Convergence is the term given when a similarity occurs in a physical characteristic of two unrelated organisms. The more the complex the characteristic and the more unrelated the organisms, the more striking the convergence.
Consider the two explanations for convergence:
Evolution: Similar environmental pressures channeled the "genetic algorithms" into finding the same optimal solution in the two organisms, given they faced the same challenges.
Intelligent Design: God reused design patterns in different species as He saw fit.
When the two organisms come from radically different environments is when the case against evolution is the strongest.
Here is one example from the Rana's article (see the link for references):
An even more remarkable example of convergence occurring in aquatic and terrestrial environments can be seen in the sandlance (fish) and chameleon (reptile), respectively. Recent experiments have uncovered an extraordinary similarity in the visual systems and behavior for these two creatures. Both the chameleon and the sandlance move their eyes independent of one another in a jerky manner, rather than in concert. While one eye is in motion the other eye is motionless. Moreover, both animals use the cornea of the eye to focus on objects. All other reptiles and fish use the lens of the eye to focus images on the retina. The chameleon and sandlance both rely on a specialized muscle (the cornealis muscle) to adjust the focusing of the cornea. The chameleon determines depth perception using a single eye. Scientists believe the sandlance also determines depth perception in this manner. Both the sandlance and the chameleon have skin coverings over their eyes to prevent them from being conspicuous to both predators and prey. The feeding behavior of both animals is also the same. The trajectory that the chameleon tongue takes when attacking its prey is the same as that taken by the sandlance when it lunges for its prey. (The sandlance buries itself in sand beds with its eyes above the surface of the sand and waits for tiny crustaceans to pass by.)
The words of the team of researchers who were among the first to discover this convergence are compelling: "When faced with a beautifully coordinated optical system such as this, it is a challenge to provide an explanation for the convergence of so many different finely-tuned mechanisms."