Under the action 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 takes place.
What is microevolution
Microevolution - population changes under the influence of evolutionary factors, which can lead to a change in the gene pool or even the emergence of a new species.
Factors of evolution can be called any processes or phenomena. Among them are mutations, isolation, genetic drift, population waves that change the genetic composition.
The size of any population is constantly changing. The reasons for this are various influences of a biotic and abiotic nature. Such population fluctuations are periodic. So, after an increase in the number of individuals in a population, it decreases. In 1905, S. S. Chetverikov called this regularity population waves. If you give examples of population waves, then these can be fluctuations in the number of predators, the reproduction of locusts or rabbits in Australia. Another example is the outbreaks of lemmings inArctic or plague epidemics that were recorded in Europe in the past.
Characteristic of the "waves of life"
These waves are characteristic of all living organisms. They can be periodic or non-periodic. Periodic are most often observed in short-lived organisms - in insects, annual plants, as well as in most microorganisms and fungi. The simplest example would 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 restructuring.
Among the changes in the number of individuals in a population, one should highlight the sudden appearance of certain species of organisms in new areas where their natural enemies are absent. We should also mention sharp non-cyclical changes in the population, which are associated with natural "catastrophes" and can be manifested by the destruction of the biogeocenosis or the entire landscape. So, several dry summer periods can change a significant area - cause the appearance of meadow vegetation in swamps and a large number of dry meadows.
If you indicate the causes of population waves, then it is worth remembering not only the relationship of living organisms with each other and with environmental factors, but also the influence of man.
Evolutionary meaning of the "waves of life"
In cases where the size of any population declines sharply, only a few individuals may remain. At the same time, their frequency of genes (alleles) is different from that which was in the original population. If after a sharp decline in the number of individuals 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 is given by a small group of organisms that has remained. That is why it can be argued that population waves affect the gene pool, since the genotype of a given group determines the genetic structure of the entire population.
At the same time, the set of mutations in the population and their concentration change dramatically by chance. So, a certain part of the mutations disappear altogether, and some suddenly grow. To summarize, we can say that population waves as an evolutionary factor are extremely important, since, under the condition of intensive selection, they are the main supplier of evolutionary material, when rare mutations are substituted for selection.
In addition, the waves of life are able to temporarily bring a number 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. You need the action of a factor that affects in one direction (this is, for example, isolation).
The impact of isolation on population size
This factor is extremely important in evolutionary terms, because it provokes the emergence of new traits in the conditions of one species and prevents the crossing of different species with each other. It is worth noting that geographic isolation is most often observed. Its essence lies inthe fact that the only area is torn, while the intersection of individuals from different parts of it becomes impossible or difficult.
It is worth noting that in an isolated population, mutations randomly develop, and as a result of natural selection, its genotype becomes more and more diverse. In addition, there is ecological isolation and various biological mechanisms that prevent individuals of different species from interbreeding freely. An example would be different preferences regarding the place or time of crossing, as well as, for example, different behavior or different structure of the genital organs in animals, which becomes an additional obstacle to crossing.
Summarizing, different types of isolation promote the formation of new species, but at the same time help to maintain the genetic structure of the species.
Gene drift
A random change in the number of genes in any small population can have significant consequences, as it can lead to a change in allele frequency. Random changes in allele frequency are called genetic drift. This process is non-directional. It was first discovered by geneticists N. P. Dubinin and D. D. Romashov.
S. Wright received confirmation regarding the randomness of genetic drift. In the laboratory, he crossed female and male Drosophila, which were heterozygous for a particular gene. After that, offspring were obtained with a concentration of the normal and mutant gene, which was 50%. Throughfor several generations, some individuals became homozygous for the mutant gene, some lost it altogether, and another part of the individuals had both the mutant and the normal gene.
It should be noted that even with a reduced viability of mutant individuals and under the influence of natural selection, the mutant allele could completely replace the normal one, causing specific population waves.
Etiology of population waves
Of all the reasons that can affect the quantitative characteristics of the population, the leading place is occupied by climatic conditions, while biotic factors are relegated to the background. With low species diversity, the number of individuals in the population depends on the weather, the chemical composition of the environment, as well as the degree of pollution.
It is worth noting that the causes of population waves, which predetermine the change in 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. The biotic influence is more dependent on it. It should be noted territorial behavior, which in the course of evolution is the most effective mechanism that restrains the growth in the number of individuals in a population. So, the activity of individuals is limited to the corresponding space. With an increase in numbers, intraspecific competition for resources or direct antagonism (attacks on competitors) develops.
Population waves also depend on behavioralreactions that, with a high population, are characterized by the appearance of an instinct for mass migration. A stress response can also develop, in which individuals develop physiological features that reduce fertility and increase mortality. So, the process of oogenesis and spermatogenesis is disturbed, cases of miscarriages become more frequent, the number of individuals in one generation decreases and the period of puberty increases. In addition, the instinct to care for offspring decreases, behavior changes - aggressiveness grows, cannibalism and an inadequate reaction to persons of the opposite sex can be observed, which ultimately reduces the population.
Features of changes in the number of populations
Many ecological processes associated with the spread of a population over an area or with a local outbreak of numbers resemble peculiar waves, which, as mentioned above, are called "waves of life". A typical example is a sudden increase in the number of insect pests in a limited area of the forest. 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 traits, one can easily calculate the propagation speed of this wave and possible methods of control.
Similarly, epidemic waves can be characterized, so this theory is successfullyused to determine the nature of the spread of various diseases and the speed of this process.
In addition, we should mention population-genetic waves, which describe the nature of the distribution of a particular gene over the area occupied by a particular population.
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 alleles P-0, 50. If we remove 10 balls at random and assume that 4 of them are black and 6 are white, then, respectively, the allele frequency will be 0.40 and 0.60.
If you increase the number of balls by 100 times by 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 whites. In this case, the allele frequency will be P-0.20 and P-0.80, respectively. If we take the third sample, then there is a chance that 9 white balls will be drawn out of 10 selected ones, or even all of them will be white.
Random fluctuations in the frequency of alleles in natural populations can be judged from this example, which can reduce or increase the concentration of a particular gene.
