Evolution is decided by selection (natural, or environmental) and all kinds of constraints such as physical, physiological, developmental and genetic constraints.
Emm.... Will those mechanisms be able to exist is still a question... Bacteria can evolve fast because they are single cell organisms, they don't really need to worry that if you change you neck then the brain had to be changed as well. Mutations can easily be adopted by the cells because of the high degree of evolution freedom. But higher organisms have their own way of adopting large genetical changes: sexual reproduction can exchange the whole genome material. This way they still have high evolvability. For those multicellular organisms which abandon sexual behavior will have short evolution future (except for one species).
Oh, I have a phone call.
I'm back. Wow, I wrote a lot.
