Evolving eyes.

Despite the incredible diversity among extant eyes, laws of physics constrain how light can be collected resulting in only eight known optical systems in animal eyes. Surprisingly, all animal eyes share a common molecular strategy using opsin for catching photons, but there are a diverse collection of mechanisms with proteins unrelated to each other used to focus light for vision. However, opsin is expressed in either one of two types of photoreceptor that differ fundamentally in their structure and tissue of origin. Taken together, this collection of observations strongly suggests that eyes have had multiple origins with remarkable convergence due to physics and molecular conservation of the opsin protein. Yet recent work has shown that a family of conserved genes are involved in eye formation despite substantial differences in their structure and origin, leading to a controversy over whether eyes evolved once or repeatedly. A likely resolution of this discussion is that particular genes and genetic programs have become associated with specific features needed for eyes and such suites of genes have been recruited as new eyes evolve. Since specific genes and their products are used repeatedly, it is somewhat difficult to conceptualize their causal relationships relative to evolutionary processes. However, detailed comparison of developmental programs may offer clues about multiple origins.