Answer:
Phenotypic plasticity
Explanation:
Humans have evolved over thousands of years to adapt to various environmental changes. For example, when we are in an environment that requires certain level of adaptation our off spring often formalizes epigenetic traits that lows them to overcome small changes in the environment. Over time certain genes that are turned on may remain on and eventually the ones that are off remains off. This give rise to some genes that "don't work" after millions of years of such evolution we end up with a complex genome with more DNA than "working genes".
It would be generation 5 in which there will be almost 100% non-singing females if there are no changes in the birds’ environment and interactions
