What environmental factors can lead to the selection of organisms in nature?

In 1859, Charles Darwin published the book The Origin of Species by Natural Selection, or the Preservation of Selected Breeds in the Struggle for Life. In it, in particular, a new view was proposed on the causes of the evolutionary development of organisms. Darwin pointed out three main factors of evolution: heredity (the ability of organisms to transmit innate traits from generation to generation), variability (the appearance of various phenotypes within a population) and selection (suppression of genotypes of organisms whose phenotypes are less adapted to external conditions than others). Selection in the population occurs due to the fact that organisms that are better adapted to external conditions survive and multiply, while worse adapted organisms more often die and / or leave fewer offspring. The role of the selection factor is played by the environment. Selection increases the adaptability of the population to environmental conditions.
With increasing population size, external conditions (for example, food) become a deterrent, which leads to competition in the population (to struggle for existence). Individuals having, due to their phenotype, an advantage in this competition will leave offspring and survive.
From the point of view of genes, selection is a process that determines which alleles will be transmitted to descendants, providing them with an advantage in competition. Changes in allele frequencies can lead to evolutionary changes, the main reason for which is the appearance of mutant alleles. Especially fast recessive mutant allele can spread in the population, being linked to some dominant allele, which is important for the life of the body. Mutant alleles associated with small changes in the phenotype can accumulate and produce evolutionary changes.
An important characteristic of selection is its pressure. The selection pressure depends on external environmental factors (this is expressed in the form of combating adverse conditions), interspecific competition (in particular, the presence of predators and parasites), as well as intraspecific competition (primarily determined by the size of the population). The intensity of selection shows the rate of evolutionary changes in the population. An increase in selection pressure (for example, as a result of a narrowing of the range of environmental conditions) is a conservative factor that helps the population better adapt to external conditions. As a result of this, a narrower specialization of the species in certain circumstances can lead to the extinction of the population when these conditions change. On the contrary, the weakening of the selection intensity, which usually occurs when the external limiting factors decrease (for example, the number of predators decreases, the species enters its new environment) contributes to an increase in species diversity.
In various circumstances, natural selection can come with different intensities. Darwin notes circumstances that favor natural selection:
• A sufficiently high frequency of manifestation of uncertain hereditary changes.
• The large number of individuals of the species, increasing the likelihood of beneficial changes.
• Unrelated crosses that increase the range of variation in the offspring. Darwin notes that cross-pollination is rare even among self-pollinating plants.
• Isolation of a group of individuals, preventing their crossing with the rest of the mass of organisms in a given population.
• Widespread distribution of the species, since at the same time individuals meet different conditions at the borders of the range, and natural selection will go in different directions and increase intraspecific diversity.
• Along with these circumstances, the main condition for the success of natural selection is its cumulative effect, which is the basis of its creative species-forming activity.