Final answer:
A species is defined as a group of organisms capable of interbreeding and producing fertile offspring. Distinct species cannot produce fertile offspring together, although closely related species can sometimes produce hybrids, suggesting that speciation is a gradual process. Defining species in practice can involve exceptions and challenges.
Step-by-step explanation:
The biological definition of a species encompasses a group of individual organisms that are capable of interbreeding and producing fertile offspring. This facet of reproductive compatibility is crucial because it ensures that genes can be passed on from one generation to the next within the species. If two different organisms can mate and have offspring that are also capable of reproducing, they belong to the same species. One species is considered distinct from another when their members cannot interbreed to produce fertile young, a common example being the inability to produce fertile offspring typically observed in different species that occasionally produce hybrid progeny.
In situations where hybrids are formed between similar species, these instances suggest a close genetic relationship that may indicate that speciation, the process where new species arise, is not entirely complete. The occurrence of such hybrids can often be observed in nature, although they usually cannot reproduce. Furthermore, defining a species has practical challenges, such as asexual reproduction or extinct species for which interbreeding cannot be observed. Thus, while the concept of producing fertile offspring is integral to the biological species concept, exceptions and practicalities of species identification can complexify this definition.