maybe yes and maybe not. but we need to check how much time it will need to this change. first- we need a lot more then a change in a bone structure to make a fish a tetrapod. even if its need only one new system to this change (lets say a system need for a land living). it will need at least 3-4 new parts (mutations). so even if each part appear one in a bilion mutations then it will need more then 10^30 mutations to make a fish into a tetrapod.