This paper reviews the debate on phenotypic plasticity and genetic assimilation in evolutionary biology. It discusses the historical context of these concepts, their theoretical foundations, and the ongoing controversy surrounding their roles in evolution. Genetic assimilation is a process where environmentally induced phenotypic variation becomes constitutively produced, no longer requiring the environmental signal for expression. It was first proposed in the late 19th century and later developed in the 1940s by Waddington and Schmalhausen. Despite initial dismissal by some evolutionary biologists, interest in genetic assimilation has increased, with new conceptual treatments and empirical studies. The paper examines criticisms of genetic assimilation, particularly from de Jong, who argues that it does not play a significant role in evolution and that proposed updates are conceptually flawed. The authors respond to these criticisms, emphasizing the importance of phenotypic plasticity in allowing organisms to survive in novel environments and the potential for genetic assimilation to occur through natural selection. They also highlight the role of quantitative genetics in understanding phenotypic evolution and the limitations of current models in predicting the adaptive role of plasticity. The paper also discusses the broader implications of phenotypic plasticity and genetic assimilation for evolutionary theory. It argues that these concepts are not in conflict with the Modern Synthesis but rather represent important extensions of it. The authors conclude that the new ideas about phenotypic plasticity and genetic assimilation represent a welcome expansion of the Modern Synthesis, not a threat to it. They emphasize the need for further empirical research to test the possibility of genetic assimilation in natural populations. The paper also highlights the importance of understanding the hierarchical relationships among phenotypic plasticity, natural selection, and genetic assimilation, and the need to clarify conceptual misunderstandings to avoid misinterpretations of their roles in evolutionary theory.This paper reviews the debate on phenotypic plasticity and genetic assimilation in evolutionary biology. It discusses the historical context of these concepts, their theoretical foundations, and the ongoing controversy surrounding their roles in evolution. Genetic assimilation is a process where environmentally induced phenotypic variation becomes constitutively produced, no longer requiring the environmental signal for expression. It was first proposed in the late 19th century and later developed in the 1940s by Waddington and Schmalhausen. Despite initial dismissal by some evolutionary biologists, interest in genetic assimilation has increased, with new conceptual treatments and empirical studies. The paper examines criticisms of genetic assimilation, particularly from de Jong, who argues that it does not play a significant role in evolution and that proposed updates are conceptually flawed. The authors respond to these criticisms, emphasizing the importance of phenotypic plasticity in allowing organisms to survive in novel environments and the potential for genetic assimilation to occur through natural selection. They also highlight the role of quantitative genetics in understanding phenotypic evolution and the limitations of current models in predicting the adaptive role of plasticity. The paper also discusses the broader implications of phenotypic plasticity and genetic assimilation for evolutionary theory. It argues that these concepts are not in conflict with the Modern Synthesis but rather represent important extensions of it. The authors conclude that the new ideas about phenotypic plasticity and genetic assimilation represent a welcome expansion of the Modern Synthesis, not a threat to it. They emphasize the need for further empirical research to test the possibility of genetic assimilation in natural populations. The paper also highlights the importance of understanding the hierarchical relationships among phenotypic plasticity, natural selection, and genetic assimilation, and the need to clarify conceptual misunderstandings to avoid misinterpretations of their roles in evolutionary theory.