What is a Species Anyway? Part 1
by Chuck Louch, PTMSC Docent
Cordilleran (at left) and Pacific Slope (at right) Flycatchers
You'd think that biologists would have answered this question long ago but they haven't; it's still a source of contention. And it's not a trivial question because the decision about whether or not to protect a local population of plants or animals depends on whether it is a "true species" or merely a local variety of a more widespread species. This is a very complex and contentious subject so, as you can imagine, biologists have enunciated many ideas about what a species is. In this, and the following essay, I will attempt to describe as simply as possible the three most widely accepted of these ideas.
The earliest descriptions of species were based almost entirely on anatomical features and led to the so-called Morphological Species Concept (MSC), which is essentially the one followed by Linnaeus. For example he used the sexual structures of flowers, among other things, to help classify plants. Incidentally, polite society of his time considered flowers to be the symbol of purity so the idea that they had sex was horrifying to them.
In its simplest form the MSC states that a species is a group of organisms that share a unique combination of morphological features distinguishing it from all other groups. Put another way; different species look different. This is not as simple as it sounds because there is always considerable variation within any group, no two individuals being identical. Therefore someone has to decide how much variation can be tolerated when defining a new species and this always involves some degree of subjectivity. In order to reduce this subjectivity as much as possible a formal protocol must be followed when proposing a new species. Thus, when a biologist describes a new species he or she deposits a reference specimen, called a "holotype", in the research collection of a museum along with a series of specimens to represent variations within the species due to age, sex, and regional variation. This collection can then be referred to by anyone working with this or similar species. Also, a detailed description of the new species, which may include features such as behavior, physiological characteristics, and geographical distribution, is published in a professional journal where people can refer to it.
The MSC is the one we use when identifying a plant or animal by comparing it with written descriptions and pictures in a field guide or when using a taxonomic key. It is important to paleontologists since they usually can't study behavioral or genetic traits but only physical remains. So it is of great practical usefulness. But it's weakness is that it treats species as static, unchanging entities rather than products and sources of evolutionary change. It says nothing about how species arise or change over time. Furthermore, despite all attempts to eliminate subjectivity, it is always there. Those who can tolerate a greater amount of diversity within their species are called "lumpers" while those with a lower toleration for diversity are "splitters". If you're a birder who keeps a life list you probably love the latter and hate the former.
After Charles Darwin upset the apple cart by showing that species are not static entities but are continually changing and giving rise to new forms, biologists felt the need for a new species concept: one that accommodated these new ideas. And so, the Biological Species Concept (BSC) was enunciated in 1942 by the population geneticist, Ernst Mayr. He stated: "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." This definition takes into account the fact that in nature many species may be broken up by geographic barriers into discrete populations that could interbreed if the barriers were removed. Thus Siberian tigers would interbreed with Bengal tigers if only they could get together so they therefore belong to the same species. By interbreeding is meant that individuals of two populations can mate freely with one another and produce fully fertile offspring.
There are two advantages to this definition. First, it provides a clear cut, apparently unambiguous criterion for deciding whether populations that differ from one another in some way belong to the same or different species and second, it is derived from one of the more important theories about how speciation occurs. Put very simply, this theory states that if two or more populations of a particular species become segregated from each other, as by geographical barriers, so that no gene exchange can occur between them (they can't interbreed) they will diverge from one another over time through the collection of genetic mutations and natural selection, until eventually they will be unable to interbreed even if the original barriers disappear. The populations will be reproductively isolated and so have become different species by the definition. This might conceivably happen to Siberian and Bengal tigers in time, provided they aren't wiped out beforehand.
Once established, reproductive isolation may be maintained by a variety of mechanisms. The most basic of these operates at the cellular level when the two sets of chromosomes possessed by a hybrid individual, one set from each parent, are so different from each other that the normal process of gamete production can't occur and the individual is sterile. Thus in a mule, one set of its chromosomes comes from its equine parent and the other set from its donkey parent so that, "In the world of mules there are no rules." Differences in courtship behavior may prevent individuals of different species from responding appropriately to each other, or the species may breed at different times, or in different habitats, or they may be anatomically incompatible. These are just a few of a large number of factors that might bring about reproductive isolation.
Although the BSC is conceptually pleasing to biologists and is the dominant concept today, it does have its problems. Most importantly, it can apply only to organisms that exchange genetic material by means of sexual reproduction and can't apply to asexually reproducing forms such as bacteria and some plants such as the common dandelion (Taraxacum officinale). The former reproduce by simple cell division while the latter does so by means of unfertilized eggs, a process called parthenogenesis. In either case, individuals cannot exchange genes with each other so that a population consists of many clones, each genetically isolated from the others. Is each of these clones a different species? Indeed, is each individual a distinct species? How many species of dandelion do you have in your lawn?
Because of this problem, and others that I will discuss in my next essay, some biologists have abandoned the BSC and have turned to other species concepts, one of which I will discuss next time. So stay tuned!