If the spotted retics were selected from a larger population of frogs, then you HAVE narrowed the gene pool. You have selected only those alleles that lead to expression of the spotted trait and thrown away the rest. The result is a line of frogs that look more like clones of each other than the variable mix you would find in nature. It is impossible to know what genes get lost in the selection process. Who knows? There could have been a gene that might have someday produced a blue retic. Variability in nature is good because it allows animals to change and adapt to changing conditions. Selective breeding goes in the opposite direction.