Frozen plasticity theory
The theory of frozen plasticity assumes that species of sexually reproducing organisms exhibit evolutionary plasticity only immediately after emergence, before genetic polymorphism builds up in their genetic pool.
The main drawback of the selfish gene theory is that it does not take into account the existence of epistatic interactions between individual genes and the consequent dependence of the phenotype manifestations of the individual alleles on their own frequency and on the frequency of alleles in other loci. The model of inter-allele selection tacitly (and erroneously) assumes that the effects of the individual genes on the phenotype of the organism and, indirectly, on the biological fitness, are simply additive. In actual fact, this is frequently not true (Chippindale & Rice 2001). Two alleles, each of which can be advantageous on its own for its bearer, can frequently substantially reduce the biological fitness of their bearer if they are in the same genome. On the other hand, two independently harmful alleles can, together, increase the fitness of their bearer. This means that the fact that the actual allele (in contrast to the overall genotype) is inherited from one generation to the next in unaltered form still does not ensure that its phenotype manifestations and their effect on the biological fitness of the individual will also be inherited (Fig. IV.13) and that they can spread through the Darwinistic mechanism of natural selection.
For example, in most individuals in the human population, the effect of loss mutations on the gene encoding the α-chain of haemoglobulin is highly negative, as inadequate production of this chain causes α-thalassemia. However, in bearers of a similar mutation in the gene for the β-chain of haemoglobulin, the same mutation is manifested in an increase in their biological fitness, as it prevents the occurrence of relative excessive production of the α-chain and thus also the occurrence of pernicious β-thalassemia (Wainscoat et al. 1983; Kanavakis et al. 1982). Thus we cannot assign any specific selection coefficient value to the individual alleles. In addition, even if we could do this, the evolutionary fate of the alleles will not be determined by the value of such a coefficient, but rather by whether the allele determines an evolutionarily stable strategy (see IV.5.1.1). Thus the solution to the problem of inadequate heritability of biological fitness suggested by Dawkins in his selfish gene theory (the model of interallelic selection) is inadequate.
As a single mutation occurs in the context of other genes in each generation as a consequence of mixing of the genomes in sexual reproduction and its effect on the properties of the bearer thus changes, a major proportion of mutations cannot become fixed in the gene pool of the species. Thus, sexually reproducing species gradually begin to exhibit an increasing degree of genetic polymorphism. Increased polymorphism again increases the probability that the new mutation will find itself in the context of a different gene pool in each generation. This viscous circle of positive feedback, together with a common phenomenon of frequency dependent selection (see chapters IV.5, IV.5.1 and IV.5.1.1), finally leads to the formation of genetic homeostasis, a phenomenon that was recognized long ago in their experiments by classical geneticists (Lerner 1958). Initially, a species changes readily under the effect of any selection pressure; however, as the frequency of the individual alleles gradually shifts away from equilibrium, the selection coefficients of the individual alleles present also gradually changes until the selected population ceases to respond to the relevant selection pressure (Fig. IV.14). This phenomenon is sometimes interpreted in that, during the selection experiment, the genetic variability in the population is exhausted and further evolution of the trait begins to be limited by waiting for a new mutation. This explanation is, however, apparently erroneous. If, at the moment when the evolution of the given trait stops, we begin to exert pressure the opposite direction, for example, instead of individuals with large dimensions of the given trait, we begin to select individuals with small dimensions of the trait, the population begins to respond quite willingly to the new pressure and the relative trait will become smaller. However, this means that, at the time when the population does not react to the original selection pressure, it still contained the genetically-dependent variability. The most probable explanation for genetic homeostasis consists in pleiotropic negative effects on the genes responsible for the given trait. Some alleles that became fixed through selection pressure of the experimenter, or at least increased their frequency, simultaneously negatively affect the biological fitness of their bearer. This means that, from a certain instance, the bearers of a particular combination of alleles continue to be at an advantage through the artificial selection of the experimenter, but are increasingly at a disadvantage in relation to natural selection. At the moment when the artificial selection and natural selection become equal, the development of the given trait stops. After termination of the selection pressure of the experimenter, a sufficiently large population will return to the original state through the action of natural selection, and the original frequency of the individual alleles and also the original phenotype (appearance and behaviour) of the individuals in the population are renewed in the population. In a small population, the return to the original state need not be complete, as some less frequent alleles disappear from the population through drift (see chapter V).
The theory of frozen plasticity (Flegr 1998), (Flegr 2008) assumes that, as a consequence of the occurrence of genetic homeostasis, clearly punctuated evolution (see XXVI.5) is characteristic for sexually reproducing species. Throughout most of their existence, species more or less do not change or change only temporarily, in spite of frequently dramatic changes in their environment. Irreversible changes in the properties of a species, i.e. anagenesis, occur only immediately following speciation, when the size of the originally small population of the newly formed species has already grown (therefore the destiny of an individual is already directed by selection, not by chance, see Chapter V.4) but the population still bears only a small fraction of the polymorphism of the parent species. The drastic reduction in the genetic polymorphism in the new species means that new mutations are present in the company of the same genes even in sexual reproduction. Until sufficient genetic polymorphism accumulates in the gene pool of the population, the species is evolutionarily plastic and can respond adaptively to selection pressures of the environment similarly to asexual species. Following accumulation of polymorphism, the species evolutionarily “freezes” (becomes evolutionary frozen on macroevolutionary time-scale and evolutionary elastic on microevolutionary time-scale) and, for the rest of its existence, only passively waits until a change in its environment causes its extinction.
Frozen plasticity may also play an important role in some processes at an intraspecies level.. Cultivated plants include both autogamous (e.g. wheat) and also heterogamous (e.g. rye) species and varieties. It follows from the theory of frozen plasticity that the properties of heterogamous species and varieties should be more stable on a micro-evolutionary scale than the properties of autogamous species or even varieties that reproduce predominantly or only vegetatively (Flegr 2002). In the former case, the alleles are in the presence of other alleles in each generation, so that their selection coefficient changes unpredictably. Thus, it is difficult for selection pressure to act consistently leading to their elimination or fixation. In contrast, for autogamous or vegetatively reproducing species, the genetic environment of the individual alleles is the same in each generation and thus the selection value remains stable from one generation to the next. Thus, the genetic composition of the population can easily submit to the effect of selection through evolutionary changes.
The different response capacity of sexual and asexual (and autogamous) species and varieties for selection pressures is apparently also very important in plant improvement and normal farming practice. It is relatively difficult to select new varieties in sexually reproducing species (or amongst the above-mentioned heterogamous plants). In order for the organisms to respond to the relevant selection pressures, it is mostly necessary to work with relatively small populations and to employ a high degree of inbreeding, to reduce as far as possible the genetic variability of the population and thus to increase the heritability of the phenotype manifestations of the relevant selected alleles. In asexual (and autogamous) species and varieties, it is possible and, because of the relative lack of genetic variability, frequently also necessary to perform selection in large populations. On the other hand, the newly obtained varieties are more stable for sexual (and heterogamous) varieties than for varieties reproducing asexually or autogamous varieties. Their advantageous properties should not gradually disappear as a consequence of the action of natural selection, which constantly increases the biological fitness of the organisms, frequently at the expense of their usefulness (Flegr 2002).
As, until recently, these phenomena had practically no support in genetic or evolutionary theory, they are very rarely described in the biological literature. The publications of the Lysenkoists constitute an exception; these results follow very well from consideration of their absurd theories. In their work, these Soviet “researchers” described the low stability of the evolutionary properties of autogamous varieties of cereals compared to heterogamous varieties and also promoted agrotechnical procedures based on intraspecies crossing of autogamous plants, which led to prolonged maintenance of the useful properties of the given varieties (Lysenko 1950). It is probable that a major part of the results published by Lysenkoists were falsified or even fabricated and it is, understandably, necessary to approach their data with a maximum of caution (Medveděv 1969). Nonetheless, it is not possible to completely ignore the fact that a certain part of the information was passed down from the experienced empirical agronomists of previous generations.
Email this page