Jour Hered 5(8): 341-347 (August, 1914)
Heredity in a Network of Descent—A Conception Based on the Normal Evolutionary Condition of Species—Characters Represent Lines of Descent Rather than Independent Units in Germ-Cells.
O. F. Cook
Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C.
THE practical value of a scientific theory lies in its use as a method of thinking. Study of the general problems of heredity should enable us to think more clearly regarding the special problems that need to be solved in the interest of agriculture and the racial improvement of mankind. The danger is always that we allow our thinking to be governed by a few facts or by facts drawn from too narrow a field of investigation. Disconnected facts command only a limited interest, but a theory that suggests relations between different kinds of facts often stimulates interest and leads to further discoveries. The utility of a general theory, as well as its probability of truth, is judged by its application to a wide range of facts.
Assumptions or analogies that may appear thoroughly justified by one class of observations or experiment often need to be modified to bring them into agreement with data of other kinds. In this way there may be a gradual attainment of a better point of view, one that facilitates the study of all of the related facts. Yet these inductive methods of biological investigation often appear very indirect in comparison with the well formulated systems of the "exact sciences."
|1Foot, Katharine, and Strobell, E. C. Preliminary Note on the Results of Crossing Two Hemipterous Species with Reference to the Inheritance of an Exclusively Male Character and Its Bearing on Modern Chromosome Theories. Biological Bulletin, 24: 187, 1913.|
The study of heredity is often narrowed to a search for a "physical basis" or "mechanism of heredity." The morphological conception of plants and animals as made up of different tissues and organs has been carried over into heredity, and has suggested the idea of finding something in the germ-cell to correspond to the characters of the adult. The first investigators looked for a complete model of the next generation in the egg, and many similar hopes of finding mechanism of heredity among the organs of cells have been cherished, all to be abandoned in turn, with the progress of cytological discovery. The sex-chromosome hypothesis, suggested by cytological studies of certain groups of insects, is the most recent of these mechanical theories, but is now challenged by other investigators familiar with the cytology of the same insects. The results of breeding experiments are also at variance with the theory that the sex characters are transmitted in the sex-chromosomes.1
MECHANISM STILL HYPOTHETICAL.
If something in the protoplasm of the germ-cell could be shown to stand in a definite causal relation to some feature of the adult organism, we might begin to understand heredity in this physical, mechanical sense. But as yet we have no such basis of observation to warrant the localization of particular characters in particular parts or organs of the germ-cell. Indeed, we have no knowledge of characters except in their external manifestation, as features or differences to which attention may be directed in the study of heredity. Theories of heredity are still useful only as analogies and have to be judged by their consistency with general biological facts and their convenience as methods of thought, in bringing the problems of heredity before the mind.
It is easy to admit in general terms that there must be something in the germ-cells to correspond to what we call characters in adult organisms, but to make this general admission is very different from forming a concrete idea of the hypothetical mechanism of heredity. We may be as sure as ever that organisms are machines, but reiteration of confidence in the general mechanical postulate does not give us increased insight into the machinery. The fact is that we are still without any definite indications of the nature of the unformed, rudimentary characters that are supposed to exist in the germ-cells. The many names that have been given to those hypothetical entities, such as pangen, determinant, id, character-unit, gen, etc., are merely symbols for the unknown quantities of biology. Their only use is for the statement of biological problems. Instead of writing our x with a single character as in algebra we use the symbol gen, with three letters. A gen is the unknown something that is assumed to represent a latent or unexpressed character; that is, a character as it is supposed to exist in the protoplasm, before it comes into expression during the development of the organism.
MENDELISM A MONOGENIC THEORY.
The so-called Mendelian theory of heredity is in reality a theory of the formation of germ-cells. To the assumption that characters are transmitted as separate particles or discrete entities of some sort, is added the idea that each germ-cell receives and transmits only a single set of these character-bearing "units."
The cells that build up the bodies of plants and animals, the so-called "somatic tissues," are supposed to contain two separate sets of character-units, or gens, derived from the two parental germ-cells that united in conjugation and initiated the development of the new organism. But with each return to the formation of germ-cells the two sets of gens are supposed to be broken up and redistributed in single sets. If the two series of gens derived from the
parent germ-cells are alike, all of the new germ-cells have the same series of gens. But if all of the gens of the two parental series were not alike, the new germ-cells receive one or the other of the unlike gens, but not both. Thus the germ-cells of hybrids between different varieties or species are supposed to remain "pure" with respect to the contrasted characters of the parental stocks.
On account of this assumption of "pure germ-cells" Mendelism may be described as a monogenic or single-gen theory of transmission. Only one gen of the kinds required to produce the various features of an adult plant or animal is supposed to be represented in the same germ-cell. The failure of a parental character to reappear in some of the members of the second and later generations of offspring is explained by assuming that the character was transmitted to only half of the germ-cells. The theory does not provide for the transmission of additional gens to represent characters that may not be brought into expression, or that may give rise to unexpected variations in later generations. To explain such variations Mendelian writers resort to the theory of De Vries, that new characters and new species arise by mutations. If a sudden change of characters appears in a member of an otherwise uniform, "pure bred" stock, it is assumed that a new character has been formed, and that such changes in the characters of uniform groups are examples of normal evolutionary progress.
ORIGIN OF NEW GENS.
The effect of these theories is to return to the pre-Darwinian doctrine of special creation, except that the species are not supposed to be made altogether de novo, but by implanting the gens of new characters in members of old species. The change of any single character is supposed to establish a new "elementary species" or "biotype." How the new gens are made and substituted for the old ones is not explained, but the mathematical simplicity of the monogenic hypothesis and the idea that the chromosomes or other internal organs of the germ-cells might represent actual, visible units of heredity, have proved very attractive, and have induced many recent writers to abandon the Darwinian conception of gradual evolutionary progress in natural species. Darwin was familiar with abrupt changes of characters, or sports, as he called them, but he did not consider them new species or examples of normal evolutionary progress. The doctrine of De Vries, supported, as it seemed to be, by the Mendelian theory of heredity, has been hailed as a great advance over Darwinism. But in some respects the new anti-Darwinian theories are logically inconsistent among themselves as well as lacking the support of pertinent facts. The more we emphasize the idea of characters as represented by definite pre-existent entities or gens, the more difficult it becomes to believe that new characters are suddenly implanted.
That workers in other fields of biology have been unable to accept the recent Mendelian and De Vriesian interpretations of the facts of heredity has not seemed a serious matter to those who were fully convinced by the statistical and cytological arguments. From this point of view it seemed a waste of time for "geneticists" to familiarize themselves with the natural species from which Darwin and his successors attempted to learn the nature of evolutionary processes. Indeed some writers have formally dismissed the idea of species as too vague and indefinite for the application of "exact methods." Instead of attempting to understand the development of new characters through the gradual evolutionary progress of species, attention is now given mostly to the changes of characters that appear in artificial pure bred strains under "experimental conditions."
But even this class of variations does not serve to support the monogenic conception of heredity. There is still no adequate proof of the most elementary assumption of the monogenic theories regarding hybrids and mutations, that one gen or character-germ can be taken out and another substituted, and leave the remaining characters the same as before. Mutations and hybrids have not been found to differ from each other by single characters as they are supposed to do in Mendelian and De Vriesian systems.
|2Cook, O. F., Mendelism and other Methods of Descent, Proc. Washington Academy of Sciences, 9: 235. 1907.|
The fact that mutations commonly differ by many characters, instead of by single characters, was noticed several years ago, and was stated in a discussion of the difference between varieties and natural species2. Since that time many additional mutations and hybrids of cotton and other plants have been examined with this question in mind, but thus far without finding a satisfactory instance of monogenic mutation or mutative reversion. In all cases where a definite change in one character was noticed equally definite differences in other characters were found. Such facts indicate that mutative changes should be thought of as polygenic, that is, as affecting several characters at once, instead of assuming that they are commonly or typically monogenic, in the sense of being limited to single characters.
ABNORMAL MENDELIAN PHENOMENA
|3Professor Castle has recently utilized this view of Mendelian characters as affording a basis for simplification of the Mendelian notation. See Castle, W. E. Simplification of Mendelian Formulae. Amer. Nat. 47: 170, 1913. .|
The nearest approach to monogenic behavior is found in cases of albinism or similar differences that may arise from simple chemical reactions. But certainly these color changes and allied phenomena afford a very narrow margin for the support of the general Mendelian idea of the existence of characters as independent units. Indeed, many Mendelian writers have virtually abandoned this idea of alternative units in adopting the "presence-and-absence" hypothesis to explain the results of Mendelian experiments. But if it be assumed that the Mendelian condition of alternative inheritance is a consequence of the dropping of normal characters out of expression, the Mendelian phenomena lose all constructive evolutionary significance and take their place among admitted abnormalities.3
|4Cook, O. F., "Dimorphic Leaves of Cotton and Allied Plants in Relation to Heredity," Bulletin 221, Bureau of Plant Industry, U. S. Department of Agriculture, 1911.|
In reality alternative inheritance is not limited to characters that show the Mendelian reactions, but extends into other fields of heredity where the theory of germinal segregation and alternative transmission of unit characters does not apply. The sudden changes of characters often shown in successive internodes of the same plant are as abrupt and as definite as any mutative change or Mendelian alternation, and yet these changes of characters in internodes take place without any opportunities for the segregation and redistribution of gens in the formation of new germ cells, as required by the Mendelian theory of alternative transmission. Such changes of character, occurring during the development of the same individual plant or animal, must represent alternative expression of characters, instead of alternative transmission. And after this distinction is appreciated it appears reasonable to expect that other sudden changes or abrupt differences among the individuals of the same stock may represent variations in the expression of characters, rather than differences of transmission. Changes of expression must be considered in the study of heredity, as well as differences of transmission.4
THE POLYGENIC INTREPRETATION.
|5Cook, O. F., "Transmission Inheritance Distinct from Expression Inheritance," Science, n.s., vol. XXV, June 7, 1907. p. 911.|
Instead of a single set of gens, as assumed in the theory of Mendelism, a multiple transmission of many alternative gens should be recognized, enough to represent the whole range of ancestral diversities. When the variations of domesticated stocks are compared with the diversities of related wild types the same general range of character is found. Many of the so-called "new characters" of mutative variations should be considered rather as suppressions or reappearances of old characters. The phenomena of reversion and recapitulation indicate that the transmission of characters is entirely independent of expression, and that characters may be transmitted in latent form for many generations and still retain their power of returning to visible expression.5
A polygenic interpretation of heredity does not conflict with the facts of Mendelism and mutation, and is in better accord with other facts of biology, both evolutionary and physiological. From a polygenic viewpoint, mutations and Mendelism appear as differences in the expression of the characters, rather than as differences of transmission. The mathematical formulae of Mendelism apply as readily to the expression of characters as to transmission and the polygenic conception of heredity enables us to avoid the misleading inferences that have been drawn from the monogenic theories. It no longer appears necessary to believe that the degenerate mutations of domesticated plants and animals represent new species, or even new characters. Uniformity in the expression of characters no longer appears to be the normal condition of heredity or of evolutionary progress. On the basis of a polygenic theory the typical condition of heredity, as of evolutionary progress, is found in normally diverse, freely interbreeding wild species, instead of in narrow-bred or self-fertilized domesticated varieties. The production of groups of uniform individuals by selective breeding or by special methods of propagation does not eliminate characters from transmission, but only regulates the expression of characters. It remains possible for the characters of remote ancestors to reappear in reversive mutations.
That characters should be transmitted in a latent condition may seem too wonderful for belief, but the fact remains that all characters are transmitted in a latent condition. The germ-cells, eggs, embryos, larvae and juvenile states do not have the characters of adults, except in the sense of transmission, nor do adults have the characters of the preliminary stages, except in the same sense of transmission to their offspring. It need not be considered more strange that a character should continue latent for a series of generations than that a seed should lie dormant for a period of years, expressing none of the characters of the plant, and yet losing none of them.
What we call characters may be, after all, only phases or stages in the development of the organism and may not be embodied in the protoplasm in any such manner as the corpuscular or genie theories have assumed. The larva or the embryo, the egg or the spermatozoon, is as truly an organism as the adult. Each form of life has its complete cycle and it is only an artificial, mechanical analogy that leads us to think of one part of the life-cycle as less complete than another. The cycle being continuous, each part can be considered as a preparation for the next, but in the very nature of the case there is no beginning or end.
If the idea of transmission seems to involve the notion of embodiment of the characters in the protoplasm of the germ-cells, we should sec that this is only a deduction from the preconceptions with which we have approached the subject, and that it affords us no additional insight into the process of transmission. Until we have a working knowledge of the constitution of the protoplasm itself, the nature of the process of transmission can only be inferred from the facts of expression. Any inferences that may be drawn regarding the numbers and relations of the gens must accord with the behavior of the visible characters, and must accommodate the facts of heredity in networks of interbreeding lines of descent in natural species, as well as in single lines of descent in specially propagated domesticated varieties.
Domestication has been supposed to change the characters of plants and animals by placing them under different environmental conditions, but this factor is probably much less important than changes in the methods of descent. Where domestication has not been accompanied by restriction of descent, the normal state of individual diversity has not been lost, or given place to the greater diversity between narrow bred strains. In such cases as the rye plant or the Guinea fowl, where narrow breeding has not been practiced, the whole species remains relatively uniform and stable, in comparison with the species where descent has been restricted to narrow groups or to individual lines.
COHERENCE OF CHARACTERS.
Merely increasing the numbers of gens does not fully provide for the fact that many characters are usually changed together instead of one character at a time. Even assuming that there are many alternatives for each of the features of a plant or animal, there would still be no reason why the substitution of one of these alternatives for another should be connected with changes among the alternative gens of other characters. Our ideas of relations of the gens to each other have to be modified, in addition to recognizing increased numbers of gens. If we think of characters as representing stages and alternative courses of development, the gens, as predetermining rudiments of the characters, must be thought of as having intimate mutual relations, instead of being considered as entirely independent.
|6Cook, O. F., Mendelism and Interspecific Hybrids, American Naturalist.|
The tendency of characters to segregate or behave as independent entities, as manifested in typical Mendelian hybrids, is often counteracted by an opposite tendency to coherence or associated expression of characters. When the tendency to coherence is strong a whole series of characters may behave more or less like a single Mendelian unit, so that examples of coherence are often reported as cases of Mendelism.6
It may be nearer the truth to say that organisms different in one respect are different throughout, than to make the Mendelian or monogenic assumption that characters are separate entities to the extent that one character can be changed without affecting the others. The idea of alternative courses of development is in better accord with the general facts of biology than the idea of alternative gens, as shown by the nature of individual differences among the members of normally diverse species, with free intercrossing of different lines of descent. We are familiar with diversity in the human species, and do not expect to find two individuals with exactly the same characters, except in cases of identical twins, and here the phenomenon of uniformity is ascribed to a division of the fertilized egg cell, analogous to vegetative propagation. As far as observation has gone, a similar diversity exists everywhere in nature, except where it is restricted by special methods of reproduction. To fit the facts of nature our theories must provide enough gens and in sufficiently intimate relations with each other to equip this infinite diversity of individual forms.
CHARACTERS AND LINES OF DESCENT.
Instead of trying to think of gens as separate entities or corpuscles of any sort we may think of them as parts of the ancestral lines that form the network of descent of the species. A better idea of the nature of the germ-plasm may be gained by considering it as a part of this network of descent. The cells that make up the bodies of plants or animals are connected into networks by delicate threads that pass through the cell walls, and the protoplasm inside the cells also has a reticular structure. Thus there is no lack of analogy for assuming a relation between characters and lines of descent. Indeed, we have all the phenomena of sexuality to warrant the suggestion of a physiological or causal relation between the network of the species and the network of the cell. As the crossing of lines of descent undoubtedly has a physiological function in increasing and maintaining the efficiency of the protoplasm, those who dwell on the idea of a mechanical basis for the phenomena of life should be the more ready to look upon the reticulum of the cell as representing the network of descent of the species. This relation or analogy allows us to consider the expression of each character as marking a stage in the development of the individual, and at the same time as corresponding to one of the many ancestral lines or alternative courses of development that are open to the members of a diverse species propagating itself in a normal network of descent.
Selective line breeding, self-fertilization, or vegetative propagation, enable us to establish definite courses through the maze of alternative paths of expression. When regularity of expression has been secured, so that all the members of a stock bring the same characters into expression, we have a uniform "pure" race. Uniformity has been accepted in the monogenic theory of Mendelism as representing the normal condition of heredity, though breeders know that uniformity is generally artificial, as well as temporary. Even in the most carefully selected stocks mutations continue to appear, the "sports," "rogues" or "black sheep" that are rejected because they do not follow the standard course of development. Thus the breeder's care is required to maintain the uniformity of a select stock, no less than to originate it in the first place.
The analogy between characters and lines of descent also allows us to think of successive appearances of the characters in the development of the individual as corresponding to their successive development in the evolutionary progress of the species. It is not necessary to suppose that the embryological record is entirely consistent or complete. New specializations, such as the larval characters of insects, may be intercalated into the early stages of the life history, instead of being added at the end. The extreme form of the recapitulation theory, that the embryonic and juvenile characters represent the exact courses of evolutionary progress, has been abandoned, but this need not interfere with the recognition of a general analogy or parallel between evolutionary development, or transmission of characters, and individual development, or expression of characters. Each character may still be thought of as a stage of development, reached by following one of the ancestral courses or lines of descent.
The conception of characters gained by this analogy is more concrete and stands in a more effective relation with general ideas of evolution and heredity than any of the character-concepts that have been based on the wholly hypothetical "units." There is no objection, of course, to particularizing the characters in any way that the facts warrant, or to recognizing any ascertained relations to chemical substances or to physical conditions, but all this can be done without abandoning the general idea of multiple descent and multiple transmission of alternative ancestral characters.
The Mendelian theory of heredity may be described as monogenic because it assumes the transmission of only one set of gens or character-units in each germ-cell. A polygenic theory is required to account for the fact that latent or suppressed characters are transmitted, as well as those that are brought into expression. The polygenic nature of heredity is also indicated by the fact that mutative changes of expression usually affect many characters at once, instead of single characters. A further advantage of the polygenic conception is that it allows characters to be brought into analogy with ancestral lines that form the network of descent of the species.
The assumption that characters are pre-existent in the protoplasm of germ-cells as discrete particles or independent units of any sort is not justified by observation or by logical necessity. Equally convenient and more truly biological methods of thinking about the problems of heredity can be developed by recognizing the relation of heredity to the network of descent of the species. This conception allows characters to be thought of as representing lines of descent instead of as discrete particles in the protoplasm.