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Some common misconceptions about evolution are evident in popular culture and the media. For one thing, evolution is often conflated with abiogenesis (organic or pseudo-organic matter coming from inorganic matter) or even the 'big bang' (a cosmological scientific theory for events extremely early in our universe). The field of evolution only deals directly with the change of living organisms, not their origins.
Another misconception is that evolution is just another hypothesis. When people say "evolution is a theory", it can be misleading. That evolution happens is an observational fact. It generally happens over very large amounts of time, but it can be witnessed even in a single human lifetime (as strains of the flu grow resistant to a vaccine, a line of farm animals is bred to be more productive, a patch of peas are selected for greenness, etc.). Evolution as defined above certainly happens, and fossil, morphological, genetic sequence and other evidence shows that it happens on a larger scale than just that we see in a single lifetime.
Evolution is a theory only in the sense that the mechanisms by which evolution happens are theoretical in nature and thus can be falsified but not necessarily proven (consider: one can state that all crows are black - this can be falsified by finding a non-black crow but cannot be proven short of a comprehensive survey of all crows in the universe). However, theoretical here is used in the sense of a scientific theory - that is, an extremely well-tested hypothesis that explains the observed phenomena better than competing hypotheses.
The most well-known mechanism by which evolutions happens is natural selection. At its heart it is an elegantly simple idea. Organisms vary, and at least some of the variations are hereditary (passed on from generation to generation) - this is hard to deny. Groups of organisms have more offspring than could all survive, so there is competition - again, hard to deny. Natural selection takes these observations and suggests that organisms which have [hereditary] variations that allow it to survive and procreate in its environment better than its conspecifics will pass on its genetic traits quicker or more profusely than others which do not survive to reproduce. Thus its traits (which are beneficial for survival in that niche) will accumulate in the population at a greater frequency over time.
Other mechanisms of evolution include sexual selection (selection via mate-competition based on secondary sex characteristics), artificial selection (from human intervention, as with breeding domestic and farm animals for traits desirable to us), gene flow (transfer of genes between populations, as from immigration) and genetic drift (a sort of sampling effect, usually in small populations, where more random sampling occurs in the production of offspring).
Together these mechanisms drive the evolution of organisms, leading to changes both small and profound. This includes speciation, where successive generations in the population of one species diverge into subpopulations - through geographical redistribution or otherwise - which eventually become separate species with a common ancestor.
Thus the theoretical details of evolution provide a framework around which we can interpret and understand the mounds of fossil, genetic and other data which we find around us as we explore our world. It allows us to build the knowledge we have today, from which many modern applications have come, such as advances in medicine and farming. Our understanding of evolution provides a framework within which we can make sense of the biological world and how it changes.