In evolution, "fitness" refers to how well an organism fits into it's environment not necessarily how strong or healthy it is. Hybrid offspring from different species may or may not have better fitness. Mammalian interspecific hybrids are typically sterile (mule) and rarely viable due to basic biological incompatibilites. Plants have better luck and are known to hybridize naturally (temp changes, irradiation, chemically induced) forming new species most likely because polyploidy is pervasive in plants.
