In it's simplest form, evolution is defined by a change in allele frequencies in a population over time, It really is as simple as that. When there is a particular gene/trait that is favourable, it will be more common in the next generation. That's all evolution is. Creationists love to distinguish between microevolution and macroevolution. In reality, they are one and the same, the only difference is the amount of time that has passed.