The thesis that species occurred through natural selection

The thesis that species occurred through natural selection, Darwin derived, based on five basic observations (facts) and made three conclusions:

All species have a biological potential to increase the number of individuals to large populations.
However, populations in nature demonstrate a relative constancy of the number of individuals in time.

3. The resources necessary for the existence of species are
limited, so the number of individuals in populations is approximately
constant over time.
Conclusion 1. Between representatives of one species there is a struggle for the resources necessary for survival and reproduction. Only a small part of individuals survive and give offspring.

There are no two individuals of the same species who would have the same properties. Representatives of one species demonstrate great variability.
In general, the variability is genetically determined, so it is inherited.
Conclusion 2. The competition between representatives of one species depends on the unique hereditary properties of individuals that provide advantages in the struggle for resources for survival and reproduction. This unequal ability to survive is natural selection.
Conclusion 3. Accumulation of the most favorable properties as a result of natural selection leads to a constant change in species. This is how evolution takes place.
Based on the huge factual material and the practice of breeding work on the breeding of new varieties of plants and breeds of animals, Charles Darwin formulated the basic principles of his evolutionary theory:

the first principle postulates that variability is an inalienable property of the living;
the second principle reveals internal contradictions in the development of living nature and claims that, on the one hand, all types of organisms tend to propagate in a geometric progression, and on the other, only a small part of the offspring survive and reach maturity;
the third principle is usually called the principle of natural selection, which plays a fundamental role in the theory of evolution not only of Darwin, but of all theories that appeared later. Natural selection constantly spreads around the world the smallest changes, discarding the unadapted, preserving and forming the stable, working inaudibly and invisibly over the improvement of every organic being in connection with the conditions of his life, organic and inorganic.