One of the challenges that faces a student of the biological sciences is the seemingly endless array of unfamiliar terms that one must learn. It is a relief to come across a relatively familiar one, such as species. Although it has a scientific sound to it, the word has entered everyday language, such that when people use it, most everyone understands what is meant. Or do they? As it turns out, there is no hard and fast definition for the word. Nonetheless, it is easy enough to find examples of species, since there are many millions of them in five kingdoms of living things—a product of a phenomenon know as speciation, whereby evolutionary lines of descent diverge and new species are created. In the world today, there are many interesting groups of species, distinguished neither by evolutionary line nor by taxonomy but instead by the ways in which they interact with their environments. Among these groups are endangered species, of whose existence most people are aware, owing to the spread of the environmentalist message through media and entertainment outlets since the early 1970s. Less familiar is another broad group that in many cases threaten humans: introduced or invasive 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.
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
- Phylum (Division in botany)
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.)
An endangered species is any plant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat. Over the course of Earth's geological history, species have become extinct naturally—sometimes in large proportions, as discussed in the context of mass extinction in Paleontology. In modern times, however, species and their natural communities are threatened mostly by human activities.
The number of endangered species worldwide is not known. In the United States—a country that, unlike most, expends considerable effort on keeping track of its endangered species—there were more than 750 species and subspecies listed by the late 1990s as endangered under the federal Endangered Species Act. Additional endangered species are being added at a rate of about 50 per year, and there is a "waiting list" of an estimated 3,500 candidate species.
Efforts at monitoring endangered species in the United States have directed a disproportionate amount of attention toward larger organisms; consequently, smaller endangered species from such groups as arthropods, mosses, and lichens have received insufficient attention. The regions of the United States with the largest numbers of endangered species are in the humid southeast and the arid southwest. These areas tend to have the unfortunate combination of unique ecological communities alongside runaway urbanization and resource development.
Overdevelopment and destruction of habitats is perhaps the most well-known ways that humans endanger the survival of species. For example, the habitat of the northern spotted owl is under threat from loggers in the Pacific North-west (see Succession and Climax). Another threat is the introduction of new species, particularly predators, to an area that is not their natural habitat—a topic we discuss in more depth later in this essay.
HUNTING THE ESKIMO CURLEW.
Another way humans threaten species is by excessive hunting. An example of a species thus threatened is the Eskimo curlew (Numenius borealis ), a sandpiper (a type of bird) that was still abundant in North America during the nineteenth century. A large, friendly creature, it was hunted in vast numbers during its seasonal migrations over the prairies and coasts of Canada and the United States and during its winter seasons in South America. (See Migration and Navigation for more about birds' winter migrations.) The Eskimo curlew became very rare by the end of the nineteenth century, and the last time an Eskimo curlew nest was seen (1866), the guns of the Civil War were practically still smoking. The last time a scientific team collected an Eskimo curlew specimen was in 1922. It might seem that the bird is extinct, but this is not the case. Although it is extremely rare, there have been a few reliable sightings of individuals and small flocks of this species, mostly during migration in Texas and elsewhere but also in its breeding habitat in the Canadian Arctic. Once abundant, the Eskimo curlew now hangs on by a thread.
RIGHT WHALES AND BLUE WHALES.
More familiar is the endangerment of whales, a cause made popular by many a "Save the Whales" bumper sticker. Among endangered animals of this group are the blue whale (Balaenoptera musculus ) and various species from the genus Balaena, or right whale. The latter species gained its common name because whalers of the nineteenth century considered it the "right" whale to hunt: it swims slowly and close to shore and so could be found and slaughtered easily. In addition, it yields a large amount of oil, used for lighting lamps in the era when Herman Melville's Moby Dick (1851) was written. The estimated world population of right whales is currently about 2,000 individuals, much depleted from the historical high numbers; though it is now protected from whaling, it suffers an excessive mortality rate from ship collisions.
As for the blue whale, it occurs virtually worldwide, and with a typical weight of 150 tons (136 tonnes) and a length of 100 ft. (30 m), it is the largest animal ever to have lived on Earth. Because it is such a fast swimmer, it could not be hunted effectively by whalers in sailing ships. Once steam-powered ships were invented, however, these whales were taken in tremendous numbers and became endangered. Because of its precarious status, this species has not been hunted for several decades, but it remains rare and endangered.
The Fate of the Dodo
When a species becomes extinct, it is gone forever. It is like a family whose last member has died without leaving an heir, but in this case the impact is potentially much more profound. Several thousand species have become extinct as the result of human activities, mostly hunting, in the past few hundred years, and of these species perhaps none is more well known than Raphus cucullatus, or the dodo.
Long before the application of the term clueless in the 1990s, a person out of touch or out of step was called a dodo. How did the bird's name come to be a synonym for stupidity? Perhaps it is just the funny sound of the name, or perhaps it is the fact that the dodo looked a bit like a turkey, another bird name used for someone of less than exemplary capabilities. Or perhaps the application of the name dodo in this way carries a hint of blaming the victim—the implication that the dodo somehow played a part in its own extinction.
In fact, the dodo's only shortcoming was its inability to overcome the threat posed by an extremely dangerous predator: the human. A member of the dove or pigeon family, the dodo was flightless and lacked natural enemies until humans discovered its homeland, the Indian Ocean island of Mauritius, in the early sixteenth century. First came the Portuguese and then, in 1598, the Dutch, who made the island a colony in 1644. By 1681 the dodo had ceased to exist. Not only did sailors collect the birds for food, but introduced species, including dogs, cats, pigs, monkeys, and rats, also preyed on dodos. They were subjected to regular slaughter by sailors, but the species managed to breed and survive on the remote areas of the island for a time. After the establishment of their colony, however, Dutch settlers launched what amounted to an extermination campaign.
No part of Earth's living environment can be removed without repercussions, and the destruction of the dodo illustrates the ripple effect that occurs when one species is eliminated. As it turned out, the bird had a symbiotic relationship (see Symbiosis) with the dodo tree, or Calvaria major, whose fruit it ate, thus releasing the seeds to germinate. With the dodo gone, the dodo tree stopped being able to reproduce. Fortunately, it is a species with a long life, and some specimens of C. major continue to survive after some 300 years; when those die, however, this species, too, will be extinct.
Exotic, Introduced, and Invasive Species
An introduced species is one that has been spread to a new environment or habitat as a result of human activity. An invasive species may or may not have been spread by humans (the ones we discuss were), but as the name suggests, it threatens an aspect of the habitat to which it has been introduced. Both introduced and invasive species are examples of exotic species, or species that have been introduced to a region or continent, usually but not always through human activity.
In the case of species introduced by humans, some were introduced deliberately and were intended to improve conditions for some human activity (for example, in agriculture) or to achieve desired aesthetic results—for example, when colonists wanted to plant a flower or tree that reminded them of home. Other introductions have been accidental, as when plants were brought with soil transported as ballast in ships or insects were conveyed with timber or food.
BENEFICIAL AND HARMFUL INTRODUCTIONS.
Some introduced species have been wildly successful. In fact, most agricultural plants and animals are introduced species: for example, wheat (Triticum aestivum ) was originally native only to a small region of the Middle East, but it now grows virtually anywhere conditions are suitable for its cultivation. Likewise, corn, or maize (Zea mays ), has spread far beyond its home in Central America. The domestic cow (Bos taurus ) once lived only in Eurasia and the turkey (Meleagris gallopavo ) only in North America, but today these species can be found throughout the world. If all introduced species were like cows and corn, or turkeys and wheat, there would not be much cause for alarm. Many introduced species are invasive, however, and pose a wide variety of threats—threats to their environments or, in some cases, to human well-being. All manner of weeds and pests are among the nefarious roll-call of invasive species, a broad grouping that ranges from nuisances to serious dangers.
ACCIDENTAL AND DELIBERATE INTRODUCTIONS.
There are more than 30,000 introduced species in the United States, and most of them enhance rather than diminish the quality of life. For example, there are the many species introduced by colonists to make them feel more comfortable in their new homes, among them, the Norway maple (Acer platanoides ), linden (Tilia cordata ), horse chestnut (Aesculus hippocastanum ), and other trees as well as many exotic species of shrubs and herbaceous plants. The European settlers also introduced some species of birds and other animals with which they were familiar, such as the starling (Sturnus vulgaris ), house sparrow (Passer domesticus ), and pigeon, or rock dove (Columba livia ).
These are all deliberate introductions; on the other hand, accidental introductions are more likely to be undesirable. When cargo ships from Europe did not have a full load of goods, they had to carry other heavy material as ballast, to help the vessel maintain its stability on the ocean. Early ships to the New World often used soil as ballast, and upon arriving, sailors dumped this soil near the port. In this way, many European weeds and other soil-dwelling organisms arrived in the Americas. In addition, ships have used water as ballast since the late nineteenth century, and many aquatic species have become widely distributed by this practice. This is how two major pests, the zebra mussel (Dreissena polymorpha, discussed later) and the spiny water flea (Bythothrepes cederstroemii ) were introduced to the Great Lakes from European waters.
Several European weeds are toxic to cattle if eaten in large quantities, and when these plants become abundant in pastures, they represent a significant potential problem. Some examples of toxic introduced weeds in the pastures of North America include common Saint-John's-wort (Hypericum perforatum ), ragwort (Senecio jacobaea ), and common milkweed (Asclepias syriaca ). Several introduced insects have become troublesome pests in forests, as is the case with the gypsy moth (Lymantria dispar ), which has defoliated many trees since its introduction to North America from Europe in 1869.
Similarly, the introduced elm bark beetle (Scolytus multistriatus ) has helped spread Dutch elm disease, itself caused by an introduced fungus, Ceratocystis ulmi. It would be interesting to note the irony inherent in this affliction, which at first glance seems to involve another apparently introduced species, the "Dutch elm." There is no such tree, however; the name refers to the fact that the disease arrived in America from Holland, probably some time after World War I. Its principal victim is the American elm, or Ulmus americana.
DELIBERATE AND HARMFUL INTRODUCTIONS.
Not all harmful introduced species were introduced accidentally. Settlers from Europe deliberately brought pets, such as the domestic dog (Canis familiaris ) and cat (Felis catus ); while these pets may add greatly to the quality of human life, they can cause problems, because they are wide-feeding predators. Such creatures threaten vulnerable animals in many places, especially isolated oceanic islands. Among other predators are mongooses (family Viverridae), often introduced to get rid of snakes, as well as omnivores, such as pigs (Sus scrofa ) and rats (Rattus spp.) Meat-eating animals are not the only threat: herbivores such as sheep (Ovis aries ) and goats (Capra hircus ) also endanger plant life in some areas as a result of overgrazing.
A particularly striking example of harmful, deliberate species introduction is the Nile perch (Lates niloticus ). First introduced to Africa's Lake Victoria in the 1950s, it has proved an economically important food source, with a large worldwide market. The problem is that the Nile perch is an extraordinarily active predator and has brought about a tragic mass extinction of native fishes. Until the 1980s, Lake Victoria supported an extremely diverse community of more than 400 species of fish, mostly cichlids (family Cichlidae), with 90% of those species being endemic, meaning that they exist only in one area. About one-half of the endemic species are now extinct in Lake Victoria because of predation by the Nile perch, although some species survive in captivity, and a few are still in the lake.
KILLER BEES, ZEBRA MUSSELS, AND KUDZU.
Three notable examples of invasive species in America are Africanized honeybees (Apis mellifera scutellata ), better known as "killer bees"; the zebra mussel (Dreissena polymorpha ); and kudzu (Pueraria lobata ). The first "killer" bees were released accidentally by a Brazilian bee breeder in 1957. These aggressive insects have no more venom than domesticated honeybees (another A. mellifera subspecies, which is also an Old World import), but they attack more quickly and in great numbers. Interbreeding with resident bees and sometimes traveling with cargo shipments, Africanized bees have spread at a rate of up to 200 mi. (320 km) a year and now threaten humans, fruit orchards, and domestic bees throughout much of South and Central America and north to Texas and California.
The zebra mussel was introduced to the Great Lakes in about 1985 in ballast water dumped by a ship or ships arriving from Europe. It colonizes any hard surface, including rocks, wharves, industrial water-intake pipes, and the shells of native bivalve mollusks. A bean-sized female zebra mussel can produce 50,000 larvae (an immature form of an animal) in a single year. Growing in masses with up to 70,000 individuals per square foot, zebra mussels clog pipes, suffocate native clams, and destroy the breeding habitats of other aquatic animals. These invaders have placed a great burden not only on the environment but also on the economy of the Great Lakes region: area industries spend hundreds of millions of dollars annually to unclog pipes and equipment.
Kudzu is an integral part of culture in the southern United States, but it originated in Japan and did not arrive on American shores until 1876. In that year, numerous foreign governments sent exhibits to the Centennial Exposition, held in Philadelphia to honor the country's 100th birthday. Two generations later, during the Great Depression, the U.S. Soil Conservation Service began promoting the use of kudzu for erosion control.
At a time when work was scarce, young men in the government-sponsored Civilian Conservation Corps (CCC) earned a living by planting kudzu throughout the South. The federal government paid farmers as much as $8.00 an acre—a fabulous sum at the time—to plant kudzu fields. Before another generation had passed, in 1953, the federal government stopped promoting the use of kudzu. In 1972, just four years shy of a century after its first introduction, kudzu was officially declared a weed by the U.S. Department of Agriculture.
Obviously, something had gone wrong. The problem was that kudzu grew too well—so fast, in fact, that in the minds of many southerners, it began to possess some sort of mystical significance. This preoccupation with kudzu is reflected in the work of several Georgians, whose state has been particularly afflicted with the vine. There is the poem "Kudzu" by James Dickey as well as the cover of Murmur, the music group R.E.M.'s 1983 debut, which features a photograph of a kudzu-covered railroad trestle near the group's hometown of Athens.
Kudzu covered more than railroad tracks, and in the mid-twentieth century, it began to seem as though it would cover the entire South with its tangled vines. The plant is capable of growing by as much as 1 ft. (0.3 m) per day during the summer and can cover virtually anything that is not moving. Over the course of a good year, kudzu can grow by as much as 60 ft. (20 m), and it has proved impervious to many herbicides. One herbicide used in Auburn, Alabama, actuallymade it grow better! Thanks to the developmentof better chemical treatments, and the use of grazing animals, such as goats, kudzu no longer isperceived as such a great threat. Additionally, various entrepreneurs and scientists have set out tomake use of the vine in weaving baskets or inpreparing foods and medicines. Ground kudzu root, called kuru, has long been used in foods and medications in China and Japan.
One might wonder why Japan is not covered in kudzu and why kudzu is not crawling up the Great Wall of China. The answer is more than a little interesting from a biological standpoint. When kudzu was transplanted to America, it was taken out of its native environment and thus away from the local insects that threatened its growth. In its new home there were no threats to its spread, and with no obstacles in its way, it began to take over the South. (For more about the development of species, see Speciation. See also the discussion of keystone and indicator species in Food Webs.)
WHERE TO LEARN MORE
All Species Foundation (Web site). <http://www.allspecies.org/>.
"Endangered Species on EE-Link." EE-Link (Environmental Education Link), North American Association for Environmental Education (Web site). <http://eelink.net/EndSpp/>.
Integrated Taxonomic Information System (ITIS), United States Department of Agriculture (Web site). <http://www.it is.usda.gov/>.
Levy, Charles K. Evolutionary Wars: A Three-Billion-Year Arms Race—The Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999.
Schwartz, Jeffrey H. Sudden Origins: Fossils, Genes, and the Emergence of Species. New York: John Wiley and Sons, 1999.
Species 2000 (Web site). <http://www.sp2000.org/>.
Van Driesche, Jason, and Roy Van Driesche. Nature out of Place: Biological Invasions in the Global Age. Washington, DC: Island Press, 2000.
Vergoth, Karin, and Christopher Lampton. Endangered Species. New York: F. Watts, 1999.
A system of nomenclature in biological taxonomy whereby each type of plant or animal is given a two-word name, with the first name identifying the genus and the second the species. The genus name is always capitalized and abbreviated after the first use, and the species name is lower-cased. Both are always shown in italics—thus, Homo sapiens and, later in the same document, H. sapiens.
The third most general of the obligatory taxonomic classification ranks, after phylum but before order.
Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.
Anyplant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat.
Species that exist in only one geographic region.
A cell that has a nucleus as well as organelles (sections of the cell that perform specific functions) bound bymembranes.
Species that have been introduced to a region or continent, usually but not always through humanactivity. See also introduced species and invasive species.
A condition in which all members of a taxon have ceased to exist.
The third most specific of the seven obligatory ranks in taxonomy, after order but before genus.
A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.
The sum of all the genesshared by a population, such as that of aspecies.
The second most specific of the obligatory ranks in taxonomy, after family but before species.
The product of sexual union between members of two species or other smaller and less genetically separate groups, such as two races. In the case of species hybrids, the process of hybridization involves genetic abnormalities that>lead in most cases to sterility.
A species that has been spread to a new environment or habitat, whether deliberately or accidentally, as a result of human activity. Introduced species, like invasive species, are considered exotic species.
An exotic species that threatens some aspect of the habit at to which it has been introduced.
The highest or most general ranking in the obligatory taxonomic system. In the system used in this book there are five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.
Structure or form, or the study thereof.
The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.
The act or process of naming or a system of names—particularly one used in a specific science or discipline. See also binomial nomenclature.
The middle of the seven obligatory ranks in taxonomy, more specific than class but more general than family.
The second most general of the obligatory taxonomic classificationranks, after kingdom and before class.
A cell without a nucleus.
Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.
The divergence of evolutionary lineages and creation of new species.
The most specific of the seven obligatory ranks in taxonomy. Species often are defined as a population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. Also, members of the same species share a gene pool.
A taxonomic group or entity.
The area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics.
A species is a group of closely related, physically similar beings that can interbreed freely. In practice, the dividing lines between species or between species and subspecies are sometimes unclear.
In sorting out species, biologists search for easily recognized diagnostic characteristics. For example, it is easier to recognize a giraffe by its long neck than by its blood proteins. However, species differ from one another not just by conspicuous features. Members of a species share a common gene pool and have a common geographic range, habitat , and similar characteristics ranging from the biochemical and morphological to the behavioral.
In taxonomy, the hierarchy of biological classification, species is the category just below genus. The major groups in the classification hierarchy are kingdom, phylum (for animals) or division (plants), class, order, family, genus, and species. Taxonomically, a species is designated in italics by the genus name followed by its specific name, as in Felis domesticus, or domestic cat. New species are named and existing species names are altered according to the International Rules of Botanical Nomenclature, the International Rules of Zoological Nomenclature, and the International Bacteriological Code of Nomenclature.
Isolating mechanisms prevent closely related species living in the same geographic area from mating with each other. These are called sympatric species, as opposed to allopatric species, which are closely related species living in different geographical areas. Sympatric species are very common. Bullfrogs, green frogs , wood frogs, and pickerel frogs, all members of the genus Rana, may be found in or near the same pond. They are prevented from mating by reproductive isolating mechanisms.
Allopatric speciation allows new species to arise from one or more preexisting species, which is a long, slow process. Most allopatric species evolve when a small population becomes physically isolated from the main part of the species, and gene flow between them stops. Genetic differences spread and accumulate in the small, isolated group. This process is most likely to occur at the border of a species, where environmental conditions exceed the range of tolerance of members of the species and there is a barrier to the species' spread. The barrier could be a mountain range, a desert , forest, large body of water and so on.
Organisms that are part of a region's natural flora and fauna are called native or indigenous species. Those associated with the mature or climax form of community are called climax species. Fragments of a climax species that remain after a major disturbance, such as a forest fire, are referred to as relic species.
Nonnative organisms are called introduced or exotic species . They can wreak havoc when brought deliberately or by accident into a new area where they have no natural enemies or where they compete aggressively with other organisms in the environment . A classic example is the introduction of the zebra mussel (Dreissena polymorpha ) into the Great Lakes , where it reproduces wildly and has clogged water treatment facilities. In south Florida, the Australian pine (Casuarina equisetifolia ) has overtaken many coastal areas because it is extremely tolerant of salt spray and otherwise poor beach soils. Its thick foliage and quick rate of multiplication allows the Australian pine to compete with native species for light, food, and space.
All of the plants and animals found in a particular area, whether native or introduced, are called resident species. Where new species adapt well so that they do not need special help to perpetuate, they are said to be acclimatized or naturalized. Invader species are not original to a region but arrive after an area is disturbed, such as by fire or over-grazing . Native species that are the first to recolonize a disturbed area are called pioneer species. Several pioneer species quickly reappeared on and around Mount Saint Helens after the volcano's 1980 eruption.
Species that are especially adapted to a highly variable, unpredictable, or transient environment are called opportunistic species. Within a particular area, dominant species are those that because of their number, coverage, or size, strongly influence the conditions of other species in the area. For example, certain trees in old-growth forests in the Pacific Northwest are vital to the success of other species in the area. Destruction of the forests by logging also causes the decline of other species, such as the spotted owl (Strix occidentalis ). In other areas, plants that are sought after by grazing animals are called ice cream species.
So-called indicator species, or indicator organisms, are watched for warning signs that their ecosystems are ill. The decline of migratory songbirds around the world is an indication that nesting and breeding grounds of the songbirds, particularly rain forests, are in danger. Other indicator species are amphibians. Some scientists believe that the decline of many frog and toad populations is a result of environmental pollution . Since amphibians have highly permeable skins, they are particularly vulnerable to environmental degradation such as pesticide pollution. The depletion of the ozone layer, which allows more ultraviolet radiation to penetrate the atmosphere , has been blamed for the decline in other frog populations. Increased ultraviolet radiation is believed to be responsible for the destruction of frog eggs, which are deposited in shallow water. When the striped bass population in Chesapeake Bay declined in 1975, it indicated pollution problems in the water. An indicator species also may be used to show the stage of development, or regeneration, of an ecosystem . For example, land use managers may watch plant species to assess the level of grazing use in rangeland.
If the survival of other species depends on the survival of a single species, that single species is called a keystone species . Where a species plays a role in more than one ecosystem, it is said to be a mobile link species.
A species population has a dimension in space, called a range, and a dimension in time. The population extends backwards in time, merging with other species populations much like the branches of a tree. The population has the potential to extend forward in time, but various factors may prevent the perpetuation of the species. When the continued existence of a species is in question, it is regarded as an endangered species . This causes great concern to conservationists, especially when the species is the only representative of its genus or family, as is the case with the giant panda (Ailuropoda melanoleuca ). A species found in a very restricted geographic range is endemic or narrowly endemic, and is at special risk of becoming extinct. Extinction results from an imbalance between a species and the environment in which it is living.
Many species exist today in name only. On a 1991 United States Fish and Wildlife Service report on species, the following were removed from the endangered species list because they all became extinct since the 1980 report was issued: the Amistad gambusia fish (Gambusia amistadensis ), the Tecopa pupfish (Cyprinodon nevadensis calidae ), Sampson's pearly mussel (Epioblasma sampsonii ), the blue pike (Stizostedion vitreum ), and the dusky seaside sparrow (Ammodramus maritimus nigrescens ).
Species richness varies with the type of region, and it is especially diverse in areas of warm or wet places, such as tropical forests. Many biologists are concerned that the unprecedented destruction of tropical forests is forcing to extinction species that have not even been discovered. When a species is lost, it is lost forever, and with it the tremendous potential, especially in medicine, of that species' contributions to the world.
[Linda Rehkopf ]
Villee, C., et. al. General Zoology. New York: Saunders College Publishing, 1984.
The most widely accepted definition of a species is the biological species concept proposed by Ernst Mayr in the 1940s. A species is a population of individual organisms that can interbreed in nature, mating and producing fertile offspring in a natural setting. Species are organisms that share the same gene pool, and therefore genetic and morphological similarities.
The Linnaean classification system places all organisms into a hierarchy of ranked groups. The genus includes one or more related species, while a group of similar genera are placed in the same family. Similar families are grouped into the same order, similar orders in the same class, and similar classes in the same phylum. The Linnean classification of species involves a binomial name made up of two parts; the first is the genus name and the second is the species name. For example Homo sapiens, the name for humans is the combination of the genus Homo and the species sapiens.
Organisms are assigned to the more specific ranks of the Linnaean classification scheme largely on the basis of shared similarities (syna pomorphisus). Although species are defined as interbreeding populations, taxonomists rarely have information on an organism’s breeding behavior and therefore often infer interbreeding groups on the basis of reproductive system morphology, and other shared characters.
In the last half a century, modern molecular techniques such as DNA hybridization have allowed biologists to gain extensive information on the genetic distance between organisms, which they use to construct hypotheses about the relatedness of organisms. From this information researchers hypothesize as to whether or not the populations are genetically close enough to interbreed.
While the biological species concept has historically been the most widely used definition of a species, more recently the phylogenetic and ecological species concepts have taken the forefront as a more inclusive and useful definition. Whereas the biological species concept defines a species as a group of organisms that are reproductively isolated (able to successfully breed only within the group), the phylogenetic species concept considers tangible (and measurable) differences in characteristics. This idea, also called the cladistic species concept, examines the degree of genetic similarity between groups of related individuals (called clades) as well as their similarities in physical characteristics. For instance, the biological species concept might group coyotes and wolves together as one species because they can successfully breed with one another.
In contrast, the phylogenetic concept would definitively split coyotes and wolves into two species based upon the degree of divergence in genetic characters and larger observable traits (e.g., coat color). In contrast to these, the ecological species concept might classify wolves and coyotes as different species by comparing the differing environmental resources that they
exploit, called adaptive zones. Currently, the precise definition of a species is a topic under constant scientific debate and likely will never fully be resolved. Rather, the definition may change with the perspectives and needs of each sub-discipline within biology (ecology versus zoology, for example). A pluralist approach combines some or all of these species concepts to arrive at a more inclusive definition.
Speciation is the process whereby over time a single species develops into two distinct reproductively isolated species. Speciation events are of two types–either allopatric or sympatric. Allopatric speciation results from the division of a population of organisms by a geographical barrier. The isolation of each of the two populations slowly results in mutations in the gene pools until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA from the two populations. The early stages of allopatric speciation are often evident when one examines the same species of fish from different ponds. Fish from the two ponds may not appear to be morphologically different, but there may be slight differences in the gene pools of each population. If the two fish populations remain separated for enough generations, they may eventually become reproductively isolated species.
Sympatric speciation is less frequent than allopatric speciation and occurs when a group of individuals becomes reproductively isolated from the larger population occupying the same range. This type of speciation may result from genetic changes (or mutations) occurring within individuals that inhibits them from interbreeding with others, except those in which the same mutation has occurred. Polyploid plant species, that is, species with more than two copies of each chromosome, are thought to have arisen by sympatric speciation.
More than 1.5 million species have been described and it is estimated that there are between 10-50 million species currently inhabiting Earth.
Allopatric speciation —Speciation resulting from a population being geographically divided.
Linnean system —Classification scheme used by taxonomists which places organisms into a hierarchy of groups.
Morphology —The physical properties possessed by an organism.
Polyploid —An organism with more than two copies of each chromosome.
Sympatric speciation —Speciation that occurs when a subpopulation becomes reproductively isolated from a larger population occupying the same range.
Cambell, Neil A. and Jane B. Reece. Biology. San Francisco, CA: Benjamin Cummings, 2004.
Cockburn, Andrew. An Introduction to Evolutionary Ecology. Boston: Blackwell Scientific Publications, 2001.
Mayr, Ernst, and Peter Ashlock. Principles of Systematic Zoology. 2nd ed. New York: McGraw-Hill, 1991.
Wilson, Edward. The Diversity of Life. New York, NY: W.W. Norton and Company, Inc., 1999.
The most widely accepted definition of a species is the biological species concept proposed by Ernst Mayr in the 1940s. A species is a population of individual organisms that can interbreed in nature, mating and producing fertile offspring in a natural setting. Species are organisms that share the same gene pool, and therefore genetic and morphological similarities.
All organisms are given two names (a binomial name); the first is the genus name and the second is the species name, for example Homo sapiens, the name for humans. The Linnaean classification system places all organisms into a hierarchy of ranked groups. The genus includes one or more related species, while a group of similar genera are placed in the same family. Similar families are grouped into the same order, similar orders in the same class, and similar classes in the same phylum.
Organisms are assigned to the higher ranks of the Linnaean classification scheme largely on the basis of shared similarities (syna pomorphisus). Species are identified on the basis of an organism's ability to interbreed, in addition to its morphological, behavioral, and biochemical characters. Although species are defined as interbreeding populations, taxonomists rarely have information on an organism's breeding behavior and therefore often infer interbreeding groups on the basis of reproductive system morphology, and other shared characters.
In the last 20 years, modern molecular techniques such as DNA hybridization have allowed biologists to gain extensive information on the genetic distance between organisms, which they use to construct hypotheses about the relatedness of organisms. From this information researchers hypothesize as to whether or not the populations are genetically close enough to interbreed.
While the biological species concept has historically been the most widely used definition of a species, more recently the phylogenetic and ecological species concepts have taken the forefront as a more inclusive and useful definition. Whereas the biological species concept defines a species as a group of organisms that are reproductively isolated (able to successfully breed only within the group), the phylogenetic species concept considers tangible (and measurable) differences in characteristics. This idea, also called the cladistic species concept, examines the degree of genetic similarity between groups of related individuals (called clades) as well as their similarities in physical characteristics. For instance, the biological species concept might group coyotes and wolves together as one species because they can successfully breed with one another. In contrast, the phylogenetic concept would definitively split coyotes and wolves into two species based upon the degree of divergence in genetic characters and larger observable traits (e.g., coat color ). In contrast to these, the ecological species concept might classify wolves and coyotes as different species by comparing the differing environmental resources that they exploit, called adaptive zones. Currently, the precise definition of a species is a topic under constant scientific debate and likely will never fully be resolved. Rather, the definition may change with the perspectives and needs of each sub-discipline within biology (ecology versus zoology, for example). A pluralist approach combines some or all of these species concepts to arrive at a more inclusive definition.
Speciation is the process whereby a single species develops over time into two distinct reproductively isolated species. Speciation events are of two types—either allopatric or sympatric. Allopatric speciation results from the division of a population of organisms by a geographical barrier. The isolation of each of the two populations slowly results in differences in the gene pools until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA from the two populations. The early stages of allopatric speciation are often evident when one examines the same species of fish from different ponds. Fish from the two ponds may not appear to be morphologically different, but there may be slight differences in the gene pools of each population. If the two fish populations remain separated for enough generations, they may eventually become two separate reproductively isolated species.
Sympatric speciation is less frequent than allopatric speciation and occurs when a group of individuals becomes reproductively isolated from the larger population occupying the same range. This type of speciation may result from genetic changes (or mutations) occurring within individuals that inhibits them from interbreeding with others, except those in which the same mutation has occurred. Polyploid plant species, that is, species with more than two copies of each chromosome , are thought to have arisen by sympatric speciation.
More than 1.5 million species have been described and it is estimated that there are between 10-50 million species currently inhabiting Earth .
Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed. Menlo Park: Benjamin Cummings, Inc. 2000.
Cockburn, Andrew. An Introduction to Evolutionary Ecology. Boston: Blackwell Scientific Publications, 1991.
Mayr, Ernst, and Peter Ashlock. Principles of Systematic Zoology. 2nd ed. New York: McGraw-Hill, 1991.
Wilson, Edward. The Diversity of Life. Cambridge, Massachusetts: Belknap Press, 1992.
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Allopatric speciation
—Speciation resulting from a population being geographically divided.
- Linnean system
—Classification scheme used by taxonomists which places organisms into a hierarchy of groups.
—The physical properties possessed by an organism.
—An organism with more than two copies of each chromosome.
- Sympatric speciation
—Speciation that occurs when a subpopulation becomes reproductively isolated from a larger population occupying the same range.
From the Greek ε[symbol omitted]δος, having several meanings: (1) a logical relation of universality, the second of the predi cables enumerated by Porphyry in his Isagoge; (2) a unit of taxonomy in biological classification; (3) the natural species as the quiddity of sensible things and the proper object of the human mind; (4) the intentional species, a likeness or representation of the object in which the thing is made present to the knowing power; and (5) one of several names for Plato's unchanging, eternal Ideas. This article considers only the first three meanings; for the others, see species, intentional; idea.
Logic. Species meant first the shape or visible form, and since anything was best recognized by its visible form, the same term was extended to designate its common intelligible character. In logic it meant the universal said in answer to "What is it?" of many that differ in number ("man," of Plato and Socrates). The species whose immediate inferiors are individuals is species only and is called the infima species (Gr. ε[symbol omitted]δος εἰδικώτατον). The species is also related to the genus, which can be said of it as a direct quidditative predicate. Species in each category that fall between the supreme genus and the lowest species are, from alternate points of view, either species or genera; they are called subaltern species.
More recent logic speaks of classes rather than species. In mathematics especially they are called sets. Thus, a class is a group of individuals each having certain properties in virtue of which they are said to be its members. The attributes constitute the intention of the class; the members determine the extension. Classes of individuals are called classes of the first order, and these are grouped in classes of the second order, etc.
The question remains whether such classes are real entities independent of the human mind, or only common names, or concepts in the mind. A. N. whitehead and B. russell describe them as "merely symbolic or linguistic conveniences, not genuine objects as their members are if they are individuals" [Principia Mathematica (3 v. Cambridge, Eng. 1910–13) 1:75]. The problem dates from the classic questioning in Plato's Parmenides on how the universal idea that is one can be in its many instances. (see universals.)
Classification and Natural Species. In the natural sciences, species are units of classification between the genus and the subspecies. aristotle used this term in natural history. A common criterion for determining a natural class is descent from a common stock and indefinite fertility among the members. Problems in this area are those of the fixity and the evolution of species. (see evolution)
Distinct from the logical ordering of the relations of species and genera is the question of the objective fundament of these species. If there is a diversity of species in the corporeal world, manifested by the activity of bodies, to what extent can these be known? Does man have quidditative knowledge, real definitions of the species of any natural substance? Of man only? Of a relatively small number? Of very many? At what point in classification do the differences become merely accidental ones? These questions are commonly debated among scholastics and Thomists, with little consensus on the answers given.
See Also: intentionality; definition; quiddity.
Bibliography: g. dalla nora, Enciclopedia filosofica (Venice-Rome 1957) 4:846–852. m. j. adler, Problems for Thomists (New York 1940). a. c. cotter, Natural Species (Weston, Mass.1947). i. m. bocheŃski, A History of Formal Logic, ed. and tr. i. thomas (Notre Dame, Ind. 1961).
There is little agreement among scientists about the definition of the word "species." However, most biologists would agree that a species is a detectable, naturally occurring group of individuals or populations that is on an evolutionary path independent from other such groups. Several more detailed definitions have been articulated over the years; two that have gained prominence are the biological species concept (BSC) and the phylogenetic species concept (PSC).
In 1942 biologist Ernst Mayr defined the biological species concept as follows: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." This definition places emphasis on restriction of gene flow among groups. Reproductive isolation means that individuals from two groups are unable to interbreed successfully, that is, produce healthy, fertile offspring. So according to this definition, an individual is a member of a particular species if it can breed successfully with members of that species but not with members of other species.
Interbreeding between two different groups is called hybridization and is viewed differently by different scientists. In animals, hybrid offspring of two different species are thought to be unhealthy or infertile as adults, but in plants hybrid offspring are often thought to be more vigorous than their parents. As a result, plant biologists and animal biologists differ regarding the significance of interbreeding in answering species questions, and most plant biologists are not proponents of the BSC.
Objections to the BSC include the fact that the extent of hybridization can range from very little to extensive, making its interpretation subjective. Also, it requires guesswork regarding the species status of groups that do not occur in the same place and thus have no opportunity to interbreed, and it cannot easily be applied to organisms in the fossil record or to those that lack sexual reproduction. Furthermore, it is now known that hybridization can occur between two independent groups that are not each other's nearest relatives. Thus, putting two hybridizing groups into one species could misrepresent evolutionary history by excluding other more closely related (and often reproductively isolated) groups.
A phylogenetic species concept was articulated by Joel Cracraft in 1987 as follows: "a species can be defined as an irreducible cluster of organisms, within which there is a parental pattern of ancestry and descent, and which is diagnosably distinct from other such clusters." This definition views a species as being the smallest possible grouping of organisms in time and space that can be differentiated from other groupings, with the basis for the differentiation being inherited. So an individual is a member of a species if it shares the inherited characteristics of the species, irrespective of whether it can hybridize with a member of another species. The primary objection to this definition is that it is too vague.
MAYR, ERNST (1904–)
German-born U.S. evolutionary biologist who helped found the "modern synthesis," the melding of evolutionary theory with genetics. Mayr's greatest contribution was to explain how new species can arise. When a population is isolated, on an island, for example, it can evolve separately from the rest of the species. Mayr's views have defined evolutionary biology for nearly three-quarters of a century, and he has won two prestigious prizes, the Balzan Prize and the Japan Prize.
These are but two examples of the numerous definitions from a century of ongoing debate about the definition and meaning of species. Scientists often approach the species question differently depending on what organisms they are studying and the way in which they are studying them. Traditionally, organisms have been grouped into species based on aspects of their appearance or particular behaviors. More recently, analysis of deoxyribonucleic acid (DNA) has joined the list of techniques for differentiating or grouping organisms. Additionally, there are specific criteria used for different groups. In plants, for instance, plant chemistry, insect associations, and number of chromosomes may be important indicators of species status. As another example, scientists studying bacteria may use such characteristics as shape, biochemistry, and conditions favoring growth to help them answer species questions. Thus, there is no simple, universally agreed-upon definition of species.
see also Biodiversity; Buffon, Count; Evolution; Speciation; Taxonomy, History of
Ann E. Kessen and Robert M. Zink
Keller, Evelyn Fox, and Elisabeth A. Lloyd, eds. Keywords on Evolutionary Biology. Cambridge, MA: Harvard University Press, 1992.
Otte, Daniel, and John A. Endler. Speciation and Its Consequences. Sunderland, MA: Sinauer Associates, 1989.
The term species, in the most general sense, refers to the various kinds of living things. Thus, species are generally recognized as distinct, fully differentiated groups of organisms. However, most modern definitions of species also recognize them as reproductive communities and acknowledge that mating occurs between members of each species but does not occur (or occurs only rarely) among members of different species. Species are therefore generally recognized as genetically differentiated, reproductive communities within which there is a pattern of ancestry and descent among organisms. Although most scientists accept this general definition, there are two somewhat different criteria that are often employed in the recognition of species, and the application of the two criteria does not always lead to the same conclusions.
The Role of Interbreeding
The first major criterion for the designation of species is the actual occurrence of interbreeding among the various organisms and populations within a species and the absence of such interbreeding between species. However, patterns of interbreeding are difficult to observe directly, particularly among plants that may live for hundreds of years. For this reason, indirect evidence regarding patterns of interbreeding is usually provided by the study of the patterns of differentiation among populations in genetically determined characteristics. In a trivial sense, oak trees and daisies are regarded as belonging to different species because they are distinguished by numerous characters and because hybrids between them are never observed. Thus, it is reasonable to conclude that they are not part of the same reproductive community. However, there are many recognized species of daisies, and many recognized species of oaks, and these species often are delimited by subtle differences. Consequently, one might examine several populations of daisies and observe that the plants are identical except for one character: the occurrence of a line of hairs along the undersides of the leaves. If all of the individuals in some populations have the line of hairs, and all of the individuals in other populations lack these hairs, there is evidence that two population systems exist and that there is no gene flow between them. In contrast, if each population that is examined includes individuals with the line of hairs and other individuals without the line of hairs, it can be concluded that this is simply a character that varies within a single species, like blood types in humans. Under this criterion, two species can be recognized even when the differences between them are not readily observable. For example, there are many documented cases in which two or more population systems differ from each other by genetically determined differences that can only be detected by biochemical tests. Though they are difficult to distinguish, the species are recognized as distinct genetic communities.
Reproductive Isolating Barriers
The second major criterion of species status is the existence of a genetically determined barrier to gene flow between species. Such a barrier, known as a reproductive-isolating mechanism or a reproductive isolating barrier (RIB), prevents members of two different species from interbreeding, even if they occur in the same location. For example, the pollen that is produced by plants of one species may not germinate when placed on the stigmas of plants of another species and, thus, there can be no reproduction or gene flow between them. In this case, the RIB is the pollen/stigma incompatibility. Because this barrier is genetically determined, the two are regarded as reproductively isolated, and, as a result, two species are recognized.
Generally, any species boundary due to a reproductive isolating barrier also serves to prevent interbreeding as defined by the first criterion, but there are many instances in which the first criterion is satisfied while the second is not. The line of hairs on the underside of the leaves, which distinguishes two species of daisies under the first criterion in the example just described, does not by itself prevent interbreeding from occurring between the two kinds of daisies. The two daisy species may fail to exchange genes not because of a genetic mechanism but because they occur on different sides of a mountain range. Some biologists argue that if a RIB is not identified, the two kinds of daisies (one with the line of hairs, the other lacking it) should be grouped together and recognized as belonging to the same species. Others argue that the two populations are, indeed, persisting as separate and fully differentiated reproductive communities. Although they may have the potential to interbreed, the available evidence suggests that this does not occur, so they should be recognized as different species. Whatever position one takes on this matter, it should be noted that most species that have been recognized by science have, in fact, been delimited according the first criterion.
see also Cultivar; Hybrids and Hybridization; Speciation; Taxonomy; Variety.
Jerrold I. Davis
Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1997.
Mayr, E. The Growth of Biological Thought. Cambridge, MA: Belknap Press of Harvard University Press, 1982.
A species is a group of organisms of the same type. The term species is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. Species is the last and smallest complete group. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species.
Species can be considered the basic unit of scientific classification. It is used to describe a group of closely related, physically similar organisms that can breed with one another. Members of the same species often are so close in features that it is difficult to tell them apart. For example, although a wolf and a dog look similar, they do not belong to the same species. The wolf belongs to the species lupus (Canis lupus), while the dog to the species familiaris (Canis familiaris). Some organisms, however, are more difficult to differentiate. This is the case with sunflowers of different species. Some species of organisms may differ very little in their conspicuous features, shape, behavior, and habitat. However, only members of the same species will share a common gene pool.
The only correct way to identify one species from another is by the binomial (two-word) scientific name. For example, the correct way to distinguish between two species of seabirds would be to use their binomial names: Sula sula and Sula nebouxii. In these examples, Sula is the genus name, while sula and nebouxii are the species names. (The species name is always the lowercase Latin word that follows the uppercase Latin genus word.) Very often, however, it is the common name for the species that is most used to refer to one particular type. So, for the example above, most people would refer to the two species of seabirds as the red-footed booby and the blue-footed booby instead of using the binomial scientific name.
spe·cies / ˈspēsēz; -shēz/ • n. (pl. same) 1. (abbr.: sp., spp.) Biol. a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial, e.g., Homo sapiens. ∎ Logic a group subordinate to a genus and containing individuals agreeing in some common attributes and called by a common name. ∎ a kind or sort: a species of invective at once tough and suave. ∎ used humorously to refer to people who share a characteristic or occupation: a political species that is becoming more common, the environmental statesman. ∎ Chem. & Physics a particular kind of atom, molecule, ion, or particle: a new molecular species. 2. Christian Church the visible form of each of the elements of consecrated bread and wine in the Eucharist. ORIGIN: late Middle English: from Latin, literally ‘appearance, form, beauty,’ from specere ‘to look.’