Evolution happens under certain conditions. This is not a theory but an observation of how the world works. "Evolution" is just the name we give to a particular process. Consider the idea of acceleration. No one says acceleration is just a theory, because it describes a common phenomena of objects in motion changing their velocity. Acceleration only enters into the realm of theory when it is part of a hypothesis about the way things work, like Newton's theories of motion (now considered fact, since they have been so well demonstrated).
So what is evolution?
Evolution is what happens when four conditions exist.
1) There exists a large population of entities (not necessarily living organisms) which are similar in most respects, but not identical.
2) These entities get replaced over time with copies of themselves, copies which are similar but not necessarily identical to the original.
3) Some condition exists which causes entities to reproduce or replicate to different degrees based on some difference in their makeup.
4) Those entities that reproduce more readily gradually come to dominate the population, in terms of numbers, and over time, the overall characteristics of the population shift.
This can be demonstrated in a number of ways. The most important way is in the human project of breeding domesticated animals with certain traits. Horses have been bred so that some are huge, strong draft animals, while others are lithe and fast, and others are diminutive. This was done by selective breeding (condition number 3). It is the same process that biologists know happens in nature, it having been observed in the wild and in the lab. It can also be demonstrated with computer programs, manufactured items, and ideas.
So evolution happens. That which is commonly called the "theory of evolution" is really a theory about the diversity of the species of plants and animals found on earth now and in the past. This theory posited that wherever the four conditions exist, plants and animals can be expected to change because of this process of evolution.
Scientists have found so much evidence for natural selection or evolution of species that they no longer consider it to be "just a theory." It is an established fact. The only questions remaining are how the process works. This will come up again and again in this blog because evolution is how the universe works. It will have to be taken into account to have the possibility of the development of religion that can effectively link the moral issues facing us to the eternal truths of the cosmos.