SPECIATION

CONCEPT

One of the defining characteristics of a species is its reproductive isolation: the fact that among animals and plants that reproduce sexually, it is impossible for members of two different species to mate and produce fertile offspring. Speciation is the process whereby a single species develops over time into two distinct, reproductively isolated species. It is one of the key evolutionary processes and is responsible for the diversity of life that exists on Earth. In the following essay we explore not only the basic facts of speciation and biological diversity but also an example of adaptive radiation, in the form of the wide range of species within the mammalian order.

HOW IT WORKS

Species and Speciation

The concept of species, discussed in the article devoted to that subject, is an extraordinarily complex one. Owing to limitations of space, that essay only hints at the many details, the competing schools of thought, and the varying definitions of species. Likewise, in the present context, it is possible to examine the concept of speciation only in the most cursory fashion. In addition to consulting the essay on Species for more information, the reader is encouraged to review the article on Taxonomy.

Taxonomy is the area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics. It uses a wide array of specialized rankings for grouping animals, but only seven of them are essential to most biology students. These seven, known as the obligatory hierarchy, are kingdom, phylum, class, order, family, genus, and species. In the case of mammals, it is also useful to refer to subphylum, which in this case is Vertebrata (see the classification of humans in Species), but for the most part it is enough for the beginning student to attain at least some mastery of the obligatory ranks.

Note that species is the most specific of these ranks, which is fitting, because species and specific come from the same Latin root, specie, or "kind." Nonetheless, it is difficult to define species beyond a reference to its place among the categories of the obligatory taxonomy. According to the biological species concept, discussed briefly in Species, a species is any population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. This is far from the only definition, however.

INTERSPECIFIC MATING.

Occasionally, it is possible to produce an infertile hybrid, such as a mule, which is created by the mating of a male donkey and a female horse, or a hinny, the product of the less common union between a male horse and a female donkey. The infertility is due to genetic disorders that arise when mating takes place between distinct species, and even this imperfect product is possible only by mating two species that are very closely related. Donkeys and horses, for instance, both belong to family Equidae, which makes them very closely connected.

In the taxonomic ranking of humans, this would be equivalent to a human mating with a fellow hominid, or member of family Hominidae. If the long-extinct genus Australopithecus were still around, it is not inconceivable that humans could mate with them and produce at least sterile offspring. Of course, it is unlikely that many humans would want to mate with Australopithecus, the most famous example of which was named "Lucy" after the Beatles' song "Lucy in the Sky with Diamonds." Standing about 3.5-5 ft. (1-1.5 m) tall, Australopithecus was very close in appearance to a modern ape who lived about four million years ago.

All of humans' close relatives are extinct, and today our nearest relatives are members of the order Primates: apes, monkeys, and marsupials. It is impossible to imagine a human mating with one of these animals and producing offspring of any kind. Likewise, it is extremely unlikely that a horse or donkey could mate with a tapir or rhinoceros, which are about as distant in relation to them as other primates are to us. (These species all belong to the order Perissodactyla, herbivorous mammals possessing either one or three hoofed toes on each hind foot. We discuss this group, along with all other mammalian orders, later in this essay.)

THE PROBLEM OF DEFINING SPECIES.

Although the biological species concept is accepted widely, it has its shortcomings, not least of which is the fact that not all species reproduce sexually. Although sexual reproduction is the case with a wide array of animals and even plants, quite a few organisms reproduce by some asexual means: for example, single-cell organisms reproduce by splitting.

Among the competing definitions of species is the phenetic (or morphological) species concept, which relies in part on common sense. According to the phenetic species concept, a species is the smallest possible population of organisms that consistently and continually remains distinct and distinguishable by ordinary methods of observation. There are also a variety of definitions that fall under the heading "phylogenetic species concepts," all of which maintain in one way or another that taxonomic classifications should incorporate the most widely recognized hypotheses regarding the evolutionary lines of descent that produced the organisms in question.

The Process of Speciation

Clearly, there is no hard and fast definition of species, but in general terms, everyone who has some familiarity with the concept has at least a basic knowledge of what does and does not qualify as a species. We will leave finer distinctions to trained taxonomists and other biologists and move on to a fact regarding which there is no disagreement: a wide array of species exists in the world today. Some estimates calculate the number of species in the five kingdoms—animals, plants, monerans, protista, and fungi (see Taxonomy for a very brief identification of each)—at about 1.5 million.

This is only the number of identified species, however. Other figures, based on the probable numbers of unidentified species in the world, put the sum total in the tens of millions. Whatever the case, it is obvious that over the course of evolutionary history (discussed in Evolution and Paleontology), there has been a widespread adaptive radiation—that is, a diversification of species as a result of specialized adaptations by particular populations of organisms.

Speciation events are described as either allopatric or sympatric. Allopatric ("different places") speciation occurs when a population of organisms is divided by a geographic barrier, a great example being the division of squirrel species caused by the formation of the Grand Canyon (see Evolution). Another example is the speciation of the black-throated green warbler, which today consists of one species in the eastern United States, along with three others in the western part of the country. Some scientists speculate that there may once have been a single species of black-throated green warbler, whose population was split by the formation of a glacier during the Pleistocene epoch. The latter was the period of the last ice age, which ended about 10,000 years ago, but the end of the ice age was a slow process. It may be that glaciers, formed in the latter part of that time, helped to separate what became three different western species.

Species share the same gene pool, or the sum of all genetic codes possessed by members of that species. The isolation of two populations slowly results in differences between gene pools, until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA between the two populations. (Deoxyribonucleic acid, or DNA, contains genetic codes for inheritance. See Genetics for more on this subject.) More rare than allopatric speciation, sympatric ("same place") speciation happens when a group of individuals becomes reproductively isolated from the larger population of the original species. This type of speciation typically results from mutation, or alterations in DNA that result in a genetic change.

Studies of three-spined sticklebacks, a variety of freshwater fish, in British Columbia have revealed what appears to be a fascinating example of sympatric speciation. Evolutionary biologist Dolph Schluter and others have discovered that the region contains two species of stickleback, one with a large mouth that feeds on large prey close to shore, the other with a small mouth that feeds on plankton in open water. Both species jointly inhabit five different lakes. Through DNA analysis, scientists have determined that the lakes were colonized independently by common marine ancestors, meaning that the process of sympatric speciation between the two varieties had to have occurred independently at least five times. This seems to indicate a situation of competition for resources that favored stickleback species at either extreme of size, as opposed to those of medium size and medium-sized mouths.

RATE OF EVOLUTIONARY CHANGE.

Closely tied to speciation is the rate of evolutionary change, or the speed at which new species arise. This is a long process, one that is usually not observable within a human lifetime or even the span of many lifetimes, though bacteria at least have shown some evolutionary change in their growing resistance to antiobiotics (see Infection). DNA analysis (see Genetics and Genetic Engineering for more about DNA) has been used to examine the rate of evolutionary change. To perform such analysis, it is necessary first to determine the percentage of similarity between the organisms under study: the greater the similarity, the more recently the organisms probably diverged from a common stock. Data obtained in this manner then must be corroborated by information obtained from other sources, such as the fossil record and comparative anatomy studies.

At certain times the rate of evolutionary change can be very rapid, leaving little fossil evidence of intermediate forms, a phenomenon known as punctuated equilibrium. This is contrasted with phyletic (that is, evolutionary) gradualism. Of course, the term rapid in this context is relative, since we are talking about vast spans of time. Life on Earth has existed for about 3,000 million years, and the fossil record goes back some 1,000 million years. This is the case, in part, because to leave fossilized remains, an organism must have "hard parts" that can become mineralized to turn into fossils. (See Paleontology for more on these subjects.)

REAL-LIFE APPLICATIONS

The Diversity of Mammals

One of the most interesting examples of speciation is that which has produced the vast array of species, including humans, that fall within the mammalian class. Mammals began evolving before the dawn of the Cenozoic era about 65 million years ago. The Cenozoic era, which started with a catastrophic event that brought about the mass extinction of the dinosaurs and the end of the Mesozoic era (see Paleontology), is truly the age of the mammal. Just as dinosaurs dominated the Mesozoic, today the world belongs to mammals as to no other class of creature.

Since its humble beginnings in the shadow of the dinosaurs, class Mammalia has undergone a massive radiation to the point that today some 4,625 species of mammal, in about 125 families and 24 orders, are recognized. (That number is changing, as noted later in the context of elephants.) This diversity is tied closely to mammals' enormous mobility, which facilitated their spread throughout the world. Aside from much less complex life-forms, such as arachnids and insects (see Parasites and Parasitology), mammals are believed to be distributed more widely throughout the world than any other comparable taxonomic grouping. Insects may be the most diverse of all animal classes, with numbers of species that may be many times greater than the number of mammals, but considering mammals' much-greater level of physical development and complexity, the diversity of their species is astounding.

MAMMALS' EARLY EVOLUTION.

In the next section we list the orders of mammals and give very brief descriptions of each. The purpose here is not to provide anything like a comprehensive discussion but rather to illustrate the enormous range of species in a class that includes anteaters, dolphins, humans, elephants, and bats. The fact that all these diverse creatures, and many more, emerged from a common evolutionary lineage is almost as amazing as the fact that this common ancestor was a reptile.

Mammals are believed to have come from the reptilian order Therapsida, which emerged during the Triassic period (from about 245 to 208 million years ago) in the early part of the Mesozoic era. Over the course of many millions of years, these creatures began to develop a number of mammal-like qualities—in particular, endothermy, or the ability to maintain internal temperature regardless of environmental conditions. In other words, these cold-blooded creatures became warm-blooded. This evolutionary process was as complex as it was lengthy. Nor was there a clean break with the past—no moment when the therapsids faded away or when it would have been clear that mammals had taken the place of their reptilian ancestors. Rather, in what must have been a fascinating taxonomic situation, for many millions of years, species that combined aspects of both reptiles and mammals walked the earth.

Mammalian Orders

The listing of the 20 orders of living mammals that follows is arranged not alphabetically but in the probable order in which these groups evolved. (This is not to imply that the process was orderly or linear; it was not.) Very few dates are given, simply because there is much dispute in most cases. Numbers of species within each order are also a subject of debate among taxonomists, and therefore these numbers are not always precise.

In the essay, Species, there is a taxonomic listing of the obligatory ranks for humans; included within that listing is a short description of the kingdom (Animalia), phylum (Chordata), and subphylum (Vertebrata) to which mammals belong. Mammal itself is defined as a vertebrate (an animal with a spinal column) that feeds its young from special milk-secreting glands, termed mammae, located on the mother's body. Mammals are warm-blooded or endothermic, meaning that their internal temperatures remain relatively stable, and their bodies usually are covered with hair. They have other distinguishing characteristics as well, such as a relatively large cranium (skull) with a hinged lower jaw attached to it.

MONOTREMES.

Order Monotrema consists of primitive, egg-laying mammals spread throughout parts of the region known as Oceania, which includes Australia, New Zealand, and islands of the southeastern Pacific. The habitat of this order lies specifically in Australia, Tasmania, and New Guinea. Monotremes, as they are called, are distinguished further by the fact that their mammary glands are without nipples, that teeth are present only in the young, and that adults have horny beaks.

The monotremes illustrate the fact that to be constituted as an order or family, a taxon, or taxonomic group, need not have large numbers. The entire order consists of a single existing species, the duck-billed platypus, which constitutes a family of its own, and two species of echidnas, creatures that look like a cross between a platypus and a porcupine. (The "porcupine" look comes from the fact that their bodies are covered in spines, or spiky protrusions.)

MARSUPIALS.

The marsupials, or order Marsupialia, include two other famous animal citizens of Oceania: the kangaroo and its close relative, the wallaby. Marsupials' young are poorly developed at birth and must continue to grow while attached to their mothers' nipples. For this reason, they must remain close to the mother, and therefore natural selection for marsupials favored those strains in which females possess a pouch bearing four teats.

Immediately after birth, the young marsupial (a kangaroo baby is called a joey) installs itself in the mother's pouch. Given this situation, marsupials can support only one offspring a year and thus are not given to the large litters that characterize another order, Carnivora, which we discuss later. Kangaroo offspring remain in the pouch until the age of about 7-10 months, by which time the mother has conceived again; the female kangaroo goes into heat just a few days after giving birth. The embryonic kangaroo remains in a state of dormancy, or arrested development, until the older sibling has left the pouch.

The marsupial order (some authorities call it a superorder, with numerous subordinate orders) consists of some 240 species. The greatest number of these, including many species of kangaroo, wallaby, wombat, and koala, are found exclusively in Australia. Some 70 additional species are scattered across parts of Oceania, including Australia, Tasmania, New Guinea, and smaller islands. There are an additional 70 species in the Americas, including four species of the genus Didelphis —the large American opossum, better known in the southern United States as possums.

Why the preponderance of marsupials in Australia and Oceania as a whole? The reason lies in Earth's geologic history, which has seen regular collisions and divisions of the continents, which are even today shifting slowly under our feet. It appears that prior to about 70 million years ago, at a time when most of Earth was united in a single supercontinent called Pangaea, marsupials originated in what is now North America and migrated to the land masses that became Australia and Oceania. Because the bulk of marsupial species remained on Australia and nearby areas when Pangaea began to break apart, marsupials underwent much greater speciation there than in North America.

XENARTHRANS, INSECTIVORES, SCANDENTIA, AND DERMOPTERA.

Known variously as xenarthrans and edentates, members of order Xenarthra either lack teeth or have very small ones. Evolutionary development has adapted the forward limbs of these creatures for digging or for holding on to the branches of trees. Included in this order are some 30 species of sloth, anteater, and armadillo. Sloths are herbivores, armadillos are omnivores (i.e., they eat plants and small animals), and anteaters, as their name would suggest, are hard-core insectivores.

The term insectivore can refer to any organism that lives by eating insects, but it is also the name for members of the order Insectivora, which includes shrews, hedgehogs, moles, and various other, less well known groups. Some 400 species, of which about 300 are shrews in a single family (Soricidae), make up this order. Not only their diet but also their pointed snouts and rodent-like appearance distinguishes this group. Many, but not all, are diggers, like the xenarthrans. Like all mammals, they have the pentadactyl limb (an appendage with five digits, like the human arm and hand)—in their case, a foot with five toes.

Order Scandentia, which is identical with the family Tupaiidae, or tree shrews, is sometimes grouped with order Insectivora. Despite their name, tree shrews, of which there are five genera and between 15 and 19 species, may live either on the ground or in the trees. Squirrel-like in appearance, they have strong claws on all their toes and are excellent climbers.

Another very small order of tree-dwellers is Dermoptera, which consists of just two species of flying lemur. Found in Indonesia and the Philippines, these creatures are equipped with skin flaps adapted for gliding. This aspect of their morphological makeup calls to mind the "flying" squirrel, but their nocturnal habits are more like those of lemurs (discussed later, with other primates); hence their common name.

CHIROPTERANS.

Among the most fascinating of mammalian orders is Chiroptera, better known as bats. This order, which consists of about 900 species in some 175 genera, is the only group of truly flying mammals, as opposed to the "flying" lemurs we just discussed. Yet they, too, have the pentadactyl limb, only in their case the forelimb has been adapted as a wing. Among the intriguing features of bats is their use of acoustic orientation, or echolocation, to find their way through the dark caves and nocturnal exteriors that make up their world. Contrary to popular belief, they are not blind, but they do have very small eyes, simply because vision is not important for bat navigation. (See Migration and Navigation for more about this subject.)

As befits an order with such a wide array of species, bats run the gamut with respect to their eating habits. The majority of bat species are insectivores that consume many thousands times their weight in insects each year. Many others are fruit bats, important members of the ecosystems they occupy, because they consume fruit and spread seeds, helping assist in seed dispersal. (Some bats also aid in pollination; see Reproduction). Then there are the three species of vampire bat, which are largely responsible for bats' unfortunate reputation with humans.

Members of subfamily Desmodontinae, species of vampire bat include Diaemus youngi and Diphylla ecaudata. However, the best-known variety is the common vampire bat, Desmodus rotundus, which is native to an area that stretches from the southwestern United States to the northwestern third of South America. As one would suspect of a creature named vampire, they live off blood, which they suck at night from birds or other mammals, including humans. These creatures are livestock pests and strike fear in humans both because of the imaginary association with vampires and for the quite real threat of contracting rabies from them. The vast majority of bats, however, are creatures that cause no harm to humans and often are unfairly persecuted as the result of human prejudices. Even in the case of the vampire bat, there is a strong possibility that it may one day help save human lives. Scientists have discovered that the saliva of Desmodus rotundus is better than any other known substance for keeping blood from clotting; therefore, vampire bat saliva may one day be adapted for use in treating heart attacks and strokes.

PRIMATES.

The order of humans, Primates, falls approximately in the middle of the mammalian class in terms of evolutionary order. This is an interesting aspect of speciation, evolution, and taxonomy: even though humans themselves are the most advanced of all creatures, it is not a logical necessity that we should come from the most recently evolved order. In fact, the opposite would seem to be the case. To produce a species whose intelligence dwarfs that of all other animals, the line of descent should be a long one. Where primates are concerned, that is certainly the case. The oldest primate samples date back some 75 million years, or long before the end of the Mesozoic.

Because it is from primates that humans draw their lineage, more has been written about primate evolution than on that of all other mammalian orders combined. The subject is such a vast one that we will not attempt to approach it here, except to encourage the reader to study in more detail elsewhere the process by which the human lineage emerged from order Primates, family Hominidae, and genus Homo.

Primates consist of two broad groups, suborders Prosimii and Anthropoidea. The first, the prosimians, includes five families (or six, since tree shrews are sometimes included) of lemurs, lorises, and tarsiers. The other suborder, known as the higher primates, encompasses another six families: marmosets and tamarins; South American monkeys other than marmosets; African and Asian monkeys; lesser apes, or siamangs and gibbons; great apes, or orangutans, gorillas, and chimpanzees; and humans, both living and extinct. (Most orders contain extinct members, but for the most part they are not discussed here.)

Most primates are tree dwellers, and among the approximately 230 species, there is enormous variation in eating habits. Many lemurs are insectivores, while great apes tend to be fruit eaters. Quite a few are omnivores, though no primate other than humans is known for eating large mammals, such as cows, sheep, and pigs. The pentadactyl limb (an appendage with five digits) is a significant feature for primates, which alone have the advantage of the opposable thumb for grasping. Humans and a few other primate species are also the only animals with four limbs who are not only capable of standing upright but also function best in this way.

CARNIVORES.

As with insectivore , carnivore is a name both for an eating preference—in this case, meat—and for members of a primate order, Carnivora. Most members of this extraordinarily varied group eat meat, including, in some cases, the "meat" of insects. Bears and some other species are omnivorous, meaning that they also eat plants, and hyenas and jackals are classic examples of detritivores, or animals who feed on the remains of other creatures.

The distinction between detritivore and carnivore relates not to the materials each consumes but to their place in the food web. Rather than consume live creatures, hyenas and jackals feed on the carcasses of dead ones. Usually these creatures are artiodactyls (discussed later), such as antelopes, which have been killed by other carnivores—big cats, such as the lion or cheetah. After the big cats have fed on the fleshy parts of the prey, hyenas come to consume the flesh that remains, and they are followed by jackals and vultures, swoop in to pick the bones. These detritivores help process the remains of formerly living things, which ultimately return to the soil. (See Food Webs for more on this subject.)

Clearly, all members of order Carnivora eat meat, though in different ways and sometimes in combination with fruit or other vegetation. Natural selection has equipped them for this purpose with sharp claws and teeth. Carnivora includes some 270 species grouped into ten families, listed here:

• Canidae (dogs, wolves, jackals, and foxes)
• Felidae (cats)
• Hyaenidae (hyenas)
• Mustelidae (skunks, mink, weasels, badgers, and otters)
• Otariidae (eared seals)
• Odobenidae (walrus)
• Phocidae (earless seals)
• Procyonidae (raccoons)
• Ursidae (bears)
• Viverridae (mongooses and civets).

Note that Felidae is a particularly varied family of some 36 species: lions, lynxes, tigers, leopards, and even ordinary domesticated cats. Thirty-five species belong to a single subfamily, Felinae, which is native to most parts of the world other than Australia, Madagascar, most oceanic islands, and, of course, Antarctica. The last species, the cheetah (Acinonyx jubatus ), is segregated into another subfamily, Acinonychinae, primarily because this cat, native to Africa and southwest Asia, is a daytime hunter, unlike its nocturnal cousins.

CETACEANS AND SIRENIANS.

Orders Cetacea and Sirenia include the majority of aquatic mammals, as opposed to the many amphibious mammals, such as seals, sea lions, sea elephants, and walruses, that belong variously to families Otariidae, Odobenidae, and Phocidae of the order Carnivora. Cetaceans include whales, dolphins, and porpoises, while sirenians are made up of just three species of manatee and one of dugong. Sirenians are large, friendly creatures that inhabit the Atlantic coast and tributary rivers (manatee) or the Indian and Pacific coastlines (dugong), but cetaceans are much more familiar.

With cetaceans, two questions, one specific and one general, often arise. The answer to the first of these, "What is the difference between a dolphin and a porpoise?," is that a porpoise is smaller and more chubby and has a blunt snout, whereas a dolphin has a beaklike snout. Some taxonomists and marine biologists put porpoises in the same family as dolphins, whereas others treat them as two different families. The more general, and much more important, question is "Why are these mammals living in the water?" In fact, life itself first appeared in the sea, so perhaps the question should be "Why or how did anything start living on land?" The transition from water to land took place long before the age of the dinosaurs, much less the emergence of mammals, but later, some mammals began to return to the water, probably about 70 million years ago.

Certainly, there is no question that cetaceans are mammals, a fact first recognized by Aristotle (384-322 b.c.), a Greek thinker who is noted not only as one of the greatest philosophers of all time but also as the father of the biological sciences. (See Taxonomy for more about Aristotle's contributions.) As Aristotle observed, whales and dolphins bear live young and suckle them with milk-producing glands; their bodies have hair, albeit only very small strands; and they possess lungs, breathing air through a blowhole.

As evidence of their terrestrial, or land-based, origins, consider that a whale fetus possesses the remnants of four limbs, each with five fingers (the pentadactyl limb), like any land mammal. Adult whales and dolphins have the streamlined, fishlike morphological appearance that is necessary for life underwater, but their resemblance to fishes is of the superficial, analogous variety discussed in Taxonomy. They have maintained and modified key terrestrial features; for example, a blowhole atop the head—one in dolphins, two in whales—replaces the nostrils, and thus the passageways for food and air are completely separate. This differs from the situation with most terrestrial mammals, which take in food and air through the same opening.

PROBOSCIDEANS, PERISSODACTYLS, AND HYRAXES.

Moving from the largest aquatic mammals, the whales, to the largest terrestrial variety, we come to the order Proboscidea, which includes elephants. Our discussions of most orders in class Mammalia have illustrated particular aspects of taxonomy and speciation, and so it is with proboscideans, which give evidence of the many species from the past that are gone forever. The order is a large one, with three suborders and some 300 species, but anyone who searches for most of those species will search in vain. All but three species are extinct. These three are the Asian elephant (Elephas maximus ) and two varieties of African elephant (Loxodonta africana or African savanna elephant and Loxodonta cyclotisare, the African forest elephant). Until 2001, taxonomists believed that there were only two living species of elephant. (See Taxonomy for more on this subject.)

Another 16 species belong to the order Perissodactyla, herbivores whose hind feet bear either one or three hoofed toes. Included among perissodactyls is another large animal from the grasslands of Africa and tropical Asia: the rhinoceros. The group also encompasses donkeys, zebras, and tapirs, but by far the most important in human terms is Equus caballus, the domesticated horse. Described by the French zoologist Comte Georges de Buffon (1707-1788) as "the proudest conquest of man," the horse was domesticated (adapted so as to be useful and advantageous for humans) some 6,000 years ago. Nonetheless, feral or wild horses remain an important subspecies.

Order Hyracoidea also consists of hoofed mammals: seven species of hyrax, a primarily herbivorous creature native to Africa and the far southwestern extremities of Asia. Hyraxes are sometimes lumped in with pikas under the term rock rabbit, but, in fact, pikas are lagomorphs, a group we discuss later. To further the confusion, hyraxes are probably the animal called a coney in the Bible, even though there is an animal called a cony (no "e") that is actually a lagomorph. This is just one of many examples of confusion resulting from the complexities of the animal world and humans' attempts to name and classify its members.

TUBULIDENTATES.

For most orders, the lowercase adjectival name (e.g., primates, carnivores, insectivores, and so on) is commonly used. On the other hand, the names hyracoidean and tubulidentate are seldom used for more obscure groups, such as Hyracoidea and Tubulidentata. If any order of mammal is obscure, it is Tubulidentata, which emerged some 60 million years ago and which consists of a single species: the African ant bear (Orycteropus afer ). The latter creature is better known by the name aardvark, which in the Afrikaans language means "earth pig."

Aardvarks at first glance might seem to belong with anteaters, sloths, and armadillos in the order Edentata, and that is what taxonomists thought for a long time. The latter half of the name tubulidentate, however, suggests the area of differentiation: the teeth. Aardvarks' teeth are unique among those of all mammals. Viewed from the top, the aardvark's jawbone is V-shaped, with the teeth midway along either side of the V. The teeth themselves are not fixed to the jaw but rest in the flesh attached to it, and instead of being covered with enamel, they are protected with a cementlike substance. The substance comes from tubules that run under the teeth.

ARTIODACTYLS.

Like many other mammalian orders we have discussed, members of order Artiodactyla are ungulates, or hoofed animals. Whereas perissodactyls have odd-numbered toes, artiodactyls have even-numbered toes—either two or four—on each foot. This large group, consisting of some 220 species, comprises a wide variety of well-known species in nine families. Among them are cows, pigs, sheep, goats, deer, antelope, bison, camels, giraffes, hippopotamuses, and numerous less well known varieties, such as okapi, pronghorn, peccaries, and deerlike chevrotains.

The camel family is particularly widespread geographically, including as it does many varieties—the llama, alpaca, and vicuña of South America—whose home is far from the habitats in the Near East that are associated with the camel. In this family is what may be a previously undiscovered species, whose existence the British Broadcasting Corporation (BBC) reported in early 2001. Living on a former nuclear weapons testing range in a remote region of Chinese central Asia, these creatures drink saltwater, which in itself is an unusual characteristic.

Although it is extraordinarily hardy, even by the standards of camels, the central Asian camels are threatened; fewer than 1,000 remain. As the BBC reported, this makes them more endangered than the more well known giant panda. The creatures survived nuclear testing in the area, which ceased in 1996, but they continue to be threatened by much less spectacular varieties of explosive: dynamite and land mines, planted by hungry locals. John Hare of the Wild Camel Protection Foundation told the BBC, "We found land mines put by the saltwater springs. So when the camels come to drink, they step on them. Bang! They are blown to pieces and picked up as meat."

As to whether the camels constitute a separate species, the molecular geneticist Olivier Hanotte told the BBC: "There are two possibilities here. One is that the domestic camel was bred from these wild ones some time back in history. The second is that the domestic camel we see today was bred from another species that has disappeared. This would mean that these wild camels are a totally separate species." As of early 2002 the camels' fate, both practically and taxonomically, remained undecided.

PHOLIDOTA/PANGOLINS.

Order Pholidota consists of seven species of scaly anteater, or pangolin. Members of this order are normally called pangolins, rather than an adjectival form of Pholidota. The word pangolin comes from a Malay term meaning "rolling over," a reference to the fact that when it is threatened, the animal curls into a little ball. As with the aardvark, members of this order once were grouped with Edentata but now are considered a separate order. Pangolins are also like aardvarks in thesense that their evolutionary relationship toother mammals is not clear.

RODENTS, LAGOMORPHS, ANDMACROSCELIDEANS.

Rodents, or members of order Rodentia, are familiar to us as both pests and pets as well as aids to research through their use as test subjects in laboratories. They are also the most abundant of all mammalian orders: about one-fourth of all families, 35% of all genera, and 50% of all living species of mammal are rodents. The group consists of some 2,205 species, among them mice, rats, squirrels, beavers, gophers, and porcupines. The name rodent comes from the Latin rodere, meaning "to gnaw," and, indeed, the defining characteristic of rodents is their chisel-like upper front teeth.

Whereas rats typically are despised creatures, mice (distinguished from rats simply because they are smaller) often are considered cute—that is, if the mouse in question is a pet or a laboratory mouse, rather than a pest chewing up the insulation or electrical wiring in someone's house. The fact that rodents are so often pests and pets arises in part from rodents' close association with humans. This is a distinction in itself, since few mammal orders manage to live successfully in such close proximity to humans. Not only do squirrels often live around human dwellings, but other species (for better or worse) often enter structures where humans live or work. Particularly notorious in this regard are black rats (Rattus rattus ) and Norway rats (R. norvegicus ), which are just two of some 500 rat species.

The two remaining orders of mammal also are composed of small, furry creatures. Lagomorphs, or members of the order Lagomorpha, are small mammals with large upper incisors (front teeth) but no canines or eyeteeth and with molars that lack roots. The 80-odd species of lagomorphs include rabbits, hares, and their lesser-known cousin the pika, or mouse-hare. The difference between rabbits and hares relates to their conditions at birth: rabbits are furless, blind, and helpless, whereas hares are furry, have open eyes, and are capable of hopping within minutes.

Finally, 28 species of elephant shrew, or jumping shrew, make up the order Macroscelidea, a collection of species known for their long, flexible, sensitive snouts. Some authorities group macroscelideans with order Insectivora, whereas others place them in another order, Mentophyla, with tree shrews. The latter often have been placed variously in orders Scandentia, Insectivora, or Primates, indicating that many areas of mammalian taxonomy remain in dispute.

WHERE TO LEARN MORE

Boxhorn, Joseph. "Observed Instances of Speciation." Talk. Origins (Web site). <http://www.talkorigins.org/faqs/faq-speciation.html>.

Kirby, Alex. "'New' Camel Lives on Salty Water." British Broadcasting Corporation (Web site). <http://news.bbc.co.uk/hi/english/sci/tech/newsid_1156000/1156212.stm>.

"Mammalia." Animal Diversity Web, The University of Michigan Museum of Zoology <http://animaldiversity.ummz.umich.edu/chordata/mammalia.html>.

Mammal Species of the World (MSW). Smithsonian National Museum of Natural History, Department of Systematic Biology—Vertebrate Zoology (Web site). <http://www.nmnh.si.edu/msw/>.

Marks, Jonathan. Human Biodiversity: Genes, Race, and History. New York: Aldine de Gruyter, 1995.

Norton, Bryan G. The Preservation of Species: The Value of Biological Diversity. Princeton, NJ: Princeton University Press, 1986.

Patent, Dorothy Hinshaw. The Challenge of Extinction. Hillside, NJ: Enslow Publishers, 1991.

——. Biodiversity. Illus. William Muñoz. New York: Clarion Books, 1996.

Schilthuizen, Menno. Frogs, Flies, and Dandelions: Speciation—The Evolution of New Species. New York: Oxford University Press, 2001.

Speciation (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/S/Speciation.html>.

UCMP Hall of Mammals, University of California, Berkeley, Museum of Paleontology (Web site). <http://www.ucmp.berkeley.edu/mammal/mammal.html>.

KEY TERMS

ADAPTIVE RADIATION:

A diversification of species over time as a result of specialized adaptations by particular populations of organisms.

ALLOPATRIC SPECIATION:

A type of speciation that occurs when a population of organisms is divided by a geographic barrier.

ANALOGOUS FEATURES:

Morphologic characteristics of two or more taxathat are superficially similar but not as a result of any common evolutionary origin.

CARNIVORE:

A meat-eating organism, or an organism that eats only meat (as distinguished from an omnivore ).

CLASS:

The third most general of the obligatory taxonomic classification ranks, after phylum but before order.

DETRITIVORES:

Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers;however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.

DNA:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.

DOMESTICATE:

To adapt an organism, whether plant or animal, so as to be useful and advantageous for humans.

FAMILY:

The third most specific of the seven obligatory ranks in taxonomy, after order but before genus.

GENE:

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.

GENE POOL:

The sum of all the genesshared by a population, such as that of aspecies.

GENUS:

The second most specific of the obligatory ranks in taxonomy, after family but before species.

HERBIVORE:

A plant-eating organism.

HYBRID:

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.

INSECTIVORE:

An insect-eatingorganism.

KINGDOM:

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.

MORPHOLOGY:

Structure or form, or the study thereof.

MUTATION:

Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.

NATURAL SELECTION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

NUMERICAL TAXONOMY:

An approach to taxonomy in which specific morphological characteristics of an organism are measured and assigned numerical value, so that similarities between two types of organism can be compared mathematically by means of an algorithm. Numerical taxonomy also is called phenetics.

OMNIVORE:

An organism that eats both plants and other animals.

ORDER:

The middle of the seven obligatory ranks in taxonomy, more specific than class but more general than family.

PENTADACTYL LIMB:

An appendage with five digits, like the human arm and hand. This appendage is common to allmammals, though it may take very different forms—for example, the dolphin's flipper.

PHENETICS:

Another name for numerical taxonomy.

PHYLOGENY:

The evolutionary history of organisms, particularly as that history refers to the relationships between life-forms, and the broad lines of descent that unite them.

PHYLUM:

The second most general of the obligatory taxonomic classificationranks, after kingdom and before class.

SPECIATION:

The divergence of evolutionary lineages and creation of new species. See allopatric speciation and sym patric speciation.

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.

SYMPATRIC SPECIATION:

A type of speciation that occurs when a group of individuals becomes reproductively isolated from the larger population of the original species. This type of speciation typically results from mutation.

TAXON:

A taxonomic group or entity.

TAXONOMY:

The area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics.

VERTEBRATE:

An animal with a spinal column.

Speciation

Speciation can be defined, in a general way, as the various processes by which new species arise. Speciation mechanisms can be categorized in several ways. Some species arise by the divergence of two or more new species from a single common ancestral species (divergent speciation), while others arise from hybridization events involving two parental species (hybrid speciation). When a hybrid speciation event occurs, the newly derived species may have the same chromosome number as its parents (homoploid hybrid speciation), or it may have a higher number (polyploid hybrid speciation). In the latter case, the chromosome number of the newly derived species is usually the sum of those of its parents. In fact, polyploid hybrid speciation is one of the most frequent speciation mechanisms in plants.

Criteria for Recognizing Species

Because two major criteria for the recognition of species are in wide use in biology, any discussion of speciation processes should refer to the criteria under which species are recognized. Criterion 1 is known as the phylo-genetic species concept. Criterion 2 is known as the biological species concept. Under either criterion (genetically distinct groups or reproductively isolated groups), species exist as one or more local populations . Most mating events involve individuals from just one population, and if local populations are small, all of the individuals within each one are closely related. However, there is often some degree of migration of individuals between populations by processes such as seed dispersal. Thus, the various local populations of a species may be loosely connected—by occasional interbreeding among them—into a larger population system, or reproductive community. Regardless of whether species are regarded as fully differentiated population systems (criterion 1), or population systems between which there is a genetically based barrier to reproduction (criterion 2), speciation processes involve the generation of two or more population systems from a common ancestor (divergent speciation) or the generation of a population system following one or more mating events involving individuals of different species (hybrid speciation). Because the two major species concepts recognize species on the basis of different criteria, a speciation event may be regarded as being complete when evaluated under criterion 1 while it is still in progress when evaluated under criterion 2.

Allopatric Speciation

Divergent speciation events can be categorized as those in which the populations that are separating into different species are geographically separated from each other (allopatric speciation) and those in which they are in close proximity (sympatric speciation). An allopatric speciation event begins with a single species that has allopatric (other land) populations, that is, populations that are geographically separated from each other and therefore in little or no contact with each other. This situation may arise when a long-distance dispersal event results in the founding of a new population that is distant from other populations of the species. An example of this would be an allopatric distribution of a species on two islands following the dispersal of one or more airborne seeds of a species from one island to the other. Allopatric distributions also arise when a widespread population system becomes fragmented into two or more allopatric systems by the formation of barriers to dispersal or by changes in climate. The rise of the Rocky Mountains and the corresponding formation of deserts and prairies in the North American interior have created unsuitable habitats for many plant species of temperate forest environments. These species, which at one time had continuous distribution ranges across the continent, became restricted to the forests of eastern and western North America. Once a species comes to have an allopatric distribution, the separate populations begin to evolve independently, and eventually they may become differentiated from each other. This differentiation may have an adaptive basis, as the populations evolve in response to different environmental conditions. Alternatively, the differentiation may be nonadaptive and simply reflect the random origins (by mutation) and genetic fixation of new characteristics that have little or no adaptive value. This is particularly likely to occur when one of the populations is very small, as is likely to be the case when a new population is founded by a long-distance dispersal event. Although the newly formed population may grow quickly, all individuals are descended from the small number of individuals that founded the new population (possibly just one individual). A population of this sort is described as having experienced a founder event, in which it goes through a "genetic bottleneck" and characteristics that were rare in the ancestral population but happened to occur among the founding individuals of the new population may occur in all individuals of the new population under these various circumstances.

As differentiation of allopatric populations proceeds, the point is eventually reached at which two species are recognized. Under the phylogenetic species criterion (criterion 1), speciation can be regarded as having been completed as soon as there are one or more genetically determined differences between the two populations, such that all individuals of one population are distinct from all individuals of the other. For example, all individuals on one island may have teeth on the margins of their leaves, while all of those on another island may lack such teeth. An intermediate stage in this process could be recognized when a particular characteristic is present in only one of the populations (possibly having arisen as a new mutation in that population), but this characteristic does not occur in all individuals of that population.

Reproductive Isolating Barriers

In the example just presented, the two populations still may not be recognized as separate species under the biological species criterion (criterion 2) even after complete differentiation in one or more characteristics has occurred. Because criterion 2 involves the presence of a genetically determined reproductive isolating barrier (RIB), genetic differentiation in any number of characters, if maintained only by the geographic isolation of the populations, generally is regarded as an insufficient basis for the recognition of the two populations as separate species. Therefore, application of criterion 2 involves the assertion that genetically distinct and allopatric population systems may belong to the same species, and what is recognized as a speciation event under criterion 1 may be regarded as only the initial stages of a speciation event under criterion 2. However, if the two distinct populations later occur in sympatry—for example, following disperal of one of them into the range of the other—a test of sympatry occurs, and the presence or absence of RIBs can be evaluated. If these exist, criterion 2 is satisfied.

RIBs can be categorized in several ways. One useful distinction is between those that operate prior to fertilization (pre-fertilization RIBs) and those that operate afterward (post-fertilization RIBs). Fertilization is a critical point in the reproductive cycle of plants, because this is the point at which an ovule either begins to develop into a seed or is lost to the population. One example of a pre-fertilization RIB is the establishment of a different floral structure so that pollinating insects do not place the pollen from individuals of one plant species on the stigmas of individuals of another species, even if they visit plants of different species in succession. If a visit to the flower of one species results in the placement of pollen on the bee's back, but the stigma of another species is touched only by the underside of a visiting bee, cross-pollination and cross-fertilization will not occur. There are many cases of differing floral structure in the orchid family (Orchidaceae), in which natural pollinators do not cross-pollinate two closely related species, even when individuals of the two species grow side by side, but hybrids are easily generated when human investigators transfer the pollen from one species to the stigma of the other. In a natural setting, the two species are genetically isolated by a pre-fertilization RIB, and thus are recognized as separate species under criterion 2 as well as under criterion 1. Another form of pre-fertilization RIB is temporal isolation (isolation by time). In this case, two closely related plant species may flower at different times of the year or day and cross-pollination therefore does not occur. However, under controlled environments, with appropriate day-lengths and temperatures, plants of two different species may be induced to flower at the same and cross-pollination may occur.

Hybridization

An example of a post-fertilization RIB is hybrid inviability. In this case, interspecific hybridization may occur under natural settings, but the off-spring of such crosses may die soon after seed germination. Another example of a post-fertilization RIB is hybrid sterility. In this case, the hybrid individuals may be viable yet they fail to produce gametes, and therefore fail to reproduce. In this case, natural hybrids may be present and even abundant in natural settings, but the two parents of these hybrids are recognized as belonging to separate species under criteria 1 and 2. However, pollen and ovules are wasted by both species. This is a particular problem when two species occur in close proximity to each other and are isolated only by a post-fertilization RIB. In such cases, each of the two species is wasting some of its pollen and seeds and the reproductive potential of both species therefore is lessened. Furthermore, hybrid individuals, even if they are sterile, may compete with one or both of the parental species for habitat and pollinators. In such cases there will be selection against those individuals that cross with individuals of the other species, and any mutation that arises in one of the parental species and contributes to a pre-fertilization RIB is likely to become established in addition to the post-fertilization RIBs that already exist. In this manner, reproductive isolation can be reinforced between two species that are in geographic proximity yet are able to hybridize.

Hybrid speciation events must, of course, involve species that occur in sympatry. Although it may seem contradictory to speak of hybridization between species when (at least under criterion 2) species are reproductively isolated population systems, it is often the case that reproductive isolation is strong but not absolute, and in such cases viable, fertile hybrids may occasionally arise between sympatric species. In homoploid hybrid speciation events, hybridization occurs between two species and thereby generates plants with a combination of characteristics that does not occur in either of the parental species. The hybrids may be better adapted than either parental species to a particular habitat, and a new and successful lineage therefore may be initiated and may spread into a habitat that is unoccupied by the parental species. Eventually, if RIBs develop, a new species can be recognized under both criteria 1 and 2. At least three cases of homoploid hybrid speciation have been documented in native sunflowers of North America.

Polyploid hybrid speciation is extremely common in many plant groups, notably in ferns, the grass family (Poaceae), and the sunflower family (Asteraceae or Compositae). Like homoploid hybrid speciation, a polyploid speciation event is initiated by a hybridization event. However, reproductive isolation between the new species and both of its parents is usually established immediately, in the form of hybrid sterility, a post-fertilization RIB. Polyploid species usually have two complete sets of chromosomes from each parent, and any hybrids that are formed between the new species and either of the parental forms are likely to experience irregular meiosis and thus to be sterile.

see also Evolution of Plants; Hybrids and Hybridization; Polyploidy; Species.

Jerrold I. Davis

Bibliography

Avise, J. C. Molecular Markers, Natural History, and Evolution. New York: Chapman & Hall, 1994.

——. Phylogeography: The History and Formation of Species. Cambridge, MA: Harvard University Press, 2000.

Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1997.

Grant, V. Plant Speciation, 2nd ed. New York: Columbia University Press, 1981.

Niklas, K. J. The Evolutionary Biology of Plants. Chicago: University of Chicago Press, 1997.

Stebbins, G. L., Jr. Variation and Evolution in Plants. New York: Columbia University Press, 1950.

Speciation

Speciation refers to the genesis of a new species from an ancestral species. There are two basic ways this can happen. Anagenesis involves one species evolving into a different species. Cladogenesis occurs when one species splits into two or more species. Cladogenesis is of greater interest in terms of biodiversity and is the type of speciation discussed here. Speciation has two primary components: diversification and genetic isolation. The two principal types of cladogenesis are allopatric speciation and sympatric speciation.

Allopatric Speciation

Allopatric speciation is the better understood of the two types of cladogenesis. It occurs when one species is separated into two groups by some physical barrier, resulting from, for example, climate change, a geological event, or a human-induced change in the environment. For example, the uplift of a new mountain range might divide an ancestral species into two isolated groups. Once the species is separated into these groups, each group may accumulate genetic changes that serve to differentiate it from the other. This accumulation of changes may result from natural selection or from random events.

If the environment on either side of the barrier is different, natural selection may favor genes that produce different traits on either side of the barrier. Even if the environment on either side of the barrier is similar, it may be that when the two groups were separated, by chance one of the groups had a different subset of the total genetic diversity present in the species, and so that group has different "raw materials" for selection to act upon. It may also happen that a neutral mutation occurs in one of the groups, meaning it is neither favorable to the organism nor unfavorable to it. Selection will not act upon such a mutation, and it may persist in the population purely by chance. Also by chance, some versions of genes may become common in a small population while others disappear. This is called genetic drift.

So by a variety of processes, involving either selection or chance, two physically separated groups may accumulate differences between them. This is the first part of the process of speciation. The second part involves the lack of gene flow between the two groups. That is, individuals from one group do not cross the barrier to mate with individuals in the other group, resulting in the genetic isolation of each group from the other. This genetic isolation permits the development of differences in the two groups.

If these differences are to persist, there must be a persistent impediment to gene flow between them. If the two groups never come into contact again, they will almost certainly accumulate enough differences over time to become separate species. If they expand their ranges and come back into contact, some other mechanism must act to "preserve" the differences they evolved in isolation (allopatry). Traditionally, this mechanism is known as reproductive isolation , and it is a byproduct of the diversification that has already taken place.

Reproductive Isolation

Reproductive isolation can operate prezygotically (premating) or postzygotically (postmating). Prezygotic reproductive isolation prevents fertilization from taking place. It may be that members of the two groups breed at different times of the day or different times of the year or in different habitats. They may have developed mechanical differences that prevent copulation, or perhaps copulation takes place, but the two groups have become chemically incompatible so that fertilization does not occur. Postzygotic reproductive isolation acts after fertilization. The embryo may not develop normally, or the offspring may be unhealthy or infertile as adults. In all these ways, reproductive isolation may prevent the gene pools of the two groups from mixing, allowing them to continue on independent evolutionary trajectories.

There is some disagreement among scientists regarding the importance of reproductive isolation in the speciation process. If, as noted above, the two groups that have accumulated differences between them remain separated by a physical barrier preventing their members from ever meeting, it may not matter whether they develop reproductive isolating mechanisms. Proponents of the phylogenetic species concept, for instance, would say that the fact that the groups have accumulated diagnostic differences and are evolving independently is sufficient evidence to say that speciation has taken place.

Sympatric Speciation

The other principal type of cladogenesis is sympatric speciation. In this type of speciation, a species splits into two groups that diversify and become genetically isolated while remaining in the same place. "Same place" typically means that individuals from both groups meet in the same habitat during the breeding season. Most of the mechanisms by which sympatric speciation may occur are poorly understood. There must be some impediment to gene flow if differentiation into two groups is going to take place.

Sympatric speciation can happen if a mutation results in an immediate reproductive barrier in a segment of the species. The most common example of this is polyploidy in plants. In this case, errors in cell division may cause a doubling of the normal number of chromosomes, which instantaneously produces a reproductive barrier.

Another possible mechanism for sympatric speciation is disruptive selection, which takes place when a species has a trait that is manifested in two very different ways, such as two different coat colors. In this case, natural selection operating in a highly partitioned environment (dark versus light background, for instance) may favor one expression of the trait in one particular portion of the habitat and the other expression of the trait in a different portion of the habitat. Selection may thus compound the differences in the trait's expression and in this way result in differentiation.

Polyploidy in plants is also an example of how quickly speciation can take place, even in a single generation. Usually, however, speciation takes longer. Just how long is dependent on many variables, such as the generation time of the organisms involved, as well as factors of chance. There are two predominant schools of thought regarding the speed of speciation. "Gradualists," on the one hand, believe groups accumulate differences slowly over hundreds of thousands or millions of years. "Punctuationalists," on the other hand, believe that speciation takes place comparatively rapidly, over thousands of years, and little change occurs between these rapid bursts of differentiation.

see also Biodiversity; Evolution; Evolution, Evidence for; Natural Selection; Population Genetics; Species

Ann E. Kessen and Robert M. Zink

Bibliography

Darwin, Charles. The Origin of Species: By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. Edited with an introduction by Gillian Beer. Oxford, England: Oxford University Press, 1996.

Gould, Stephen Jay. The Panda's Thumb. New York: W. W. Norton, 1980.

Otte, Daniel, and John A. Endler, eds. Speciation and Its Consequences. Sunderland, MA: Sinauer Associates, 1989.

Quammen, David. The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York: Simon & Schuster, 1996.

Speciation

Speciation is the process by which new species of organisms arise. Earth is inhabited by millions of different organisms, all of which likely arose from one early life-form that came into existence about 3.5 billion years ago. It is the task of taxonomists to decide which out of the multitude of different types of organisms should be considered species. The wide range in the characteristics of individuals within groups makes defining a species more difficult. Indeed, the definition of species itself is open to debate.

Concepts of Species

In the broadest sense, a species can be defined as a group of individuals that is "distinct" from another group of individuals. Several different views have been put forward about what constitutes an appropriate level of difference. Principal among these views are the biological-species concept and the morphological-species concept.

The biological-species concept delimits species based on breeding. Members of a single species are those that interbreed to produce fertile off-spring or have the potential to do so. The morphological-species concept (from the ancient Greek root "morphos," meaning form) is based on classifying species by a difference in their form or function. According to this concept, members of the same species share similar characteristics. Species that are designated by this criteria are known as a morphological species.

Organisms within a species do not necessarily look identical. For example, the domestic dog is considered to be one species, even though there is a huge range in size and appearance among the different breeds. For naturally occurring populations of organisms that we are much less familiar with, it is much more difficult to recognize the significance of any character differences observed. Therefore deciding what characteristics should be used as criteria to designate a species can be difficult.

Speciation Mechanisms: Natural Selection and Genetic Drift

Before the development of the modern theory of evolution, a widely held idea regarding the diversity of life was the "typological" or "essentialist" view. This view held that a species at its core had an unchanging perfect "type" and that any variations on this perfect type were imperfections due to environmental conditions. Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently developed the theory of evolution by natural selection, now commonly known as Darwinian evolution.

The theory of Darwinian evolution is based on two main ideas. The first is that heritable traits that confer an advantage to the individual that carries them will become more widespread in a population through natural selection because organisms with these favorable traits will produce more offspring. Since different environments favor different traits, Darwin saw that the process of natural selection would, over time, make two originally similar groups become different from one another, ultimately creating two species from one. This led to the second major idea, which is that all species arise from earlier species, therefore sharing a common ancestor.

When so much change occurs between different groups that they are morphologically distinct or no longer able to interbreed, they may be considered different species; this process is known as speciation. A species as a whole can transform over time into a new species (vertical evolution) or split into more separate populations, each of which may develop into new species (adaptive radiation).

Modern population geneticists recognize that natural selection is not the only factor causing genetic change in a population over time. Genetic drift is the random change in the genetic composition of a small population over time, due to an unequal genetic contribution by individuals to succeeding generations. It is thought that genetic drift can result in new species, especially in small isolated populations.

Isolating Mechanisms

Whether natural selection and genetic drift lead to new species depends on whether there is restricted gene flow between different groups. Gene flow is the movement of genes between separate populations by migration of individuals. If two populations remain in contact, gene flow will prevent them from becoming separate species (though they may both develop into a new species through vertical evolution).

Gene flow is restricted through geographic effects such as mountain ranges and oceans, leading to geographic isolation. Gene flow can also be prevented by biological factors known as isolating mechanisms. Biological isolating mechanisms include differences in behavior (especially mating behavior), and differences in habitat use, both of which lead to a decrease in mating between individuals from different groups.

When geographic separation plays a role in speciation, this is known as allopatric speciation, from the Greek roots allo, meaning separate, and "patric," meaning country. In allopatric speciation, natural selection and genetic drift can act together.

For example, imagine a mud slide that causes a river to back up into a valley, separating a population of rodents into two, one restricted to the shady side of the river, the other to the sunny side. Because coat thickness is a genetically inherited trait, eventually, through natural selection, the population of animals on the cooler side may develop thicker coats. After many generations of separation, the two groups may look quite different and may have evolved different behaviors as well, to allow them to survive better in their respective habitats. Genetic drift may occur especially if either or both populations remain small. Eventually these two populations may be so different as to warrant designation as different species.

It is also possible for new species to form from a single population without any geographic separation. This is known as "ecological" or "sympatric" (from the Greek root sym, meaning same) speciation, and it results in ecological differences between morphologically similar species inhabiting the same area. Sympatric speciation can occur in flowering plants in a single generation, due to the formation of a polyploid. Polyploidy is the complete duplication of an organism's genome, for example from n chromosomes to 4n. Even higher multiples of n are possible. This increase in a plant's DNA content makes it reproductively incompatible with other individuals of its former species.

see also Chromosomal Aberrations; Conservation Biology: Genetic Approaches; Mutation; Population Genetics; Selection.

R. John Nelson

Bibliography

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, 1998.

Mayr, Ernst. Evolution and the Diversity of Life: Selected Essays. Cambridge, MA: Belknap Press, 1976.

spe·ci·a·tion / ˌspēshēˈāshən; ˌspēsē-/ • n. Biol. the formation of new and distinct species in the course of evolution.DERIVATIVES: spe·ci·ate / ˈspēshēˌāt; spēsē-/ v.