Final answer:
Microevolution refers to small changes within a species' genetic structure over a brief time span, while macroevolution encompasses broader evolutionary changes that may lead to new species, generally observed over geological time periods.
Step-by-step explanation:
Microevolution consists of changes within a species occurring over a short period of time due to variations in allele frequencies in a population's gene pool. This is the level of evolution witnessed when, for example, studying Darwin's finches and observing measurable alterations within a single species' genetic structure. Macroevolution, on the other hand, involves more significant evolutionary changes over time that can be observed over geological timescales. This results in the formation of new species, a process termed speciation.
The relationship between microevolution and macroevolution is that the former, through the accumulation of small genetic changes, can lead to the latter when these changes compound across vast evolutionary periods. Factors such as nonrandom mating, migration, population variation, and natural selection all contribute to microevolution, which, if extensive enough and coupled with reproductive isolation, can result in macroevolution.