Final answer:
The Modern Synthesis is a key concept in Biology that integrates population genetics and Mendelian inheritance with Darwin's theory of evolution, defining evolution as changes in allele frequency over generations.
Step-by-step explanation:
The Modern Synthesis of evolutionary theory is the integration of Darwin's natural selection concept with Mendelian genetics, and importantly, the framework of population genetics. It explains the evolution of populations and deduces how evolutionary processes such as natural selection affect a population's genetic composition over time. This comprehensive model includes the understanding that evolutionary forces such as mutation, natural selection, genetic drift, and gene flow collectively alter allele frequencies within a population, leading to evolution.
The correct answer to the question posed is therefore: D. Population genetics and Mendelian inheritance. These two fields were crucial in establishing the modern synthesis, melding the theory of natural selection with a genetic perspective, and thus overcoming the hurdles of earlier, less accurate genetic theories.