According to the mentioned above provisions, a variety of species possessing new specific features allowing a better adaptation to the environment (habitat) can be formed within each species. If the newly acquired qualities are inherited, then genetic differences are amplified in subsequent generations due to the selection of properties that ensure the genotypes’ survival and the new mutation’s accumulation. The mentioned above variety of species adapts better to existing conditions, phenotypically moving away from the parent species. Intraspecific competition leads to the deliberate (selective) removal of the least adapted to the environment individuals and an increase in the number of individuals whose properties favour the survival and reproduction in this habitat. According to the evolutionary theory proponents, such a natural selection is the main mechanism for the new species’ occurrence. Evolution is a process of long-term and gradual qualitative changes that ultimately result in the new species’ occurrence.

According to some evolutionary theory’s advocates, the palaeontological, biogeographic, systematics, plant and animal breeding, morphological, comparative embryology and comparative biochemistry data confirm the evolutionary occurrence of species through natural selection, although they are not indisputable evidence. Three groups of facts are used to confirm the theory. The first group represents examples of species’ minor changes observed within a geologically short period of time in the wild nature, as well as the results of agricultural plants and domestic animals’ breeding. The second group is the fossil record, which indicates a significant variability of life throughout the history of Earth. The third group of evidences deals with the similarity of the morphological features of the all organisms’ structure, which may indicate their common origin. All explanations of the facts discovered are reduced to a spontaneous random process without any intellectual interference in the programming system.

Charles Darwin formulated his theory of evolution in accordance with the morphological and physiological characteristics, using the natural selection ideas, but did not determine the source of variability in the population. A synthetic theory of evolution combining the Darwinian natural selection’s idea with the laws of heredity and population genetics was developed in the middle of the last century. Currently, it is the most developed system of ideas regarding the speciation’s processes. The hypothesis of the new genes’ recessivity was the impetus for its development; according to it mutations constantly arise in each reproducing group of organisms during the gamete maturation as a result of errors in the DNA replication. Therefore, the mutation process is the most important evolutionary factor, and the bulk of evolutionary material is represented by various forms of mutations, which are manifested by means of changes in the hereditary properties of organisms that occur naturally or are caused by artificial means. After the different types of isolation’s occurrence between populations, they begin to evolve independently, and as a result, genetic differences are gradually accumulated between them, and with time genetic incompatibility is achieved, and crossing becomes impossible.

Julian Huxley, the English biologist and naturalist, indicates in his famous book «Evolution: The Modern Synthesis» (1942) that the species is a system of populations that are reproductively isolated from other species’ populations, and each species is ecologically isolated; speciation lies in the genetic isolating mechanisms’ occurrence and is carried out mainly in conditions of geographical isolation. According to the J. Huxley’s ideas, reproductive isolation is the main criterion that indicates the speciation’s completion.

According to the synthetic theory of evolution, the formation of new species occurs as a result of the separation of individuals of one species into groups that do not interbreed, and the very evolution is defined as the populations’ genetic structure change over time. The allele’s frequency changes thus becoming more or less common compared to other forms of this gene. The acting evolution forces lead to changes in the allele frequency in this or that direction. The change disappears when the new allele reaches the fixation point − it completely replaces the ancestral allele or disappears from the population. Mutations increase the population’s variability due to the emergence of new genes’ allelic variants − mutational variability. If any allele increases the organism’s adaptability more than other ones of the given gene, then the share of this allele in the population will increase with each generation, i.e., selection takes place in favour of this allele. Evolution through the natural selection is a process in which mutations increasing the organisms’ adaptability are fixed. As a result, three processes are necessary for the evolution’s implementation: mutational (to generate new variants of genes with a low phenotypic expression); recombination (to create new phenotypes of individuals), and selection (to determine the compliance of these phenotypes with the given living conditions or growth).

Judging by the palaeontological chronical and by the mutations’ speed indexes, this concept advocates believe that it takes in average 3 million years to reach the complete incompatibility of genomes, which makes crossing impossible. Therefore, it is a rear event to witness the new specie’s formation in natural environment.

In addition to mutational, there is also the combinatorial variability distinguished, which is determined by recombination, but it leads not to the allele frequencies’ changes but to their new combinations.

The gene drift is one more factor contributing to the allele frequencies’ changes.

The synthetic theory of evolution differs from the Ch. Darwin’s evolution in the following points:

1. It distinguishes a population in which the same species individuals are able to interbreed, and but not an individual or a separate species.

2. It considers a steady change in the population genotype’s change the process of evolution.

3. Mutational processes and isolation are treated as the leading factors.

4. The mutational and recombinative variability are the material for evolution.

5. Natural selection is considered as the main reason for the adaptations and speciation’s development.

In accordance with the synthetic theory of evolution speciation is a time-consuming process. However, J. B. S. Haldane found a discrepancy between the real speciation speed and the expected one based on the population genetics models (Haldane’s dilemma) by means of mathematical calculations. He published an article «Cost of Natural Selection» on the basis of his research in the «Journal of Genetics» scientific edition. D. Haldane calculated the mathematical relationship between the intensity of selection and rate of the existing alleles’ substitution in the population by other, more adapted ones. He also evaluated the mortality rate caused by the positive natural selection, while maintaining the mutant gene. According to his calculations, the speciation would have taken much more time for the stage-to-stage formation than it has actually taken (according to the paleontological data) for the implementation of speciation would require much more time than is actually observed (according to palaeontological data).

Later, M. Kimura, while studying the rate of amino acid substitutions in proteins, found that for mammals the substitution rate for the genome per generation was several hundred times higher than the Haldane’s estimation. Kimura showed that in order to maintain a constant population size while preserving mutational substitutions, the rate of occurrence of which stands to one substitution in two years, each parent should produce 3.27 106 descendants so that one of them survives and begins to breed. The mismatch of this impressive number with real data served as the basis for the «Neutral Molecular Evolution Theory» development.

Another argument in favour of this theory’s development was the fact that the assumption regarding more frequent occurrence of favourable mutations (in reality, such mutations are rather rare compared to the adverse ones) was necessary in order to explain the molecular evolution rate provided it proceeds under the natural selection influence.

The «neutral evolution» hypothesis’ main content lies in the point that majority of the changes at the macromolecular level are not controlled by natural selection as Darwin’s theory states, but are determined by the random drift of neutral mutations. According to the authors, this theory is proved by a number of direct and indirect arguments. At the same time, the neutral molecular evolution theory does not turn down the role of natural selection in the development of life on Earth, but emphasizes the proportion of mutations that possess adaptive significance. This theory has demonstrated that the processes associated with speciation are still far from a final explanation based on population-genetic models.

Modern synthetic theory of evolution has a number of disadvantages. It makes no distinction between macroevolution and microevolution, considering one the continuation of other one on a larger scale. Such view’s advocates have detected genetic changes from the population’s original composition from generation to generation, in various laboratory scientific experiments, including the model organisms’ development (drosophila, mice, and bacteria). It gave the grounds to assume that both microevolution and macroevolution are based on the same mechanisms and, in their view, minor changes can lead to significant ones over time, but the authors provide no evidences to prove it. Experts have no doubts regarding the microevolutionary processes within the species, for example, the hair colour or skin colour change, the beak shape, etc., due to which many types of breeds and sorts can exist within the same species. However, microevolution is determined by the alleles frequency’s change in a population (i.e. genetic variability due to such processes as selection, mutation, genetic drift), while the macroevolution involves changes at the species level or higher.

Fred Hoyle argued in his books «The Intelligent Universe» (1983), «Mathematics of Evolution», «Evolution from Space» (1981) and «Why Neo Darwinism Does Not Work» (1982), that the calculation results state that the Neo-Darwinism theory either does not work at all, or works only partially. Many of the common arguments that biologists use to confirm evolutionary theory turn out to be doubtful.

Academician Yu. P. Altukhov (Алтухов Юрий Петрович), a Director of the Russian Academy of Sciences’ Institute of General Genetics named after N. I. Vavilov, the «Successes of Modern Biology» journal editor-in-chief, points out in his book «Genetic Processes in Populations» (2003) criticizing the synthetic theory of evolution:

«In any case, it is becoming more and more obvious that evolutionary consequences are not necessarily concluded from the results of a study of both natural and experimental populations with their systemic organization taken into account. On the contrary, it confirms once again that the genetic variability of the simplest populations, traditionally regarded as the evolutionary process’s elementary units, is nothing but a stabilization mechanism for the species’ hierarchical, historically formed structure. Such a conclusion contrasts with the population genetics’ tradition, which has always been focused on the dynamics of populations, identifying it with the very evolutionary process».

However, the synthetic theory of evolution still remains the only scientific model to explain the known facts of the species’ origin and development. At the same time, the contradictions accumulated require the development of other theoretical concept of the of life occurrence on Earth.