Flexi Says: The modern definition of evolution considers different mechanisms other than natural selection for evolution to occur. The current theory of evolution considers that genes are inherited by individuals from their parents. Variation in genes known as alleles can form due to mutations. The modern theory of evolution suggests that gradual accumulation of smaller changes at the gene level can lead to evolutionary changes. In other words, microevolution leads to macroevolution. Quantitatively, microevolution is measured and described by changes in allele frequencies in a population over time.