The article "Phenotypic flexibility and the evolution of organismal design" by Piersma and Drent, published in *Trends in Ecology and Evolution* in 2003, explores the concept of phenotypic flexibility and its role in understanding organismal design and adaptation. The authors argue that while developmental plasticity, which refers to irreversible changes in traits during development, has received significant attention, reversible phenotypic variation within an individual (phenotypic flexibility) has been underexplored in evolutionary biology. They propose a framework to categorize different types of phenotypic plasticity, emphasizing the importance of reversible variations for understanding organismal design and adaptation. The article highlights several examples of reversible phenotypic changes in various organisms, such as sea urchins adjusting their body size based on food availability, crustaceans reducing their body size under high energy expenditure, and birds changing the size of their gizzards in response to diet. These examples illustrate how phenotypic flexibility can help organisms optimize their physiological and behavioral responses to environmental conditions. The authors also discuss the nature of plasticity in environments with varying predictability, suggesting that in unpredictable environments, rapid and reversible phenotypic changes offer fitness advantages. In predictable environments, organisms can anticipate and adjust their phenotypes through life-cycle stages. They propose a quantitative genetic framework that includes both reversible and irreversible components of phenotypic variance, which can help quantify the importance of these factors in shaping the phenotype. Finally, the article emphasizes the potential of intra-individual phenotypic variations in experimental studies of phenotypic design, similar to how behavioral ecologists have successfully studied behavioral traits. The authors conclude that phenotypic flexibility is a crucial aspect of organismal design and should be better integrated into evolutionary models and theoretical frameworks.The article "Phenotypic flexibility and the evolution of organismal design" by Piersma and Drent, published in *Trends in Ecology and Evolution* in 2003, explores the concept of phenotypic flexibility and its role in understanding organismal design and adaptation. The authors argue that while developmental plasticity, which refers to irreversible changes in traits during development, has received significant attention, reversible phenotypic variation within an individual (phenotypic flexibility) has been underexplored in evolutionary biology. They propose a framework to categorize different types of phenotypic plasticity, emphasizing the importance of reversible variations for understanding organismal design and adaptation. The article highlights several examples of reversible phenotypic changes in various organisms, such as sea urchins adjusting their body size based on food availability, crustaceans reducing their body size under high energy expenditure, and birds changing the size of their gizzards in response to diet. These examples illustrate how phenotypic flexibility can help organisms optimize their physiological and behavioral responses to environmental conditions. The authors also discuss the nature of plasticity in environments with varying predictability, suggesting that in unpredictable environments, rapid and reversible phenotypic changes offer fitness advantages. In predictable environments, organisms can anticipate and adjust their phenotypes through life-cycle stages. They propose a quantitative genetic framework that includes both reversible and irreversible components of phenotypic variance, which can help quantify the importance of these factors in shaping the phenotype. Finally, the article emphasizes the potential of intra-individual phenotypic variations in experimental studies of phenotypic design, similar to how behavioral ecologists have successfully studied behavioral traits. The authors conclude that phenotypic flexibility is a crucial aspect of organismal design and should be better integrated into evolutionary models and theoretical frameworks.