Evolution

1. Mutation occurs
2. This mutation is most likely harmful (, thus reducing the chance of reproduction therefore halting the propagation of the gene
3. If the mutation is beneficial to survival or encourages more reproduction, (ie more nerves in the vagoo) then the gene will be propogated
4. give this process millions of years and..

>>1
I believe the most common mutations are neutral, actually. Then harmful/fatal, then beneficial.