There are a lot of simple things out there that aren’t really simple once you start trying to understand and explain them. The concept of “species” is one of those non-simple concepts. I had assumed that I had a good gut understanding of “species” until I read an article called “Speciation,” by Andrew P. Hendry, published in the March 12, 2009 edition of Nature (available online only to subscribers).  Hendry suggests that the term “species” as a technical classification in the field of biology is “ambiguous and amorphous.”  He starts by quoting Darwin, from on the origin of species:

In short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merrily artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be free from the vain search for the undiscovered and undiscoverable essence of the term species.

Hendry suggests that modern biological research has proved Darwin. No universal or

easily applicable concept of “species” exists; instead, more than two dozen approaches exist with regard to “species.”  The most common version is the “biological species concept” (BSC). This definition holds that species are “groups of actually or potentially interbreeding individuals that are reproductively isolated from other such groups (that is, they exchange few genes). Hendry elaborates:

The BSC is sometimes interpreted to imply the extreme situation where two groups are separate species only when successful hybrids cannot ever be produced-and any two such groups certainly are separate species. But many other groups that are widely accepted to represent separate species frequently violate the strict criteria; for example, some estimates hold that 25% of all plant species and 10% of all animal species hybridize successfully with at least one other species. Probably for this reason, the BSC is often relaxed to the point that different groups are considered separate species if they can maintain their genetic integrity and nature. This more useful, albeit more ambiguous, criterion allows for some genetic exchange (gene flow) between species as long as they do not become homogenized.

Hendry then goes on to discuss various challenges to BSC. How can you apply it to groups that are geographically separated (“allopatric”)? If they never attempted to breed (became “sympatric”), we can’t know whether they would be capable of collapsing back into a single species. This problem suggests testing by bringing disparate groups into artificial contact in a laboratory (with its many attendant hurdles).

The second main challenge for the BSC?

BSC cannot be applied to most organisms, including those without sexual reproduction, such as viruses and some microbes, and those that are now extinct. For these reasons, and others, an alternative set of species concepts revolves around the magnitude of morphological or genetic differences between groups. The difficulty for these concepts is in deciding just how big a difference is sufficient to elevate to groups to the status of separate species.

Hendry further suggests that speciation “is best thought of not as a specific endpoint … but rather as an accumulation of reproductive isolation and of morphological/genetic differences through time. This emphasis on speciation as a process has helped to refocus research…”

There are currently 1.5 million species that have been tagged with Linnaean binomials (e.g., Homo sapiens), but this could be the tip of the iceberg; there might be as many as 10 million species on earth. Hendry also mentions that speciation has traditionally been thought of as a slow process, but many species have arisen in less than 15,000 years (cichlid fish species of Lake Victoria, Africa; and numerous northern Hemisphere fish and bird species arising since the last glaciation).

Hendry also examines the engines of speciation. The dominant view has been that speciation was caused by random genetic differences accruing gradually between allopatric populations, eventually giving rise to genetic barriers to reproduction. As Darwin suggested, however, speciation sometimes results when traits undergoing “adaptive divergence might influence mating preferences, such that individuals adapted to different environments now reject each other as mates.” In these cases, the hybrid offspring don’t thrive because they aren’t well adapted to either environment, thus pointing to adaptive divergence as an “engine” of speciation (e.g., Darwin’s finches, stickleback fishes and the many varieties of cichlids of Africa). Another way for species to come into existence is through fusion: “Hybrids between existing species sometimes embarked on separate evolutionary trajectories from their ancestors.” Hendry explains that perhaps more than 10% of plant species have come into existence through fusion.

There is much more information where that came from in Hendry’s fine article. At a minimum, I’ve learned that I don’t know nearly as much as I thought I did when it comes to the “simple topic” of speciation.