Final answer:
If fleas develop a preference for cattle blood and mate with others that prefer cattle, sympatric speciation could occur, leading to a more specialized and efficient flea population adapted to cattle as their host.
Step-by-step explanation:
In this hypothetical scenario, if a certain species of flea develops a preference for cattle blood and begins to mate exclusively with fleas that share this preference, we would expect to see a process akin to sympatric speciation over time. This type of speciation occurs when a subset of a population develops distinct ecological or behavioral traits that lead to reproductive isolation from the main population within the same geographical area. As the new population of fleas becomes more specialized to their chosen host, the cattle, they diverge genetically from the original flea population that prefers pronghorn antelopes. This specialization provides them with a survival advantage when feeding on cattle, leading to higher reproductive success among those with the trait preference. This is supported by the observation that host-specialist ectoparasites often perform more efficiently on their chosen host compared to generalist ectoparasites which may have lower performance across multiple host species.