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The Species Problem Part 2

by Chuck Louch, PTMSC Docent

Resident Orca	Trascient Orca

 

At the end of my last essay I was discussing the Biological Species Concept or BSC which states, to refresh you memory, that species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. The chief advantage of this definition is that it correlates well with the major theory about how species arise. But as I pointed out, it doesn't work very well with organisms that reproduce asexually.

Another more serious problem with the BSC is that hybridization between described species is not at all uncommon in plants and animals. Thus, about 10% of North American bird species are known to hybridize with other species to produce fertile offspring. A glaring example of this is found in the herring gull group (Genus Larus). The Herring Gull itself hybridizes extensively with the Lesser Black-backed Gull over much of its range. It also hybridizes with the Glaucous Gull in Iceland, the Glaucous-Winged Gull in Alaska, and the Slatey-Backed Gull in Siberia. Furthermore the Icelandic, Western, and Mexican Yellow-footed Gulls form an interbreeding group. And we all know about our local Western and Glaucous-Winged Gulls! So, if you' re confused by gull identification, cheer up, so are the gulls!

Another kind of problem is presented by our Pacific salmon in which each individual goes back to its natal stream or tributary to reproduce. This means that we have many local breeding populations of, say, Chinook salmon that are reproductively isolated by their migratory behavior. Does the BSC say that each of these populations is a distinct species? Apparently not since all chinook are classed as (Oncorhynchus tshawyitscha). But, according to the BSC does that make sense?

Botanists don't find this concept very useful because a great many plant species can hybridize quite freely to produce fertile offspring.

Finally, this concept might be difficult to apply if you were a field biologist and found what you thought was a new species because it could take years of research for you to discover whether or not it could interbreed with other species. And if you were a paleontologist, the concept would be absolutely worthless because all you have to work with are long dead fossil remains.

For all of these reasons, and because of new ideas about how speciation occurs, many biologists are turning away from the BSC and to other concepts, one of which I will discuss here. This is the Phylogenetic Species Concept (PSC) that was first promulgated in the1980's and is becoming increasingly acceptable to taxonomists and students of evolutionary theory despite the fact that there is still a good deal of confusion about what it says. For instance, Joel Cracraft, one of its early proponents, has made several attempts to define the concept. One of his definitions states: "A species is the smallest diagnosable cluster of individuals within which there is a parental pattern of ancestry and descent." In other words, a species is a group of individuals with a unique combination of traits derived from a common ancestor that sets it apart from all other groups. Stated in still another way, a species is a group of organisms that have a common, unique evolutionary history. Thus all Western Gulls share a unique set of characteristics produced by a common ancestry that sets them apart from all other gulls. The same can be said for Glaucous Winged Gulls. And so they are separate species despite their ability to hybridize.

With this definition it doesn't matter whether a putative species can or cannot interbreed with other similar groups. What matters is that it is, and remains, differentiated and distinct from all other groups. Also, this concept is not directly concerned with the mechanisms of speciation but only with the results and so doesn't have to be modified as ideas about speciation change, as they inevitably will.

At first glance the MSC (the Morphological Species Concept of Linnaeus) that we discussed last time) and PSC seem pretty much the same. But while the former isn't necessarily associated with evolutionary theory the latter is. As with the MSC, the criteria used to describe a species can be morphological, physiological and behavioral. In addition, new techniques for studying base sequences in DNA have provided a powerful tool for characterizing a species and tracing its evolutionary history. This has made it possible to tease out evolved differences and relationships between groups that were not evident with more traditional criteria.

But still the element of judgement remains because the researcher has to decide how much genetic variation she or he can tolerate in a species. Also, how are the differences between species maintained if not through reproductive isolation?

Nevertheless bird listers like this concept because it has enabled them to enlarge their species lists. For instance, in our area the Western Flycatcher has been split into the Pacific Slope and Cordilleran Flycatchers on the basis of DNA evidence. And recently it has been proposed that the Pacific Slope population residing on the Channel Islands be declared a third species. Are these real species or just varieties of a single species? Well it depends on your concept of what a species is.

Thus I hope you can see that the species concept is a slippery beast at best and hard to pin down. This is necessarily so because organic evolution is a continuing process that we don't fully understand. Nothing is static. Everything is constantly changing. Old species are continually giving rise to new ones or disappearing, and new species are in the process of diverging or maybe even coming back together again. Therefore some biologists feel that what we call "species" are simply points on an evolutionary continuum. They are ephemera, here today and gone tomorrow. So maybe we should follow Darwin's lead and agree that species are not real entities in nature but simply convenient categories for helping us in our studies of the natural world.

Reams have been written on this subject and it is too much to hope that this very brief exposition can cover it adequately, but I hope that I have at least managed to give you some idea of the problem and how biologists have tried to solve it. Is it important? That's for you to decide.

And now for a quiz: As you know there are two distinct populations of orcas (Orcinus orca) living in our coastal waters, the Transients and the Residents. They differ from each other in a number of ways, which I will list very briefly. Residents eat fish; vocalize a lot; are organized into fairly large family groups called pods; travel around in fairly restricted and predictable areas; move directly from one point to another seldom entering small bays or coves as they travel. Transients eat marine mammals; are mainly silent; live in smaller family groups; swim close to shore entering small bays or coves as they travel. In addition to these behavioral differences the two groups can be distinguished by means of DNA, while morphological differences make it possible to differentiate them in the field. As shown in the following diagrams, the dorsal fins of Resident females are more rounded while those of the Transients are more pointed. Furthermore, the saddle patch (the gray or white area at the base of the dorsal fin) is further forward on the Transients than on the Residents. Finally, when Transient and Resident pods meet, they tend to avoid each other.

So, on the basis of what you know about the species concept, do you think we have two species of orcas in our region or just one? (We're ignoring the newly identified Offshore population).

References: Osborne, Calambokidis, Dorsey. A Guide to Marine Mammals of Greater Puget Sound. Island Publishers. 1988. Cracraft, Joel. 1989. Speciation and its Ontology. Sinauer Associates. David Allen Sibley, el al. The Sibley Guide to Bird Life and Behavior. Alfred A. Knopf, 2001 Joel Welty. The Life of Birds. Saunders College Publishing, 1973. Ernst Mayr. Systematics and the Origin of Species. Harvard University Press, 1942 The Golden Trashery of Ogden Nashery There is a tremendous amount of information on the Internet.