Part 7 (1/2)
_The Evidence from Non-Disjunction_
In the course of the work on Drosophila exceptions appeared in one strain where certain individuals did not conform to the scheme of s.e.x linked inheritance. For a moment the hypothesis seemed to fail, but a careful examination led to the suspicion that in this strain something had happened to the s.e.x chromosomes. It was seen that if in some way the X chromosomes failed to disjoin in certain eggs, the exceptions could be explained. The a.n.a.lysis led to the suggestion that if the Y chromosome had got into the female line the results would be accounted for, since its presence there would be expected to cause this peculiar non-disjunction of the X chromosomes.
That this was the explanation was shown when the material was examined. The females that gave these results were found by Bridges to have two X's and a Y chromosome.
The normal chromosome group of the female is shown in figure 52 and the chromosome group of one of the exceptional females is shown in figure 69.
In a female of this kind there are three s.e.x chromosomes X X Y which are h.o.m.ologous in the sense that in normal individuals the two present are mates and separate at the reduction division. If in the X X Y individual X and X conjugate and separate at reduction and the unmated Y is free to move to either pole of the spindle, two kinds of mature eggs will result, viz., X and XY. If, on the other hand, X and Y conjugate and separate at reduction and the remaining X is free to go to either pole, four kinds of eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are expected: viz. many XY and X eggs and a few XX and Y eggs.
[Ill.u.s.tration: FIG. 69. Figure of the chromosome group of an XXY female, that gives non-disjunction.]
These four kinds of eggs may be fertilized either by female-producing sperms or male-producing sperms, as indicated in the diagram (fig. 70).
[Ill.u.s.tration: FIG. 70. Scheme showing the results of fertilizing white bearing eggs (4 kinds) resulting from non-disjunction. The upper half of the diagram gives the results when these eggs are fertilized by normal red bearing, female producing sperm, the lower half by normal, male producing sperm.]
If such an XXY female carried white bearing Xs (open X in the figures), and the male carried a red bearing X (black X in the figures) it will be seen that there should result an exceptional cla.s.s of sons that are red, and an exceptional cla.s.s of daughters that are white. Tests of these exceptions show that they behave subsequently in heredity as their composition requires. Other tests may also be made of the other cla.s.ses of offspring.
Bridges has shown that they fulfill all the requirements predicted. Thus a result that seemed in contradiction with the chromosome hypothesis has turned out to give a brilliant confirmation of that theory both genetically and cytologically.
HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL
In pa.s.sing I invite your attention to a speculation based on our maps of the chromosomes--a speculation which I must insist does not pretend to be more than a guess but has at least the interest of being the first guess that we have ever been in position to make as to how many factors go towards the makeup of the germ plasm.
We have found practically no factors less than .04 of a unit apart. If our map includes the entire length of the chromosomes and if we a.s.sume factors are uniformly distributed along the chromosome at distances equal to the shortest distance yet observed, viz. .04, then we can calculate roughly how many hereditary factors there are in Drosophila. The calculation gives about 7500 factors. The reader should be cautioned against accepting the above a.s.sumptions as strictly true, for crossing-over values are known to differ according to different environmental conditions (as shown by Bridges for age), and to differ even in different parts of the chromosome as a result of the presence of specific genetic factors (as shown by Sturtevant). Since all the chromosomes except the X chromosomes are double we must double our estimate to give the _total_ number of factors, but the half number is the number of the different kinds of factors of Drosophila.
CONCLUSIONS
I have pa.s.sed in review a long series of researches as to the nature of the hereditary material. We have in consequence of this work arrived within sight of a result that seemed a few years ago far beyond our reach. The mechanism of heredity has, I think, been discovered--discovered not by a flash of intuition but as the result of patient and careful study of the evidence itself.
With the discovery of this mechanism I venture the opinion that the problem of heredity has been solved. We know how the factors carried by the parents are sorted out to the germ cells. The explanation does not pretend to state how factors arise or how they influence the development of the embryo. But these have never been an integral part of the doctrine of heredity. The problems which they present must be worked out in their own field. So, I repeat, the mechanism of the chromosomes offers a satisfactory solution of the traditional problem of heredity.
CHAPTER IV
SELECTION AND EVOLUTION
Darwin's Theory of Natural Selection still holds today first place in every discussion of evolution, and for this very reason the theory calls for careful scrutiny; for it is not difficult to show that the expression ”natural selection” is to many men a metaphor that carries many meanings, and sometimes different meanings to different men. While I heartily agree with my fellow biologists in ascribing to Darwin himself, and to his work, the first place in biological philosophy, yet recognition of this claim should not deter us from a careful a.n.a.lysis of the situation in the light of work that has been done since Darwin's time.
THE THEORY OF NATURAL SELECTION
In his great book on the _Origin of Species_, Darwin tried to do two things: first, to show that the evidence bearing on evolution makes that explanation probable. No such great body of evidence had ever been brought together before, and it wrought, as we all know, a revolution in our modes of thinking.
Darwin also set himself the task of showing _how_ evolution might have taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to everyone, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and to leave most offspring. In consequence their descendants should in time replace through compet.i.tion the less well-adapted individuals of the species. This is the process Darwin called natural selection, and Spencer the survival of the fittest.
Stated in these general terms there is nothing in the theory to which anyone is likely to take exception. But let us examine the argument more critically.
[Ill.u.s.tration: FIG. 71. Series of leaves of a tree arranged according to size. (After de Vries.)]
If we measure, or weigh, or cla.s.sify any character shown by the individuals of a population, we find differences. We recognize that some of the differences are due to the varied experiences that the individuals have encountered in the course of their lives, i.e. to their environment, but we also recognize that some of the differences may be due to individuals having different inheritances--different germ plasms. Some familiar examples will help to bring home this relation.
If the leaves of a tree are arranged according to size (fig. 71), we find a continuous series, but there are more leaves of medium size than extremes.
If a lot of beans be sorted out according to their weights, and those between certain weights put into cylinders, the cylinders, when arranged according to the size of the beans, will appear as shown in figure 72. An imaginary line running over the tops of the piles will give a curve (fig.