What is Species?

Today I’m going to be killing two birds with one stone, so to speak, answering two related questions from the website. Mr. Neil and Asimov both wrote in to ask about the concept of “species.” This is an excellent question because this cuts right to the heart of what most people think about when they consider evolution- the formation of new species. And if you remember from the Darwin Day podcast, the mechanism of natural selection was what Darwin proposed as the way for new species to originate. But the title of his book, “On the Origin of the Species,” assumes the concept of species, so what exactly is it?

Although you’d think that it’d be a simple matter to distinguish between two species (for example, a cat from a dog), like most aspects of science, this is not the case. As a matter of fact, there does not yet exist a universal, objective definition for species. There do exist several imperfect methods of classification that have been used a various times, and though taken together, they are able to come very close to a truly objective standard, on their own they are lacking.

The first category is typology. This means that all members of a species conform to a set of fixed characteristics, and goes back to the classical standard of defining a new species. Once a new organism was discovered, the first example was killed, stuffed, or fixed in some manner, and then brought back to a museum and placed in a drawer. This was called the “type specimen,” and any other organisms that were thought to belong to its species would have to be compared to it. However, we know now that different physical characteristics can exist in members of a population without necessarily implying a difference in species.

The next category is Morphometry. This means that all members of a species will share certain physical characteristics. This is similar to what I just described with typology, with the subtle difference that instead of beginning with a type specimen, individual organisms are grouped by the way they look. While this seems to be common sense, the study of genetics has shown us that some organisms can look very similar without being closely related.

This next category is probably the most well-recognized- that of sexual isolation. This means that all members of a species are able, or are potentially able to interbreed. As some of you may be thinking, this completely precludes those organisms which reproduce asexually. This is true, and I’ll address that later. Sexual isolationism depends on the fact that usually there exists some kind of geographical barrier that separates two groups of one population for an extended period of time, during which both groups evolve independently of the other, and are eventually sexually incompatible for any number of reasons. The most glaring problem with this category are hybrids. Hybrids are the progeny of a sexual union between two organisms which are classified as separate species, but which are still closely related. Mules, for example, are the result of a cross between a horse and a donkey. Tigers and lions are also somewhat famously known to hybridize. In most cases, the hybrid progeny is sterile, and so the sexual isolationist could say that hybrids are no challenge to this definition of species, however, a small percentage of hybrids are able to breed back to one of the parental species, and so it would theoretically be feasible to generate a fertile, true-breeding hybrid species given enough time and resources.

The only other way to define a species is by the category of phylogeny. This means that all members of a species have a common ancestor, and that the lineage of the species is continuous. Now, taken on its own, it would be difficult to distinguish one species from another, since all organisms ultimately have a common ancestor, and so divergence between different groups are assumed based on evolutionary mechanisms. The most common way to do this now is through genomic analysis- by comparing the DNA sequences of different organisms we can determine phylogenetic relationships and thus distinguish between species.

But where does that leave asexual organisms? Well, we just do the best we can, and omit the third category I mentioned, that of sexual isolationism. This would include animals which reproduce by parthenogenesis, plants which reproduce by apomixis, as well as all bacteria, archaea, and protists. For these organisms, the concept of “species” is more like a temporary tag than anything stamped in metal. Evolutionary forces exerted on these organisms make their genomes more plastic through time, so they seem to be constantly in phenotypic flux. Sexual organisms, by comparison, have traded vertical plasticity for horizontal plasticity, and have populations with a high amount of genetic variation instead.

However, sometimes this potential for genetic variation can lead to situations where even sexual isolationism isn’t truly a cut-and-dry distinction. The evolution of new species often involves geographic variables, such as altitude, latitude, or bodies of water. Remember, the selective force that drives evolution is the ability of a population to adapt to a particular environment- if confronted with a different environment, a population has to adapt different characteristics. If those geographic variables exist linearly- that is, in a straight line- then we would expect to see a linear range of different species. We would also expect that similar species existing close to each other along that line would be able to interbreed or hybridize to a certain extent. What would happen, though, if instead of using a straight line, a population evolved along a circle? Let’s say, around a mountain range, or around a lake? This would then be called a “ring species.” What makes a ring species troublesome is that, if you start at the beginning of the ring and follow the different species in one direction, you find that each neighboring species can interbreed, until you come “full circle” and are back at the original species, which can not interbreed with the final species. There are several good examples of this, including the Ensatina salamanders of the California Simi valley, the Greenish warbler of Eurasia, and the Larus gulls which circle the Arctic.

I realize this is hard to visualize through a podcast, but try to imagine a circle drawn with the letters A through E written clockwise along the circle. If each of these letters represents a different subspecies, then A can interbreed with B, B can interbreed with C, C can interbreed with D, and D can interbreed with E. So far, this group of subspecies would be considered one big species, since in totality they can interbreed. But if you pair A and E, they cannot interbreed. So this poses a problem for those who would argue that sexual isolationism is the best objective criterion for defining a species: Clearly A and E are separate species by that definition, but there exists a continuous range of other populations that can interbreed between the two extremes. In this situation, the entire group of subspecies is considered a ring species. If, in the future, the intermediate subspecies B, C, and D go extinct for any number of reasons, then we’d be left with only subspecies A and E, in which case there’d be no problem calling them separate species.

Now, the concept of a species, and the various criteria that go into forming that concept, contain within them the idea of species organization. Because after all, what good is it dividing different organisms into species if you can’t arrange them in groups? And in fact, it was a man named Carl Von Linne (or Linneus) who first began to systematically divide organisms into species as part of his classification of all living creatures. It because of him that the scientific name of all organisms involved two names- the genus followed by the species name, usually in Latin or Greek. This organization, called taxonomy, is based on the principle of hierarchical organization. As you go up the hierarchy, you encompass more individual organisms. So, a species includes all populations of one organism, a genus includes all related species, a family includes all related genuses, all the way up to Kingdom, which Linneus considered the highest classification possible.

The problem with Linnean taxonomy is that it only organizes according to shared characteristics. While shared characteristics often accompany common descent, this is not always the case. A more meaningful method of taxonomic organization is called cladistics, in which taxonomic relationships are defined based on shared characteristics derived from ancestral population. A written example of this organization is called a cladogram, and looks something like a family tree, which is essentially the same kind of relationship that is implied. In a cladogram, however, each branch point only contains two branches, so that the organisms which are descendent from that stem can be divided into two groups based on one characteristic that one group of organisms share that the other group does not. In this way, evolutionary relationships can be inferred- two species that are separated by only two branch points on a cladogram are more closely related than two species that are seaparated by five branch points. In molecular biology, these branch points are determined by nucleotide or amino acid sequence similarity, and one can predict evolutionary relationships based on the cladistic hierarchy of a species’ genome.

So, just to review everything I’ve talked about- the concept of species is not a clear-cut as most people assume- in fact in some instances, like that of a ring species, it can be downright paradoxical. But regardless, there still exist many ways, through morphology and genetics, that we can distinguish between different types of organisms and even confirm evolutionary relationships between them.

That’s it for this episode of the Evolution 101 podcast. I hope you’ve noticed the new logo that was designed by our listener Sasha Richey.