Final answer:
The controversy around the occurrence of complex characters like eyes stems from their development involving complex inheritance patterns and their evolution being a subject of multidisciplinary study and debate.
Step-by-step explanation:
The occurrence of complex characters, such as eyes, is highly controversial because their development involves complex inheritance patterns that deviate from the simpler ones first described by Gregor Mendel. Understanding eye formation demands a multidisciplinary approach, spanning from evolutionary biology to genetics. For instance, the variety of eye colors, from green and blue to hazel and violet, showcases how non-Mendelian inheritance can result in a rich diversity within a single physiological feature.
Additionally, discussions on the evolution of eyes can become contentious due to differing interpretations on how such complex structures could evolve through natural selection. Some see the eye's complexity as a challenge to evolutionary theories, while evolutionary biologists find ways to explain this complexity through gradual changes over millions of years. Debates also revolve around the roles of genetic mutations, environmental influences, and the potential for modular development, where parts of the eye evolve independently and later combine to form a fully functional organ.