multiple choice question\nwhich is left unexplained by the biological species concept, thereby weakening the…

Brief Explanations:

The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. Viable but sterile hybrids like mules cannot produce fertile offspring, which challenges the concept's focus on inter - breeding and fertility. Natural and captive hybridization can still be related to the concept in terms of potential gene flow and reproductive isolation in some cases, but sterile hybrids directly conflict with the core idea of the biological species concept.

Answer:

The existence of viable, but sterile hybrids (such as the mule)