Answer: a. change in allele frequencies over time
Explanation:
Biological evolution is defined as the genetic changes from generation to generation.
Biological evolution can also defined as the changes in the inherited traits in a population over successive generation of organisms. Changes in the allele frequencies over a time causes changes in the genetic expression of an organism, which is a key characteristics of biological evolution.
The genetic expression changes from generation to generation during reproduction process and hence different characteristics tend to exist within any given population and shows genetic variation.
Hence, the correct option is a. "change in allele frequencies over time".
