Final answer:
Microevolution refers to genetic changes within populations, while macroevolution describes broader evolutionary changes over time. Populations are the units of evolution because their genetic composition changes over time. The gene pool represents all the genetic diversity available for evolution.
Step-by-step explanation:
Microevolution refers to the changes in the genetic structure (allele frequency) of a population over time, while macroevolution refers to larger scale evolutionary changes seen over paleontological time.
Populations, rather than individuals, are the units of evolution because it is the genetic composition of a population that changes over time due to factors like natural selection, genetic drift, migration, and mutation.
A gene pool is the sum total of all the alleles present in a population. It represents the genetic diversity available for evolution to act upon.