Why are there three separate groups of fish instead of one group for all fish?

Fish are classified into three separate groups—jawless fish (Agnatha), cartilaginous fish (Chondrichthyes), and bony fish (Osteichthyes)—based on their distinct anatomical and evolutionary traits. These groups reflect significant differences in their skeletal structures, reproductive methods, and adaptations to their environments. This classification helps scientists better understand the evolutionary relationships and ecological roles of various fish species. By organizing fish into these categories, researchers can study their diversity and evolutionary history more effectively.