Charles Darwin (1809-1882), an English naturalist, developed an evolutionary theory that underlies the modern synthetic theory: the theory of natural selection.
According to Darwin, organisms better adapted to the environment have a better chance of survival than those less adapted, leaving a larger number of descendants. The best adapted organisms are therefore selected for that environment.
The basic principles of Darwin's ideas can be summarized as follows:
Individuals of the same species have variations on all characterstherefore not identical with each other.
Every organism has great reproductive capacity, producing many offspring. Meantime, only some of the descendants reach adulthood.
O number of individuals of a species is kept more or less constant over the generations.
Thus, there is a great "struggle" for life among the descendants, because although many individuals are born few reach maturity, which keeps the number of individuals in the species constant.
In the "fight" for life, organisms with variations favorable to environmental conditions where they live are more likely to survive compared to organisms with less favorable variations.
- Bodies with these advantageous variations are more likely to leave offspring. Since there is character transmission from parents to children, they have these advantageous variations.
- Thus, over the generations, the acting of natural selection on individuals maintains or improves their degree of adaptation to the environment.
The Synthetic Theory of Evolution
THE Synthetic theory of evolution or Neodarwinism It has been formulated by many researchers during years of study, drawing on Darwin's notions of natural selection and incorporating current notions of genetics.
The most important individual contribution of genetics, extracted from Mendel's work, replaced the old concept of inheritance through mixing blood with the concept of inheritance through particles: genes. Synthetic theory considers, as Darwin had already done, the population as an evolutionary unit. THE population can be defined as grouping of individuals of the same species that occur in the same geographical area, in the same time interval.
To better understand this definition, it is important to know the biological concept of species: grouping of naturally or potentially intercrossing natural populations reproductively isolated from other groups of organisms. When in this definition it is said potentially intercrossingmeans that a species may have populations that do not naturally cross because they are geographically separated. However, artificially placed in contact, there will be crossing between individuals, with fertile descendants.
Therefore, they are potentially intercrossing. The biological definition of species is only valid for organisms with sexual reproduction, since, in the case of organisms with asexual reproduction, the similarities between morphological characteristics define species groupings.
Looking at the different populations of individuals with sexual reproduction, it can be noted that there is no one equal to the other. Exceptions to this rule could be univiteline twins, but even they are not absolutely identical, although the initial genetic heritage is the same. This is because somatic changes may occur due to the action of the medium. The enormous diversity of phenotypes in a population is indicative of the genetic variability of this population, and it can be noted that it is generally very wide.
Understanding the genetic and phenotypic variability of individuals in a population is fundamental for the study of evolutionary phenomena, since evolution is, in fact, the statistical transformation of populations over time, or changes in the frequency of the genes of that population. population. Factors that determine changes in gene frequency are called evolutionary factors. Each population has a gene set, which subject to evolutionary factors, can be changed. The gene pool of a population is the pool of all genes present in that population. Thus, the greater the genetic variability.
The evolutionary factors that act on the gene set of the population can be grouped into two categories:
Factors that tend to increase population genetic variability: gene mutation, chromosomal mutation, recombination;
Factors that act on the already established genetic variability: natural selection, migration and genetic oscillation.
The integration of these factors together with geographic isolation may lead, over time, to the development of mechanisms for reproductive isolation, when, then, new species emerge. In the following chapters, these topics will be covered in more detail.