Final answer:
Specialized tissues in complex organisms are a result of cellular differentiation, which allows for efficient task division among cells and the formation of complex organ systems.
Step-by-step explanation:
Complex organisms develop specialized tissues as a direct result of cellular differentiation. The process of multicellularity entails cells becoming more efficient by performing fewer tasks, leading to the development of specialized cells and tissues that form organ systems, such as the circulatory and respiratory systems. Surface area-to-volume ratio is also essential in other aspects of animal development and specialization.