10.6: Species are not always easily defined.

Biologists, like all humans, can be biased. When investigating the natural world, for example, they often focus on plants and animals, to the exclusion of the rest of the earth’s rich biodiversity. This gets them into trouble when it comes to an idea such as the biological species concept. While the biological species concept is remarkably useful when describing most plants and animals, it falls short of representing a universal and definitive way of distinguishing many life forms (FIGURE 10-10).

Difficulties in classifying asexual species. The biological species concept defines species as populations of interbreeding individuals. But this is a useless distinction for the asexual species of the world. Recall from Chapter 6 that asexual reproduction, common among single-celled organisms (including all bacteria), many plants, and some animals, is a form of reproduction that doesn’t involve fertilization or even two individuals. Rather, the cell (or cells) of an individual simply divides, creating new individuals. Because asexual reproduction does not involve partners or interbreeding, the concept of reproductive isolation is not meaningful, and it might seem that every individual should be considered a separate species. Clearly, that’s not a helpful rule to follow.

Difficulties in classifying fossil species. When classifying fossil species, differences in the size and shape of fossil bones from different individuals can never definitively reveal whether there was reproductive isolation between those individuals. This makes it impossible to apply the biological species concept.

Difficulties in determining when one species has changed into another. Based on fossils, it seems that modern-day humans, Homo sapiens, probably evolved from a related species called Homo heidelbergensis about 250,000–400,000 years ago. This seems reasonable, until you consider that your parents—who are in the species H. sapiens—were born to your H. sapiens grandparents, who were born to your H. sapiens great-grandparents, and so on. If humans evolved from H. heidelbergensis, at what exact point did H. heidelbergensis turn into H. sapiens? It may not be possible to identify the exact point at which this change occurred.

Difficulties in classifying ring species. Living in central Asia are some small, insect-eating songbirds called greenish warblers. Unable to live at higher elevations of the Tibetan mountain range, the warblers live in a ring around it. At the southern end of the mountain range, in northwest India, the warblers interbreed with each other. Along either side of the range, the warbler population is split, and warblers on one side do not interbreed with those on the other, because the mountain range separates them. Where the two “side” populations meet up again at the northernmost end of the mountain range, in the forests of Siberia, they can no longer interbreed.

What happened? Gradual changes in the warblers on each side of the mountain range accumulated so that the two populations that meet up in Siberia are sufficiently different, physically and behaviorally, that they have become reproductively incompatible. But because the two non-interbreeding populations in the north are connected by gene flow through other populations farther south, there is no exact point at which one species stops and the other begins. So where do you draw the line? The greenish warblers are just one example of the more than 20 such ring species that have been described.

Difficulties in classifying hybridizing species. Increasingly, hybridization—the interbreeding of closely related species—has been observed among plant and animal species. This phenomenon fits with the biological species concept as long as postzygotic barriers have evolved, so that the hybrids are unable to reproduce. But in some cases, such as among butterflies in the genus Heliconius, the hybrids have high survival rates and are fertile, whether interbreeding with one another or with individuals of either of the two parental species. This suggests that the borders between the species are not clear-cut.

All of these shortcomings have prompted the development of several alternative approaches to defining what a species is. These alternatives tend to focus on aspects of organisms other than reproductive isolation as defining features. The most commonly used alternative is the morphological species concept, which characterizes species based on physical features such as body size and shape. Although the choice of which features to use is subjective, an important aspect of the morphological species concept is that it can be used effectively to classify asexual species. And because it doesn’t require knowledge of whether individuals can actually interbreed, the morphological species concept is a bit easier to use when observing organisms in the wild.

When it comes to species definitions, although the biological species concept is the most widely used and can be applied without difficulty to most plants and animals, we should not expect one size to fit all, and there will probably never be a universally applicable definition of what a species is. From asexual species to ring species to hybridizing species, the diversity of the natural world is simply too great to fit into neat, completely defined and distinct little boxes. Nonetheless, scientists can generally use a species definition that is satisfactory for a particular situation.