Species

Species is the basic, most fundamental unit of the entire taxonomic hierarchy — every higher rank is ultimately built from species. A species is a group of organisms sharing similar morphological features (traits) that are reproductively isolated, meaning they can interbreed among themselves to produce fertile offspring.

This reproductive-isolation criterion is what makes species biologically meaningful rather than a description of appearance alone, but there is an important exception: some closely related species can still mate, and while this occasionally happens, the resulting hybrid offspring are typically sterile rather than fertile. The textbook illustrates this with four crosses: a male horse crossed with a female donkey produces a sterile Hinny; a male donkey crossed with a female horse produces a sterile Mule (the horse's 64 chromosomes and the donkey's 62 chromosomes cannot pair up properly during meiosis in the 63-chromosome hybrid, so it cannot form viable gametes); a male lion crossed with a female tiger produces a Liger; and a male tiger crossed with a female lion produces a Tigon. Because …