In nature, there are constantly a lot of events that affect the gene pool of any population of organisms. And they all relate to the driving forces of evolution. In their capacity, Charles Darwin singled out natural selection and the struggle for existence.
Modern biologists include gene drift, population waves, and mutation frequency as the driving forces of evolution. Refinements and additions to the theory of evolution became possible after the development of molecular biology and decoding of genomes. What factors relate to the driving forces of evolution, according to modern synthetic theory, we consider in this article.
Heredity: nuclear and cytoplasmic
The property of all living organisms to transmit traits from generation to generation (heredity) is not in vain attributed to the driving forces of evolution. It is heredity that ensures the continuity and consolidation of valuable adaptations for the survival, reproduction and discreteness (individuality and diversity) of species. In this case, the entire set of chromosomes (genotype) in the nucleus of an organism’s cell acts as a material of evolution. In addition, some organelles of the cell have their own ring-shaped DNA, which are inherited independently from the mother to the descendants (plastids in plants and mitochondria in all living organisms).
Variability is the key to species diversity
The driving forces of evolution also include the ability of descendants to acquire characteristics that parental forms did not have. But not all variability leads to the consolidation of new features in the genotype. Phenotypic variability, as a factor of adaptability to the environment, does not affect the gene apparatus, but is a form of manifestation of the genotype in the phenotype and is within the limits of the norm of the response of the trait. It does not belong to the driving forces of evolution. In the context of our article, genotypic variability (mutational and combinative), with changes in the genotype, is of interest.
Variability combinative
This type of variation is directly related to the sexual process and is expressed in independent chromosome divergence and crossing-over processes (exchange of sites between homologous chromosomes) as a result of meiosis during the formation of germ cells (gametes). It is the various combinations of genes and their alleles in the gamete genome and the appearance in sexual reproduction that accelerated evolutionary processes on the planet and have become a significant gain in facilitating adaptability to environmental conditions for panmicic (sexually reproducing) organisms.
Genomic Level Mutations
The largest type of mutation process that changes the entire genome (a set of genes) without affecting the structure of the chromosomes.
- Polyploidy is an increase that is a multiple of the haploid (n) set (3n, 4n, 5n, 6n, 7n, and so on) of the number of chromosomes in the body. This type of mutation is inherent in many plants and simple animals.
- Aneuploidy is the appearance of extra or loss of chromosomes as a result of violations in the passage of meiosis. As a result, an organism with a complete set of chromosomes (2n) develops monosomy (2n-1), trisomy (2n + 1), or nullisomy (2n-2). Most often, such individuals are not viable or are carriers of severe genetic diseases (Down syndrome in humans is associated with the presence of a third chromosome in 21 pairs).
Chromosomes and their mutations
In this case, as a result of violations in the passage of gametogenesis (gamete formation), rearrangements occur in the structure of the chromosomes themselves. Such mutations alter the functioning of gene combinations, less often of individual genes, but do not affect the change in the number of chromosomes. There are a lot of types of mutations at this level. We call only duplication (doubling) and deletion (loss) of the chromosome region.
Gene level mutations
These are mutations of the smallest scale - a point change in one gene. It is this type of mutation that is most often referred to as the driving forces of evolution, since they contribute to an increase in the number of new alleles in the genotype and diversity within the species. Changes in one gene leads to a change in one or more (with multiple effects) traits, increasing the variability of phenotypes. As such mutations accumulate in the population, they become an evolutionary factor.
Waves of numbers
A sharp increase in the number of individuals or its catastrophic reduction is called the waves of life or population waves. Changes in numbers can occur as a result of many factors (fires, volcanoes, epidemics, the disappearance of natural enemies). But all of them are random in nature and lead to changes in the gene pool of the entire population, when outsiders can be at the forefront and vice versa.
Isolation as a factor and driving force of biological evolution
Isolation as a factor in restricting free cross-breeding between populations of one species of panmictic organisms is a clear sign of the effect of this evolutionary factor. Most species on the planet appeared due to the emergence of reproductive isolation of populations. The following types are distinguished:
- Spatial (geographical, man-made).
- Biological (environmental, morphological, ethological, genetic).
In any case, when a barrier between free populations arises between populations, we can talk about the beginning of speciation.
The struggle for existence as a tool of natural selection
The instrument of natural selection is the struggle for existence, when only an organism more adapted to these conditions will survive and leave the prolific offspring. Their struggle for existence happens:
- Inside - the most cruel and implacable. The competition of representatives of one species for food resources, territory, better living conditions and the ability to leave offspring leaves no chance for weak and unadapted individuals.
- Between representatives of different species, but occupying the same ecological niche. As an example, competition for plant foods of giraffes and zebras led to physiological features during the evolution process, minimizing competition.
Fighting organisms with adverse conditions. Example: fat humps of a camel and fleshy leaves of succulents as adaptive mechanisms of life in the desert. Or the luminous organs of deep-sea fish.