Irreducible complexity is the hallmark of many anti-evolutionist arguments as it stipulates that many biological structures are too complex to have evolved from simpler, less-complex precursors. A common straw man is the mousetrap, whereby it is claimed that removing any part of the mousetrap makes it useless. (Biologist John McDonald elegantly dismembers this argument by describing a reducibly complex mousetrap). Anyway, such irreducible complexity is supposed to be proof of an intelligent designer (i.e., an immortal creator). Flagella, whip-like projections from some cells that are used for locomotion (see photo), are often claimed to be irreducibly complex biological structures. This view has been thoroughly discredited, even in a court of law (see Kitzmiller v. Dover).
A paper by Liu and Ochman in PNAS compiles further evidence against the hypothesis of irreducible complexity. They took advantage of full genome sequence for 41 bacterial species to trace the phylogenetic occurrence of flagellum proteins. The results show, amazingly, that the "core components of the bacterial flagellum originated through the successive duplication and modification of a few, or perhaps even a single, precursor gene." Flagella don't sound so irreducibly complex, then, do they? The take-home message is that evolutionary theory is sufficient to explain even the origin of complex organs and organelles and we needn't postulate intelligent designers. My compliments to Renyi Liu and Howard Ochman for a fine piece of work.