Population waves as an evolutionary factor. Causes of population waves

Under the influence of elementary factors, the frequency of certain genes changes in the gene pool, which leads to a change in the genotype and in the phenotype of the population, and with the long-term effect of natural selection, its differentiation passes.

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 the appearance of a new species.

Evolution factors can be called any processes or phenomena. These include mutations, isolation, gene drift, population waves that alter the genetic makeup.

The size of any population is constantly changing. The reasons for this are various effects of biotic and abiotic nature. Such fluctuations in population size are of a periodic nature. So, after an increase in the number of individuals in the population, it decreases. In 1905 SS Chetverikov called this pattern the population waves. If we cite examples of population waves, then this may be fluctuations in the number of prey victims, the breeding of locusts or rabbits in Australia. An example is also the outbreaks of lemmings in the Arctic or plague epidemics that were recorded in Europe in the past.

Characteristic of "waves of life"

These waves are typical for all living organisms. They can be periodic and 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 can 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 species of living organisms in the biogeocoenosis, therefore they can lead to radical restructuring.

Among the changes in the number of individuals in a population, one should single out the sudden appearance of certain species of organisms in new areas where their natural enemies are absent. Also, mention should be made of sharp non-cyclic changes in numbers, which are associated with natural "catastrophes" and can be manifested by the destruction of the biogeocoenosis or the whole landscape. So, several dry summer periods can change a significant territory - to 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 interconnection of living organisms with each other and with environmental factors, but also the influence of man.

The evolutionary meaning of the "waves of life"

In cases where the population of any population is sharply reduced, only a few individuals may remain. In this case, the frequency of genes (alleles) differs from that of the original population. If, following a sharp decline in the number of the population, the population grows sharply, then the beginning of a new outbreak of population growth in the population is caused by a small group of organisms that has remained. That is why it can be argued that population waves affect the gene pool, as 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 change quite accidentally. So, a certain part of the mutations disappears altogether, and some suddenly grow. If we generalize, then we can say that population waves as an evolutionary factor are extremely important, since under the condition of intensive selection are the main supplier of evolutionary material, when rare mutations are substituted for selection.

In addition, the waves of life can 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 an action of a factor that affects one direction (this is, for example, isolation).

The effect of isolation on population size

This factor is extremely important in the evolutionary plan, since it provokes the appearance of new characters under conditions of the same species and does not allow the cross-breeding 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 area is torn, and the intersection of individuals from different parts of it becomes impossible or difficult.

It should be noted that in an isolated population random mutations develop, and as a result of natural selection, its genotype becomes more and more diverse. In addition, there is an ecological 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 different structure of the genital organs in animals, which becomes an additional obstacle to crossing.

If we generalize, we can say that different types of insulation contribute to the formation of new species, but at the same time help preserve the genetic structure of the species.

Drift of genes

A random change in the number of genes in any small population can have noticeable consequences, since it can lead to a change in the frequency of alleles. Random changes in the frequency of alleles are called gene drift. This process has an undirected character. It was first discovered by the geneticists NP Dubinin and DD Romashov.

Confirmation of randomness of gene drift was obtained by S. Wright. He crossed the female and Drosophila males in the laboratory, which were heterozygous for a particular gene. After this, a progeny with a normal and mutant gene concentration was obtained, which was 50%. After several generations, some individuals became homozygous for a mutant gene, some even lost it, 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 subject to the influence of natural selection, the mutant allele was able to completely displace the normal, 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, biotic factors are assigned to the second plan. With a low species diversity, the number of individuals in a population depends on the weather, the chemical composition of the environment, and the degree of contamination.

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. Biotic influence depends more on it. It should be noted territorial behavior, which in the course of evolution is the most effective mechanism, which restrains the growth of the number of individuals in the population. So, the activity of individuals is limited to the corresponding space. With an increase in the number of developing intraspecific competition for resources or direct antagonism (attack on competitors).

Population waves also depend on behavioral responses, which, with a high population size, are characterized by the emergence of a mass migration instinct. A stress response can also develop in which individuals develop such physiological characteristics that reduce fertility and increase mortality. So, the process of ovogenesis and spermatogenesis is disrupted, the incidence of miscarriages increases, the number of individuals in one generation decreases and the period of puberty increases. In addition, the instinct of caring for the offspring decreases, behavior changes - aggressiveness grows, cannibalism and inadequate response to people of the opposite sex can occur, which ultimately reduces the population size.

Peculiarities of changes in the number of populations

Many environmental processes associated with the distribution of the population in the area or with a local outbreak resemble the kind of waves that, as mentioned above, were called "waves of life". A typical example is a sudden increase in the number of insect 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 increasing their density or the spread of the so-called population wave. Knowing the characteristics of mobility and certain population characteristics, you can easily calculate the speed of propagation of this wave and possible methods of struggle.

Similarly, epidemic waves can be characterized, so this theory is successfully used 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 specific gene in the range that a particular population occupies.

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 alleles P-0.50. If you delete at random 10 balls and assume that 4 of them have a black color and 6 - white, then, respectively, the allele frequency will be 0.40 and 0.60.

If you increase the number of balls by a factor of 100 by adding 400 blacks and 600 whites, and then again randomly picking any 10, then the likelihood that their color ratio will differ significantly from the original, for example 2 black and 8 white. In this case, the frequency of the alleles will be respectively Р-0,20 and Р-0,80. If we take the third sample, then there is a probability that 9 white balls will be extracted from the 10 chosen ones, or even all of them will have a white color.

In this example, one can judge random fluctuations in the frequency of alleles in natural populations that can reduce or increase the concentration of a particular gene.