Q.Why mule is sterile in nature?

Species is the basic, most fundamental unit of the entire taxonomic hierarchical system — every higher rank is ultimately an assemblage built up from species. A species is defined as a group of organisms that share similar morphological features (traits) and, critically, are reproductively isolated from other such groups in the sense that they can interbreed among themselves to produce fertile offspring.

This reproductive-isolation criterion is what makes species biologically meaningful rather than just a description of appearance, but the textbook also flags an important exception: some closely related species can still mate with one another, and while this occasionally happens in nature or under human management, the resulting hybrid offspring are typically sterile rather than fertile. Classic examples include the cross between a male horse and female donkey, which produces a sterile Hinny, and the reverse cross between a male donkey and female horse, which produces a sterile Mule; similarly, a male lion crossed with a female tiger produces a Liger, while a male tiger crossed with a female lion produces a Tigon. Because these hybrids cannot themselves reproduce fertile offspring in turn, the parent lines remain, by definition, separate species rather than merging into one. …