Final answer:
Biological evolution is defined as a change in the frequency of an allele in a population over time. This is a key concept in population genetics and reflects how populations evolve, not individuals, often in response to natural selection and other evolutionary forces.
Step-by-step explanation:
In biology, particularly in the context of population genetics, biological evolution is defined as a change in allele frequency over time within a population's gene pool. Allele frequencies measure the variability of a specific allele within a population relative to all other alleles of that gene. For example, the ABO blood type system can show changes in allele frequencies, which represent evolutionary changes in the population. When natural selection, genetic drift, or mutations cause these frequencies to shift, this leads to a population's evolution.
It's important to clarify that individuals do not evolve; evolution acts on the population level over generations. For instance, the observed bill size change in medium ground finches does not suggest that individual birds' bills are changing, but rather that the average bill size of the population changes over time due to selective pressures. Thus, when a population exhibits a consistent change in allele frequencies that affects a genetic trait, this is the process of evolution unfolding.