Last updated on 5 Apr 2014
One of the fundamental aspects of evolution is speciation. This is the process by which more species come into being, and there are many different definitions and mechanisms that have been proposed by biologists in the last couple of centuries. I aim to write an occasional series on what it is supposed to be at various times in the history of biology, as well as the theoretical and, if I get to it, professional aspects.
For there to be speciation, however, there needs to be the possibility of new species. The common view is that this requires a theory of evolution, but in fact, biologists from Linnaeus onwards have posited the generation of new species, even in the absence of anything resembling evolution. For example, during the middle ages, it was commonplace to think new species arose by spontaneous generation (that was the main method that writers about the Ark proposed, along with hybridisation). As I have argued, the notion of “species” itself arose from consideration of the Ark story, as more and more species were reported by travellers and colonisers.
Linnaeus was a creationist, as were nearly all naturalists during the 18th century. He held that varieties within species, and possibly even some species themselves, were but local forms caused by the action of soil, climate and weather. However, he allowed, later in his life, for a kind of speciation by hybridisation. First of all is the famous comment of the species Thalictrum lucidum:
Is the plant sufficiently distinct from T. flavum? It seems to me a daughter of time. [Species plantarum]
What he meant by this is unclear. It is not enough to base a speciation theory on. But he then described four species of Scorpiurus and says
It is beyond all doubt, that all these formerly arose from a single species, and the alteration in the environment is not sufficient for their creation: what commingling has then given rise to the constant plants?
He repeats this about species of Geranium, Calendula, Sonchus, and Campanula and the suggestion is they formed by hybridism. As Ramsbottom (1938) from whom I get this, says:
Five varieties of Solanum nigrum appear to be the offspring of hybrids. … he states that the varieties between Fumaria spicata and F. capreolata, judging from their flowers, might be considered as F. oficinalis and queries whether they are hybrids.
Perhaps equally striking is the treatment of varieties in ‘Species Plantarum’ when we bear in mind the definitions repeated two years previously. Far from being merely variations in non-essential characters, they are treated in the same way as species,and as may be seen from some of the quotations already given it is sometimes queried whether what is described as a species is only a variety or vice versa.
Rowbottom doesn’t think Linnaeus has changed his mind from the earlier Philosophica botanica. Instead he thinks this is something Linnaeus had always allowed. Linnaeus’ student Daniel Rudberg in 1744 had discussed the possibility of hybrids forming. And in 1746, another student, Johannes Gustavus Wahlbom, had discussed hybridisation in tulips. He explained it as degeneration: related species were a degradation of the original species, a view Rowbottom ascribes to Aristotle’s student Theophrastus. A modern botanist would assign this to plesiomorphic (underived) developmental systems, which is not so far removed. In 1751, his student Johannes J. Haartman described a hundred species thought to be hybrids on taxonomic grounds. Several other students made similar comments, quoted by Ramsbottom.
Although Linnaeus famously supposed that a genus, Peloria, was the result of hybridism between a flower of Linaria and some unknown plant, which he published in 1744 after Gmelin had responded to a letter from Linnaeus with news that he had found some hybrids too (Gardiner 2001) , he finally made his views explicit in a tract, Disquisition on the sex of plants, in 1760, in which he wrote:
There can be no doubt that these are all new species produced by hybrid generation. And hence we learn, that a mule offspring is the exact image of its mother in its medullary substance, internal nature, or fructification, but resembles its father in leaves. This is a foundation upon which naturalists may build much. For it seems probable that many plants, which now appear different species of the same genus, may in the beginning have been but one plant, having arisen merely from hybrid generation. … these Geraniums, I say, would almost induce a botanist to believe, that the species of one genus in vegetables are only so many different plants as there have been different associations with the flowers of one species, and consequently a genus is nothing else than a number of plants sprung from the same mother by different fathers. But whether all these species be the offspring of time; whether, in the beginning of all things, the Creator limited the number of future species, I dare not presume to determine. I am, however, convinced, this mode of multiplying plants does not interfere with the system or general scheme of nature
So Linnaeus held that from an initial plant with a variety of possible forms and parts, hybrids could generate some, but not an open-ended number, of new species. In a tract published two years after this, his student Johannes Mart. Gråberg wrote:
We imagine that the Creator at the actual time of creation made only one single species for each natural order of plants, this species being different in habit and fructification from all the rest. That he made these mutually fertile, whence out of their progeny, fructification having been somewhat changed, Genera of natural classes have arisen as many in number as the different parents, and since this is not carried further, we regard this also as having been done by His Omnipotent hand directly in the beginning; thus all Genera were primeval and consisted of a single Species. That as many Genera having arisen as there were individuals in the beginning, these plants in course of time become fertilized by others of different sort and thus arose Species until so many were produced as now exist. … That also some Genera multiplied into very numerous Species…. That these Species were sometimes fertilized out of congeners, that is other Species of the same Genus, whence have arisen Varieties.
Todays genera are the original creations of God. Ramsbottom says
The same theory of progress from simple to compound, from few to many (e simplice progressus ad composita; e paucis ad plura!) was repeated in the sixth edition of ‘Genera Plantarum’, 1764.
Linnaeus fixism was widely adopted, although it was in part based upon an artificial system, by Linnaeus’ own admission. He wanted a natural system – one that explained the underlying causal relationships between plants – but never was able to produce it. As late as 1830, John Lindley was calling Linnaeus’ system “natural”, remarking
Nature herself, who creates species only (Lindley 1830, xvi).
The genera were God’s creation here, too. Linnaeus’ ideas that species were generated was a commonplace. His ideas of the diversification of species by hybridism, however, while it was not used as the foundation for much research, became part of the botanist’s mental toolkit. This is not surprising, though, as naturalists had used hybridism as an explanation of new and deviant species since Aristotle had written about it for animals in the Historia Animalium, and Theophrastus in his Enquiry into plants. Later, hybridism was the foundation of Mendel’s researches, to which we shall return.
Gardiner, Brian G. 2001. “Linneaus’ species concept and his views on evolution.” The Linnean 17 (1):24–36.
Lindley, John. 1830. An introduction to the natural system of botany: or, A systematic view of the organisation, natural affinities, and geographical distribution, of the whole vegetable kingdom: together with the uses of the most important species in medicine, the arts, and rural or domestic economy. London: Longman, Rees, Orme, Brown, and Green.
Ramsbottom, John. 1938. “Linnaeus and the species concept.” Proceedings of the Linnean Society of London 150 (192-220)
1. It was not. It is an epigenetic mutation, neither concept of which was available to Linnaeus.