What term describes hybrids that are sterile or have low fertility?

The term that best describes hybrids that are sterile or have low fertility is hybrid infertility. This phenomenon occurs when two different species interbreed, and the resulting hybrids cannot produce viable offspring. Hybrid infertility is a crucial concept in the study of speciation and reproductive barriers, as it highlights a postzygotic mechanism that reduces gene flow between species.

This understanding is particularly important in natural contexts, such as when two species occupy overlapping habitats but do not fully merge into one due to infertility of hybrids. This situation maintains distinct species even in the presence of hybrid offspring. The term captures the essence of the biological challenges that arise when different species attempt to reproduce, leading to a breakdown in the continuation of those hybrids.

Hybrid vigor, on the other hand, refers to the phenomenon where hybrids exhibit improved biological qualities compared to their parents, which is the opposite of infertility. Postzygotic isolation is a broader term that includes various mechanisms that prevent hybrid offspring from becoming viable or fertile, but it doesn't specifically highlight the sterility of hybrids, while behavioral isolation pertains to the prezygotic differences in mating behaviors that prevent hybridization altogether.