Are all organisms with similar physical characteristics related to one another? If not, why?

Yes, all organisms with similar physical characteristics are related to one another.

Explanation:

Species that have common characteristics are closely related. They can even interbreed with one another. The ones with similar physical characteristics tend to be closer than others. Species may share same physical characteristics because they had common ancestors.

Scientists make a comparison of the DNA of the organisms and find that species with same DNA are related to each other. Two similar organisms may also evolve with time to adapt to their surroundings. There would be different reasons for their evolution. In a nutshell, they are closely related contributing to the ecosystem.