Final answer:
Male lions reaching sexual maturity alter allele frequencies in the lion population through gene flow as they migrate to new prides and contribute genetic material.
Step-by-step explanation:
When male lions reach sexual maturity, they leave their group in search of a new pride. This movement of mature males can alter the allele frequencies of the population primarily through gene flow, which is the transfer of genetic variation from one population to another. As these males enter new prides and mate, they bring with them alleles from their natal pride's gene pool, thus changing the genetic makeup of the population they join. This process can introduce new genetic variation to a population and is a significant mechanism of evolutionary change.