This is one reproductive isolation and speciation.
In other words how we go from one species to two. and we do that through reproductive isolation them reproductively. example of this,a really good example of this is Diane Dodd in the 1980s she took a group of fruit flies and she just fed them different things. So this group ate just starch and these ones ate just maltose. And after eight generation,when they were done,a group of individuals