Facilitated variation
The theory of facilitated variation demonstrates how seemingly complex biological systems can arise through a limited number of regulatory genetic changes, through the differential re-use of pre-existing developmental components.
[1][2] Organisms are built from a set of highly conserved modules called "core processes" that function in development and physiology, and have remained largely unchanged for millions (in some instances billions) of years.
Different core processes become linked, through differential regulation, in different combinations, and operate in different amounts, states, times, and places, to generate new anatomical and physiological traits.
Examples include: Exploratory processes first generate a very large amount of physiological variation, often at random, and then select or stabilize the most useful ones, with the rest disappearing or dying back.
The vertebrate embryo is organized spatially into perhaps 200 compartments, each uniquely defined by its expression of one or a few key genes encoding transcription factors or signaling molecules.
An example of compartmentation is found in the developing spine: all vertebrae contain bone-forming cells, but those in the chest form ribs, whereas those in the neck do not, because they arose in different compartments (expressing different Hox genes).
Then, as noted previously, axons in large numbers extend into the bud from the nerve cord; some fortuitously contact muscle targets and are stabilized, and the rest shrink back.
[3][4][5][6][7] Parter et al.[3] demonstrate how key elements of facilitated variation theory, such as weak regulatory linkage, modularity, and reduced pleiotropy of mutations, evolve spontaneously under realistic conditions.
In the classical Darwinian view, a large number of successive mutations, each selected for its usefulness to the survival of the organism, is required to produce novel structures such as wings, limbs, or the brain.
[14] Creationists and advocates of Intelligent Design have argued that complex traits cannot evolve through successive small modifications to pre-existing functional systems.
The theory of facilitated variation challenges this idea of irreducible complexity by explaining how random mutation can cause substantial and adaptive changes within a species.
Different species of Heliconius butterfly have independently evolved similar patterns, apparently both facilitated and constrained by the available developmental-genetic toolkit genes controlling wing pattern formation .
