The article by Professor Th. Dobzhansky explores the process of speciation, which is the formation of discrete non-interbreeding arrays of organisms. Dobzhansky identifies two main components of evolution: the increase in diversity through genetic variation and the development of discontinuity through isolation. He discusses the two meanings of "speciation": one focusing on the formation of discrete arrays and the other including the production of variability. The traditional view holds that species formation is a gradual process, but Dobzhansky argues that isolating mechanisms may develop independently of genetic differences. Dobzhansky emphasizes that the maintenance of species as discrete entities requires isolation, which can be geographical or physiological. He notes that polyploidy, a clear-cut method of speciation, often results in physiological barriers. However, the more common method involves gradual accumulation of genetic changes, and the origin of these isolating mechanisms remains a mystery. The article discusses the genetic basis of isolating mechanisms, suggesting that they may arise from the accumulation of specific genes that prevent hybridization. Dobzhansky proposes a theory where each species, genus, and geographical race is an adaptive complex adapted to a specific ecological niche. Speciation is seen as the fixation of these patterns through the development of physiological isolating mechanisms, which typically occur at the boundaries between races. Dobzhansky provides evidence supporting his theory, such as the occurrence of hybrid zones and the selective advantage of genes that prevent hybridization. He also addresses challenges, such as the development of isolating mechanisms in species isolated on oceanic islands. The article concludes by emphasizing the importance of isolating mechanisms in preventing interbreeding and their role in speciation.The article by Professor Th. Dobzhansky explores the process of speciation, which is the formation of discrete non-interbreeding arrays of organisms. Dobzhansky identifies two main components of evolution: the increase in diversity through genetic variation and the development of discontinuity through isolation. He discusses the two meanings of "speciation": one focusing on the formation of discrete arrays and the other including the production of variability. The traditional view holds that species formation is a gradual process, but Dobzhansky argues that isolating mechanisms may develop independently of genetic differences. Dobzhansky emphasizes that the maintenance of species as discrete entities requires isolation, which can be geographical or physiological. He notes that polyploidy, a clear-cut method of speciation, often results in physiological barriers. However, the more common method involves gradual accumulation of genetic changes, and the origin of these isolating mechanisms remains a mystery. The article discusses the genetic basis of isolating mechanisms, suggesting that they may arise from the accumulation of specific genes that prevent hybridization. Dobzhansky proposes a theory where each species, genus, and geographical race is an adaptive complex adapted to a specific ecological niche. Speciation is seen as the fixation of these patterns through the development of physiological isolating mechanisms, which typically occur at the boundaries between races. Dobzhansky provides evidence supporting his theory, such as the occurrence of hybrid zones and the selective advantage of genes that prevent hybridization. He also addresses challenges, such as the development of isolating mechanisms in species isolated on oceanic islands. The article concludes by emphasizing the importance of isolating mechanisms in preventing interbreeding and their role in speciation.