Under the influence of elementary factors in the gene pool, the frequency of certain genes changes, which leads to a change in the genotype and phenotype of the population, and with prolonged exposure to natural selection, its differentiation proceeds.
What is microevolution?
Microevolution - changes in the population under the influence of evolutionary factors, which can lead to a change in the gene pool or even to the emergence of a new species.
Factors of evolution can be called any processes or phenomena. Among them are mutations, isolation, gene drift, population waves that change the genetic composition.
The sizes of any population are constantly changing. The reasons for this are various biotic and abiotic influences. Such fluctuations in the population are periodic. So, after an increase in the number of individuals in the population, it decreases. In 1905, S. S. Chetverikov called this regularity population waves. If you give examples of population waves, it may be fluctuations in the number of predator victims, breeding locusts or rabbits in Australia. An example is the outbreak of lemmings in the Arctic or the plague epidemics that were recorded in Europe in the past.
The characteristic of the "waves of life"
These waves are characteristic of all living organisms. They can be periodic and non-periodic. Periodic are most often observed in short-living organisms - in insects, annual plants, as well as in most microorganisms and fungi. The simplest example may be seasonal changes in numbers.
Non-periodic population waves depend on a combination of several complex factors. As a rule, they concern not one, but several types of living organisms in a biogeocenosis, therefore, they can lead to radical reorganizations.
Among the changes in the number of individuals in the population, the sudden appearance of certain types of organisms in new areas where their natural enemies are absent should be highlighted. We should also mention the sharp non-cyclic changes in numbers that are associated with natural "disasters" and can manifest themselves as a destruction of the biogeocenosis or the whole landscape. Thus, several arid summer periods can change a significant territory - cause the appearance of meadow vegetation in the swamps and a large number of dry meadows.
If we indicate the causes of population waves, then it is worth recalling not only the relationship of living organisms with each other and with environmental factors, but also the influence of man.
The evolutionary meaning of “waves of life”
In cases where the population of any population decreases sharply, only a few individuals may remain. Moreover, their frequency of genes (alleles) is different from that in the original population. If after a sharp decline in the population in the population there is a sharp rise, then the beginning of a new outbreak of growth in the number of individuals in the population gives a small group of organisms that remain. That is why it can be argued that population waves affect the gene pool, since the genotype of this group determines the genetic structure of the entire population.
In this case, the set of mutations in the population and their concentration abruptly change dramatically. So, a certain part of the mutation disappears altogether, and some suddenly grow. To summarize, we can say that population waves as an evolutionary factor are extremely important, since, subject to intensive selection, they are the main supplier of evolutionary material when rare mutations are substituted by selection.
In addition, the waves of life can temporarily remove a series of mutations or genotypes into another abiotic or biotic environment. Despite this, even a combination of population waves and mutations does not ensure the evolutionary process. We need the action of a factor that affects in one direction (this, for example, isolation).
Effect of isolation on population size
This factor is extremely important in the evolutionary plan, since it provokes the appearance of new characters in the conditions of one species and does not allow the crossing of different species among themselves. It is worth noting that most often there is geographical isolation. Its essence lies in the fact that the only range is torn, while the intersection of individuals from different parts of it becomes impossible or difficult.
It is worth noting that mutations accidentally develop in an isolated population, and as a result of natural selection, its genotype is becoming more diverse. In addition, there is environmental isolation and various biological mechanisms that prevent individuals of different species from freely interbreeding. An example may be different preferences regarding the place or time of crossing, as well as, for example, excellent behavior or the different structure of the genitals in animals, which becomes an additional obstacle to crossing.
To summarize, we can say that different types of isolation contribute to the formation of new species, but at the same time help maintain the genetic species structure.
Gene drift
A random change in the number of genes in any small population can have significant consequences, since it can lead to a change in allele frequency. Random changes in allele frequency are called gene drift. This process is non-directional. It was first discovered by geneticists N.P. Dubinin and D.D. Romashov.
Confirmation regarding the randomness of gene drift was received by S. Wright. In laboratory conditions, he crossed females and male Drosophila, which were heterozygous for a particular gene. After that, the offspring was obtained with a concentration of the normal and mutant gene, which amounted to 50%. After several generations, some individuals became homozygous for the mutant gene, some lost it altogether, and another part of the individuals had both a mutant and a normal gene.
It should be noted that even with reduced viability of mutant individuals and under the influence of natural selection, the mutant allele was able to completely replace the normal one, causing specific population waves.
Etiology of population waves
Of all the reasons that can affect the quantitative characteristics of a population, climatic conditions occupy a leading place, and biotic factors are relegated to the background. With a low species diversity, the number of individuals in a population depends on the weather, the chemical composition of the environment, and also on the degree of its pollution.
It is worth noting that the causes of population waves, which determine the change in the population size, depend on its density or influence independently of this parameter.
Abiotic and anthropogenic factors, as a rule, do not depend on population density. Biotic influence is more dependent on it. It should be noted territorial behavior, which during evolution is the most effective mechanism that inhibits the growth of the number of individuals in the population. So, the activity of individuals is limited by the corresponding space. With an increase in numbers, intraspecific competition for resources or direct antagonism develops (attack on competitors).
Population waves also depend on behavioral reactions, which, with a high population size, are characterized by the appearance of the instinct of mass migration. A stress reaction may also develop in which individuals develop physiological characteristics that reduce fertility and increase mortality. So, the process of ovogenesis and spermatogenesis is disrupted, cases of miscarriages are becoming more frequent, the number of individuals in one generation decreases and the period of puberty increases. In addition, the instinct of caring for offspring decreases, behavior changes - aggressiveness grows, cannibalism and an inadequate reaction to people of the opposite sex can be observed, which ultimately reduces the population size.
Features of population changes
Many ecological processes associated with the distribution of the population over the range or with a local outbreak of the population resemble peculiar waves, which, as mentioned above, are called “waves of life”. A typical example is the sudden increase in the number of pests in a limited forest area. Under favorable conditions, insects are able to capture more and more new territories, which is a typical picture of an increase in their density or the spread of the so-called population wave. Knowing the characteristics of mobility and certain population characteristics, one can easily calculate the speed of propagation of this wave and possible methods of control.
Similarly, epidemic waves can be characterized, therefore, this theory is successfully used to determine the nature of the spread of various diseases and the speed of this process.
In addition, mention should be made of population-genetic waves, with the help of which they describe the nature of the distribution of a particular gene over the range occupied by a particular population.
The mechanism of action of population waves
Population waves can be characterized using a model example. So, in a closed box there are 500 black and the same number of white balls, which corresponds to the frequency of the P-0.50 alleles. If you randomly remove 10 balls and assume that 4 of them are black in color and 6 are white, then, accordingly, the frequency of alleles will be 0.40 and 0.60.
If you increase the number of balls by 100 times, adding 400 black and 600 white, and then again randomly pick up any 10, then it is likely that their color ratio will differ significantly from the original, for example 2 black and 8 white. The frequency of alleles will be respectively P-0.20 and P-0.80. If we take the third sample, then it is likely that 9 white balls from 10 favorites will be extracted, or even all of them will have a white color.
From this example, one can judge the random fluctuations in the frequency of alleles in natural populations, which can reduce or increase the concentration of a particular gene.