Irreducible Complexity
Irreducible complexity is the proposal that some functional biological systems contain several interacting parts such that removal of a necessary part destroys the system's basic present function.
The Investigative Question
Does the dependence of a biological system upon multiple interacting components provide evidence concerning how that system originated?
What We Observe
Some biological systems cease to perform their normal function when an essential component is removed. Intelligent Design advocate Michael Behe used the term "irreducible complexity" to focus attention on systems whose present function depends upon several coordinated parts.
Scientific Background
The concept concerns functional dependency in an existing system. A crucial distinction must be made between demonstrating that a modern system requires several parts and demonstrating that no evolutionary pathway to that system is possible.
Why It Matters
This distinction lies at the center of the debate. Present-day dependency is experimentally testable. Historical claims about whether simpler ancestors could perform different functions require additional evidence.
What Is Known
Biologists can experimentally identify components required for particular present functions. Many molecular systems also contain optional, redundant, replaceable, or lineage-specific components, so dependency must be evaluated for each system rather than assumed.
What Is Proposed
Evolutionary critics of irreducible-complexity arguments propose that complex systems can arise through changes of function, recruitment of existing components, duplication and divergence, scaffolding, redundancy followed by loss, and other historical pathways.
What Remains Uncertain
For ancient systems, complete step-by-step histories are often unavailable. The existence of homologous components and evidence for possible intermediate functions can support evolutionary models without necessarily identifying every historical step.
Key Numbers
The number of essential components varies by organism, system, environment, and the particular function being tested.
Design Relevance
Irreducible complexity is directly relevant to Intelligent Design because it asks whether coordinated functional systems display characteristics that are difficult to produce through selectable incremental changes. The argument is strongest when it addresses actual evolutionary pathways rather than merely noting that a modern system has necessary parts.
Assessment
Irreducible complexity identifies a legitimate question about the origin of integrated function, but present-day part dependency alone does not settle the historical question. Competing explanations must be compared with the biological evidence.