How is the modern idea of speciation different from Darwinian?

C. Darwin believed that the formation of new species occurs in nature on the basis of existing ones. However, according to modern concepts of evolution, the formation of a new species occurs within the population, which is an elementary unit of evolution. Populations are genetically open systems. And while a gene flow occurs between them as a result of the migration of individuals, the species remains a single genetically closed system. However, the occurrence of isolation (barrier) between the two populations leads to the accumulation of hereditary differences in them that impede the crossing of individuals of these populations in subsequent encounters. This proves that populations are becoming genetically closed systems – and therefore new species. So, there was a process of speciation.