GOOD QUESTION
Question: Human designers reuse designs that work well. Life forms also repeat the use of certain structures (the camera eye, for example). Is this evidence for common descent, evolutionary convergence, common design, or a combination of these?
Answer: Within evolutionary biology, there are only two ways to explain similar biological structures. The first is to attribute them to common descent. Thus two organisms share a structure because they inherited it from a common evolutionary ancestor. The option is to attribute similar structures to convergence. Thus two organisms share a structure because it evolved more than once (separate evolutionary pathways “converged on it”). By adopting an engineering approach to biological structure, intelligent design explains similar structures in terms of common design. Note that this is not to preclude that a repeated structure arose via an evolutionary process. But in that case it would be a guided evolutionary process and not a blind, purposeless evolutionary process as in Darwinism. Common design, perhaps expressed through evolutionary convergence, accounts for the repetitions of many biological structures (like the camera eye in humans and squids) far better than common descent or blind evolutionary convergence.