by
V.A. Shevchenko
Institute of General Genetics
Academy of Sciences
Moscow, U.S.S.R.
Heredity is usually defined as the ability of parents to transmit their characteristics and peculiarities of development to their offspring. Every animal and plant species maintains its characteristic features in a number of generations, and under whatever conditions it may be placed will reproduce its peculiarities provided it still maintains its ability to reproduce.
Heredity ensures material and functional succession between generations of organisms, while it also maintains a definite order in the variability of living organisms. The multitude of various organic forms are grouped in definite systematic units, such as species, genera, families or orders. This systematic pattern of existence of organisms is possible only because of the existence of the mechanism of heredity which ensures the maintenance of not only the traits of resemblance within every group of animals or plants but also the distinctions between them. Heredity is inseparably connected with the process of reproduction, and reproduction is related to the division of the cell and the reproduction of its structure and functions. The ensuring of the succession of properties is only one aspect of heredity; another aspect is the ensuring of the accurate transmission of a specific type of development, the formation in the process of ontogenesis of definite characters and properties, a definite type of biosynthesis and metabolism.
Whatever their type of reproduction, in most of the organisms (except unicellular ones), separate somatic or sex cells do have properties and peculiarities characteristic of the multicellular organism. These characters and properties are formed in strictly consecutive order in the process of individual development under particular environmental conditions. The clear-cut pattern of the individual development of every organism is determined by its heredity.
The hereditary constitution is formed by a number of various genes. The entire set of these genes is called a genotype. Consequently, the concept of genotype is identical to that of genetic constitution. The term phenotype implies the outward appearance and the state of the individual at a given moment. This state is a result of the interaction between the genotype and the environment. The entire process of the development of an individual from the fertilized ovum to the adult organism takes place under the controlling influence of the genotype, this influence interacting continuously with the multitude of environmental conditions under which the growing organism finds itself. Thus the properties of an individual depend on two main factors, viz., the hereditary constitution (the genotype) and the environment in which the organism occurs and with which its genotype interacts.
Individuals belonging to any species, either animal or plant, differ from each other in a great number of individual peculiarities. The analysis of these distinctions reveals some regularities in their distribution among the individuals descending from particular parent forms as well as among individuals living under particular environmental conditions.
An experimental analysis allows a deeper understanding of the very essence of these distinctions. Some of them, once appearing in a certain individual, are again similarly expressed in the offspring; others, appearing in all individuals under particular conditions, disappear in the progeny if the latter develops under different environmental conditions.
In the first case it is not possible to establish any apparent relationship between the environmental factors and the specific hereditary reaction of the organism. Darwin believed this to be mainly determined by the individual properties of every individual and called these changes individual and “indefinite”. Now they are known as mutations.
In the second case a relationship may easily be established between particular environmental factors and the pattern of changes in the organism. The specific reactions are evidently determined mainly by the organism itself, and Darwin called these mass or “definite” changes. Now they are called modifications. Their expression undoubtedly depends on the hereditary properties of an organism - on the general hereditary properties of the particular species as a whole rather than on properties of the individual organism. Usually modifications are of an adaptive nature and they are replicated in different individuals of a particular species. The ability to form particular adaptive modifications is the result of a long historical development of organisms under particular environmental conditions.
For a long time a dispute has been going on about what is more important for the formation of an individual - the environment or the genetic constitution. Those working in the field of genetics are often reproached for underestimating the role of environment. However, this reproach is absolutely groundless for the main thesis of genetics is, as has been mentioned earlier, that a phenotype is the result of the interaction between a genotype and the environment. Thus it is claimed that there always exists an interaction between environment and heredity.
By conducting investigations on suitable material it is possible to reveal to some extent the relative role of the environment and the genotype. For this, two methods are used. The first consists of studying genotypically different individuals under as similar environmental conditions as possible. For example, several different kinds of the same plant species may be grown side by side on an experimental plot and the distinctions observed may be studied, which in this case may be considered to be genetic distinctions.
The other method is based on studying genetically identical individuals or varieties placed under different environmental conditions. In this case it is seen that different species change to varying extents under the influence of different environmental conditions, i.e. they have different plasticity. Modifications appearing under such different conditions are purely individual and transient properties which are not inherited by the offspring.
By way of illustrating the relative roles of the environment and the genotype we may take identical twins. It was established that such twins have identical genotypes for they develop due to fission of a single fertilized ovum. Unlike non-identical twins (i.e., those developing from two different ova fertilized simultaneously) identical twins always belong to the same sex and bear a very strong resemblance to each other both physically and psychically. An example is provided by the five well-known Dion quintuplets in Canada who are all identical and belong to the same clone.
The main result obtained from the study of twins is that hereditary constitution plays an extremely important part in determining not only purely outward traits but also psychic properties as well as resistance to various diseases. This applies even to those cases when the twins are separated from each other at an early age and grow up under entirely different environmental conditions. Generally, the resemblance between twins is rather more evident in respect to physical traits than in respect to mental faculties which are more susceptible to the effect of environment.
Generally, we are able to distinguish the characters which are mainly dependent upon the genotype and which are not likely to be affected by environment (e.g., the colour of eyes) as well as such states which are conditioned by purely external effects (e.g., suntan or an infectious disease). However, even in these states which are undoubtedly conditioned by the environment, the role of the genotype appears to be more important than might be expected. Dark-haired people are more easily suntanned than blond ones; as for the susceptibility to infectious diseases, there is hardly any disease the contraction of which would be altogether independent of the genotype.
Thus, it is not particular characters that are inherited but the norms of reactions, i.e., the specific type of reactions to various environmental effects. For example, two cows which have equal weights at a certain level of feeding may respond differently to an increase in the quantity of food, for it may happen that one of them may not be able to utilize the increased quantity of food.
Formerly, when variability on the basis of recombinations was something which still remained to be learned, it was believed that biological variability was always determined by the direct influence of environmental conditions on the properties of individuals. These concepts were most fully developed in 1809 by the French biologist J. Lamarck. Lamarck emphasized that organs which were not used by an individual would become poorly developed and weak, whereas those organs which were often used would improve more and more. According to Lamarck such individual adaptations, both direct and indirect, which occur due to exercising or not exercising particular organs, are to some extent inherited.
Darwin (1867) accepted Lamarck's ideas of the inheritability of individual adaptations and supplemented them with his own theories about the hereditary trend of organisms towards non-directional variability. This means that in some of the progeny of one individual some character will deviate to one side, whereas in others it will deviate to the other side as compared to its state in parent individuals. If natural selection affects such mixed material and the effect is favourable so that this particular character is strengthened, then the average significance of this character will gradually increase.
Recent investigations confirm that natural selection is an extremely important factor. On the other hand it appears that Darwin's (op.cit.) ideas about the inherent trend of organisms towards non-directional variability and the consequent ability to change continuously and unlimitedly are erroneous.
One of the principal achievements in the field of genetics was the discovery that biological variability was an intricate phenomenon depending on several absolutely different causes. This was particularly well shown by the Danish scientist, W.Johannsen (1908) His selection experiments with populations and pure lines of French beans are among classical investigations in the field of genetics. Johannsen's chief achievement consists in ascertaining that selection affects only the population, i.e., the genetically heterogenous material, whereas within pure lines selection brings no results because variability within pure lines is restricted to modifications. Thus, for example, within a pure line we can select the biggest or the smallest seeds in any number of generations, but the average size typical of this line will nevertheless remain unchanged.
In populations of cross-fertilizing individuals there are better possibilities for selection in a definite direction than in populations of self-fertilizing individuals. This is related to the fact that cross-fertilizing individuals are characterized by a higher degree of heterozygosis and intensive variability conditioned by recombinations. Selection in such populations often brings good results. However, in spite of the large number of possible combinations of genes in such populations, we also have some limits here which cannot be exceeded. These limits are determined by the fact that in a population there exists a finite number of original genes which are subject to selection. When the entire gene pool has been used in forming combinations which are favoured by selection, the selection in this direction is terminated.
However, there is one more possibility of further changes. Completely new genes may be formed as a result of mutations, i.e., changes in the hereditary constitutions which are not recombinations of genes. (Mutations are considered in detail elsewhere in this seminar.
Thus, variation is conditioned by three different causes: (i) environmental effects; (ii) recombinations, and (iii) mutations.
Both in mutations and in modifications we deal with changes in the occurrence and expression of hereditary characters of the organism. In the development of any individual environmental factors mainly act as agents engaging particular form-developing processes. This is to be considered as a result of the historical development of an organism under particular environmental conditions. Evolution proceeds under gradual liberation of a developing organism from the power of random environmental phenomena.
According to Schmal'gauzen (1946) this is accompanied by the development of internal regulatory mechanisms controlling the processes of individual development. “The liberation of the organism from the determinant role of environmental factors is indeed the establishment of the internal factors of development determining the specific course of form-developing processes” (Schmal'gauzen, op.cit.)
In the early stages of their origin living creatures were entirely controlled by random changes in environmental factors in determining elementary performance of their natural functions. The evolution of living creatures was the result of the gradual liberation from this dependence. The effect of external factors was gradually directed into a certain channel and transformed in the organism according to its specific responses. The organism developed means of passive and active defence against adverse effects and made positive use of favourable ones; various environmental factors getting more and more assimilated by the organism by the differentiation of the latter. The organism's responses to the assimilated environmental factors are always strictly specific and this specificity is determined by the historically conditioned characters of the organism itself, its evolution under particular environmental conditions.
The non-assimilated factors of environment, provided they reach the organism and are able to affect it, do not find any prepared basis in the organism. The organism developed historically without interacting with these particular factors (e.g., atomic radiation) and therefore it is not able to respond expediently to such effects. Changes in the organism under the influence of a non-assimilated factor will be indefinite - these might be either mutations or modifications appearing for the first time. Such modifications will not be of an adaptive nature and they are usually called morphoses. Among those which are well known at present are the so-called roentgenomorphoses, chemomorphoses, thermomorphoses, etc. All these reactions are at best indifferent; often they are displayed as apparent disformities characterized by vital capacity too low to exist under natural conditions. A great number of such morphoses appear under the effect of atomic radiation or chemical mutagens in the generation exposed to a mutagen.
Morphoses may also occur if the organism's reaction norm is upset due to a mutation. Such morphoses are extremely labile. They easily change their manifestation with a change in the intensity of environmental factors. By way of illustrating such morphoses, we can take variable non-adaptive manifestations of many mutations of the Drosophila, e.g., the mutation of its ribbonlike eyes (Fig.1 - bar AA) is displayed by an increase in the number of eye facets at higher ambient temperatures. The allelomorphic mutation (Fig.1 - infra-bar AA', A'A') shows a reverse reaction, i.e., the number of eye facets decreases at lower temperatures. There is nothing adaptive in these reactions.
Modifications are usually said to be non-hereditary. This is not quite so. The very ability to form particular modifications is due to the genetic make-up of the individual. The concept of modification implies differences in the manifestation of normal organization (or particular mutation) depending, for their development, on various environmental factors.
Fig.1 The number of eye facets in homozygous females Bar (A'A) and infra-bar (AA) and the corresponding heterozygote (AA') as related to temperature.
Sometimes modification relates to particular characters or organs. An example is provided by fish changing their colour in relation to the colour of the bottom. At low temperatures, mammals develop thicker and longer fur. These changes are of an adaptive nature.
In other instances modifications involve, if not to the same degree, the entire organism, e.g., differences in the size of the adult animal.
Sometimes the ability to form modifications is displayed in the apparent transformation of the whole organization, the changes being of the nature of complete adaptive transformations. The organization appears to be polymorphic in relation to different environmental conditions. Such an organism may be said to have not one norm but several complete adaptive norms. Sometimes different adaptive norms correspond to different seasons (seasonal polymorphism).
Modifications may also be distinguished by the form of reaction. Sometimes the reaction is of the nature of an immediate response to a particular irritator. Such is apparently the direct effect of temperature on the process of pigmentation. However, in most cases there appears to be a rather indirect effect of factors through a more or less intricate chain of interdependent physiological processes. There may exist quite a significant time lag between the time of perception of irritation and the time when this effect is displayed in the morphogenetic reaction. Thus in some trees the light and the shady forms of leaves are predetermined by the conditions of illumination in the preceding summer rather than in spring when these buds begin to develop.
Generally, there is no proportional relationship between the intensity of the external factor and the quantitative manifestation of the modification. At the lower threshold of irritability of the organism's tissues a typical reaction immediately sets in and is fully displayed. The pattern of the organism's reaction does not change with a further increase in the intensity of the irritation stimulus up to the maximum sustainable level which corresponds to the upper threshold of the irritability of the tissues.
The reversibility of modifications may be expressed in different ways. Sometimes modifications seem to be easily reversible within one individual and may be repeated any number of times in any direction. For example, some animals change their colour very quickly. These are the so-called physiological reactions (the colour of flatfish in relation to the colour of the bottom, change of colour in chameleons). An example of slowly reversible reactions is provided by the results of exercising and not exercising the organs.
Other modifications, once they are developed, are retained by the individual to the end of its existence. For example, differences in the structure of queen bees are entirely dependent on their feeding at the larval stage and they retain their characteristic traits to the end of their life. Seasonal forms of daphnia, rotifers and a number of other planktonic organisms depend on feeding and temperature; however, once they are developed, they do not undergo change.
Reversibility of modifications, and particularly adaptive ones, is an important feature. If the ability to form adaptive modifications is the result of a historically developed adaptation to variable environmental factors, then its full significance lies in its reversibility. Without reversibility, i.e., non-inheritance, there would be no individual adaptation.
As has already been emphasized above, the organism's reaction to particular environmental factors is always specific, and this specificity is determined by the hereditary characters of the organism. Therefore, geneticists say that any genotype is generally characterized by its particular reaction norm. This norm includes also individual reactions of the organism under different environmental conditions, i.e., the ability to form different modifications.
The concept of reaction norm is very important to general problems of biology, for it is one of the few strictly defined concepts which help in clarifying debatable questions concerning variable forms and their role in the process of evolution. Any modification is a peculiar form of individual response which is a part and a characteristic feature of the reaction norm of this particular organism. On the other hand a mutation is a change in this reaction norm, and this is completely characteristic of any mutation.
A mutational change in the reaction norm usually means a change in the process of individual development. Much is known about mutations in Drosophila. Here we are aware of a wide variety of mutations manifested by changes in body colour, in the length and shape of setae, in the size and shape of the eye, by different fecundity, higher or lower growth rate of the larvae, etc. Mutations differ in the depth to which they penetrate into the very basis of the organization, in the degree to which its different sides become involved and the extent to which they are manifested. We know many mutations in Drosophila which are manifested in the underdevelopment of wings. Different mutations are related to different manifestation of this underdevelopment, from a very slight decrease in the size of the wingplanes or the formation of small notches in the wings, to the complete rudimentation of the wings with all the various transitional stages between the normal wing and its complete absence (Fig.2). Sometimes the manifestation of the mutation is localized; in other instances it involves many characters. In considering the numerous mutations which may serve as material for progressive evolution, we must make a special note of those mutations which are manifested by an increasing number of similar formations (polymerization). Such changes prepare the background for a further complication of the organization by way of division of functions and corresponding differentiation of the originally identical parts.
A great number of mutations represent a more or less sharply expressed disturbance of the normal structure of the organism and its functions. This is quite natural, for due to the intricate interrelationships between the organism's parts and between the organism and environment, almost any change in the organization on the whole proves to be unfavourable - it always upsets the historically established inter-relationships both in the organism itself and their relation to environmental factors. However, the harmfulness of mutations is but relative. We can only speak about the relative detriment or the relative usefulness of mutations. With a change in environmental factors a formerly detrimental mutation may assume a positive significance and vice versa. In this connection it should be noted that normally individuals belonging to any species live under different and variable environmental conditions. What is harmful under certain conditions may prove to be indifferent or even useful under different conditions. It should also be noted that since any mutation has multiple manifestations, the generally detrimental mutations may still appear to have indifferent or, sometimes, relatively favourable manifestations. These latter may assume a positive significance under certain conditions of the organism's existence, and due to this, may be maintained in the population through selection.
Fig.2 Some mutations of Drosophila melanogaster manifested by changes in the wing structure.
| 1. | Notch | 2. | Delta |
| 3. | Vestigial | 4. | Antlered |
| 5. | Curled | 6. | Apterous |
The main factor involved in evolution is natural selection; it appears that in selection the decisive role is played by the concrete manifestation of individual peculiarities under particular environmental conditions, namely the phenotype of the organism. Therefore the significance of modifications in the process of evolution must not be overlooked. If they are adaptive they determine the survival of these particular individuals and thus have a bearing on their future existence and evolution. However, adaptive modifications result from the historical development of the forms of reaction. Their changes as well as the appearance of new forms of reaction are related to changes in the hereditary basis, i.e., reaction norms. Any change in the reaction norm is a mutation. Hence, evolution is based only on mutations. Mutations combine but are not absorbed. This is of great importance in the process of evolution, since it makes possible the formation of new, more favourable vital combinations. The manifestation of mutations is variable and depends not only on environmental conditions but on the genetic background as well. The manifestation of mutations may be obscured by the wide norms of the organism's individual adaptability to environmental factors and by the power of mutual adaptation of the organism's parts. This suppression of the manifestation of mutations is of no less importance in the process of evolution than the expression of their manifestation. If the latter allows various combinations, the former provides the necessary prerequisites. Most mutations are largely unfavourable. The suppression of their manifestation provides the possibility of their maintenance and accumulation in a population in a cryptic way. At the appearance of successful combinations, their manifestation may become more pronounced in the same process of individual adaptation, in the transition to the homozygous state.
The possibility of the occurrence of inheritable adaptability is easy to understand if we make a reference to the studies of Nilson-Ele, according to whom quantitative properties, such as winter-resistance, early ripeness, specific type of growth, etc., are conditioned by polymeric genes. Polymeric genes provide material for numerous recombinations, which in their turn result in the appearance of an unlimited number of various biotypes. Genetic adaptation occurs more easily in cross-fertilizing species since these organisms continuously form great numbers of new genetic combinations. Cross-fertilization is the most reliable way of creating genetic variation, and this accounts for the existence of a great number of various mechanisms in nature which favour cross-fertilization and hence recombinational variability.
Numerous examples are available of genotypic adaptations to different environmental conditions. For example, in the silk-worm (Lymantria dispar) the time when the moth emerges from the chrysalis depends on the time of the onset of winter. Goldschmidt's experiments demonstrated that the comparison of northern and southern forms showed genetic variations in the period of development which seem to have resulted from recombinations of genes and natural selection. The action of natural selection in such instances must be very effective: the moth that would emerge from the pupa with a too short resting stage would produce the offspring which would immediately be killed by low temperatures.
Under different environmental conditions the significance of particular forms varies to a considerable extent. With regular variations in the factors of the heterogeneous environment depending on local conditions or seasons, adaptive modifications acquire great importance. Under such conditions genetic variations may not be very pronounced. When random and short-lived changes prevail in environmental factors genetic variations may easily become manifest; a number of variants occur in which certain genetic variations may largely be suppressed. In this way, a foundation is laid for intraspecific differentiation.
Thus, hereditary adaptation to different environmental conditions is a fundamental biological phenomenon which is of importance not only to intraspecific differentiation, but plays an important part in the formation of new species and, hence, in all those changes, which are usually called evolution, in the living creatures inhabiting the globe.
Darwin, C., 1867 The origin of species. (Proiskzhdenie vidov)
Goldschmidt, R., 1932 Untersuchungen sur Genetik der geographischen Variation III. Abschliessendes über die Geschlechtrassen von Lymantria dispar. Arch.EntwMech. Org., 126:277–324
Johannsen, W., 1908 Über Erblichkeit in Populationen und in reinen Linien. Z.indukt.Abstamm. -u.VererbLehre, 1:1
Lamarck, J., 1809 Philosophie zoologique. Paris
Nilsson-Ehle, H., 1909 Kreuzungsuntersuchungen an Hafer und Weizen. Lund Univ. Arsskr., ser 2, 5(2):1–122
Shmal'gauzen, I.I., 1946 Factors of evolution (Faktory evolyutsii). M. Publ.Akad.Nauk SSSR
