Fundamentals of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 5. Population Waves

5.2. Evolutionary Significance of Population Waves

Population Waves (referred to as waves of life by S.S. Chetverikov), much like the mutation process, are considered providers of elementary evolutionary material.

At the same time, it should be understood that they do not create new material on their own; rather, their evolutionary significance lies in

altering the genotypic composition of a population. They are sometimes accompanied by a relatively rapid expansion or contraction of the species' geographic range, or a significant part of it.

Extensive experimental research has established that different genotypes possess distinct ecological characteristics. A case in point is the previously mentioned Adaliabipunctata; according to N.V. Timoféev-Ressovsky, its dark morph exhibits higher winter mortality, yet this is compensated for by a higher fecundity compared to the light morph. Similar results have been obtained for other organisms, such as the mutant form of the red flour beetle (Tribolium castaneum). As population density increases, survival drops more sharply than in the wild type. However, the reduction in fertility among mutants occurs more gradually, thereby maintaining population polymorphism under any circumstances.

The heritability of traits depends entirely on the Specific features of a population—it transforms in response to changing environmental conditions and population states. Differences among individuals and populations alike do not necessarily indicate any genetic advantage (as mentioned earlier, the "best" variant in one environment may prove to be the "worst" in another).

Allele frequency fluctuations depend largely on population size. Constancy of allele frequencies across generations is possible only when population numbers are sufficiently large. The smaller the population size, the greater the probability of random fluctuations, which can lead to phenomena such as Genetic Drift (shifts in allele frequencies). Driven by this process, genetic differences among small isolated populations may increase, while within each population, homozygosity rises As a result of Inbreeding. Consequently, the stability of a population's genotypic Structure depends directly on its size.

Thus, even population size fluctuations (particularly population crashes) can inherently trigger unpredictable shifts in the frequency distribution of individual alleles across generations through genetic drift.

To this, one must also add Changes in the concentration of particular traits driven by the waves themselves. For instance, if a population of 100 individuals contained only two individuals with a new trait (corresponding to 2%), and the concentration remained unchanged during population growth, then out of 1,000 individuals, that

trait would be found in 20 individuals. Conversely, if a population wave receded, bringing numbers back to their original level while all 20 individuals bearing the new trait survived, the concentration of this trait would surge from 2% to 20%, thereby making natural Selection acting upon this trait significantly more effective.

In populations experiencing catastrophic bottlenecks, allele frequency concentrations invariably diverge from their initial baseline. In the process, certain traits previously present in negligible concentrations may vanish altogether or, conversely, become widespread as the surviving individuals reproduce. In other words, random fluctuations in genotype concentrations occur, allowing previously rare traits to enter the evolutionary arena. It is well established that traits present in low concentrations within a population are virtually impervious to natural selection. Therefore, population waves are viewed as providers of elementary evolutionary material because they expose genotypes of initially low frequency to the action of natural selection.

The Evolutionary Significance of population waves may also lie in the fact that population growth typically coincides with a relaxation of the Struggle for Existence, whereas a population decline intensifies it, which to some extent modulates the intensity of natural selection. When population waves are accompanied by fluctuations (regular or semi-regular oscillations in both positive and negative directions) or a dramatic expansion of the geographical range, certain genotypes are temporarily introduced into novel environmental conditions. Although this exposure is transient, it facilitates the testing of these genotypes and their subsequent integration into the evolutionary process. Nevertheless, precisely because population waves are random and non-directed in their action, they cannot be considered a self-sufficient driving force of evolution.