Chapter 1 - Anatomical and Functional Diversity of Animal Eyes

Abstract

Looking at different eyes, it is tempting to be judgmental and to label different designs as better or worse. However, it is important to understand that the elaboration of an eye does not necessarily lead to an adaptive advantage for that specific animal. Evolution can only improve an eye if it is developmentally possible and if its bearer gains overall fitness benefits from doing so. Therefore, any advantage from better eye performance must exceed any costs arising from the new design. These include costs from underlying brain structures as much as from the eye itself; a sophisticated eye can only evolve in conjunction with sophisticated visual processing centers. While it may be difficult to judge whether a particular eye design is better or worse for its bearer, it is relatively easy to judge its quality against the limits of physics. Some of the simplest visual organs are limited to the detection of major changes in light levels, failing to resolve spatial information that arguably is the most important visual quality of eyes.