I'm not disagreeing with that as such, nor am I agreeing with it, simply because you can't call that evidence without a lot more detail as to why calves are starting to be born without tusks. Also, evolution is nothing to do with 'triggering' genes, as I think Tormod mentioned before, evolution is more like an error, mutation or such like that is beneficial that manages to stay in the gene pool long enough to become common. The most likely cause of tusks not being present in new borns is probably because the previous use of tusks ceased to exist, I don't know enough about elephants to say what the previous use was but thats my best explanation. (probably all seemed a bit hazy but I'm tired sorry )