What does it mean if a hybrid is described as sterile?

When a hybrid is described as sterile, it means that the organism is viable and can live after birth, but it cannot produce offspring. This commonly occurs in hybrids formed from two different species, where differences in chromosomal structures or reproductive mechanisms prevent successful reproduction. A well-known example of this phenomenon is seen in mules, which are hybrids of a horse and a donkey; they are healthy and live normal lifespans, but they are unable to reproduce.

In this context, the other options do not accurately describe sterility. The first option implies that the hybrid cannot survive at all, which is not true since a sterile hybrid can still live. The second option suggests that the hybrid can reproduce but produces weak offspring, which contradicts the definition of sterility. The last option states that the hybrid has a higher fitness than its parents, which does not relate to their ability to reproduce, making it irrelevant in the context of what sterility means. Thus, the correct understanding of sterility in hybrids centers on their viability without the capability for reproduction.