Indeed it is up to people that are trained in the Biological Sciences to attempt to explain to people like him processes involved in evolution that can be complex and difficult to fully fathom for even well informed scienctists. Other wise misinformation will become epidemic in our society. Mutations are the only known source of new alleles that current evolutionary theory allows. Also the possibility of acquired characteristics should always be considered although it has never been proved on any significant scale.
Chance plays the defining role here.
If one studies Mendel Genetic theory one can gather better how this works. 3.Gene flow-This is a change in trait frequencies brought about by immigration or emigration of a species. This can encourage or discourage certain traits being prevalent in a species over time. Change in trait frequencies due to chance events that can create favorable survival traits for a member of a species. There are perhaps other forces at work causing evolution that we have not fathom.