Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. The majority of toolkit genes are components of signaling pathways and encode for the production of transcription factors,
Pax6/eyeless, which controls eye formation in all animals. It has been found to produce eyes in mice and Drosophila, even if mouse Pax6/eyeless was expressed in Drosophila.[2]
This means that a big part of the morphological evolution undergone by organisms is a product of variation in the genetic toolkit, either by the genes changing their expression pattern or acquiring new functions. A good example of the first is the enlargement of the beak in Darwin's large ground-finch (
Geospiza magnirostris), in which the gene BMP is responsible for the larger beak of this bird, relative to the other finches.[3]
The loss of legs in
Distal-less is very under-expressed, or not expressed at all, in the regions where limbs would form in other tetrapods.[4]
Toolkit genes, as well as being highly conserved, also tend to evolve the same function
in parallel. Classic examples of this are the already mentioned Distal-less gene, which is responsible for appendage formation in both tetrapods and insects, or, at a finer scale, the generation of wing patterns in the butterflies Heliconius erato and Heliconius melpomene. These butterflies are Müllerian mimics whose coloration pattern arose in different evolutionary events, but is controlled by the same genes.[8]
This supports
John C. Gerhart's theory of Facilitated Variation, which states that morphological evolutionary novelty is generated by regulatory changes in various members of a large set of conserved mechanisms of development and physiology.[9]
