A Critique of the Theory of Evolution Part 3

[Ill.u.s.tration: FIG. 23. Cross between pea and rose combed fowls. (Charts of Baur and Goldschmidt.)]

A fourth case is shown in the fruit fly, where an ebony fly with long wings is mated to a grey fly with vestigial wings (fig. 24). The offspring are gray with long wings. If these are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).

[Ill.u.s.tration: FIG. 24. Cross between long ebony and gray vestigial flies.]

The possibility of interchanging characters might be ill.u.s.trated over and over again. It is true not only when two pairs of characters are involved, but when three, four, or more enter the cross.

[Ill.u.s.tration: FIG. 25. Diagram to show the history of the factors in the cross shown in Fig. 24.]

It is as though we took individuals apart and put together parts of two, three or more individuals by subst.i.tuting one part for another.

Not only has this power to make whatever combinations we choose great practical importance, it has even greater theoretical significance; for, it follows that the individual is not in itself the unit in heredity, but that within the germ-cells there exist smaller units concerned with the transmission of characters.

The older mystical statement of the individual as a unit in heredity has no longer any interest in the light of these discoveries, except as a past phase of biological history. We see, too, more clearly that the sorting out of factors in the germ plasm is a very different process from the influence of these factors on the development of the organism. There is today no excuse for confusing these two problems.

If mechanistic principles apply also to embryonic development then the course of development is capable of being stated as a series of chemico-physical reactions and the "_individual_" is merely a term to express the sum total of such reactions and should not be interpreted as something different from or more than these reactions. So long as so little is known of the actual processes involved in development the use of the term "individuality", while giving the appearance of profundity, in reality often serves merely to cover ignorance and to make a mystery out of a mechanism.

THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.

Darwin based many of his conclusions concerning variation and heredity on the evidence derived from the garden and from the stock farm. Here he was handicapped to some extent, for he had at times to rely on information much of which was uncritical, and some of which was worthless.

Today we are at least better informed on _two_ important points; one concerning the _kinds_ of variations that furnish to the cultivator the materials for his selection; the other concerning the modes of inheritance of these variations. We know now that new characters are continually appearing in domesticated as well as in wild animals and plants, that these characters are often sharply marked off from the original characters, and whether the differences are great or whether they are small they are transmitted alike according to Mendel's law.

Many of the characteristics of our domesticated animals and cultivated plants originated long ago, and only here and there have the records of their first appearance been preserved. In only a few instances are these records clear and definite, while the complete history of any large group of our domesticated products is unknown to us.

Within the last five or six years, however, from a common wild species of fly, the fruit fly, Drosophila ampelophila, which we have brought into the laboratory, have arisen over a hundred and twenty-five new types whose origin is completely known. Let me call attention to a few of the more interesting of these types and their modes of inheritance, comparing them with wild types in order to show that the kinds of inheritance found in domesticated races occur also in wild types. The results will show beyond dispute that the characters of wild types are inherited in precisely the same way as are the characters of the mutant types--a fact that is not generally appreciated except by students of genetics, although it is of the most far-reaching significance for the theory of evolution.

A mutant appeared in which the eye color of the female was different from that of the male. The eye color of the mutant female is a dark eosin color, that of the male yellowish eosin. From the beginning this difference was as marked as it is to-day. Breeding experiments show that eosin eye color differs from the red color of the eye of the wild fly by a single mutant factor. Here then at a single step a type appeared that was s.e.xually dimorphic.

Zoologists know that s.e.xual dimorphism is not uncommon in wild species of animals, and Darwin proposed the theory of s.e.xual selection to account for the difference between the s.e.xes. He a.s.sumed that the male preferred certain kinds of females differing from himself in a particular character, and thus in time through s.e.xual selection, the s.e.xes came to differ from each other.

[Ill.u.s.tration: FIG. 26. Clover b.u.t.terfly (Colias philodice) with two types of females, above; and one type of male, below.]

In the case of eosin eye color no such process as that postulated by Darwin to account for the differences between the s.e.xes was involved; for the single mutation that brought about the change also brought in the dimorphism with it.

In recent years zoologists have carefully studied several cases in which two types of female are found in the same species. In the common clover b.u.t.terfly, there is a yellow and a white type of female, while the male is yellow (fig. 26). It has been shown that a single factor difference determines whether the female is yellow or white. The inheritance is, according to Gerould, strictly Mendelian.

[Ill.u.s.tration: FIG. 27. Papilio turnus with two types of females above and one type of male below.]

In Papilio turnus there exist, in the southern states, two kinds of females, one yellow like the male, one black (fig. 27). The evidence here is not so certain, but it seems probable that a single factor difference determines whether the female shall be yellow or black.

Finally in Papilio polytes of Ceylon and India three different types of females appear, (fig. 28 to right) only one of which is like the male. Here the a.n.a.lysis of the breeding data shows the possibility of explaining this case as due to two pairs Mendelian factors which give in combination the three types of female.

[Ill.u.s.tration: FIG. 28. Papilio polytes, with three types of female to right and one type of male above to left.]

Taking these cases together, they furnish a much simpler explanation than the one proposed by Darwin. They show also that characters like these shown by wild species may follow Mendel's law.

[Ill.u.s.tration: FIG. 29. Mutant race of fruit fly with intercalated duplicate mesothorax on dorsal side.]

There has appeared in our cultures a fly in which the third division of the thorax with its appendages has changed into a segment like the second (fig.

29). It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference.

A factor that causes duplication in the legs has also been found. Here the interesting fact was discovered (Hoge) that duplication takes place only in the cold. At ordinary temperatures the legs are normal.

[Ill.u.s.tration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'

normal eye.]

In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost (fig. 30). This change also took place at a single step. All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye--indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in gla.s.s milk bottles, by a change in a single factor.

I may recall in this connection that wingless flies (fig. 5 f) also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings had been blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.

The preceding examples have all related to recessive characters. The next one is dominant.

[Ill.u.s.tration: FIG. 31. Mutant race of fruit fly called bar to the right (normal to the left). The eye is a narrow vertical bar, the outline of the original eye is indicated.]

A single male appeared with a narrow vertical red bar (fig. 31) instead of the broad red oval eye. Bred to wild females the new character was found to dominate, at least to the extent that the eyes of all its offspring were narrower than the normal eye, although not so narrow as the eye of the pure stock. Around the bar there is a wide border that corresponds to the region occupied by the rest of the eye of the wild fly. It lacks however the elements of the eye. It is therefore to be looked upon as a rudimentary organ, which is, so to speak, a by-product of the dominant mutation.

The preceding cases have all involved rather great changes in some one organ of the body. The following three cases involve slight changes, and yet follow the same laws of inheritance as do the larger changes.

[Ill.u.s.tration: FIG. 32. Mutant race of fruit fly, called speck. There is a minute black speck at base of wing.]

At the base of the wings a minute black speck appeared (fig. 32). It was found to be a Mendelian character. In another case the spines on the thorax became forked or kinky (fig. 52b). This stock breeds true, and the character is inherited in strictly Mendelian fas.h.i.+on.

[Ill.u.s.tration: FIG. 33. Mutant race of fruit fly called club. The wings often remain unexpanded and two bristles present in wild fly (b) are absent on side of thorax (c).]

In a certain stock a number of flies appeared in which the wing pads did not expand (fig. 33). It was found that this peculiarity is shown in only about twenty per cent of the individuals supposed to inherit it. Later it was found that this stock lacked two bristles on the sides of the thorax.

By means of this knowledge the heredity of the character was easily determined. It appears that while the expansion of the wing pads fails to occur once in five times--probably because it is an environmental effect peculiar to this stock,--yet the minute difference of the presence or absence of the two lateral bristles is a constant feature of the flies that carry this particular factor.

In the preceding cases I have spoken as though a factor influenced only one part of the body. It would have been more accurate to have stated that the _chief_ effect of the factor was observed in a particular part of the body.

Most students of genetics realize that a factor difference usually affects more than a single character. For example, a mutant stock called rudimentary wings has as its principle characteristic very short wings (fig. 34). But the factor for rudimentary wings also produces other effects as well. The females are almost completely sterile, while the males are fertile. The viability of the stock is poor. When flies with rudimentary wings are put into compet.i.tion with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. The hind legs are also shortened. All of these effects are the results of a single factor-difference.

[Ill.u.s.tration: FIG. 34. Mutant race of fruit fly, called rudimentary.]

One may venture the guess that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance.

It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whose _chief_ influence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters.

I am inclined to think that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase "unit character" has done much to mislead the uninitiated as to the effects that a single change in the germ plasm may produce on the organism.

Fortunately, the expression "unit character" is being less used by those students of genetics who are more careful in regard to the implications of their terminology.

There is a cla.s.s of cases of inheritance, due to the XY chromosomes, that is called s.e.x linked inheritance. It is shown both by mutant characters and characters of wild species.

For instance, white eye color in Drosophila shows s.e.x linked inheritance.

If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. The grandfather has transmitted his peculiarity to half of his grandsons, but to none of his granddaughters.

[Ill.u.s.tration: FIG. 35. Diagram showing a cross between a white eyed male and a red eyed female of the fruit fly. s.e.x linked inheritance.]

The reciprocal cross (fig. 36) is also interesting. If a white eyed female is bred to a red eyed male, all of the daughters have red eyes and all of the sons have white eyes. We call this criss-cross inheritance. If these offspring are inbred, they produce equal numbers of red eyed and white eyed females and equal numbers of red eyed and white eyed males. The ratio is 1: 1: 1: 1, or ignoring s.e.x, 2 reds to 2 whites, and not the usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in entire accord with Mendel's principle of segregation.