The protostomes encompass a staggering number of different body forms and lifestyles. The following aspects of protostome evolution have contributed to this enormous diversity:
The evolution of segmentation permitted some groups of protostomes to move different parts of the body independently of one another. Species in some groups gradually evolved the ability to move rapidly over and through the substrate, through water, and through air.
Complex life cycles with dramatic changes in form between one stage and another allow individuals of different stages to specialize on different resources.
Parasitism has evolved repeatedly, and many protostome groups parasitize plants and animals.
The evolution of diverse feeding structures allowed protostomes to specialize on many different food sources. Specialization on food sources undoubtedly contributed to reproductive isolation and further diversification.
Predation was a major selection pressure favoring the development of hard external body coverings (exoskeletons and shells). Such coverings evolved independently in many lophotrochozoan and ecdysozoan groups. In addition to providing protection, these coverings became key elements in the development of new systems of locomotion.
Better locomotion permitted prey to escape from predators, but also allowed predators to pursue their prey more effectively. Thus the evolution of animals has been, and continues to be, a complex “arms race” among predators and prey.
Animation 31.1 An Overview of the Protostomes
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Many major evolutionary trends among the protostomes are shared by the deuterostomes, which include the chordates, the group to which humans belong. We will turn to the deuterostomes in the next chapter.