Final answer:
Evolution is the change in the genetic makeup of a population over time, a concept that encompasses changes in allele frequencies due to natural selection, genetic drift, and other evolutionary forces.
Step-by-step explanation:
Evolution is defined as the change in the heritable characteristics of biological populations over many generations. Specifically, in terms of population genetics, it is a change in the frequency of an allele in a population. For instance, using the ABO blood group system, the frequency of an allele, such as IA, is the number of copies of that allele divided by all the copies of the ABO gene in the population. A change in this frequency over time is what constitutes evolution in that population.
It is important to note that evolution refers to the genetic composition of a population changing over time, and not the changes that occur in an individual's lifetime, which is referred to as development. An example of an evolutionary change is when the average value of a characteristic in a population, like the bill size of finches, changes over time due to natural selection—not because individual finches' bills are changing, but because the proportions of individuals with different bill sizes shift within the population.
The modern synthesis of evolutionary theory incorporates Darwin's, Wallace's, and Mendel's contributions on evolution and heredity using the framework of population genetics. It defines evolution as a change in allele frequency over generations and explains that allele frequencies change due to processes such as mutation, natural selection, genetic drift, and migration, barring which a population would reach what is known as Hardy-Weinberg equilibrium, where allele frequencies do not change.