Part 4 (1/2)

[Ill.u.s.tration: FIG. 36. Diagram ill.u.s.trating a cross between a red eyed male and white eyed female of the fruit fly (reciprocal cross of that shown in Fig. 35).]

It has been shown by Sturtevant that in a wild species of Drosophila, viz., D. repleta, two varieties of individuals exist, in one of which the thorax has large splotches and in the other type smaller splotches (fig. 37). The factors that differentiate these varieties are s.e.x linked.

[Ill.u.s.tration: FIG. 37. Two types of markings on thorax of Drosophila repleta, both found ”wild”. They show s.e.x linked inheritance.]

Certain types of color blindness (fig. 38) and certain other abnormal conditions in man such as haemophilia, are transmitted as s.e.x linked characters.

[Ill.u.s.tration: FIG. 38, A. Diagram ill.u.s.trating inheritance of color blindness in man; the iris of the color-blind eye is here black.]

[Ill.u.s.tration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]

In domestic fowls s.e.x linked inheritance has been found as the characteristic method of transmission for at least as many as six characters, but here the relation of the s.e.xes is in a sense reversed. For instance, if a black Langshan hen is crossed to a barred Plymouth Rock c.o.c.k (fig. 39), the offspring are all barred. If these are inbred half of the daughters are black and half are barred; all of the sons are barred. The grandmother has transmitted her color to half of her granddaughters but to none of her grandsons.

[Ill.u.s.tration: FIG. 39. s.e.x-linked inheritance in domesticated birds shown here in a cross between barred Plymouth Rock male and black Langshan female.]

[Ill.u.s.tration: FIG. 40. Reciprocal of Fig. 39.]

In the reciprocal cross (fig. 40) black c.o.c.k by barred hen, the daughters are black and the sons barred--criss-cross inheritance. These inbred give black hens and black c.o.c.ks, barred hens and barred c.o.c.ks.

There is a case comparable to this found in a wild species of moth, Abraxas grossulariata. A wild variation of this type is lighter in color and is known as A. lacticolor. When these two types are crossed they exhibit exactly the same type of heredity as does the black-barred combination in the domestic fowl. As shown in figure 41, lacticolor female bred to grossulariata male gives grossulariata sons and daughters. These inbred give grossulariata males and females and lacticolor females. Reciprocally lacticolor male by grossulariata female, (fig. 42) gives lacticolor daughters and grossulariata sons and these inbred give grossulariata males and females and lacticolor males and females.

[Ill.u.s.tration: FIG. 41. s.e.x-linked inheritance in the wild moth, Abraxas grossulariata (darker) and A. lacticolor.]

[Ill.u.s.tration: FIG. 42. Reciprocal of Fig. 41.]

[Ill.u.s.tration: FIG. 43. Four wild types of Paratettix in upper line with three hybrids below.]

It has been found that there may be even more than two factors that show Mendelian segregation when brought together in pairs. For example, in the southern States there are several races of the grouse locust (Paratettix) that differ from each other markedly in color patterns (fig. 43). When any two individuals of these races are crossed they give, as Nabours has shown, in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there are here at least nine characters, any two of which behave as a Mendelian pair. These races have arisen in nature and differ definitely and strikingly from each other, yet any two differ by only one factor difference.

[Ill.u.s.tration: FIG. 44. Diagram ill.u.s.trating four allelomorphs in mice, viz. gray bellied gray (wild type) (above, to left); white bellied gray (above, to right); yellow (below, to right); and black (below, to left).]

Similar relations have been found in a number of domesticated races. In mice there is a quadruple system represented by the gray house mouse, the white bellied, the yellow and the black mouse (fig. 44). In rabbits there is probably a triple system, that includes the albino, the Himalayan, and the black races. In the silkworm moth there have been described four types of larvae, distinguished by different color markings, that form a system of quadruple allelomorphs. In Drosophila there is a quintuple system of factors in the s.e.x chromosome represented by eye colors, a triple system of body colors, and a triple system of factors for eye colors in the third chromosome.

MUTATION AND EVOLUTION

What bearing has the appearance of these new types of Drosophila on the theory of evolution may be asked. The objection has been raised in fact that in the breeding work with Drosophila we are dealing with artificial and unnatural conditions. It has been more than implied that results obtained from the breeding pen, the seed pan, the flower pot and the milk bottle do not apply to evolution in the ”open”, nature ”at large” or to ”wild” types. To be consistent, this same objection should be extended to the use of the spectroscope in the study of the evolution of the stars, to the use of the test tube and the balance by the chemist, of the galvanometer by the physicist. All these are unnatural instruments used to torture Nature's secrets from her. I venture to think that the real ant.i.thesis is not between unnatural and natural treatment of Nature, but rather between controlled or verifiable data on the one hand, and unrestrained generalization on the other.

If a systematist were asked whether these new races of Drosophila are comparable to wild species, he would not hesitate for a moment. He would call them all one species. If he were asked why, he would say, I think, ”These races differ only in one or two striking points, while in a hundred other respects they are identical even to the minutest details.” He would add, that as large a group of wild species of flies would show on the whole the reverse relations, _viz._, they would differ in nearly every detail and be identical in only a few points. In all this I entirely agree with the systematist, for I do not think such a group of types differing by one character each, is comparable to most wild groups of species because the difference between wild species is due to a large number of such single differences. The characters that have been acc.u.mulated in wild species are of significance in the maintenance of the species, or at least we are led to infer that even though the visible character that we attend to may not itself be important, one at least of the other effects of the factors that represent these characters is significant. It is, of course, hardly to be expected that _any_ random change in as complex a mechanism as an insect would improve the mechanism, and as a matter of fact it is doubtful whether any of the mutant types so far discovered are better adapted to those conditions to which a fly of this structure and habits is already adjusted.

But this is beside the mark, for modern genetics shows very positively that adaptive characters are inherited in exactly the same way as are those that are not adaptive; and I have already pointed out that we cannot study a single mutant factor without at the same time studying one of the factors responsible for normal characters, for the two together const.i.tute the Mendelian pair.

And, finally, I want to urge on your attention a question that we are to consider in more detail in the last lecture. Evolution of wild species appears to have taken place by modifying and improving bit by bit the structures and habits that the animal or plant already possessed. We have seen that there are thirty mutant factors at least that have an influence on eye color, and it is probable that there are at least as many normal factors that are involved in the production of the red eye of the wild fly.

Evolution from this point of view has consisted largely in introducing new factors that influence characters already present in the animal or plant.

Such a view gives us a somewhat different picture of the process of evolution from the old idea of a ferocious struggle between the individuals of a species with the survival of the fittest and the annihilation of the less fit. Evolution a.s.sumes a more peaceful aspect. New and advantageous characters survive by incorporating themselves into the race, improving it and opening to it new opportunities. In other words, the emphasis may be placed less on the compet.i.tion between the individuals of a species (because the destruction of the less fit does not _in itself_ lead to anything that is new) than on the appearance of new characters and modifications of old characters that become incorporated in the species, for on these depends the evolution of the race.

CHAPTER III

THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM