Introductory Chapter: Population Genetics

1. Introduction

Population genetics is a science that studies the genetic composition and distribution, as well as its effects, through mathematical formulas and indicators for measuring genetic diversity. It aims to evaluate allele, phenotypic and genotypic frequencies in populations of living beings; allowing to understand the origin and dynamics of genetic variation, making it possible to make predictions about the influences of one or more evolutionary processes on these compositions over generations. In this context, population genetics seeks to find an evolutionary meaning to explain genetic variation in living beings and better understand the evolutionary mechanisms that act on it.

Understanding the genetic diversity of populations has several uses, such as monitoring pathogens and vectors, conservation studies and species management, genetic improvement of plants and animals, genetic counselling, monitoring of hereditary diseases, etc. The mechanisms associated with changes in allelic and genotypic frequencies are 1) Mutation; 2) Natural selection; 3) Migration (with gene flow); 4) Genetic drift [1].

2. Mutation

Mutations are the primary sources of genetic variation, responsible for avoiding genetic homogeneity between populations, as they result in new alleles. By definition, mutations are changes in the DNA sequence, which may result from spontaneous errors in DNA replication during cell division or due to external factors such as exposure to mutagenic chemicals, radiation and viral infections [2].

3. Natural selection

Natural selection, initially proposed by Darwin, advocated that those with characteristics that increased the chance of survival or reproduction of individuals tended to settle in populations. In this context, it can be said that natural selection works favouring the advantageous alleles, genotypes and phenotypes and eliminating the unfavourable ones. There are two types of natural selection: positive or Darwinian selection, which acts on an adaptive mutation by increasing its frequency in the population; and negative or purifying selection, which acts in the opposite direction, reducing the frequency or even eliminating deleterious mutations from populations [3].

4. Migration (with gene flow)

The migration of individuals between different populations with consequent reproductive success results in gene flow, which is the transfer of alleles between populations. The outcome of these gene transfers depends on the difference between allelic frequencies in populations and the number and proportion of migrant individuals. It is a crucial evolutionary mechanism for conservation geneticists since gene flow is essential to minimise the effects of inbreeding and genetic drift on natural populations [4].

5. Genetic drift

Of all the evolutionary mechanisms, genetic drift is the one that most randomly alters the gene and allelic frequencies of populations. As it is a completely stochastic process, it is impossible, at first, to predict the allelic frequencies in the face of a drift event. This mechanism is associated with the loss of genetic variation in populations, which may make them more vulnerable in subsequent generations. Genetic drift is a consequence of a drastic alteration of natural and casual order, such as earthquakes, tsunamis, tornadoes, floods, fires, avalanches and other processes, affecting a large population contingent [5].

6. Hardy–Weinberg equilibrium

The Hardy–Weinberg equilibrium is one of the fundamental principles of population genetics. Assuming that a population is panmictic and mated at random and that there is no interference from evolutionary mechanisms (Mutation, Natural Selection, Migration and Drift), allelic and genotypic frequencies remain the same across generations. The Hardy–Weinberg theorem can be applied mathematically to a pair of alleles (a gene) through Newton’s binomial, according to the following expression in Eq. (1):

$$p^2+2pq+q^2=1$$E1

Where:

p is the frequency of the A allele.

q is the frequency of the a allele.

p² is a frequency of AA homozygotes.

q² is a frequency of aa homozygotes.

2pq is the frequency of Aa heterozygotes.

When populations are not in Hardy–Weinberg equilibrium, it means that evolutionary forces are acting and changing their allele and genotypic frequencies [6].

Analysing population genetics, through scientific investigations, is one of the best ways to understand the evolutionary history of living beings, with diverse applications in various sectors (health, agriculture, genetic improvement, biotechnology, etc.). In this sense, population genetics is an integrative and challenging science, with great discoveries and challenges to be achieved and a great and promising future in the post-genomic era.