Final answer:
Species are defined under the Biological Species Concept by their ability to interbreed and produce fertile offspring, though this is not always observable in practice. Alternative definitions, such as the phylogenetic species definition, use unique traits to distinguish a new species.
Step-by-step explanation:
Under the Biological Species Concept (BSC), a species is defined by the presence of reproductive compatibility. This means that members of the same species are similar enough to each other to produce fertile offspring together. For instance, all human beings belong to the Homo sapiens species because we can potentially interbreed with each other but not with members of any other species. However, there are limitations to this definition, as it is not always practical or possible to determine the breeding behaviors of organisms, especially in the case of extinct species or those that reproduce asexually. In the case of organisms where interbreeding cannot be observed, such as fossils, species are often identified by morphological features or genetic characteristics. The presence of unique traits within a set of organisms can distinguish them as a new species under the phylogenetic species definition. If the same species shows different colors, this variation typically reflects intraspecific diversity, which is the normal range of variation found within a single species. This kind of biodiversity is essential for the resilience and adaptability of a species in changing environments.