchable, version of the 1902 printed document of Bateson's “Mendel's Principles of Heredity: A Defence” which is available at https://www.biodiversitylibrary.org/item/100035#page/64/mode/1up. The recent translation by Staffan Müller-Wille and Kersten Hall on the website of the British Society of History of Science was offline while we were writing the manuscript.]

This article considers whether Mendel’s Pisum research was related to the question of speciation by hybridization, as Olby (1979) suggests. Only a few biologists have addressed this question. Hartl and Orel (1992) pointed out that Mendel considered it completely immaterial whether the Pisum forms should be classified as species or varieties and that this did not fit well with the question of the role of hybridization in species formation. Orel and Hartl (1994) and Orel (1996) disagreed with the species multiplication interpretation of Mendel’s work but did not present additional arguments for a rebuttal. The rejection in these two publications was based on the belief that the 1866 paper clearly indicated Mendel’s interest in the transmission of traits from parents to offspring. The present paper explicitly scrutinizes Olby’s arguments that Mendel’s main concern was species multiplication. The second question raised by Olby (1979), to what extent Mendel’s interpretation differed from Mendelian genetics after 1900, will be discussed in a forthcoming article in Hereditas. We here also provide new relevant historical information and conclude that Olby’s claims about the speciation objective of the Pisum experiments do not hold.

Olby’s interpretation

We first present the core of Olby’s ideas and, in the next section, discuss the hybridists in whose tradition Olby believed Mendel to stand. In “Mendel no Mendelian?” Olby concluded:

Mendel’s overriding concern was with the role of hybrids in the genesis of new species. Are hybrids variable or constant?—for if constant they might mark the first stage in the genesis of new species. He approached the subject with his conception of constant and independently transmitted characters. The laws of inheritance were only of concern to him in so far as they bore on his analysis of the evolutionary role of hybrids.

It is striking that Olby does not provide a species definition, but he comments on Callender’s opinion from a summary of an unpublished thesis:

indeed it would seem, on Callender’s view, that the behaviour of Pisum was a disappointment to him, only migitated by his discovery of constant forms among the hybrid progeny which showed new combinations of characters.

[However, Callender (1988) denies having said this: “Neither this view, nor anything approximating to it, was contained in the very brief summary of my conclusions which I sent to Dr Olby in 1974.” (endnote 1).]

This suggests that Olby apparently proposed that Mendel considered constant forms with new combinations of characters arising from crosses as new pea species. Mendel studied seven traits with two contrasting forms. Mendel (1866) stated:

All constant combinations which in peas are possible by the combination of the said 7 differentiating characters were actually obtained by repeated crossing. Their number is given by 2⁷ = 128.

If Mendel considered this to be 128 different species, it would mean he was using the strictest species concept. As we will argue later, this does not follow from the 1866 article. Olby (1985a) again did not clarify Pisum by discussing the combined results of Pisum and Hieracium:

Mendel failed to arrive at a decisive conclusion as to whether or not species in general can multiply by crossing. He knew, of course, that true-breeding forms showing new combinations of characters were yielded in some of the progeny of hybrids, but as to whether first-generation hybrid forms could in some cases be perpetuated without any “splitting” or reversion of their progeny he could not offer a straight answer. His two chief research organisms—Pisum and Hieracium—behaved differently. Pisum did not show this phenomenon of hybrid constancy, Hieracium did.

We disagree with Olby that Mendel did not arrive at a decisive conclusion; he concluded that Pisum did not multiply by crossing, whereas Hieracium did. A differentiated conclusion can be a decisive conclusion. In later publications on Mendel and species multiplication, Olby hardly writes about Pisum anymore and emphasizes the constant hybrids of Hieracium (Olby 1985b, 1997). In this context, Olby (1997) only remarked:

….. his study of the edible pea did not lead him to reject the concept of species multiplication by hybridization. Instead it led him to study other species, especially in the genus Hieracium, in the expectation of finding evidence in support of that theory.

The theory of species multiplication by hybridization

Olby (1985b) pointed out that in the 18th and 19th centuries, there was a theory of speciation by hybridization that went back to the great Swedish systematist Carl Linnaeus (1707–1778). Linnaeus had originally (1737) assumed that all existing species were created by God almighty, but gradually changed his opinion. At the end of his life (1774), he assumed that all species within a genus had arisen from hybridization of a small number of originally created species (Callender 1988). Therefore, this theory is called “species multiplication by hybridization”. Already in the first edition of “Origins of Mendelism”, Olby (1966) had placed “Mendel within the continuing tradition of plant hybridizers from Koelreuter to Gaertner to Mendel.” The early hybridists Kölreuter and Gärtner rejected this idea of species multiplication because of their religious belief. Their artificial crosses served to disprove species multiplication by hybridization. These crosses showed that hybrids reverted to the parent species when self-pollinated. In addition, their pollen was often of poor quality, and they were easily pollinated by the parent species. As a result of repeated backcrossing, successive hybrid generations more and more resembled the parental species. Olby (1985b) stated:

Even in the eighteenth century, informed opinion was against the possibility of hybridisation between very distant forms—most intergeneric and some interspecific crosses— as a source of new species. Debate long continued however, over the extent of crossing between closely related species and the achievement of species multiplication thereby (p. 264).

According to Olby (1985b), in contrast to Kölreuter and Gärtner, Mendel believed in the idea of species multiplication through hybridization and wanted to test this with his Pisum and Hieracium experiments.

The theory of species multiplication by hybridization suited such a framework and Mendel, as a physicist, devised a rigorous experimental programme to test it. The resulting theory of hybridisation accounted for the evidence of both variable and constant hybrids. (p. 266).

Olby (1985b) referred to Franz Unger, the professor of botany, whose lectures and practical courses Mendel had followed at the University of Vienna in the winter-semester 1852/1853:

In his textbook of 1855 Unger rejected, once more, the belief in the stability of species. Instead he held that variants arise in natural populations and that the slight variants give rise to varieties and sub-species whilst the larger variants form specific differences. This view was at variance with the opinion of Koelreuter and Gaertner and it would appear that Mendel carried out his experiments in order to decide the issue. (p. 97)

According to Unger (1855), the established assumption that species were immutable had to be rejected, and it had to be accepted that the diversity of plants resulted from the process of reproduction. Under what circ*mstances and how new species arose were both still completely unknown, according to Unger. Incidentally, it is important to note that Olby speaks of slight variants, not hybridization. Unger (1855) actually did not believe that hybridization played an important role in species formation:

The hybrids are then to be found in the vicinity of the parental plants, but since they are generally unable to survive through reproduction, they disappear again unless they are replaced by new hybrids (p. 392).

An important argument for Olby (1979), Brannigan (1979), and Müller-Wille and Orel (2007) to place Mendel in the tradition of the hybridizers, who were concerned with the question of whether species could arise through hybridization, was that Mendel explicitly mentioned these hybridists in the introduction and again in the discussion of the 1866 article. Müller-Wille (2021) wrote:

Helpfully, Mendel himself was quite explicit in placing himself within such traditions. Not only did he list predecessors in the first paragraph of his paper, but he also concluded it with a critical review of their experiments.

Mendel (1866) wrote in the introduction:

To this object [to follow up the developments of the hybrids in their progeny] numerous careful observers, such as Kölreuter, Gärtner, Herbert, Lecoq, Wichura, and others, have devoted a part of their lives with inexhaustible perseverance.

These researchers have in common that they made artificial crosses, but their motives were quite different. Kölreuter and Gärtner were what Mayr (1982) calls “species hybridizers” (p. 641). Herbert and Lecoq were, in Mayr’s terminology, “plant hybridizers”, who made crosses to produce new ornamental plants. A better name for the latter group is perhaps “horticultural hybridizers”. Wichura was a naturalist, a contemporary of Mendel, interested in natural hybridization between willow (Salix) species. However, the mentioning of previous hybridizers does not mean that Mendel asked the same questions. In 1849, Gärtner had published his magnum opus, “Experiments and observations on hybridization in the plant kingdom”, which summarized the more than 10,000 artificial plant crosses he had made and discussed these with respect to the work of others. Of course, Mendel consulted Gärtner’s book just as his contemporaries Darwin, Nägeli, and Wichura did; it was the most important source of crossbreeding results at the time.

Moreover, Mendel’s sentence, quoted above, including the mention of “Kölreuter, Gärtner,” is so similar to one from the German translation of “The Origin of Species” (Darwin 1863) that this seems to be the source [Translated: “…..Kölreuter and Gärtner, who have devoted almost their whole lives to this subject, …” (p. 275)]. It is, therefore, difficult to maintain that this sentence places Mendel in the old tradition of the species hybridizers Kölreuter and Gärtner. According to Mayr (1982), “It is totally misleading to say that Mendel’s conceptual framework was that of the hybridizers. It is precisely the breaking away from the tradition of the hybridizers that characterizes Mendel’s thinking and constitutes one of his greatest contributions.” (p. 713). This seems to be correct to us.

Mendel’s species concepts and Pisum systematics

Mendel (1866) wrote about the taxonomic status of the 34 Pisum accessions that he obtained from seed dealers:

Their systematic classification is difficult and uncertain. If we adopt the strictest definition of a species, according to which only those individuals belong to a species which under precisely the same circ*mstances display precisely similar characters, no two of these varieties could be referred to one species. According to the opinion of experts, however, the majority belong to the species Pisum sativum; while the rest are regarded and classed, some as sub-species of P. sativum, and some as independent species, such as P. quadratum, P. saccharatum, and P. umbellatum. The positions, however, which may be assigned to them in a classificatory system are quite immaterial for the purposes of the experiments in question. It has so far been found to be just as impossible to draw a sharp line between the hybrids of species and varieties as between species and varieties themselves.

As Hartl and Orel (1992) noticed before, that Mendel considered the taxonomic rank of the Pisum forms immaterial is of course problematic if he was interested in species multiplication by hybridization as Olby suggested. Mendel took the definition of the strictest species concept verbatim from Schleiden’s (1846) book, of which he had a copy (Iltis 1924). However, Schleiden felt that with the state of science of that day, the discussion of what constituted a species was pointless and led to much wasted paper. No doubt Mendel read Schleiden’s critical opinion on this when he copied his definition. According to Mendel, the strictest species definition, “however”, was not the view of experts, who considered most accessions to belong to the species P. sativum and the others to subspecies or a few independent species.

Müller-Wille and Orel (2007) attempted to reconcile Olby’s conception of Mendel’s work in the sense of species multiplication in the hybridist tradition with that of Mendel’s work as an investigation of trait inheritance. To this end, they assumed that Mendel “adopted” the strictest species concept. However, Mendel’s phrasing of applying the strictest species definition is clearly that of a hypothetical one (“Wollte man; If we adopt”). Moreover, the last line we quoted above from Mendel’s paper shows that Mendel regarded species and varieties as real entities but as extremes of a continuum.

Müller-Wille and Orel (2007) also argued that the German word “Arten”, which is frequently used in Mendel (1866), should be translated as “species” and not as “variety”, “kind”, or “strain” as was done in the translations by Bateson (1902) and Stern and Sherwood (1966). However, when Mendel describes the 34 accessions he had bought, he renders them in a single paragraph as “Sorten”, “Formen”, and “Art[en]”. Clearly, for Mendel, these words had a similar meaning. According to 1860s dictionaries, “Sorten” and “Arten” can both be translated into English as “kind”, “species”, or “sort” (Lucas 1863; Schröer 1863). Therefore, Mendel’s frequent use of the word “Arten” does not mean he accepted the strictest species concept.

Probably what prompted Mendel to come up with the strictest species definition was a publication in die Botanische Zeitung by Hermann Hoffmann, professor in Giessen, in 1862 (“An attempt to determine the value of species and variety”, Hoffmann 1862). The Botanische Zeitung was in the Natural Sciences Society (NSS) library in Brünn, of which Mendel was a member. Hoffmann had grown a bean variety for seven years and found that the seed color and size showed some variation but always reverted to the original type the next generation. Since the differences between bean lines were stable, they had to be seen as different species. The same strictest species concept was disseminated in France by Alexis Jordan, who cultivated the Linnean species Draba verna in his experimental garden and split it into over 200 new species that constantly differed in minor characteristics (Bonneuil 2002). Because of their belief in the constancy of species, Jordan and Hoffmann were anti-Darwinists.

Linnaeus had assumed the constancy of species and used the concept of variety for environmentally induced variation. In Mendel’s time, the existence of non-environmentally induced variation within a species was generally accepted. Mendel’s teacher, Franz Unger, regarded the constancy of species as an unproven assumption (Unger 1855). In a review of a book by Alexis Jordan, Eduard Regel, the influential German director of the Botanical Garden in St. Petersburg and publisher of the Garten Zeitung in 1856, characterized Jordan as “known by the many and largely untenable species and his extreme views on genus and species” (Regel 1856). The Linnaean school admitted variability of species within limits (Decaisne 1866). During the 1860s, it was clearly backward to assume the constancy of the species in the German-speaking scientific world (Junker 2011). In the early 1870s, Čelakovský (1873) and Focke (1875) further spoke out against the application of the strictest species definition by Hoffmann and Jordan. These botanists, according to Focke, had elevated what in the Linnean system was a variety to a species, which led to unnecessary confusion because the concept of species was already in use in the Linnean system for a higher group of forms. Calling any variety with a constant flower color a species was, according to Čelakovský (1873), a species concept ad absurdum (p. 234). In modern terms, Jordan and Hoffmann were extreme “splitters”, whereas most botanists in the 1860–1870s were “lumpers”.

In the same volume (1862) of the Botanische Zeitung as Hoffmann’s article, Friedrich Alefeld protested against the application of strictest definition because this would mean that the 80 different pea forms he cultivated would be 80 different species (Alefeld 1862). Alefeld was the most prominent legume taxonomist in Germany and had two years earlier, in 1860, divided the genus Pisum into just two species: P. sativum and P. frigidum (Vavilovia formosa) (Alefeld 1860). Alefeld (1862) argued that it was not only a stable difference that distinguished species but also that the difference should be sufficiently large (without specifying how large). Alefeld belonged to the experts (“Fachgelehrten”) who saw the different Pisum forms as subspecies of P. sativum; Moritz Willkomm, the author of the Flora from which Mendel would copy Pisum species notes (see below), distinguished four Linnean species with Latin binomials. In 1866 Alefeld published his “Agricultural Flora” (Alefeld 1866), in which he classified cultivated forms into the framework of scientific systematics with Latin nomenclature. According to him, all garden peas belonged to one species, P. sativum, with two subspecies and eight variety groups comprising 102 cultivars.

In 1862 the NSS published a flora of Brünn and surroundings (Flora des Brünner Kreises) written by Mendel’s friend Alexander Makowsky (Makowsky 1862). Mendel wrote the climatological part of this Flora. Gustav von Niessl and Carl Theimer, other members of the NSS and friends of Mendel, were also contributors. This Flora used the species criterion of A. Neilreich’s “Flora of lower Austria” (Neilreich 1859) (p. 31). Neilreich was a convinced lumper (Neilreich 1846), and his species criterion was based on the absence of intermediate forms between two species in nature (Neilreich 1870). Makowksy’s Flora described P. sativum as a single Linnean species with three varieties: the garden peas Var. hortense and Var. quadratum and the field pea Var. arvense.

In conclusion, Mendel’s (1866) text and the broad species concept adhered to generally and, notably among those in Mendel’s direct environment, leads to a rejection of the idea that Mendel was an extreme splitter who had adopted the strictest species concept.

Gärtner, laying the foundation for Mendel?

In the back of Mendel’s copy of Gärtner’s book are handwritten notes about several Pisum species and their characteristics (Fig. 1).

Olby (1985b) translated these notes:

“Pisum arvense: flowers solitary, wings red.

Pisum arvense et sativum: pods almost cylindrical, in Pisum umbellatum Mill. cylindrical and straight; in saccharatum Host. straight, ensiform, constricted on both sides. (var. flexuosum Willd. sickle-shaped, seeds small, angular); in Pisum quadratum Mill. straight, ensiform, not constricted, seeds pressed tightly together. In Pisum sativum and arvense, the bases of the stipules rounded and denticulate-crenate, stipules cordate. In saccharatum and quadratum, stipules obliquely incised, pods pressed flat. In sativum, saccharatum and umbellatum, seeds round.”

Olby (1985b) discussed the probable meaning of these notes, referring to them in the index as “Gärtner, laying foundation for Mendel” (p. 303):

These notes are important because they show Mendel at work, hunting for clearly-marked character differences between the various forms of peas. Hence it is reasonable to assume that these notes were written prior to the purchase of the 34 varieties of peas for testing in 1854. If this is so, Mendel must have purchased his copy of Gärtner’s book, Versuche and Beobachtungen Ueber die Bastarderzeugung im Pflanzenreich, before he had worked out the detailed plan of his experiments with Pisum.

Clearly no definite conclusion can be drawn on this point, but I hold it as very probable that Mendel learnt of Gärtner’s work from Unger in Vienna, that Mendel looked at the book during his botanical studies there in 1852, and that he subsequently purchased a copy which he read in more detail in 1853–54, before he chose the 34 varieties of peas. At the back of the book he noted down some of the character differences between the various varieties and species of Pisum, presumably from published descriptions. Later he chose two of these distinguishing characters for the experiments reported in 1865; i.e. form of the seed angular or round (character difference No. 1 of the 1865 [sic] paper), and shape of the pod constricted or inflated (character difference No. 4 of the 1865 paper) (p. 213).

If we could determine from which Flora these features were taken, it would be possible to infer the earliest possible date that these notes were made. Therefore, we checked the Pisum descriptions of eleven digitized German-language Floras published between 1832 and 1866 that we could get hold of [Reichenbach (1830–1832), Sailer (1841), Mössler (1843), Koch (1846), Petermann (1849), Kittel (1853), Neilreich (1859), Alefeld (1860), Maly (1860), Makowsky (1862), and Willkomm (1863)]. It turns out that the only Flora containing all the words of Mendel’s notes is Moritz Willkomm’s (1863) Flora (“Führer in’s Reich der deutschen Pflanzen”). The variety flexuosum Willd. could not be found in any of the other Floras. Makasheva (1979) also mentions Willkomm (1863) as the only reference to this pea variety. The Willkomm Flora dealt with a large number (364) of cultivated plants. Willkomm classifies the garden peas into four different Linnean species, like Mendel (1866), noting that P. sativum and P. saccharatum have many varieties and forms.

Although all words match Willkomm’s Flora, they are not in the same order. This is not surprising because Willkomm’s text is a determination key. According to the NSS catalog, the Willkomm Flora was not in the NSS library (Anonymous 1875). Perhaps Mendel copied the Pisum notes in another library in Brünn or Vienna. Mendel’s handwriting looks hasty compared to the other annotations referring to the passages in Gärtner’s book and includes abbreviations (for example P. sativ. for P. sativum), a correction (insertion of an extra “c” in saccharatum) and a strikethrough.

Willkomm wrote the preface of his Flora at the end of May 1863. Unless an older flora with the complete Pisum notes is found, we must assume that the notes were copied after the summer of 1863. The scenario that Olby considered probable, i.e. that Gärtner’s text laid the foundations for Mendel’s detailed plans before he started his experiments, does not concur with these dates. It is plausible that Mendel intended to give a Linnean Latin binomial to the Pisum varieties following the everyday use by naturalists when he held the first of his two Pisum lectures at the NSS monthly meeting in February 1865.

There are additional problems with Olby’s suggestion that Gärtner laid the foundation for Mendel’s pea experiments regarding species multiplication by hybridization. Kölreuter and Gärtner primarily worked with hybrids between Linnean species, which is a necessary condition for Mendel’s experiments to be considered as concerning species multiplication rather than varietal diversification. However, according to Gärtner (1849), there were few natural (interspecific) hybrids in the Leguminosae family. He even stated specifically that successful crosses between pea varieties (such as those Mendel made) should not be considered as evidence for hybridization in the legume family: “Indeed, the combinations of the different varieties of P. sativum succeed easily and perfectly…..but these are mere varieties and not pure species (p. 173)”. Although Gärtner himself also performed crosses with Pisum, it was to study the effects of the pollen on the fruit and seed characteristics (xenia), not regarding the hybrids from crosses and their descendants. Concerning xenia, Gärtner considered Pisum an exception to the general rule that the pollen did not influence the seed morphology (p. 89) [This concerns the color and shape of the cotyledons, so properties of the offspring, already visible in the seed]. Gärtner also commented critically on Pisum pollination experiments conducted by Thomas Andrew Knight (1759–1838): “But since these experiments deal with varieties whose nature differs significantly from that of the pure species in relation to their fertilization and reproduction, … ” (p. 54) [“Da es sich aber bei diesen Versuchen von Varietäten handelt, deren Natur in Beziehung auf ihre Befruchtung und Fortpflanzung von der der reinen Arten bedeutend abweicht,….”]. The reservations made by Gärtner make it unlikely that Mendel started a research program on species multiplication in Pisum in 1854 based on Gärtner’s book.

A newspaper article in the Brünner Zeitung from July 1861 describes Mendel as a plant breeder who, in addition to peas, also bred beans and cucumbers (Van Dijk et al. 2018). This applied research is hard to reconcile with a fundamental species multiplication research program. Species multiplication was a question about hybridization between wild species and not between varieties of cultivated species. The introduction to Alefeld’s “Agricultural Flora” (1866) is telling about how botanists thought about cultivated plants: “the botanists, mostly look down on the cultivated forms and consider their systematic study almost beneath their dignity”.

The importance of the concluding remarks

Olby and other historians of science attach great importance to the Concluding Remarks of the 1866 article. In this section, Mendel showed how his Pisum findings could explain observations made by Kölreuter and Gärtner. This is also the section where Mendel wrote about constant hybrids as being new species. According to Müller-Wille (2021), Mendel explicitly placed himself in the tradition of the [species] hybridists with the Concluding Remarks.

Several reprint versions of Mendel’s (1866) paper have omitted this section. Olby ended “Mendel no Mendelian?” saying that his reinterpretation was long-overdue; “a reinterpretation which restores meaning and purpose to the whole text of the Versuche.” (our emphasis). Along a similar vein, Olby (1985b) wrote: “Those who have reprinted Mendel’s paper without the final section have done a disservice to the history of genetics” (p. 265).

However, reports in the Brünn newspaper the Brünner Zeitung cast serious doubt about whether the concluding remarks were included in the two lectures in February and March 1865 (Van Dijk and Ellis 2022). Instead, the statement in the Brunner Zeitung, that “the Legume family, according to well-known researchers, [was] not well suited for hybridisation”, is remarkable because if Mendel wanted to show how his findings could explain the results obtained by Kölreuter and Gärtner, this remark would have been irrational. It is also noteworthy that the newspaper reports of the lectures do not mention speciation or constant hybrids. Furthermore, it is hard to see how the whole text of the 1866 paper, with the explanation of the developmental series, could be read in one long and one short lecture. Moreover, the original manuscript of the 1866 article contains two broad horizontal lines, dividing it into three sections, the first two covering the lectures. Together this strongly suggests that the Concluding Remarks were added after the lectures, between the spring of 1865 and the summer of 1866, the deadline for submission. If so, then the Concluding Remarks cannot be used to interpret Mendel’s original motivation for the Pisum crossing experiments.

Pisum and Hieracium one research program on species multiplication?

The revisionist’s view of Mendel’s work assumes that Mendel’s Pisum and Hieracium experiments were part of an extensive hybridization program on species multiplication that he started in 1854 and ended in 1873. It seems implausible to us that Mendel’s interests did not develop while conducting his Pisum experiments in the 12 years after 1854. A lot changed during these years, for example, the publication of the German translation of Darwin’s “Origin of Species” (Darwin 1863), the establishment of the NSS in Brünn to promote pure scientific research, and important discoveries in the cell and reproductive biology of those days. There is no reason to assume that the original motivation for the execution of the two breeding programs was the same.

We have suggested that Mendel, in 1854, started with plant breeding experiments to develop better pea varieties and, when conducting crosses, became curious about the recurrent segregation patterns of seed traits (van Dijk and Ellis 2018; van Dijk et al. 2022). This led to an investigation into the inheritance of other traits. On New Year’s Eve in 1866, Mendel sent a reprint to Carl Nägeli with a long covering letter, explaining his Pisum studies and discussing his plans with supposed constant hybrids (Hieracium, Cirsium, and Geum). Nägeli had written that hybridization could provide insight into the transmission of traits from parents to offspring. Nägeli replied, interpreting Mendel’s paper as an inheritance study. In contrast, Mendel’s letter to Anton Kerner von Marilaun, professor of botany in Innsbruck, dated one day later, only contained the first formal paragraph of the letter to Nägeli. This reveals the importance Mendel attached to Nägeli’s judgment of his work compared to that of Kerner. A few years later, Kerner supported the idea of the origin of new species from hybridization in a paper titled “Can Bastard Forms become Species?” (Kerner 1871). However, Kerner did not cite Mendel’s (1866) paper on Pisum, nor his 1870 paper on Hieracium (Mendel 1870), as might be expected if Mendel’s work was about species multiplication by hybridization.

A further argument against the idea that the Pisum and Hieracium experiments were part of one research program on species multiplication is that in the title and throughout the text of his 1866 Pisum paper, Mendel used the German word “Hybriden”, but the word “Bastarden” in the 1870 Hieracium paper. Both words are commonly translated as “hybrids” in English. However, “Hybriden” should be translated as intraspecific hybrids, whereas “Bastarden” should be translated as interspecific hybrids as we have discussed before (Van Dijk and Ellis 2022). Mendel made the same distinction between Pisum Hybriden and Hieracium Bastarden in his letters to Nägeli (Stern and Sherwood 1966). It is evident that Mendel considered the hybrids obtained in Pisum and Hieracium crossing experiments to be of a different kind.

Conclusion

Olby’s 1979 paper is a very influential publication among historians of science (Müller-Wille 2021). One of the conclusions of Olby (1979), which we investigated here, was that Mendel was not primarily interested in inheritance but in speciation. While geneticists have little awareness of the revisionist view on Mendel’s work, Olby’s account of Mendel’s “real” motivation has found its way to several popular science books, such as Waller (2002), Endersby (2007), Theunissen (2013), Kampourakis (2015), and Bowler (2015). The revisionist interpretation also combines with other calls to reduce or remove Mendel’s pea experiments from the genetics curriculum (e.g. Allchin 2003; Campanile et al. 2015; Sparks et al. 2020; Bapty 2022). This is unfortunate because when we examine Olby’s arguments critically, it turns out that regarding the Pisum experiments, which are the essence of Mendel’s genetics, as we have shown here, little of Olby’s views can hold.

Acknowledgments

THNE gratefully acknowledges receipt of an Institute Strategic Fellowship from the John Innes Centre. PJVD thanks KeyGene, for providing a supportive environment for research into the early history of genetics. We are grateful to Adrienne Jessop for critically reading the original manuscript and to two reviewers for their valuable feedback and recommendations.

Funding

This research received no external funding.

Literature cited

Author notes

Conflicts of interest The authors declare no conflict of interest.

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