BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.1. Classification

2.1.4. Taxonomic Hierarchy

Of all the hierarchical levels, the term "species" has the most precise definition. A species can be defined as a group of closely related organisms capable of interbreeding to produce fertile offspring. In some cases, the mating of two closely related organisms results in sterile offspring. For example, the mule—a hybrid produced by crossing a female horse (mare) and a male donkey (jack)—is sterile. Consequently, by definition, the donkey and the horse belong to different species.

Mules differ from their parents by possessing greater stamina, which results from inheriting robust traits from both parents (hybrid vigor).

There are exceptions to the rule regarding offspring fertility. For instance, lions and tigers belong to different species. However, while offspring produced by crossing a male tiger with a lioness can sometimes produce fertile offspring, the offspring of a male lion and a tigress are sterile. But since tigers naturally inhabit forests and lions inhabit grasslands, interbreeding between them occurs almost exclusively in captivity.

Each species possesses distinct morphological, behavioral, and ecological characteristics (Fig. 2.2; see also Ch. 27). As we ascend the taxonomic hierarchy, we find that the number of shared traits among members of a group decreases. For example, members of the same genus share a greater number of similarities than members of the same family or order.

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Fig. 2.2. A. Canis familiaris, the domestic dog. Because all dog breeds successfully interbreed with one another, they are classified as a single species. B. Canis latrans (coyote). This canine is widespread across North America and feeds primarily on carrion. C. Canis lupus (wolf). Widespread throughout the Northern Hemisphere, its range overlaps with those of the other two species. There are documented cases of successful interbreeding between coyotes, wolves, and dogs, with the resulting offspring being fertile. This example illustrates the difficulty of establishing a precise definition of a species. It is frequently even more challenging to accurately define broader taxonomic groups, such as genera and orders. All canids belong to the Order Carnivora.

As this Structure/133.html">Discussion shows, formulating an exact definition of a species is practically impossible. This is hardly surprising, given that species undergo continuous change (evolution) over time. According to the theory of natural Selection, this process is driven by the survival of the fittest—that is, the individuals best adapted to a particular environment. When environmental changes occur, selection favors these resilient individuals, ultimately leading to the gradual transformation of the species over generations. When different populations of the same species become isolated from one another by ecological or physical barriers, such as oceans or mountain ranges, their subsequent evolutionary paths may diverge to the point where interbreeding becomes impossible, eventually giving rise to distinct new species.

In some cases, genetic differences between distinct species may not be sharply defined. For example, the herring gull and the lesser black-backed gull are classified as separate species because they differ in Morphology and behavior and typically do not interbreed. Nevertheless, they occasionally nest in the same areas and form pairs (Ch. 27).