A scientist is studying a population of crickets and finds that there is variation in the songs used by males to call for mates. Some males call with high-pitched three-chirp songs and some call with low-pitched four-chirp songs. However, offspring from the two types of males are able to interbreed so the scientist classifies the population as a single cricket species and goes on to another project. One year later a mutation arises in a female cricket that makes her unable to hear high frequencies. Ten years later a graduate student returns to the same cricket population and discovers that there are now two distinct cricket species in the area although they appear morphologically identical. What is the most likely explanation?