Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Wednesday, December 08, 2004

Did Darwin explain the origin of species in On the Origin of Species?

There is a repeated canard that Darwin did not account for the origin of species in his book (repeated by, among others, Futuyma). He did.

There is a similar canard that Darwin had no definition of "species" and indeed that he denied that species were real. This, too, is false.

On the latter, allow me to quote from Michael Ghiselin's Triumph of the Darwinian Method (1984 edn, chapter IV):

It is not difficult to find passages in Darwin's writing suggesting that he considered species to be purely artificial constructs. For instance: "In short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species."[26] On the basis of statements such as this, Mayr and others have come to the conclusion that Darwin upheld a morphological, as opposed to a biological, species concept.[27] That is, they maintain that he looked upon species as merely classes of organisms having a given degree of similarity and difference in the observed properties of their members. But there are certain other passages in Darwin's writings which show that the problem is more complicated. In his second notebook on the transmutation of species, Darwin says: "As species is real thing with regard to contemporaries - fertility must settle it."[28] The reality of species is affirmed in a letter to Gray written in 1860, in which Darwin severely criticizes some assertions of Louis Agassiz. The following statement is particularly relevant to the question at issue: "How absurd that logical quibble - 'if species do not exist, how can they vary?' As if any one doubted their temporary existence."[29] It appears that there is at least one sense in which species are thought to be real, although it is evident that there is a sense in which they are held to be not real. This being the case, any citation of statements by Darwin in support of his holding one or another point of view must be buttressed by a demonstration of the sense which he intended.

Ghiselin's footnote references are:
26 Origin, p. 485.

27 Mayr, "Isolation."

28 Second Notebook, p. 99. (The view expressed here was later modified.)

29 Life and Letters, II, 333.

Ghiselin, Michael T. 1969. The triumph of the Darwinian method. Berkeley: University of California Press, 1984 edition.

I have around 37 pages of Darwin's comments on species - so far as I can tell he always thought that species were contemporaneously real things, that were temporary. Interfertility is not, as Ghiselin suggests, a view Darwin later abandoned, but it is only a test, and not the cause of species in his later view.

He thought that species were formed from selection favouring varieties within a species. Although he flirted with geographical isolation as a cause before the Origin, after it, he thought selection, and only selection, caused new species, and he had a debate with Moritz Wagner, who proposed geographical isolation, on the matter.

Darwin appears to have been correct that selection can cause species, but incorrect in thinking that other factors such as geographical isolation cannot, and that all speciation is driven by selection. However, it does seem like all speciation is followed by selection as novel species adapt to different circumstances (including different mates).

Darwin explained speciation by what we would now call sympatric speciation (that term was invented much later, by EB Poulton). His mechanism was abandoned for a long time after around 1904 (Poulton's paper) in favour of Wagner's geographic isolation hypothesis (JT Gulick's work was pretty convincing to most).

Poulton, Edward Bagnall. 1903. What is a species? Proceedings of the Entomological Society of London: 1946-1994, reprinted in Poulton, Edward Bagnall. 1908. Essays on Evolution. Oxford: Clarendon.

However there are the following points to make:

1. Darwin did give an answer to the origin of species in The Origin of Species.

2. Recent work shows that in fact his mechanism does, realistically, generate new species from time to time (reviewed nicely by Schilthuizen 2001).

Schilthuizen, Menno. 2001. Frogs, flies, and dandelions: the making of species. Oxford: Oxford University Press.

3. Many new species are formed either by polyploidy or in allopatry, in fact.


4. Darwin explained some, but not all speciation.

The myth seems to be that Darwin did not even try to explain new species in TOoS. He did. He gave a good explanation - it just happens that it does not explain most speciation.

OK, now to what Darwin did say on the subject (6th edition used - so far as I can tell he does not change his views on this from 1859-1882. Page numbers in original edition):

Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully see in the next chapter, follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. [Chapter III, p51f]

And in chapter IV, he notes

In order that any great amount of modification should be effected in a species, a variety, when once formed, must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before; and these must be again preserved, and so onward, step by step. [p66]

Reproductive isolation maintains the subsequent species:

Intercrossing plays a very important part in nature by keeping the individuals of the same species, or of the same variety, true and uniform in character. It will obviously thus act far more efficiently with those animals which unite for each birth; but, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants. Even if these take place only at long intervals of time, the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind; and thus in the long run the influence of crosses, even at rare intervals, will be great. With respect to organic beings extremely low in the scale, which do not propagate sexually, nor conjugate, and which cannot possibly intercross, uniformity of character can be retained by them under the same conditions of life, only through the principle of inheritance, and through natural selection which will destroy any individuals departing from the proper type. If the conditions of life change and the form undergoes modification, uniformity of character can be given to the modified offspring, solely by natural selection preserving similar favourable variations. [p79]

Darwin at the time of the sixth edition thought that the formation of species did not rely on isolation and here he takes Moritz Wagner's view to task. Isolation does, he thought, make species formation easier, but he cannot agree it is required, as Wagner thought. And if the isolated population is too small, then isolation can in fact prevent speciation from occurring due to a lack of variation. The founder effect or drift through biased sampling has not occurred to him, as it later did to Weismann.

Isolation, also, is an important element in the modification of species through natural selection. In a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be almost uniform; so that natural selection will tend to modify all the varying individuals of the same species in the same manner. Intercrossing with the inhabitants of the surrounding districts will, also, be thus prevented. Moritz Wagner has lately published an interesting essay on this subject, and has shown that the service rendered by isolation in preventing crosses between newly-formed varieties is probably greater even than I supposed. But from reasons already assigned I can by no means agree with this naturalist, that migration and isolation are necessary elements for the formation of new species. The importance of isolation is likewise great in preventing, after any physical change in the conditions such as of climate, elevation of the land, &c., the immigration of better adapted organisms; and thus new places in the natural economy of the district will be left open to be filled up by the modification of the old inhabitants. Lastly, isolation will give time for a new variety to be improved at a slow rate; and this may sometimes be of much importance. If, however, an isolated area be very small, either from being surrounded by barriers, or from having very peculiar physical conditions, the total number of the inhabitants will be small; and this will retard the production of new species through natural selection, by decreasing the chances of favourable variations arising. [p79f]

Although isolation is of great importance in the production of new species, on the whole I am inclined to believe that largeness of area is still more important, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely. Throughout a great and open area, not only will there be a better chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms. Moreover, great areas, though now continuous, will often, owing to former oscillations of level, have existed in a broken condition; so that the good effects of isolation will generally, to a certain extent, have concurred. Finally, I conclude that, although small isolated areas have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas; and what is more important, that the new forms produced on large areas, which already have been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. They will thus play a more important part in the changing history of the organic world. [p80f]

Ironically, as Kottler (1978: 285-288) noted, Darwin in the Notebooks believed that isolation was a sine qua non for speciation, in part following the views of Leopold von Buch. By this later stage, Darwin appears to have made Natural Selection the primary cause of species, requiring that variation needs to occur in situ as it were, and so needing larger populations to give it opportunity to do so.

Kottler, Malcolm J. 1978. Charles Darwin's biological species concept and theory of geographic speciation: the Transmutation Notebooks. Annals of Science 35:275-297.

[Thanks to Mike Syvanen and John Harshman for discussion]