How about [the Italian wall lizards](https://en.wikipedia.org/wiki/Italian_wall_lizard) that [evolved new muscles around their intestines](http://news.nationalgeographic.com/news/2008/04/080421-lizard-evolution_2.html) after being introduced to an island? ([other article](http://scienceblogs.com/pharyngula/2008/04/23/still-just-a-lizard/), [actual paper](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2290806/)). The paper reports that these cecal valves are "a structure previously unreported for this species and rare in this family and scleroglossan lizards in general"; indeed, "The fact that <1% of all currently known species of squamates have cecal valves ... illustrates the unusual nature of these structures in this population.".

It is possible that these values are an ancestral structure that was reactivated by natural selection; however, we know that natural selection can act very quickly [to change the size of morphological structures](http://news.nationalgeographic.com/news/2006/07/060714-evolution_2.html), so it's also possible that existing muscles quickly changed size in response to selective pressure. Figuring out the developmental mechanism of cecal valves would sort that out.