Because of these problems, much work has been put into modifying and improving the BSC and many alternative definitions of species have been suggested. In general, these efforts highlight how difficult it is to make all species fit easily into one definition. The natural world truly defies neat categorization!

Several useful ideas, however, have come out of this literature. One of these is the notion that a species can sometimes be characterized by its ecological niche, which, as we will discuss further in Chapter 47, is a complete description of the role the species plays in its environment—its habitat requirements, its nutritional and water needs, and the like. It turns out that it is impossible for two species to coexist in the same location if their niches are too similar because competition between them for resources will inevitably lead to the extinction of one of them. This observation has given rise to the ecological species concept (ESC), the idea that there is a one-to-one correspondence between a species and its niche. Thus, we can determine whether or not asexual bacterial lineages are distinct species on the basis of differences or similarities in their ecological requirements. If two lineages have very different nutritional needs, for example, we can infer on ecological grounds that they are separate species.

Another species concept is the phylogenetic species concept (PSC), which is the idea that members of a species all share a common ancestry and a common fate. It is, after all, species rather than individuals that become extinct. The PSC requires that all members of a species are descended from a single common ancestor. It does not specify, however, on what scale this idea should be applied. All mammals derive from a single common ancestor that lived about 200 million years ago, but there are thousands of what we recognize as species of mammals that have evolved since that long-ago common ancestor. But, under a strict application of the PSC, would we consider all mammals to be a single species? Similarly, siblings and cousins are all descended from a common ancestor, a grandmother, but that is surely not sufficient grounds to classify more-distant relatives as a distinct species. The PSC can be useful when thinking about asexual species, but, given the arbitrariness of the decisions involved in assessing whether or not the descendants of a single ancestor warrant the term “species,” its utility is limited.

449

It is worth bearing these ecological and phylogenetic considerations in mind when thinking about species. Although the BSC remains our most useful definition of species, the ESC and PSC broaden and generalize the concept. For example, in the case of a normally asexual species that is difficult to study because conjugation is so infrequent, we can jointly apply the ESC and PSC. We can use the ESC to loosely define the species in terms of its ecological characteristics (for example, its nutritional requirements), and we can refine that definition by using genetic analyses to determine whether the group is indeed a species by the PSC’s standard (that is, that all its members derive from a single common ancestor).

Despite the shortcomings of the BSC and the usefulness of alternative ideas, we stress that the BSC is the most constructive way to think about species. In particular, by focusing on reproductive isolation—the inability of two different species to produce viable, fertile offspring—the BSC gives us a means of studying and understanding speciation, the process by which two populations, originally members of the same species, become distinct.

Quick Check 1 Why haven’t we been able to come up with a single, comprehensive, and agreed-upon species concept?