The modern synthesis of evolutionary theory grew out of the cohesion of Darwin’s, Wallace’s, and Mendel’s thoughts on evolution and heredity, along with the more modern study of population genetics. It describes the evolution of populations and species, from small-scale changes among individuals to large-scale changes over paleontological time periods.

To understand how organisms evolve, scientists can track populations’ allele frequencies over time. If they differ from generation to generation, scientists can conclude that the population is not in Hardy-Weinberg equilibrium, and is thus evolving

modern synthesis — overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today. population genetics — study of how selective forces change the allele frequencies in a population over time. founder effect — event that initiates an allele frequency change in part of the population, which is not typical of the original population. frequency-dependent selection — selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).