In the debate between intelligent design advocates and their Darwinian opponents, the claim is made by the IDers that there are systems in living things that are irreducible complex, i.e. they could not have evolved step by gradual step because they cannot function until all the parts are in place and operating. The bacterial flagellum is considered the paradigmatic example of IC but there are numerous others.
This
site, for example lists a couple dozen systems or structures that are alleged to be irreducibly complex. Whether they are or not may be debated, but I'll leave that debate to others more qualified than I. Here are some examples for which the article offers brief explanations:
- Bacterial Flagellum
- Eukaryotic Cilium
- Aminoacyl-tRNA Synthetases (aaRS)
- Blood clotting cascade
- Ribosome
- Antibodies and the Adaptive Immune System
Students of biology will find the complete list interesting reading.