What term describes a system that appears too complex to have naturally evolved from simpler systems?

The term that describes a system that appears too complex to have naturally evolved from simpler systems is "irreducibly complex system." This concept was popularized in discussions about intelligent design and evolution. Irreducible complexity refers to biological systems that are composed of multiple interdependent parts, where the removal of any one part would cause the system to cease functioning effectively.

The idea behind irreducible complexity is that such systems could not have evolved in a stepwise manner, as gradual changes would likely eliminate the functionality of the system at various stages, making it seem improbable that these complex structures arose through natural processes alone.

This concept is often used to challenge the theory of evolution by natural selection, suggesting that certain biological systems are too intricate to have appeared through evolution alone without the intervention of an intelligent designer.