Species are distinct lineages on the tree of life. Speciation is usually a gradual process as one lineage divides into two. Over time, lineages of sexual species remain distinct from one another because they have become reproductively isolated.

Question 1

Biologists are interested in many aspects of species, and their emphases differ depending on the questions they are asking about species. Biologists who are interested in the processes that led to lineage-splitting emphasize the mechanisms that allow lineages to diverge and to remain isolated from one another after divergence (i.e., how species come into existence and remain distinct from one another). Biologists who are mostly concerned with identifying species and understanding their distribution in time and space are more likely to emphasize historical aspects and how we can tell species apart (i.e., how we recognize species in nature). Still other biologists may be interested primarily in how species co-exist in communities, and these biologists are likely to emphasize the ecological role of species (how do species differ in their adaptations and niches?). Each of these emphases leads different biologists to emphasize different attributes of species.

Question 2

Sexual lineages must have a substantial degree of reproductive isolation or they cannot remain distinct from one another. So even if a biologist is primarily using a morphological species concept, the morphological differences between species cannot be maintained in time and space unless the two species are largely reproductively isolated from each other. Obviously, reproductive isolation is central to the biological species concept. The many versions of lineage species concepts conceptualize species as distinct lineages in the tree of life. If these lineages are to remain distinct from one another through time, they must be substantially reproductively isolated from one another. Thus, reproductive isolation is important (either directly or indirectly) to all of the various species concepts.

Question 3

The biological species concept limits its scope to “actually or potentially interbreeding natural populations.” Clearly, asexual organisms do not form such populations. Nonetheless, they do form lineages that exist in time and space, and these lineages may be maintained by other factors, such as available niche space. Most biologists who emphasize the biological species concept are interested primarily in the mechanisms that lead to reproductive isolation and its maintenance over time. The lineage concepts of species are not in conflict with the biological species concept, but they represent a more inclusive way to think about all species, regardless of their sexuality. The lineage concepts also allow biologists to study species across geological time (where reproductive interactions are not possible).