Irreducible Complexity:

In his landmark book, "Darwin's Black Box", Michael Behe introduced the notion of irreducible complexity as a challenge to neo-Darwinian theory:

"By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. " [page 39]

Why is irreducible complexity a challenge to Darwinian theory?

In the "Origins of Species" Charles Darwin wrote:
"If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." [Sixth Edition, New York University Press, page 154]

Irreducible complexity is a challenge to Darwinian theory because it shows that some biological structures cannot be built by "numerous, successive, slight modifications" as Darwin first proposed.

A prime example of Michael Behe's "irreducible complexity"
is the bacteria flagellum. With over 40 essential parts, the
flagellum is a rotary motor used to propel a bacteria in liquid.
Spinning at 17,000 rpms, the motor is acid driven, liquid cooled
and self-replicating.

--- Flagellum image created by Discovery Media Productions