The species is the lowest generally recognized division of the Linnaean hierarchy of life.
Thus every species belongs to a genus, while genera are division of families, and so forth. Every species that has been discovered and named by biologists is called by both its generic and specific name: for example, our own species is called Homo sapiens. Homo is the genus to which our species "sapiens" belongs (along with other fossil species, such as Homo erectus). Generally, scientific names of species are italicized; once the name has been spelled out completely in a text, it is permissible to write it in abbreviated form—for example, H. sapiens.
Thus species are a category of life, and every particular example—again, such as Homo sapi-ens—is an actual historical entity, referred to as a taxon (pl.: taxa) by systematists. Taxa can be of any rank: thus a family in the Lin-naean system is another category, higher than the species; Family Hominidae, to which our species belongs, is also a taxon—a particular collection of related species belonging to several different genera.
Biologists are still uncertain how many species of plants, animals, fungi, and microorganisms currently inhabit the earth. So far, just under 2 million species have been discovered and named. Although the great majority of bird and mammal species are thought to have been discovered and already named, many fishes and marine invertebrates, as well as tropical plants and insects, are thought to remain undiscovered. Bacteria and other microbes are probably the least well known of all the major divisions of life.
In view of the rapid loss of species currently underway (see Sixth Extinction), biologists have redoubled their efforts to find and name as many species as possible before they are lost to extinction. And they have tried to sharpen their estimates about the total number of species currently in existence. According to one estimate (based primarily on the rate
of discovery, plus estimates of the number of species per hectare typically found in places like tropical rain forests and coral reefs), there are at least 10 to 12 million species on earth. Other estimates range even higher—more than 100 million. That we have traveled to the moon but still do not know within an order of magnitude how many species there are on earth is both testimony to the state of our biological ignorance and a direct signal of how much exploratory and analytic work needs to be done on species.
Most of the species that have existed on earth are already extinct—and, of course, we have no precise idea how many species have existed since the inception of life more than 3.5 billion years ago. But the process of evo lution continually produces new species (through the process of speciation), and species diversity is considered to be at least as high now as it ever was during the history of life.
Species occupy a special place in evolutionary biology. Unlike the other categories of the Linnaean hierarchy, which are simply ever-larger collections of related species, species are the bottom-line division. Species are composed of organisms, not taxa, and organisms within a species are capable of interbreed-ing—of producing offspring with one another. Thus, from the standpoint of evolutionary biology, species can be defined as "groups of organisms capable of interbreeding." This definition (a shortened version of the so-called biological species concept) is important because it stresses the genetic connectedness that holds a species together. This species concept is most directly applicable to sexually reproducing organisms, such as most animals (for example, mammals, birds, insects) and higher plants; it is somewhat more problematic when it comes to strictly asexual organisms and microbes—perhaps especially bacteria. Yet bacteria and many forms of life that reproduce asexually are also capable of exchanging, and occasionally do exchange, genes between one another, so in an extended sense, the notion of species as the greatest collection of breeding individuals applies to all of life.
As reproductive entities, species can be thought of essentially as packages of genetic information. Like each individual organism, every species has an origin (a beginning), a history, and an end (through extinction). Species tend to be very long-lived: The average age of a marine species is between 5 and 10 million years, and some species last a lot longer than that. Life on land seems a bit more precarious, and rates of evolution (origination of species, or "speciation") and extinction are on average faster on land than in the sea. Terrestrial mammals, for example, seem to last only about 1 to 3 million years.
Yet such longevities of species came as something of a surprise to evolutionary biologists. Darwin himself originally felt that species, in a sense, do not exist as real, stable entities. Darwin's task was to convince the world that life had evolved; the biology of his day saw species as immutable—permanent entities each created separately by a Divine Creator. To show that there could be connections—evolutionary transitions—between species, Darwin essentially argued that species are transient entities: The different kinds ("species") you might see visiting the bird feeder in your backyard might look sharply different from each other, but those differ ences, Darwin thought, are bound to change. More closely related species resembled each other still more closely in the not-so-distant geological past, and they are bound to deviate from one another still further as time goes on and evolution keeps working.
Thus, to Darwin and most other early evolutionary biologists, species are ephemeral entities—almost like progress reports of a continual process of evolutionary transformation. However, we now understand from the fossil record that species have discrete origins and persist, usually recognizably unchanged ("stasis"), and eventually become extinct.
An important key to the puzzle of species— that is, what species are and the role they play in the evolutionary process—came in the work of the geneticist Theodosius Dobzhan-sky and the bird systematist Ernst Mayr (see also Evolution; Speciation). Previously, biologists looking at modern species thought of them as collections of similar organisms that happen to interbreed. They did, however, realize that sometimes females of different species, such as some American warblers and finches, look more like one another than they do the springtime males with which they pair up, occupy a territory, and produce one or more broods of offspring. Naturally, biologists were forced to keep males and females together in the same species, even though, strictly speaking, sometimes some members of a species looked more like members of other species.
Dobzhansky and Mayr, in effect, simply reversed the logic. Species, they said, are breeding communities—the largest group of animals who share adaptations allowing them to interbreed. For that reason, the individuals within a species share a pool of genetic information— and that is why the members of a species tend to resemble one another more closely than they do members of other, even closely related, species. That males from different closely related species may appear more different from one another than females, at least among some birds, simply reflects the reproductive adaptations that ensure that females will mate with appropriate males and not waste time selecting mates with which their reproductive efforts would end in failure (though occasionally between-species matings are successful—a phenomenon known as hybridization).
In addition to developing the biological species concept, Dobzhansky and Mayr thought that, ironically, Darwin never did adequately address the "origin of species" in his book of that name. Because Darwin saw species as transitory stages of a continual evolutionary stream—lineages constantly being modified gradually by natural selection—he failed to address the question of why most species most of the time do appear to be discrete (that is, noticeably different from one another). Dobzhansky thought that such discontinuities between species must be a direct result of the evolutionary process—and not, for example, simply a matter of the extinction of intermediates that once, in the past, bridged the gap between two particular species.
Thus Dobzhansky and Mayr developed the notion of allopatric speciation (see Specia-tion)—the idea that new species evolve essentially by budding off from their ancestral species when a portion of a species becomes physically isolated from the main section of the species. If natural selection modifies the features (and, of course, the genetic information underlying those features) far enough in the isolated population, the ability to interbreed could easily be lost, as there would now be a genetic mismatch.
Although it is still a somewhat controversial matter, the data of paleontology strongly suggest that most adaptive evolutionary change occurs in conjunction with the origin of new reproductive communities—in other words, new species. But why would the attributes of organisms in a species devoted to such things as energy procurement (for example, finding and consuming prey, in the case of a carnivore such as a lion) change at the same time as new reproductive adaptations are evolving?
One plausible explanation was given in Mayr's idea of peripheral isolates. Species have definite ranges in space as well as in time. Only a very few species are known to exist all over the earth—and even then not in absolutely all habitats. Homo sapiens—our own species—is one example. Most other species are restricted to portions of continents. For example, the red-bellied woodpecker is an Eastern bird species of the United States, moving up North in recent years from its ancestral southerly climes as global temperature has been on the rise. From the Rocky Mountains westward, other closely related species of woodpeckers replace the red-bellied woodpecker. The question then becomes: What restricts the ranges of species?
Two factors in general govern the geographic distribution of species. One is simply history: A species might very well be able to occupy an area, but its ancestors simply never got there. There are no bears, modern or ancient, in Africa, yet there are bears in India and many other places where habitats seem rather similar to those known in Africa. Bears simply never got there. Moreover, we know that the movements of humans, both inadvertently and deliberately, have transported species to places far from their native habitats— and many of them have thrived, often to the detriment of species native to their new homes (see Alien Species, Introduction of).
On the other hand, the map of the distribution of any modern species has boundaries—boundaries subject to change as environments change (as in the case of the red-bellied woodpecker, above; see also Habi tat Tracking). The reason why those boundaries are there at any particular moment, though, is that the organisms of each species have environments to which they are adapted—in terms of available food, temperature, rainfall, and the like. Things are less lush for a species as you approach its boundaries— for the simple reason that the environment itself is marginal for a species as you observe it from its center to its edge.
Now consider Mayr's peripheral isolates, a population living near the margin of a species' range, where life is more difficult than it is for the organisms of that species living near the center of the range. If something happens— a river changes course, or a section of land becomes too arid, or the like—thus cutting off the marginal population from the members of the species nearer the center of the geographic range of the species, and if there is the appropriate genetic variation, natural selection is likely to quickly modify the adaptations of the organisms in this peripheral population, in effect "redefining" the relatively harsher conditions at the periphery of the ancestral range as the new optimum, preferred habitat. Rapid adaptive change is likely to occur in the process of budding off a new species from the ancestral species.
After their origin, and assuming that the new species survives (many newly evolved species are thought to go extinct quickly—a sort of species-level analogue to infant mortality), species are apt to remain very stable for long periods of time—contrary to Darwin's original supposition. Two factors seem to underlie this phenomenon of so-called stasis: habitat tracking, and the geographic structure of species themselves.
Habitat tracking occurs when the environment changes and, instead of natural selection constantly modifying a species to keep pace with that environmental change, the range of a species shifts as familiar habitat spreads to new locales that the species can easily reach. The recent change in the range of red-bellied woodpeckers, for example, in response to global warming is an example of such habitat tracking.
But it is the geographic structure of the internal genetics of species that seems to be most important in causing the relative lack of evolutionary change that most species exhibit throughout their multimillion-year histories. Most species are broken up into local populations that are parts of different ecosystems. Consider, for example, the American robin, the species Turdus migratorius, a species of thrush. This species is found throughout North America, extending its range far to the north to forage and breed in the summer months. In the Adirondack Mountains of New York state, this bird lives in dense, moist forests. It is also found in sandy, sunny coastal habitats of the Atlantic Coast—and rocky habitats of the Pacific coast. In between, it can be found in suburban gardens and in the arid, desertlike environments of the Southwest. It is as at home in high altitudes as it is at the coastal shorelines. In short, robins are ecological jacks-of-all-trades.
Imagine, then, the different sort of existence a robin leads in the moist, cool woodlands of the American Northeast—as compared, say, with life at an elevation of 6,000 feet in the arid Southwest. The conditions— amount of rainfall, foodstuffs, potential predators, even potential diseases—will be very different in those places, a reflection of the very different physical environments, hence ecologies, of those regions. Natural selection will be acting very differently in those places—and in all the other different ecological settings where robins are found. In other words, the very patchiness of species distribution—divided up as nearly all species are into local populations integrated into very different local ecosys-tems—virtually ensures that no species is destined to develop gradual evolutionary change all in the same direction across the entire range of a species. Rather, local populations each have their semi-independent histories, which condition is bound to lead to little or no net genetic (that is, evolutionary) change for the species as a whole.
Species also become extinct. As we have seen, the vast majority of species that have ever lived have already become extinct—died of natural causes. Although any species might independently dwindle to the brink of extinction for a variety of reasons, the kinds of environmental changes that in essence go too far too fast (so that habitat tracking, and hence survival, are impossible) are generally apt to drive a number of species living in a region to extinction at more or less the same time. The history of life as revealed in the fossil record suggests that most species come into existence with others in its regional ecosystems at about the same time (joining those already there); all of these species survive for roughly the same interval of time (often for millions of years), but then many tend to disappear at about the same time—victims of extinction as environmental change disrupts and modifies the habitat. The cycle then starts over again—in episodes that have been called "turnover pulses," or periods of "coordinated stasis." Thus species do not live in a vacuum: the evolutionary history of each species is usually closely connected to the origins, histories, and extinctions of other species living in the same general region.
See also: Alien Species; Evolution; Evolutionary
Biodiversity; Extinction, Direct Causes of; Habitat
Tracking; Human Evolution; Linnaean Hierarchy;
Punctuated Equilibria; Sixth Extinction; Speciation
Eldredge, Niles. 1998. Life in the Balance. Princeton:
Princeton University Press; Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Futuyma, Douglas J. 1997. Evolutionary Biology. Sunderland, MA: Sinauer; Maynard Smith, John. 1993. The Theory of Evolution. Cambridge: Cambridge University Press; Mayr, Ernst. 2001. What Evolution Is. New York: Basic Books; Wilson, Edward O. 1993. The Diversity of Life. Cambridge: Harvard University Press.
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