What is defined as evolutionary change within a species or small group of organisms, especially over a short period?

The term that refers to evolutionary change within a species or a small group of organisms, particularly over a short time frame, is microevolution. This concept captures the small-scale changes that occur at or below the level of species, encompassing alterations in allele frequencies, genetic drift, mutation, migration, and selection within populations.

Microevolution highlights the gradual process through which populations adapt to changing environments or circumstances. Examples can include changes in coloration, body size, or resistance to disease among populations. These changes can result from evolutionary pressures and can often be observed over relatively short time scales, distinguishing it from broader evolutionary patterns that might take place over much longer time periods.

In contrast, terms like macroevolution refer to larger-scale evolutionary changes that can occur over much longer periods, often resulting in the emergence of new species or groups. Speciation specifically describes the process by which new species arise, while natural selection is a mechanism that drives evolutionary change by favoring individuals with advantageous traits. Understanding microevolution is critical to grasp how species adapt and evolve in response to their environments on a more immediate basis.