Selective Pollination
Donald Forsha Jones (1928)

Chapter 7

ASSORTATIVE MATING (p 119-130)

In fact there is a greater difference between the assortative mating in Protozoa and in Metazoa than between selective pairing in the higher animals and the differential fertilization in the seed plants. In the Protozoa the individuals themselves correspond to the gametes in a certain sense. In these forms a selective action has been demonstrated. Where there is a discrimination among the gametes produced by one individual in the seed plants, the differential action usually involves only the male gametes and not the female.

In the Protozoa, reproduction commonly takes place by division of each individual into two parts which subsequently grow to normal size. Under certain conditions, owing to both internal and external influences, two individuals come together and undergo an exchange of nuclear material. This process, called conjugation, corresponds to sexual reproduction in the higher animals. Working with Paramecium, Pearl (1907) has shown that there is a marked tendency for the pairing to be between individuals of approximately the same size. This seems to be due to the necessity of the anterior ends and the mouths of a pair to come together reasonably well for a successful conjugation. If they do not, the two individuals separate or die.

The correlation in length of the members of a conjugant pair for all the series measured was found to be between +0.5 and +0.6 This association was thought not to be due to any environmental factor tending to make all the conjugating individuals alike. No correlation was shown when the individuals were paired at random. Neither was the similarity in size thought to be due to an equalization process during which, by an exchange of cell contents, the two individuals adjusted their sizes to each other, since the correlations were not greater when the pairs were measured in a late stage of conjugation than in an early.

Fig. 20. Assortative mating in Paramecium. (From Jennings in Journal Experimental Zoology.)

When the two species, Paramecium caudatum and P. aurelia, were present in the same culture, the coefficient of correlation was very high, being +0.94. In fact no pairing of members of the different species was observed. This was due in part to a tendency for the two species to conjugate at different times. All of the possible reasons for this similarity in the members of each pair were carefully considered, and the conclusion was reached that there was a definite tendency toward assortative mating in these organisms. In another ciliate, Blepharisma undulans, Watters (1912) observed a similar mating of like with like. While individual members of the same culture differed as much as 14.5 mm., no two members of a conjugating pair differed more than 5.5 mm. in length. Out of 279 pairs, 35 were equal in length and only 7 pairs differed by more than 3.5 mm. Going somewhat higher in the animal scale, there is evidence from the mollusks of an assortative mating. The hermaphroditic nudibranch, Chromodoris zebra, has been carefully studied by Crozier (1917, 1918). Sexually mature individuals mating at one time in a restricted area were found to vary from 4 to 18 cm. in total length. The coefficient of correlation between the members of a pair collected under natural conditions was found to be +0.6. Matings under laboratory conditions gave a correlation of +0.7. Observations showed that in most cases large individuals mated successfully with large, and small individuals with small, at different seasons of the year. One of the principal advantages to the organism, Crozier considers, is a conservation of eggs and sperm resulting in a larger total number of offspring than if random mating occurred. Large animals deposit many more eggs than small ones, but it was not shown that a small individual does not produce sufficient spermatozoa to fertilize all the eggs of a large individual. In most animals there is such a great excess of male germ cells over the female that this would be the natural assumption with this animal unless the contrary were proved. The assortative mating in this hermaphroditic organism seems to be largely a mechanical adaptation necessary to secure the proper interchange of germinal elements. It principally involves similarity in size, a somatic character depending perhaps more upon age and environmental influences than upon germinal differences. At the same time, should germinal differences arise altering size or other characters affecting the mating proclivities, it can hardly be doubted that the tendency to homogamy would set the two types apart even though living in the same situation.

Other species beside Chromodoris zebra are found in the water about Bermuda, but interspecific matings have not been observed. One species, Chromodoris roseapicta, is about the same size as C. zebra, but they have never been seen to hybridize under natural conditions; and even when confined in small dishes the two species could not be induced to mate. It is thought that the mutual attraction of members of different species depends upon reactions to specific substances secreted by the mature individuals. If so, assortative mating takes place with respect to these characters as well as to size. When Crozier (1918, p. 287) says that "assortative mating can play no part in the prevention of interspecific crossing," he is evidently restricting his definition of assortative to size characters.

Fig. 21.—Relation between length of male and of associated female in Dikerogammarus. (From Crozier and Snyder in Biological Bulletin.)

The writers call attention to one way in which selective pairing may have an evolutionary significance. They say:

The imaginal size of certain parasitic insects is known to be determined by the size and rate of development of the particular species of host insect in which their larvae grow and pupate. This is due to a developmental correlation between the pupation of the larval host and of its contained larval parasites. If adult insects, of the same species, differing in size through this means, practice selective mating and are by some agency compelled to oviposit in a host of the type from which they are themselves reared, a basis is clearly afforded for the foundation of divergent types. (W. J. Crozier and L. H. Snyder, "Selective Coupling of Gammarids," Biological Bulletin, XLV (1923), 102.)

Among insects, evidence of assortative mating has been obtained from at least two species. In Leptinotarsa a marked tendency to pair according to size is shown by the data collected by Tower (1906) and given in Table XII. There seemed to be no attempt on the part of beetles differing in size to avoid each other, but mating was not successful unless the partners were of nearly the same size. This similarity of mates with respect to size was not found to hold for color.

Kellogg (1906) states that pairing is entirely at random with respect to color pattern in a rather small number of matings observed in Hippodamia. Contrary to this conclusion, Pearl (1907) points out that Kellogg's data actually do indicate a possible assortment.

In a study of two very similar species of Drosophila which give sterile hybrids, Sturtevant (1920) observed that the females of either melanogaster or simulans are far more likely to mate with males of their own species than with those of the other. The males court females of either species apparently without discrimination. The hybrids of both sexes have been seen to mate with both parental species, but it was not determined as to whether any preference was shown.

Scanty as the evidence is, the results carefully compiled from a few widely separated species among the lower organisms show a definite tendency for like to mate with like. This is particularly noticeable with respect to size characters and is largely an automatic mechanical adjustment involving somatic differences which may or may not have a germinal basis.

The situation among the higher animals is indicated by the following quotation from Darwin's Animals and Plants under Domestication (II, 102-4).

Certain domestic races seem to prefer breeding with their own kind; and this is a fact of some importance, for it is a step towards that instinctive feeling which helps to keep closely allied species in a state of nature distinct. We have now abundant evidence that, if it were not for this feeling, many more hybrids would be naturally produced than is the case. We have seen in the first chapter that the Alco dog of Mexico dislikes dogs of other breeds; and the hairless dog of Paraguay mixes less readily with the European races, than the latter do with each other. In Germany the female Spitzdog is said to receive the fox more readily than will other dogs; a female Australian Dingo in England attracted the wild male foxes. But these differences in the sexual instinct and attractive power of the various breeds may be wholly due to their descent from distinct species. In Paraguay the horses have much freedom, and an excellent observer believes that the native horses of the same colour and size prefer associating with each other, and that the horses which have been imported from Entre Rios and Banda Oriental into Paraguay likewise prefer associating together. In Circassia six sub-races of the horse are known and have received distinct names; and a native proprietor of rank asserts that horses of three of these races, whilst living a free life, almost always refuse to mingle and cross, and will even attack each other.

It has been observed, in a district stocked with heavy Lincolnshire and light Norfolk sheep, that both kinds, though bred together, when turned out, "in a short time separate to a sheep"; the Lincolnshires drawing off to the rich soil, and the Norfolks to their own dry light soil; and as long as there is plenty of grass, "the two breeds keep themselves as distinct as rooks and pigeons." In this case different habits of life tend to keep the races distinct. On one of the Faroe islands, not more than half a mile in diameter, the half-wild native black sheep are said not to have readily mixed with the imported white sheep. It is a more curious fact that the semimonstrous ancon sheep of modern origin "have been observed to keep together, separating themselves from the rest of the flock, when put into enclosures with other sheep." With respect to fallow deer, which live in a semi-domesticated condition, Mr. Bennett states that the dark and pale coloured herds, which have long been kept together in the Forest of Dean, in High Meadow Woods, and in the New Forest, have never been known to mingle: the dark-coloured deer, it may be added, are believed to have been first brought by James I. from Norway, on account of their greater hardiness. I imported from the island of Porto Santo two of the feral rabbits, which differ, as described in the fourth chapter, from common rabbits; both proved to be males, and, though they lived during some years in the Zoological Gardens, the superintendent, Mr. Bartlett, in vain endeavoured to make them breed with various tame kinds; but whether this refusal to breed was due to any change in instinct, or simply to their extreme wildness; or whether confinement had rendered them sterile, as often occurs, cannot be told.

Whilst matching for the sake of experiment many of the most distinct breeds of pigeons, it frequently appeared to me that the birds, though faithful to their marriage vow, retained some desire after their own kind. Accordingly I asked Mr. Wicking, who has kept a larger stock of various breeds together than any man in England, whether he thought that they would prefer pairing with their own kind, supposing that there were males and females enough of each; and he without hesitation answered that he was convinced that this was the case. It has often been noticed that the dovecot pigeon seems to have an actual aversion towards the several fancy breeds; yet all have certainly sprung from a common progenitor. The Rev. W. D. Fox informs me that his flocks of white and common Chinese geese kept distinct.

These facts and statements, though some of them are incapable of proof, resting only on the opinion of experienced observers, show that some domestic races are led by different habits of life to keep to a certain extent separate, and that others prefer coupling with their own kind, in the same manner as species in a state of nature, though in a much less degree.

Our interest now turns to man himself. It is obvious that human marriages are highly selective. Real racial differences are so seldom brought together in wedlock that the occurrence of miscegenation is a matter of comment. The influences which tend to prevent interracial unions involve much more than physical differences. The whole social inheritance, customs, and folkways tend to separate not only races but different political and religious groups. Wealth and social position are notable factors in determining conjugal ties. Since these have some basis in mental ability, inherited intellectual capacity is an important agency in determining the partners in matrimony.

That human marriages are also assortative with respect to minor physical characters and constitutional strength will seem surprising to many, as there is a general conviction that opposites tend to attract each other. Pearson and Lee (1903) find a definite assortative mating of like with like in such characters that could be easily measured as height, span of arms, length of forearm, color of eyes, and longevity. Surprising as it may be, they found that husband and wife are more alike than first cousins. The size measurements were made upon the parents of college students from widely separated districts, and the correlation is considered not to be affected unduly by marriages occurring within different local races which might show an apparent relation when none actually existed. This factor was also carefully ruled out in their investigation of longevity. For this, the ages were taken from tombstones of two rural communities where the people belonged to the same racial stocks and seldom moved from the community. Data were also taken from the very complete records of the Society of Friends. There is a tendency, which was fully recognized, for the husband and wife to be buried in different localities when their deaths occurred at widely separated intervals, and for that reason the larger differences would not be included in the tabulation. This was borne out by the figures obtained from a London cemetery which showed a much higher correlation than from the rural communities, and this was undoubtedly due to the failure to include the more negative pairs. Environmental factors which might give a correlation not based on similarity in constitutional strength were carefully considered and ruled out. Length of life is delinitely inherited, as shown by Beeton and Pearson (1899) and by Pearl (1922). Longevity is an indication of general constitutional strength, and the fact that there is a definite assortative mating of like with like with respect to this and other physical traits within similar groups of people is certainly significant. Pearson and Lee have also shown that the similarity between husband and wife is greater among the parents of adult children than in the population as a whole. Homogamy is therefore a factor in fertility and consequently tends to perpetuate itself.

In conclusion it can be said that there is evidence from some animals from the lowest to the highest that sexual reproduction does not take place between random pairs but on the contrary is selective. In its effect this assortative mating of like with like is comparable to the differential fertilization among plants whereby like forms take part more frequently in reproduction than unlike forms. To a certain extent, at least, the similarities involved in these relationships have a basis in common heredity; and in so far as they do, this phenomenon has an important bearing on the future progeny. Since positive evidence is at hand for those characters which can be measured, it is probable that a similar tendency exists with respect to other less tangible features which may be even more concerned with physiological isolation.