Darwin's Assessment of Hybridization
(Continued from the previous page)
Certainly, he made conflicting statements regarding the significance of hybridization, just as he did regarding the meaning of species, and with respect to the importance of saltation. Thus, in the first chapter of the Origin (1859), Darwin strongly expresses the opinion that new forms of life rarely arise through hybridization:
When in any country several domestic breeds have once been established, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of the crossing of varieties has, I believe, been greatly exaggerated, both in regard to animals and to those plants which are propagated by seed. In plants which are temporarily propagated by cuttings, buds, &c., the importance of the crossing both of distinct species and of varieties is immense; for the cultivator here quite disregards the extreme variability both of hybrids and mongrels, and the frequent sterility of hybrids; but the cases of plants not propagated by seed are of little importance to us, for their endurance is only temporary. Over all these causes of Change I am convinced that the accumulative action of Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant Power.
So here he expresses the opinion that hybridization is of very little importance in producing new sexual forms. In a letter to Hooker dated July 13, 1856, he had also dismissed hybridization: "With respect to crossing, … I think you misunderstand me. I am very far from believing in hybrids: only in crossing of the same species or of close varieties." Also in the first chapter of the Origin he says:
There can be no doubt that a race may be modified by occasional crosses, if aided by the careful selection of those individual mongrels, which present any desired character; but that a race could be obtained nearly intermediate between two extremely different races or species, I can hardly believe. Sir J. Sebright expressly experimentised for this object, and failed. The offspring from the first cross between two pure breeds is tolerably and sometimes (as I have found with pigeons) extremely uniform, and everything seems simple enough; but when these mongrels are crossed one with another for several generations, hardly two of them will be alike, and then the extreme difficulty, or rather utter hopelessness, of the task becomes apparent. Certainly, a breed intermediate between two very distinct breeds could not be got without extreme care and long-continued selection; nor can I find a single case on record of a permanent race having been thus formed.
Here, too, he discounts hybridization, but only to the extent of dismissing the feasibility of producing a breed intermediate between two parent forms. He does, however, say the parents can be modified by hybridization. But in the eighth chapter he makes statements that seem directly to contradict the opinions just quoted. There he says
A doctrine which originated with Pallas, has been largely accepted by modern naturalists; namely, that most of our domestic animals have descended from two or more aboriginal species, since commingled by intercrossing. On this view, the aboriginal species must either at first have produced quite fertile hybrids, or the hybrids must have become in subsequent generations quite fertile under domestication. This latter alternative seems to me the most probable, and I am inclined to believe in its truth.
If this quotation were read alone and one had not seen those quoted before it, it would seem clearly to show that Darwin was convinced that intermediate breeds can be produced by hybridization. But the eighth chapter of the Origin is not a discussion of possible sources of natural variation. It is a single long polemic against the idea that God specially endows hybrids with sterility in order to keep the "species" immutable. Darwin introduced Pallas' doctrine, not to support the idea that breeds can be produced by hybridization, but to attack the idea of special endowment. Apparently, he failed to notice the inconsistency. For elsewhere, in his discussion of the probable sources of the variation on which natural selection acts, he makes no mention of Pallas. At the time of the first publication of the Origin. Darwin wrote Lyell and assured him that he largely discounted Pallas with regard to the production of new forms through hybridization. The passage in the letter, dated October 31, 1859, reads as follows:
That you may not misunderstand how far I go with Pallas and his many disciples I should like to add that, though I believe that our domestic dogs have descended from several wild forms, and though I must think that the sterility, which they would probably have evinced, if crossed before being domesticated, has been eliminated, yet I go but a very little way with Pallas & Co. in their belief in the importance of the crossing [i.e., hybridization] and blending of the aboriginal stocks. You will see this briefly put in the first chapter.
In a subsequent letter to Lyell (January 1865), Darwin continued to avow his belief that only the accumulation of minor variation was significant: "The more I work, the more I feel convinced that it is by the accumulation of such extremely slight variations that new species arise."
And yet, it seems Darwin did in fact come to attribute more significance to hybridization. In a letter to Huxley dated December 22, 1866, at the time that he was completing work on Variation of Animals and Plants under Domestication (1868), Darwin confides "Now that I have worked up domestic animals, I am convinced of the truth of the Pallasian view of loss of sterility under domestication, and this seems to me to explain much." In Variation he makes strong statements affirming the efficacy of hybridization in producing new breeds. For example, the following extended quotation from Variation shows that he was well aware that new breeds had been obtained from hybridization:
There can be no doubt that crossing, with the aid of rigorous selection during several generations, has been a potent means in modifying old races, and in forming new ones. Lord Orford crossed his famous stud of greyhounds once with the bulldog, in order to give them courage and perseverance. Certain pointers have been crossed, as I hear from the Rev. W. D. Fox, with the foxhound, to give them dash and speed. Certain strains of Dorking fowls have had a slight infusion of Game blood; and I have known a great fancier who on a single occasion crossed his turbit-pigeons with barbs, for the sake of gaining greater breadth of beak.
In the foregoing cases breeds have been crossed once, for the sake of modifying some particular character; but with most of the improved races of the pig, which now breed true, there have been repeated crosses,—for instance, the improved Essex owes its excellence to repeated crosses with the Neapolitan, together probably with some infusion of Chinese blood. So with our British sheep: almost all the races, except the Southdown, have been largely crossed; "this, in fact, has been the history of our principal breeds." To give an example, the "Oxfordshire Downs" now rank as an established breed. They were produced about the year 1830 by crossing "Hampshire and in some instances Southdown ewes with Cotswold rams:" now the Hampshire ram was itself produced by repeated crosses between the native Hampshire sheep and Southdowns; and the long-woolled Cotswold were improved by crosses with the Leicester, which latter again is believed to have been a cross between several long-woolled sheep. Mr. Spooner, after considering the various cases which have been carefully recorded, concludes, "that from a judicious pairing of cross-bred animals it is practicable to establish a new breed." On the continent the history of several crossed races of cattle and of other animals has been well ascertained. To give one instance: the King of Wurtemburg, after twenty-five years' careful breeding, that is, after six or seven generations, made a new breed of cattle from a cross between a Dutch and a Swiss breed, combined with other breeds. The Sebright bantam, which breeds as true as any other kind of fowl, was formed about sixty years ago by a complicated cross. Dark Brahmas, which are believed by some fanciers to constitute a distinct species, were undoubtedly formed in the United States, within a recent period, by a cross between Chittagongs and Cochins. With plants there is little doubt that the Swede-turnip originated from a cross; and the history of a variety of wheat, raised from two very distinct varieties, and which after six years' culture presented an even sample, has been recorded on good authority.
Until lately, cautious and experienced breeders, though not averse to a single infusion of foreign blood, were almost universally convinced that the attempt to establish a new race, intermediate between two widely distinct races, was hopeless "they clung with superstitious tenacity to the doctrine of purity of blood, believing it to be the ark in which alone true safety could be found." [Here, Darwin is quoting Spooner ] Nor was this conviction unreasonable: when two distinct races are crossed, the offspring of the first generation are generally nearly uniform in character; but even this sometimes fails to be the case, especially with crossed dogs and fowls, the young of which from the first are sometimes much diversified. As cross-bred animals are generally of large size and vigorous, they have been raised in great numbers for immediate consumption. But for breeding they are found utterly useless; for though they may themselves be uniform in character, they yield during many generations astonishingly diversified offspring. The breeder is driven to despair, and concludes that he will never form an intermediate race. But from the cases already given, and from others which have been recorded, it appears that patience alone is necessary; as Mr. Spooner remarks, "nature opposes no barrier to successful admixture; in the course of time, by the aid of selection and careful weeding, it is practicable to establish a new breed." After six or seven generations the hoped-for result will in most cases be obtained; but even then an occasional reversion, or failure to keep true, may be expected. [italics added]
The final, italicized passage makes his attitude clear. Here, in Variation, he is saying new breeds can in fact be produced by hybridization. This claim directly contradicts the views he expresses in the first chapter of the Origin to the effect that new breeds cannot be obtained in this way. Indeed, on the next page Darwin asserts that
the several kinds of dogs are almost certainly descended from more than one species, and so it is with cattle, pigs and some other domesticated animals. Hence the crossing of aboriginally distinct species probably came into play at an early period in the formation of our present races. From Rutimeyer's observations there can be little doubt that this occurred with cattle; but in most cases one form will probably have absorbed and obliterated the other, for it is not likely that semi-civilised men would have taken the necessary pains to modify by selection their commingled, crossed, and fluctuating stock. Nevertheless, those animals which were best adapted to their conditions of life would have survived through natural selection; and by this means crossing will often have indirectly aided in the formation of primeval domesticated breeds.
Of the origin of dogs, elsewhere in Variation he says,
From the resemblance in several countries of the half-domesticated dogs to the wild species still living there—from the facility with which they can often be crossed together—from even half-tamed animals being so much valued by savages and from the other circumstances previously remarked on which favour their domestication it is highly probable that the domestic dogs of the world have descended from two good species of wolf (viz. C. lupus [i.e., Grey Wolf] and C. latrans [i.e., Coyote]) and from two or three other doubtful species of wolves (namely the European Indian and North African forms) from at least one or two South American canine species from several races or species of the jackal and perhaps from one or more extinct species.
So in Variation Darwin clearly affirms the idea that new breeds can be produced by hybridization. In his own words, "patience alone is necessary."
To some extent, it seems Darwin's contradictory statements can be attributed to his changing attitudes on hybridization. In an 1862 letter to Hooker, Darwin wrote: "I formerly thought with [i.e., concurred with] you about rarity of natural hybrids, but I am beginning to change." In Variation (1868: vol. II, 110) Darwin strongly endorses Pallas' claim that fertility can recover during the establishment of a hybrid breed.
The indirect evidence in favour of the Pallasian doctrine appears to me to be extremely strong. In the earlier chapters I have attempted to show that our various breeds of dogs are descended from several wild species; and this probably is the case with sheep. There can no longer be any doubt that the Zebu or humped Indian ox belongs to a distinct species from European cattle: the latter, moreover, are descended from two or three forms, which may be called either species or wild races, but which co-existed in a state of nature and kept distinct. We have good evidence that our domesticated pigs belong to at least two specific types, S. scrofa and Indica, which probably lived together in a wild state in South-eastern Europe. Now, a widely-extended analogy leads to the belief that if these several allied species, in the wild state or when first reclaimed, had been crossed, they would have exhibited, both in their first unions and in their hybrid offspring, some degree of sterility. Nevertheless the several domesticated races descended from them are now all, as far as can be ascertained, perfectly fertile together. If this reasoning be trustworthy, and it is apparently sound, we must admit the Pallasian doctrine that long-continued domestication tends to eliminate that sterility which is natural to species when crossed in their aboriginal state.
He even comments (1868: vol. II, 97) that he could have produced a stable hybrid breed himself if he had wished it:
I crossed some Labrador and Penguin ducks, and recrossed the mongrels with Penguins; afterwards most of the ducks reared during three generations were nearly uniform in character, being brown with a white crescentic mark on the lower part of the breast, and with some white spots at the base of the beak; so that by the aid of a little selection a new breed might easily have been formed.
This method of breeding sounds surprisingly similar to the production of a new stabilized recombinant derivative (see Chapter 4). In The Descent of Man (1871), too, with regard to interbreeding of human races, he describes a process of this same sort:
Whether a heterogenous people, such as the inhabitants of some of the Polynesian islands, formed by the crossing of two distinct races, with few or no pure members left, would ever become homogeneous, is not known from direct evidence. But as with our domesticated animals, a cross-breed can certainly be fixed and made uniform by careful selection in the course of a few generations, we may infer that the free inter-crossing of a heterogeneous mixture during a long descent would supply the place of selection, and overcome any tendency to reversion; so that the crossed race would ultimately become homogeneous, though it might not partake in an equal degree of the characters of the two parent-races.
Thus, Darwin did come to attribute more significance to hybridization in his later years. His comments in later correspondence, and in later publications (other than later editions of the Origin) show that he came to view hybridization as a viable source of variation on which subsequent selection, particularly artificial selection, could act. He clearly thought new stable forms could be obtained from such a process, and that a hybrid population could "ultimately become homogeneous" under the influence of selection. However, his new views on the subject never gained significant expression in any edition of the Origin. his most widely read book. Certainly such ideas were never canonized among the dogmas of neo-Darwinian theory.
One of Darwin's greatest talents was his remarkable ability to assuage the ire of his opponents, which he accomplished in large part by his great personal charm. However, his desire to keep on good terms with others seems to have led him to shun certain controversial issues. The best-known example is the long delay between the publication of the Origin (1859) and his public admission of his belief that humans are descended from apelike ancestors (The Descent of Man, 1871). In the Origin his references to this idea were entirely oblique. There he limited himself to a single vague statement: "Light will be thrown on the origin of man and his history." Lamarck (1809: 170) had long before described a process by which humans might have evolved from apes. Mayr (1982: 352) says Lamarck presented his views on human origins "with far more courage than Darwin fifty years later in the Origin. "
Perhaps Darwin simply felt that altering later editions of the Origin to reflect his changed assessment of hybridization would have aroused unwanted opposition. It would be interesting to know whether he ever ran across Daniel Defoe's poem The True Born Englishman (1701) and considered hybridization in connection with his own origins:
Thus from a mixture of all kinds began,
That het'rogeneous thing, an Englishman:
In eager rapes, and furious lust begot,
Betwixt a painted Britain and a Scot.
Whose gend'ring off-spring quickly learn'd to bow,
And yoke their heifers to the Roman plough:
From whence a mongrel half-bred race there came,
With neither name, nor nation, speech nor fame.
In whose hot veins new mixtures quickly ran,
Infus'd betwixt a Saxon and a Dane.
While their rank daughters, to their parents just,
Receiv'd all nations with promiscuous lust.
This nauseous brood directly did contain
The well-extracted blood of Englishmen.
1. (Histoire Naturelle, XVI, 547–548) expressed the opinion that most of the various breeds of domestic pigeons were derived from intercrossing of three naturally occurring forms. Thus, he says: “It is therefore quite possible, as we have already suggested, that the Rock Dove [Columba livia], the Wood Pigeon [Columba palumbus], and the Turtle Dove [Streptopelia turtur], species that appear to keep themselves separate in a natural state, may nevertheless unite frequently in captivity and that from their union may be produced the majority of our domestic pigeon breeds, of which some are of the size of the Wood Pigeon, and others resemble the Turtle Dove in their smallness, their shape, etc., and others resemble the Rock Pigeon, or show affinity to all three.” Translated by E. M. McCarthy. Original French: “Il est donc fort possible, comme nous l’avons déjà insinué, que les bisets, les ramiers et les tourterelles, dont les espèces paroissent se soutenir séparément et sans mélange dans l’état de nature, se soient néanmoins souvent unies dans celui de domesticité ; et que de leur mélange, soient issues la plupart des races de nos pigeons domestiques, dont quelques-uns sont de la grandeur du ramier, et d’autres ressemblent à la tourterelle par la petitesse, par la figure, etc. et dont plusieurs enfin tiennent du biset ou participent de tous trois.”
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Appendix H: Darwin's Assessment of Hybridization © Macroevolution.net