Which of the following best describes the integration of the study of genetics and population biology into evolutionary theory? a. the modern synthesis b. cladistical analysis c. the theory of endosymbiosis d. the theory of epigenetics

The cladistical analysis refers to the reconstruction of the evolutionary trees. The relationship among the organisms are hypothesized in cladistical analysis.

The theory of endosymbiosis explains the interdependent relationship of two organisms when one is ingested by the other and it result in a permanent relationship. The theory of epigenetics is concerned with the control of the gene activity in the developing complex organisms.

The modern synthesis explains the fusion of the Darwin's evolution theory, and the Mendelian genetics. Hence, the answer is 'Option A - the modern synthesis'.