Phytospreading, proposed by S.V. Meyen, depends on the role of biological evolutionary “mechanisms”. Some biological species that occupy a vast territory from the equator to the mid latitudes break up into two areas and form two populations. Later, the conditions arise for these populations (new related species) convergence at equatorial or middle latitudes, resulting in the hybridization of these species. This process takes place due to “return wave” of phytospreading during the next general cooling period or “second wave” phytospreading during the future warming period. Hybridization and the preceding heterochronies, which affect reproductive processes, create new biological species with progressive morphophysiological structures.
Biological evolution ; Nomogenesis ; Phytospreading ; Heterochrony ; Hybridization
The triumphant march of the synthetic theory of evolution (neo-Darwinism), inspired by the rapid development of genetics, was suspended in 1960s, less than half a century after the formulation of its main theses. Even the founders (e.g., E. Mayr) began to realize that the theory of STE contained flaws. Some biologists had doubts as to the reality of the specification mode, which was described by the genetics of populations. At the end of his life, E. Mayr came to the conclusion that macroevolution could not be explained by changes in gene frequencies. Macroevolution, in general, was left behind with neo-Darwinism; by the end of the twentieth century, the impossibility of macroevolution through microevolution became evident. The primary but not the sole criticism of STE is that it was unable to explain the phylogeny and to predict any act of progressive biological evolution.
What was proposed by the biologists of the XX century as an alternative for neo-Darwinism?
Nazarov, 1991 ; Nazarov, 2007 noted a number of contemporary popular non-Darwin concepts of evolution, characterized by internal consistency with the potential for further development. The following have been thoroughly analysed: 1) the group of “space” hypotheses of phasing development of the organic world (hypothesis “conjugate evolution”); 2) the hypotheses of symbiogenesis, hybridogenesis and netted evolution; and 3) nomogenesis. There are no clear conceptual borders between these hypotheses, which often act in a complementary manner rather than as alternatives. Separately, V.I. Nazarov analysed the ideas of saltation and macromutation, where an “ideological” plan merges with a nomogenesis or finalism. This idea turned out to be the most fruitful. The geneticist, R. Goldschmidt (hypothesis of systemic mutations), and the palaeontologist, O. Schindewolf (hypothesis of tipostrophe theory), proposed a plausible mechanism of progressive evolution (unfortunately, their works were not translated into the Russian language, thus the only way to become acquainted with them is through a presentation, for example, of Nazarov, 1991 ; Nazarov, 2007 ). These ideas did not receive a worthy development. Apparently, most modern biologists feel comfortable enough within the framework of orthodox neo-Darwinism, thereby saving themselves from formulating and solving complex theoretical or philosophical problems without any guarantee of success.
The ideas of R. Goldschmidt are summarised in the following: 1) the source of macroevolutionary changes are systemic mutations, implementing radical transformations of the internal structure of chromosomes, leading to the emergence of a new phenotype and a new species; 2) as a result of systemic mutations, a mass of abnormal forms emerges, some of which may be a few individual units (“hopeful monsters”), which happen to occur under favourable conditions, founding new macroevolutionary branches. O. Schindewolf, in full agreement with R. Goldschmidt, brought to his hypothesis the idea of the cyclicality of the evolutionary process and its stages and also the conditioning of macromutations by external (space) factors. In fact, the works of A. Vandel, A. Dalcq, B.L. Lichkov, V.A. Krasilov and other biologists are aligned with this same idea.
The R. Goldschmidt hypothesis did not become a theory. Achievements in molecular biology (especially data on chromosomal rearrangements that were obtained in the last decade) and palaeontology have neither denied this hypothesis nor provided conclusive evidence in its favour. Saltation, from the R. Goldschmidt perspective, is approaching nomogenesis in its “technological” aspects. Later, these ideas formed a theory of punctuated equilibrium, which is considered the most authoritative alternative to neo-Darwinism (Gould, 1986 ), although it does not break away from it entirely.
However, the mechanism of evolution, as well as its orientation, remains unresolved. There is no conclusive evidence of the effectiveness of systemic macromutations or an explanation of the irreversibility of morphophysiological complications of organisms. Even the theory of punctuated equilibrium has yet to provide any answers to these questions.
Nomogenesis (“evolution on the basis of consistent patterns”) is considered to have entered into biology due to the works of L.S. Berg (1977) in the early twentieth century and his contemporary N.I. Vavilov. Despite the evidence of realism in this idea, nomogenesis was not accepted. Moreover, in the middle of the twentieth century, nomogenesis was forgotten for a short period of time, but later, it was revived and developed thanks to B.L. Lichkov, A. Lima-de-Faria, S.V. Meyen, Y.V. Tchaikovsky and their few supporters.
Palaeobotanist S.V. Meyen, one of the most talented evolution theorists, has played the most prominent role in the studies of the mechanisms of biological evolution. Among his important achievements (in addition to introducing the concepts of meron and refrain and the creation of meronymy as a section of taxonomy) there is the hypothesis of phytospreading. It should be noted that S.V. Meyen attempted to synthesise nomogenesis and neo-Darwinism, although he failed to completely abandon the postulates of STE and, perhaps because of this conciliatory position, did not manage to create a holistic concept. However, he came the closest to answering the main question of theoretical biology: how evolution works.
S.V. Meyen showed that almost all superior generic taxa of higher plants appear in the equatorial belt at a lower stratigraphic level than outside of it (Meyen, 1987 ). In other words, they have equatorial origins. During warming periods, representatives of these taxa migrate from the equatorial belt to higher latitudes. This is the essence of phytospreading. As the distance from the equator increases, the macroevolutionary activity of the higher plants decreases, and in the Arctic zone, only speciation occurs. A more successful macroevolution of higher plants in the equatorial zone can be associated not with the strengthening but rather with the weakening of natural selection. Generally, the role of natural selection in the early days of taxon formation at the family level or higher apparently is not predominant, but rather is episodic. According to S.V. Meyen, a crucial role in evolution is played by selectively neutral saltations. Phytospreading features and many related aspects of biological evolution are described by Tchaikovsky, 1990 ; Tchaikovsky, 2008 .
At the next cold snap, in the middle and high latitudes, settlers either disappear or persist at the new location but with a slight change in their overall organization, giving rise to new species, genus, and, to a lesser degree, higher rank taxa, but not with higher orders. At that point S.V. Meyen stopped.
What happens during a cold snap? Is the process similar to phytospreading, but observed in the opposite direction?
It is obvious, that the reverse process is not necessarily symmetrical to the former. According to S.V. Meyen, during a cold snap, entire ecotopes (relatively homogeneous environments with the appropriate biocenosis), not separate forms, shift from high latitudes to low. Traces of boreal species introductions into the tropical flora have not been found. Nevertheless, the ecotope naturally shifts, and its biological communities are necessarily accompanied by representatives of flora that had ancestors that moved to mid-latitudes by phytospreading during the relative warming. At the peak of cooling, apparently, the full period of this rhythmic process (called reversible phytospreading) will terminate when the areas of ancestral and “modified” species in the middle latitudes will be connected. The process is entirely dependent on climatic rhythms; however, it will likely be a half-period for the climate.
There is an alternative (or complementary) version of phytospreading, “closure”. Once the species brought by the “first wave” of phytospreading consolidates at the new location in the middle latitudes is forced to undergo some phenotypic and genetic changes due to subsequent cooling, a new warming period will arrive. The “second wave” of phytospreading will bring in the ancestors of these species from the same latitude that, due to the more favourable conditions, remained almost at the same levels. In this case, the phytospreading period coincides with a major climatic period.
The outcome of both versions of phytospreading is hybridization (if it is indeed possible).
Analysing the behaviour and transformation of the lower taxa in the process of “reversible” or “repeated” phytospreading, it is possible to construct a model of the formation of a new, more vertebrate, lower taxon of species, building a cause-effect chain through the example of reversible phytospreading.
Life activities of organisms living in the subequatorial zone are subject to a circadian rhythm. Seasonality, of course, in one form or another appears, but it is not dominant. Referring to the modern pattern of natural areas, it appears that, in the equatorial belt (with an overall width approximately 30°, i.e. 15° to the north and south of the equator), the functioning of the biota complies with the circadian rhythm to the maximum extent.
At latitudes corresponding to the modern subtropical latitudes, the defining rhythm of all natural processes becomes annual (seasonal). Circadian rhythms also retain their influence on the life of the subtropical biota, but control only some aspects of the functioning of the organisms. In particular, the reproduction of offspring over time is largely “reoriented” to the yearly rhythms. This phenomenon, together with the initial cold spell, will lead to a change in the nature of ontogeny. The morphological characteristics and size of individuals will change; allometric changes will manifest. Finally, there will be an adjustment of the genetic apparatus, most likely in the form of consolidation of conventional adaptive mutations in the genotype due to the unfavourable environment. This refers more to the idioadaptation or the allomorphosis rather than to the aromorphosis of A.N. Severtsov (1939) . There are enough works regarding how unfavourable climatic conditions affect the life activities of organisms and there is no doubt that these climatic conditions are the cause for mass heterochronies (Raff and Kofmen, 1986 ). Heterochronies manifest themselves in different ways. As expected, the most significant evolutionary role is played by neoteny and the accompanying progenesis. In this context, it is important to emphasize that heterochrony is manifested in a certain increase in the duration of the reproductive process. For example, a well-known fact that has no direct relation to botany is that the duration of pregnancy of women of the African race is one week less than of Caucasoid women. Because this is a slow adaptive-type process, this heterochrony becomes the norm and is enshrined in the genotype.
What is the level of phylogenetic differences between these converging related groups? There is no exact answer, at least because “species” and “subspecies” do not have absolutely clear criteria. It is possible to say that the maximum level is the level of closely related species (species of the same genus) and the minimal level corresponds to subspecies or races. The prerequisite is the ability to interbreed and create hybrid offspring, with the final step of this stage being the emergence of hybrids able to produce the offspring.
To explain the present act of evolution, it is necessary to resort to contemplative modelling, which cannot be left behind by theoretical biology.
For this procedure, it is preferable to use the representatives of those species (genera, families), which, according to palaeobotanists and palaeontologists, are the most probable ancestors of the new and evolutionary, more “advanced” high-ranking taxa. However, the model of “biospreading hybridization” should be workable for all raw materials, so the choice of objects can be quite arbitrary.
A well-known homosporous bracken.
Its developmental cycle consists of two phases or states. The first, the sporophyte, and the second, the gametophyte. These formations cannot be combined. On the underside of some leaf plates of sporophyte sporangia, bracken spores, resulting from meiosis sporulation, appear. Bracken spores germinate on the ground in a small prothallus, such as in the case of Prothallia dioecious . On the bottom side of the prothallia, the genitals are formed — archegonia (female) and antheridia (male), where the fixed egg and sperm (mobile in water) are developed and insemination takes place with the participation of water. After insemination of the egg, leaf-shaped body sporophytes begin to grow and the cycle is completed.
By the beginning of the second stage, plants will have virtually no differences from their subequatorial ancestors. They will most likely form a continuous area that can be ruptured afterwards. Further, as a result of initial cooling, the so-called “zonal stratification” (the gradient pattern of climatic zones) is enhanced. Bracken individuals, which are already in the subtropical zone, undergo significant adaptive changes, including a form of heterochrony. This modified species (subspecies) begins to spread in an equatorial direction. As the geographical areas of the ancestral and modified species (subspecies) converge, a convergence of their morphological traits, caused by the increasing similarity in climatic conditions, will appear, and subsequently, it could facilitate cross-fertilization and the formation of hybrids capable of producing offspring.
Stage 3 — convergence of areas and hybridization. Gametes of individuals of the subequatorial species participate in the insemination of the eggs from individuals of the modified species. Notably, the reproductive cycle of the first group is slower and the sporulation velocity is higher. In this case, the new hybrid may have a peculiar neoteny, such as accelerated sporulation (“subequatorial scenario”) at the normal speed of vegetative growth corresponding to maternal individual (“subtropical” scenario) or normal or high-speed development of sporangia. There is no direct confirmation of the reality of this type of neoteny in the natural environment, and in the laboratory, it is impossible to artificially recreate the conditions of phytospreading hybridization. However, there is an indirect confirmation. Biologists are familiar with genomic imprinting, an epigenetic process by which the expression of certain genes is dependent on the parent sex that provided the allele. For example, the representatives of mammalian paternal genes are responsible for the formation of the placenta and female genes for the differentiation of embryonic cells (Haig and Westoby, 1989 ). This effect is also observed in insects and, in the vegetable world, in flowering plants. It is reasonable to assume that this phenomenon has a certain “fundamental” genomic basis, which is not yet entirely clear. To a certain extent, it is common to a wide range of species. V.A. Geodakyan (2012) defines this effect (in relation to placental mammals) in the following manner: in the beginning of embryogenesis, only the genes of the mother “work”, then the father genes step in and gradually progress, and in the final phase of embryogenesis, only the father genes “work”. Conclusion: the formation of a hybrid may proceed in accordance with the formula: the velocity of the development of the vegetative part corresponds to the maternal, the rate of sporangia development is higher compared with the vegetative part, and the maturation rate of sporules is higher than the rate of the development of the sporangia .
In the case that this scenario develops, instantaneous changes of the morphophysiological organization of the obtained hybrids will appear in comparison with both of the ancestral forms. First, there will be certain changes in leaf plates with sporangia on them. They will take the form of strobila. The separation (or even split) of microsporocytes and macrosporangia are likely to take place. Second, the sporules will germinate without leaving a sporophyte. The gametophyte phase of plant development sharply reduces. The outcome: a completely new species, reminiscent of a highly organized heterosporous fern-moss selaginella or a primitive gymnosperm appears. This is the result of the heterochrony that, according to R. Goldschmidt, affects the course of individual development and leads to the emergence of a new phenotype and a new species. However, its source is not a random systemic macromutation, rather a systemic hybridization.
If hybrids maintain the ability to interbreed with each other in the favourable conditions of equatorial climate, they will have a real chance of survival. Ecological and evolutionary advantages of the new species will manifest themselves afterwards. The genetic consolidation of this natural finding is a different issue. It can be solved in agreement with the traditional notions of STE.
A similar simulation can be carried out with gymnosperms with the purpose of obtaining angiosperms. However, it is no coincidence that the origin of flowering plants is recognized as one of the greatest mysteries of biology. If we take a pine or spruce as the paternal material and the outcome of the same contemplative modelling, it is possible to “transform” a fir cone into something resembling a bunch of bird cherries. Flower does not come across explicitly. More accurately, it is possible to generate a monosexual male flower from the male cones. To do this, it is necessary to assume that the maturation of gametes (pollen) is not accelerated, but is rather delayed (this effect is called retardation), and that the micro-strobilus, due to such a delay, will have no additional development, as is “prescribed” by the genome. It will appear as the outrunning growth of scales and sprouts, connecting the pollen sacs with an axis. Subsequently, these sprouts with the pollen sacs become stamens, while the scales become carpels. It is clear that such a scheme is too rough and primitive; nevertheless, its logical basis gives hope for detailing and elucidating the specific mechanisms for the conversion of strobili into flowers. However, most likely in this case (and not only), an important role is played by the reciprocal effect.
A well-known nimble or sand lizard. (Selection of the object, as in the first case, is random.)
By the initiation of the third stage (which is no longer phytospreading but rather zoospreading or biospreading), the areas of sub-equatorial and former mid-latitude species (subspecies) are enclosing. The process of hybridization involves female individuals of the subequatorial species and subtropical male individuals. Delayed expression of male genes in the final stages of embryonic development results in the incomplete formation of the hard shell of the embryo at the time of its full maturity; this is how the viviparity appears. Because the newborn hybrid organism is under stressful conditions, the long hypothesized transformation of some of the sebaceous or perspiratory glands into the lactescent glands seems very plausible. The body of the female attains a new function; a search for a special liquid to help save the life of the newborn, and this new function requires an appropriate organ. It had been forming for long enough (possible transitional form — simple monotreme mammals). Next, the adjustment and polishing of the new form goes through selection, where it will have one distinct potential advantage: the better preservation of offspring in unfavourable conditions. It is interesting that in this case, there is no need for the hypothesis of “hopeful monsters”, as proposed by R. Goldschmidt. Hybridization is going to be massive, and the problem of finding reproductive partners is not going to be as sharp as in the “monsters” hypothesis.
According to the results of contemplative modelling under such hybridization, there is the almost instantaneous and radical redesign of the process of reproduction and reproductive organs. Vegetative parts of the plants and animal soma are changing afterwards (it may be said “get adjusted”). In other words, the process of the shaping of a fundamentally new type, as a consequence of a heterochrony and hybridization, initiates with the reproductive organs, and gradually extends to the rest of the body. It does not happen in a single generation, but will be fast enough to regard the process of species emergence as a saltation.
Palaeoclimatic data generally does not contradict the hypothesis of either biospreading options. The credibility of the stratigraphic determination of gymnosperms and mammals is that it is possible to “adapt” to the cold climate phase (“reversible” biospreading) and the warm phase (“repeated” biospreading) (Konischev, 2005 ). Regarding the angiosperms, their appearance (Lower Cretaceous) is preceded apparently only by warming. However, it should be understood that the appearance of a new and innovative species does not necessarily occur at extreme climatic rhythms exclusively.
Does “reversible” or “repeated” biospreading always lead to progressive development, or in other words, to the morphofunctional complication of organisms? Certainly not. The genetic neutralization of the biospreading effect most likely occurs in convergence areas. Insufficient genetic adaptation (“overrun” or “shortage” of genetic differences between two related species by the beginning of the third stage) is plausible. Apparently, this is the primary and most difficult obstacle to overcome. Finally, even in the event the first positive biospreading effect is achieved, further progressive evolution can be disrupted by external factors (it is known there are viviparous lizards).
According to contemplative modelling, biospreading hybridization determines a sharp functional reorganization of a large group of individuals. Restructuring inevitably requires appropriate compensation in the form of similar morphofunctional changes or in the emergence of new bodies. Apparently in a crisis situation, the emergence of new organs is an optimal yield. The structure of the body responds to the sudden dissonance with proper functional complexity. The new organs appear in the area of functional “disorder”; the metamorphosis of tissue is a way to implement innovations.
From all of this, it can be concluded that the new species is likely to result in a phyletic line that will subsequently expand and thereafter will become parents for a range of lower taxa. Nevertheless, biospreading involves many species, including species that will be assigned to the same genus or even to a higher taxon. Thus, even a “new” hybrid species can comprise a community, which can be regarded by biologists as a single genus, family, and so on. Emergence of a new high-ranking taxon is likely to result in a monophyletic, but such a statement, according to V.A. Krasilov (1989) , cannot be unconditional. The ancestors of a new (morphophysiologically progressive) taxon, of a family rank or above, can be a single species (later diverging) forming various species of the genus and also species that constitute taxon of a higher rank.
Weaknesses of the proposed hypothesis (as of many others) are stipulated by the absence of irrefutable evidence. It all begins with the hypothetical character of the original process, phytospreading itself. It cannot be observed or modelled, either naturalistically or mathematically. However, the first step towards phytospreading was made by A. Wallace, a contemporary of Ch. Darwin (Tchaikovsky, 2008 ). He noted the following trend: relics that are remnants of ancient groups tend to occupy the outskirts of the former areas of their groups (deciduous forests occupied the warm countries, and coniferous were ousted to the “cold” edge of the temperate zone). After the work of S.V. Meyen, the effectiveness of phytospreading became almost self-evident. Moreover, the evidence of the zoospreading reality was disclosed (Chernov, 1988 ).
The idea proposed in this paper of a reversible or repeated biospreading, that is, the “completion” of biospreading over a full period, is even more hypothetical. However, this idea logically follows from the very nature of the change of palaeoclimates. For example, L.S. Berg used the glacial theory to explain the existence of bipolar (living in the temperate zones of both hemispheres) species of marine biota (Berg, 1922 ), which is gleaned from the assumption of reversibility of biospreading. The possibility for the morphophysiological complications of individuals of some species as a result of biospreading hybridization is a completely different matter. This mechanism cannot be proven nor refuted for the previously mentioned reason; the inability to artificially reproduce the biospreading genetic effect (i.e., reproduction of genomes of two closely related species in the beginning of the third stage). It is worth repeating that this type of hybridization is extremely rare; it most likely occurs once in a geological period.
The inability of neo-Darwinism to explain and predict macroevolution leads biology to a gradual and irreversible transformation into a purely technological discipline aimed at decoding the genome, the study of mutations and cloning, and so on. This trend is opposed by a relatively small number of theoretical biologists looking for ways to explore beyond the orthodoxy of neo-Darwinism. Directions of “breakthroughs” can be different for different ecosystems (Tchaikovsky, 2008 ), epigenetics (Nazarov, 2007 ), ad nomogenetics (Nazarov, 2007 ; Tchaikovsky, 2008 ). This work is based on the ideas of S.V. Meyen and complements the concept of nomogenesis. However, even if the facts of the existence of reversible and (or) repeated biospreading and the possibility of biospreading hybridization with the formation of morphophysiologically “advanced” species will be confirmed, other fundamental problems will remain unresolved, such as why “it” occurs and why the complication occurs.