Species - How it works

Taxonomy in Brief

The concept of species falls under the heading of taxonomy, the area of the biological sciences devoted to the identification, naming, and classification of living things according to apparent common characteristics. Taxonomy is discussed in detail within the essay on that subject, but to appreciate the topic of species in context, it is helpful to have at least some knowledge of the larger subject. At one time taxonomists were concerned most with the morphological characteristics (i.e., the structure or form) of organisms as a basis for classifying many species within a larger grouping. Today, however, shared evolutionary lineage is much more important than morphological features in determining whether taxa (plural of taxon, meaning a taxonomic group or entity) can be classed together. Organisms may be linked closely in terms of evolutionary lines of descent but differ in a particular morphological aspect as a result of the adaptive changes that accompany natural selection. The latter, a key concept in the theory of evolution put forward by the English naturalist Charles Darwin (1809-1882), is a process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

It is therefore possible for organisms in a particular environment to develop a common adaptive mechanism through generations of natural selection, even though those organisms themselves are not related to fish closely in terms of evolutionary line of descent. Thus, whales and dolphins, mammals that live underwater, evolved the ability to swim just as well as fish, but that does not mean they are connected closely. Conversely, organisms may be close, or relatively close, in terms of evolutionary lines of descent yet differ in significant morphological features. To use the whale and dolphin example again, these creatures are classified as mammals owing to certain particulars (discussed later), but they differ from the vast majority of mammals in that they have no legs. They do, however, have four appendages, just like the rest of the mammalian class; as a result of natural selection, however, theirs ceased to operate as legs (an encumbrance for life in the water) a long time ago, and today they function instead as fins.

Obligatory Ranks

The classification system used today is an outgrowth of a system developed by the Swedish botanist Carolus Linnaeus (1707-1778) in the 1730s. The realms of zoology and botany, areas of biology devoted to the study of animal and plant life, respectively, differ somewhat with regard to their classification systems, but both use international codes of nomenclature with roots in the Linnaean system. There are many possible ranks of classification, but only seven are obligatory, meaning that all species must be assigned a place in these groupings. The obligatory ranks are listed here. The entire list of rankings, including versions of obligatory ranks with such prefixes as sub-, super-, and infra-, as well as such additional ranks as cohort or tribe, are given in Taxonomy. Note the difference between the zoological and botanical names for the second rank.

Obligatory Taxonomic Ranks

• Kingdom
• Phylum ( Division in botany)
• Class
• Order
• Family
• Genus
• Species

As discussed in Taxonomy, this book uses a system of five kingdoms, whose characteristics are defined in that essay. Even at the level of kingdom, not everything is delineated precisely (see the discussion in Taxonomy), and there are significant areas of dispute. For example, some taxonomic systems include viruses. Because viruses are not cellular in structure and are not universally regarded as true organisms, however, they are not included in the five-kingdom system used here.

Below the level of kingdom, definitions become even more difficult. Organisms are grouped into phyla on the basis of body plan or organization, but there is no regular pattern for grouping within the smaller categories. For example (as noted later herein), humans are placed within their particular phylum and sub-phylum on the basis of their spinal columns and overall internal bone structure, but those specifics play no significant role in categorizing them within any of the more specific groupings to which they belong. Furthermore, the generic definitions of the categories—for example, class as opposed to class Mammalia, class Insecta, or some other class in the taxonomic system—are purely relative. In other words, class is simply the obligatory rank that is more specific than phylum but more general than order.

DESIGNATING A SINGLE SPECIES WITHIN THE RANKS.

When preparing an outline for a paper, students are taught that no topic should have only one subheading; instead, that solitary subheading should be moved up one level. Such rules do not apply in taxonomy, and it is not necessary that there be more than one subgroup within a larger group. For example, there might be only one class in a phylum. Taxonomists use detailed definitions to single out particular groups, such as class Mammalia. The following list shows the placement of humans within the larger taxonomic universe, along with brief explanations of a few (though far from all) characteristics that define each group.

• Kingdom Animalia: Multicell eukaryotic (that is, possessing cells with a nucleus and specialized compartments called organelles) organisms that obtain their nutrition solely by feeding on other organisms. (Other defining characteristics of Animalia are discussed in Taxonomy.)
• Phylum Chordata: Animals whose bodies, at least at some point in their life cycles, include a rudimentary internal skeleton with a stiff supporting rod known as a notochord. All chordates at some point also breathe through gills (in the case of a human, while still in the womb). Other characteristics set apart chordates, including a tail or the remnants of one. Humans belong to the subphylum Vertebrata, or chordates with a spinal column.
• Class Mammalia: Vertebrates that feed their young from special milk-secreting glands, known as mammae, located on the mother's body. Mammals have other distinguishing characteristics, such as a hinged lower jaw attached to the skull.
• Order Primates: A group of mammals whose characteristics may include some version of an opposable digit (e.g., the human thumb) and other features that, while they are prevalent among most primates, are not universal to them. Not every one of these traits is exclusive to primates, a group that includes prosimians (e.g., lemurs), monkeys, apes, and humans.
• Family Hominidae: Primates noted for their erect posture, large brains, rounded skulls, small teeth, bipedal locomotion (i.e., they walk on two legs), and tendency to use language for communication. Humans are the only surviving species in the family, but extinct hominids include Homo habilis (about 1.6 million years ago) and H. erectus (about two million years ago) as well as the more distant Australopithecus (about eight million years ago).
• Genus Homo : Hominids with especially large skulls as well as the features that characterize family Hominidae. Members of this genus, which included H. erectus and H. habilis as well as H. sapiens, also are known for their ability to fashion precise tools.
• Species Homo sapiens : Members of the genus Homo ("man") noted for, among other things, the ability to use symbols and writing. This category includes modern humans and the extinct Cro-Magnon and Neanderthal man.

Note that the proper name of any ranking more general than species is capitalized (e.g., phylum Chordata), with species (and subspecies) names in lowercase. Genus, species, and sub-species names are rendered in italics (e.g., Homo sapiens, or "man the wise"), whereas proper names of the more general groupings are presented in ordinary type (e.g., class Mammalia). If the same name appears a second time in the same article, the genus name usually is abbreviated: thus, H. sapiens. Another important abbreviation is spp., implying several species within a genus—for example, Quercus spp. refers to more than one species of oak.

Taxonomy makes use of a system called binomial nomenclature, in which each species is identified by a two-word name, designating genus and species proper. Beyond the species name, there may be subspecies names: humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. Additional rules govern the inclusion of a name or an abbreviation at the end of the species or sub-species name, to recognize the person who first identified it—in this case, Linnaeus. Hence the proper full name of our species is Homo sapiens sapiens Linneaus, 1758.

The Mystery of Species

If one studies the delineation of humans' place in the overall taxonomic structure, one may notice that for several groupings, the defining characteristics are a bit "fuzzy around the edges." This is true even of the animal kingdom, as noted in Taxonomy: mobility and locomotion, seemingly so integral to the definition of animal, are not prevalent among all animal species. Given the many gray areas and areas of dispute in the larger taxonomic categories, it should come as no surprise that the smallest of the obligatory rankings, that of species, lacks a precise definition.

The most widely accepted definition of species is the one put forward by the Germanborn American evolutionary biologist Ernst Mayr (1904-) in the 1940s. Mayr's idea, known as the biological species concept, defines a species as a population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. Members of two different, but closely related species in some cases can mate with one another to produce infertile offspring, the most well-known example being the mule, a sterile hybrid produced by the union of a male donkey and a female horse.

The definition offered by the biological species concept requires qualification. While many plants and animals reproduce sexually, many more do not; no single-cell life-forms reproduce in this way, yet there are certainly many different and distinct species of bacteria and protozoa. Thus, a further qualification typically is added to the definition of species : members of the same species share a gene pool, or a total sum of genes. Genes carry information about heritable traits, which are passed from parent to offspring. Whereas the gene pool is shared by members of a species, nonmembers of that species have genes that do not belong to that gene pool. To use a rudimentary example, let's say that there is a gene pool containing genes x, y, and z. Individuals that have these genes fit within the gene pool, but an individual with gene w does not.

The definition of species remains challenging, with special problems raised in the area of botany. It is also sometimes possible to confuse

species and race, a grouping that applies not only in the world of humans but also that of other animal and even plant species. Race is different from species inasmuch as races are not isolated genetically from one another; in other words, there are no biological barriers to interbreeding between races. (See Speciation for a discussion of the process whereby single species develop over time into more than one reproductively isolated species.)