by Greg Mayer
In a previous post on how giraffes got their long necks, I noted that this was a venerable question in biology. The contrast between a Lamarckian and Darwinian explanation of how giraffes got their long necks has long been a popular example in introductory textbooks. My high school textbook (Biological Sciences Curriculum Study ‘Yellow’ edition, 1968) had it, and W.T. Keeton, in the college text Biological Science (2nd ed., 1972) used it, though calling it “now rather hackneyed”. I did a little checking and found some interesting historical tidbits.
I did not consult Lamarck, but found that Wallace, not Darwin first broached the issue from the natural selection side, in his part of the famed Linnean Society paper of 1858 (all the Darwin/Wallace materials quoted here were quickly located using the marvelous The Complete Works of Charles Darwin Online). Notice that in the following extract Wallace argues that it is a process of selection among variants, with access to food being the selective factor, that drives the evolution of the neck.
The hypothesis of Lamarck—that progressive changes in species have been produced by the attempts of animals to increase the development of their own organs, and thus modify their structure and habits…. Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them.
A year later, in the first edition of the Origin, Darwin refers not to giraffe’s necks, but to their tails and their neck vertebrae.
The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as driving away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. (p. 195)
The framework of bones being the same in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse,—the same number of vertebræ forming the neck of the giraffe and of the elephant,—and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. (p. 479)
The giraffe’s tail is used to illustrate adaptation, or suitability to the conditions of existence, which comes about through the modifying effects of natural selection; the giraffe’s neck, with its seemingly incongruously low number of vertebrae, illustrates the unity of type (the same structures or organs in organisms of varying conditions of existence) which reveals common ancestry. Thus the giraffe is used to illustrate the two great unexplained phenomena faced by biology prior to Darwin– adaptation and unity of type– and Darwin’s unified explanation for thes phenomena: descent with modification by means of natural selection. These examples continue throughout all editions of the Origin. But there is no mention of the long neck as a feeding adaptation.
In the third edition of the Origin (1861), in the Historical Sketch, Darwin first mentions the neck as an adaptation for feeding. He is here explicating Lamarck, correctly noting Lamarck’s two evolutionary mechanisms, the effects of use and disuse (the inheritance of acquired characters, which Darwin, and essentially all his contemporaries, accepted; the later naming of this as ‘Lamarckian’ inheritance is unhistorical), and the innate drive for progressive development.
With respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, that is, to the effects of habit. To this latter agency he seems to attribute all the beautiful adaptations in nature;—such as the long neck of the giraffe for browsing on the branches of trees. But he likewise believed in a law of progressive development; and as all the forms of life thus tended to progress, in order to account for the existence at the present day of very simple productions, he maintained that such forms were now spontaneously generated. (p. xiii)
It is not until the sixth edition of the Origin (1872) that Darwin gives extended consideration to the long neck as an adaptation for feeding, but in response to criticisms by Mivart, not to make a contrast with Lamarck. The problems addressed are how incipient stages of adaptive structures can be selected for, and why, if a trait is adaptive, why don’t all species (or at least all similar species) evolve the trait. Here’s the full discussion (it’s a bit long for a blog, but what the heck).
All Mr. Mivart’s objections will be, or have been, considered in the present volume. The one new point which appears to have struck many readers is, “that natural selection is incompetent to account for the incipient stages of useful structures.” This subject is intimately connected with that of the gradation of characters, often accompanied by a change of function,—for instance, the conversion of a swim-bladder into lungs,—points which were discussed in the last chapter under two headings. Nevertheless, I will here consider in some detail several of the cases advanced by Mr. Mivart, selecting those which are the most illustrative, as want of space prevents me from considering all.
The giraffe, by its lofty stature, much elongated neck, fore-legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths. The Niata cattle in S. America show us how small a difference in structure may make, during such periods, a great difference in preserving an animal’s life. These cattle can browse as well as others on grass, but from the projection of the lower jaw they cannot, during the often recurrent droughts, browse on the twigs of trees, reeds, &c., to which food the common cattle and horses are then driven; so that at these times the Niatas perish, if not fed by their owners. Before coming to Mr. Mivart’s objections, it may be well to explain once again how natural selection will act in all ordinary cases. Man has modified some of his animals, without necessarily having attended to special points of structure, by simply preserving and breeding from the fleetest individuals, as with the race-horse and greyhound, or as with the game-cock, by breeding from the victorious birds. So under nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food. That the individuals of the same species often differ slightly in the relative lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; whilst the individuals, less favoured in the same respects, will have been the most liable to perish.
We here see that there is no need to separate single pairs, as man does, when he methodically improves a breed: natural selection will preserve and thus separate all the superior individuals, allowing them freely to intercross, and will destroy all the inferior individuals. By this process long-continued, which exactly corresponds with what I have called unconscious selection by man, combined no doubt in a most important manner with the inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe.
To this conclusion Mr. Mivart brings forward two objections. One is that the increased size of the body would obviously require an increased supply of food, and he considers it as “very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages.” But as the giraffe does actually exist in large numbers in S. Africa, and as some of the largest antelopes in the world, taller than an ox, abound there, why should we doubt that, as far as size is concerned, intermediate gradations could formerly have existed there, subjected as now to severe dearths. Assuredly the being able to reach, at each stage of increased size, to a supply of food, left untouched by the other hoofed quadrupeds of the country, would have been of some advantage to the nascent giraffe. Nor must we overlook the fact, that increased bulk would act as a protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck,—and the taller the better,—would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. It is from this cause, as Sir S. Baker remarks, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a means of offence or defence, by violently swinging its head armed with stump-like horns. The preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objection), if natural selection be so potent, and if high browsing be so great an advantage, why has not any other hoofed quadruped acquired a long neck and lofty stature, besides the giraffe, and, in a lesser degree, the camel, guanaco, and macrauchenia? Or, again, why has not anymember of the group acquired a long proboscis? With respect to S. Africa, which was formerly inhabited by numerous herds of the giraffe, the answer is not difficult, and can best be given by an illustration. In every meadow in England in which trees grow, we see the lower branches trimmed or planed to an exact level by the browsing of the horses or cattle; and what advantage would it be, for instance, to sheep, if kept there, to acquire slightly longer necks? In every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through natural selection and the effects of increased use. In S. Africa the competition for browsing on the higher branches of the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate animals.
Why, in other quarters of the world, various animals belonging to this same order have not acquired either an elongated neck or a proboscis, cannot be distinctly answered; but it is as unreasonable to expect a distinct answer to such a question, as why some event in the history of mankind did not occur in one country, whilst it did in another. We are ignorant with respect to the conditions which determine the numbers and range of each species; and we cannot even conjecture what changes of structure would be favourable to its increase in some new country. We can, however, see in a general manner that various causes might have interfered with the development of a long neck or proboscis. To reach the foliage at a considerable height (without climbing, for which hoofed animals are singularly ill-constructed) implies greatly increased bulk of body; and we know that some areas support singularly few large quadrupeds, for instance S. America, though it is so luxuriant; whilst S. Africa abounds with them to an unparalleled degree. Why this should be so, we do not know; nor why the later tertiary periods should have been much more favourable for their existence than the present time. Whatever the causes may have been, we can see that certain districts and times would have been much more favourable than others for the development of so large a quadruped as the giraffe. (pp. 177-179)
So, it is Wallace rather than Darwin, who has most clearly contrasted the variational mechanism of natural selection (some individuals reproduce more than others, changing the average composition of the next generation) with Lamarck’s transformational mechanism (each individual changes). And one last point: an alert reader recommends the following as a current argument for the hypothesis of long necks as a feeding adaptation: CAMERON, E. & TOIT, J. T. du. 2007. Winning by a neck: tall giraffes avoid competing with shorter browsers. American Naturalist 169: 130-5 (abstract).