Part 8 (1/2)
The result means that the general population is made up of definite kinds of individuals that may have been sorted out.
That his conclusion is correct is shown by rearing a new generation from any plant or indeed from several plants of any one of these lines. Each line repeats the same modal cla.s.s. There is no further breaking up into groups. Within the line it does not matter at all whether one chooses a big bean or a little one--they will give the same result. In a word, the germ plasm in each of these lines is pure, or h.o.m.ozygous, as we say. The differences that we find between the weights (or sizes) of the individual beans are due to external conditions to which they have been subjected.
In a word, Johannsen's work shows that the frequency distribution of a pure line is due to factors that are extrinsic to the germ plasm. It does not matter then which individuals in a pure line are used to breed from, for they all carry the same germ plasm.
We can now understand more clearly how selection acting on a general population brings about results in the direction of selection.
An individual is picked out from the population in order to get a particular kind of germ plasm. Although the different cla.s.ses of individuals may overlap, so that one can not always judge an individual from its appearance, nevertheless on the whole chance favors the picking out of the kind of germ plasm sought.
In species with separate s.e.xes there is the further difficulty that two individuals must be chosen for each mating, and superficial examination of them does not insure that they belong to the same group--their germ plasm cannot be inspected. Hence selection of biparental forms is a precarious process, now going forward, now backwards, now standing still. In time, however, the process forward is almost certain to take place if the selection is from a heterogeneous population. Johannsen's work was simplified because he started with pure lines. In fact, had he not done so his work would not have been essentially different from that of any selection experiment of a pure race of animals or plants. Whether Johannsen realized the importance of the condition or not is uncertain--curiously he laid no emphasis on it in the first edition of his ”Elemente der exakten Erblichkeitslehre”.
It has since been pointed out by Jennings and by Pearl that a race that reproduces by self-fertilization as does this bean, automatically becomes pure in all of the factors that make up its germ plasm. Since self-fertilization is the normal process in this bean the purity of the germ plasm already existed when Johannsen began to experiment.
HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS RESULTS?
If then selection does not bring about transgressive variation in a general population, how can selection produce anything new? If it can not produce anything new, is there any other way in which selection becomes an agent in evolution?
We can get some light on this question if we turn to what man has done with his domesticated animals and plants. Through selection, i.e., artificial selection, man has undoubtedly brought about changes as remarkable as any shown by wild animals and plants. We know, moreover, a good deal about how these changes have been wrought.
(1) By crossing different wild species or by crossing wild with races already domesticated new combinations have been made. Parts of one individual have been combined with parts of others, creating new combinations. It is possible even that characters that are entirely new may be produced by the interaction of factors brought into recombination.
(2) New characters appear from time to time in domesticated and in wild species. These, like the mutants in Drosophila, are fully equipped at the start. Since they breed true and follow Mendel's laws it is possible to combine them with characters of the wild type or with those of other mutant races.
Amongst the new mutant factors there may be some whose chief effect is on the character that the breeder is already selecting. Such a modification will be likely to attract attention. Superficially it may appear that the factor for the original character has varied, while the truth may be that another factor has appeared that has modified a character already present.
In fact, many or all Mendelian factors that affect the same organ may be said to be modifiers of each other's effects. Thus the factor for vermilion causes the eye to be one color, and the factor for eosin another color, while eosin vermilion is different from both. Eosin may be said to be a modifier of vermilion or vermilion of eosin. In general, however, it is convenient to use the term ”modifier” for cases in which the factor causes a detectable change in a character already present or conspicuous.
[Ill.u.s.tration: FIG. 82. Scheme to indicate influence of the modifying factors, cream and whiting. Neither produces any effect alone but they modify other eye colors such as eosin.]
One of the most interesting, and at the same time most treacherous, kinds of modifying factors is that which produces an effect _only_ when some other factor is present. Thus Bridges has shown that there is a factor called ”cream” that does not affect the red color of the eye of the wild fly, yet makes ”eosin” much paler (fig. 82). Another factor ”whiting” which produces no effect on red makes eosin entirely white. Since cream or whiting may be carried by red eyed flies without their presence being seen until eosin is used, the experimenter must be continually on the lookout for such factors which may lead to erroneous conclusions unless detected.
As yet breeders have not realized the important role that modifiers have played in their results, but there are indications at least that the heaping up of modifying factors has been one of the ways in which highly specialized domesticated animals have been produced. Selection has accomplished this result not by changing factors, but by picking up modifying factors. The demonstration of the presence of these factors has already been made in some cases. Their study promises to be one of the most instructive fields for further work bearing on the selection hypothesis.
In addition to these well recognized methods by which artificial selection has produced new things we come now to a question that is the very crux of the selection theory today. Our whole conception of selection turns on the answer that we give to this matter and if I appear insistent and go into some detail it is because I think that the matter is worth very careful consideration.
ARE FACTORS CHANGED THROUGH SELECTION?
As we have seen, the variation that we find from individual to individual is due in part to the environment; this can generally be demonstrated.
Other differences in an ordinary population are recognized as due to different genetic (hereditary) combinations. No one will dispute this statement. But is all the variability accounted for in these two ways? May not a factor itself fluctuate? Is it not _a priori_ probable that factors do fluctuate? Why, in a word, should we regard factors as inviolate when we see that everything else in organisms is more or less in amount? I do not know of any _a priori_ reason why a factor may not fluctuate, unless it is, as I like to think, a chemical molecule. We are, however, dealing here not with generalities but with evidence, and there are three known methods by means of which it has been shown that variability, other than environmental or recombinational, is not due to variability in a factor, nor to various ”potencies” possessed by the same factors.
(1) By making the stock uniform for all of its factors--chief factors and modifiers alike. Any change in such a stock produced by selection would then be due to a change in one or more of the factors themselves.
Johannsen's experiment is an example of this sort.
[Ill.u.s.tration: FIG. 83 a. Drosophila ampelophila with truncate wings.]
(2) The second method is one that is capable of _demonstrating_ that the effects of selection are actually due to modifiers. It has been worked out in our laboratory, chiefly by Muller, and used in a particular case to demonstrate that selection produced its effect by isolating modifying factors. For example, a mutant type called truncate appeared, characterized by shorter wings, usually square at the end, (fig. 83a). The wings varied from those of normal length to wings much shorter (fig. 83b). For three years the mutant stock was bred from individuals having the shorter wings until at last a stock was obtained in which some of the individuals had wings much shorter than the body. By means of linkage experiments it was shown that at least three factors were present that modified the wings.
These were isolated by means of their linkage relations, and their mutual influence on the production of truncate wings was shown.
[Ill.u.s.tration: FIG. 83 b. Series of wings of different length shown by truncate stock of D. ampelophila.]
An experiment of this kind can only be carried out in a case where the groups of linked gens are known. At present Drosophila is the only animal (or plant) sufficiently well known to make this test possible, but this does not prove that the method is of no value. On the contrary it shows that any claim that factors can themselves be changed can have no finality until the claim can be tested out by means of the linkage test. For instance, bar eye (fig. 31) arose as a mutation. All our stock has descended from a single original mutant. But Zeleny has shown that selection within our stock will make the bar eye narrower or broader according to the direction of selection. It remains to be shown in this case how selection has produced its effects, and this can be done by utilizing the same process that was used in the case of truncate.
Another mutant stock called beaded (fig. 84), has been bred for five years and selected for wings showing more beading. In extreme cases the wings have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows great variability. It is probable here as Dexter has shown, that a number of mutant factors that act as modifiers have been picked up in the course of the selection, and when it is recalled that during those five years over 125 new characters have appeared elsewhere it does not seem improbable that factors also have appeared that modify the wings of this stock.
[Ill.u.s.tration: FIG. 84. Two flies showing beaded wings.]
(3) The third method is one that has been developed princ.i.p.ally by East for plants; also by MacDowell for rabbits and flies. The method does not claim to prove that modifiers are present, but it shows why certain results are in harmony with that expectation and can not be accounted for on the basis that a factor has changed. Let me give an example. When a Belgian hare with large body was crossed to a common rabbit with a small body the hybrid was intermediate in size. When the hybrid was crossed back to the smaller type it produced rabbits of various sizes in apparently a continuous series.
MacDowell made measurements of the range of variability in the first and in the second generations.
_Cla.s.sification in relation to parents based on skull lengths and ulna lengths, to show the relative variability of two measurements and of the first generation (F_1) and the back cross (B. C.)_
CHARACTER
GENERATION
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ Length of{ F_1
skull { B.C.
3
Length of{ F_1
ulna { B.C.
1
1
2
3
1
2
4
4
_same table continued_
CHARACTER
GENERATION
0
1
2
3
4
5
6
7
8
9
10
11
12
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+ Length of{ F_1
2
2
8
5
10
7
skull { B.C.
6
4
13
18