Answer:
In biology it refers to observed changes in organisms, to their descent from a common ancestor, and at a technical level to a change in gene frequency over time; it can also refer to explanatory theories (such as Charles Darwin's theory of natural selection) which explain the mechanisms of evolution. Natural selection
There are two major components to the theory. The first is the mechanism of natural selection, which was proposed independently by Charles Darwin and Alfred Russell Wallace in 1858.
In the same way artificial selection occurs when breeders choose seeds or studs that will improve their stock in the next generation, natural selection is the process of sorting living things according to how well adapted they are to their environment.
In the case of artificial selection, humans choose which traits are desirable. In the case of natural selection, traits that increase the likelihood of survival and reproduction will become more common within a population or species over time.
In the past, natural selection has been misrepresented by calling it the survival of the fittest. This statement oversimplifies the mechanism by making it sound like a tautology: the survival of those who survive.
The truth is that individuals never survive. What survives is the process for making another individual, and this resides in genes found in populations.
Natural selection has more to do with differential reproduction than survival, and what it selects are the genes that code for desirable traits or characteristics. The interaction of individuals with their environment provides a mechanism for sorting out which traits (not which individuals) will be passed on to the next generation.
Nature of inheritance
The second major component to the theory is the nature of inheritance, which follows the insights made by Gregor Mendel in 1865 and has advanced considerably since then due to our understanding of genes, DNA and the molecular processes of life.
When natural selection was first formulated by Darwin, the nature of inheritance was not understood. Our current understanding of inheritance is very sophisticated and includes the precise mechanisms for passing genes on to the next generation, how genes are modified by mutation and how they are shared among sexual species.
If we know enough about a gene and its various forms, it is possible to accurately predict the change in the frequency of those genes over time using mathematical formulae from population and evolutionary genetics theory.
This alteration of gene frequencies is subtle and does not, at first glance, seem worthy of being called evolution. But it is precisely these small changes at the genetic level that lead to large changes in the organisms that carry them.
The sorting of genes affects the fate of populations: populations drift apart and become species, and species diverge to create whole groups of plants or animals that dominate the landscape for millions of years.
The intricate details of cellular processes are responsible for the glorious and majestic diversity of life on our planet.
The theory of evolution includes large changes over vast periods of time and tiny changes made when one cell divides into two.
These processes form a continuum that is the history of life on Earth