 Macroevolution refers (most of the time, in practice) to evolutionary patterns and processes above the species level. It is usually contrasted with microevolution, or evolutionary change within populations. This customary way of drawing the macro/micro distinction is not perfect, however, because species sometimes consist of multiple populations. Some evolutionary processes, such as the spread of a trait from one population to another, might count as within-species processes but not within-population processes.  Population genetics (see entry),  which emerged during the modern synthesis of the early- to mid-twentieth century, explains within-population microevolutionary change in terms of  natural selection,  genetic drift, mutation, and migration.