A complicated process to calculate, population genetics was based on Mendel’s work alongside Darwin’s theory of Natural selection and helps us nowadays to directly observe evolution at our doorstep.
There are five factors influencing population genetics; Natural selection, Sexual selection, the attraction to desirable traits. Mutation occurs when there is an error during DNA replication in meiosis, genetic drift is due to random chance events, and finally the constant gene flow caused by immigration and emigration.
These factors alone or together affect the allele frequency within a population, over time that can grow to favour specific traits, and if this continues, the evolution of a new species could occur.
From Crash Course: Biology on YouTube by Hank Green.