There is no universal definition of a species, there are different ones depending on usage. Darwin s: i look at the term species, as one arbitrarily given for the sake of individuals closely resembling each other . 1hawaiian honeycreepers have all evolved from a single species in only 10 million years. When we understand the origin of reproductive isolation, we understand the origin of species. Darwin didn t actually focus on speciation or even actually understand speciation at all. The two major concepts over what a species is: The taxonomic concept which is often called the morphological species concept because it s based on morphological species between concepts. But the problem is, that it s somewhat arbitrary. So, genetics came in to give a more objective definition (which can also have problems). However, when you can t cross-breed individuals to test this concept, for example with asexual or hard-to-reach organisms like deep-sea organisms, this is not a great concept.