Two legged goats and developmental variation

Pelvic musculature of a normal (A) and two-legged (B) goat. The gluteus muscle ("gt") has a long anterior extension, which has been reinforced anteriorly by novel tendons ("t"). These are new anatomical structures that were generated in the absence of any direct genetic specification.



The pelvic bones responded to the unusual stresses imposed on them with changes in shape, as well. To the left is the pelvic skeleton of a normal (A) and two-legged (B) goat ("i"=ischium). Slijper noted that the dorsoventral flattening and elongation of the ischium resembled the forms seen in kangaroos, a naturally bipedal animal.



These are not genetic changes -- the goat was ordinary domestic stock, and presumably had perfectly ordinary genes that, under normal circumstances, would have generated more typically goatish morphology. These are instead the indirect consequences of a plastic phenotype, responding to a radical change in its environment.

(...)

I think there are several important messages here. One is that genes are not "for" some feature; the absence of forelimbs did not conjure a gene "for" gluteal tendons into existence. Rather, cellular patterns of gene expression are regulated in response to the environment, and in turn modulate the environment of other cells and tissues. We have been conditioned by years of good (but selective) results in genetics and molecular biology to view gene expression as an end result. It is not. In development, gene expression is part of a process that produces an end result in collaboration with multiple other factors.

http://scienceblogs.com/pharyngula/2006/01/two_legged_goats_and_developme.php#more