Speciation is the cessation of interbreeding between one animal population and all other populations with which it formerly exchanged genes. When interbreeding between two populations is interrupted, they will become genetically differentiated because the force of gene flow can no longer maintain their similarity. The other evolutionary forces then combine to make the populations diverge:
- Genetic drift causes each population to lose alleles. Because drift is a random process, it is likely that each population will lose different alleles, making them steadily more different.
- Natural selection will cause the populations to adapt to their environments. When these environments are different, the adaptations of the populations will be different as well.
- Unique mutations that occur in individuals in each population cannot move to the other, and as these mutations grow in number, the populations become more genetically different.
If interbreeding is not restored between the populations, the accumulated amount of genetic and phenotypic changes between them eventually will make interbreeding impossible. When one population ceases to interbreed with all other populations, its evolutionary history diverges because genetic changes cannot cross the reproductive barrier. Simply put, the population resulting from a speciation is a species.
Patterns of speciation
Reproductive isolation can occur because populations become physically separated from each other. In primate evolutionary history, this scenario has occurred many times, for example, when anthropoid monkeys first arrived in South America some 50 million years ago. At that time, South America was an island continent separated by ocean from both North America, with which it is presently connected, and Africa, with which it had been connected before about 80 million years ago. The leading hypothesis for this original dispersal of monkeys to South America is that a small number of individuals crossed the then-smaller South Atlantic Ocean on storm-swept "rafts" of logs and other vegetation. Such a dispersal may have depended on the presence of volcanic islands between the continents which no longer exist, or patterns of ocean currents that were stronger when the two continents were closer together (Flynn and Wyss 1998). Like primates, rodents also apparently reached South America by this mechanism. However, the survival of such colonists and their establishment on the South American shore was a rare, unlikely event, and upon their arrival these early anthropoids were stranded with a formidable isolating barrier, the Atlantic Ocean, preventing subsequent mate exchange with their African relatives. Many similar colonizations, usually less spectacular in scope, have occurred throughout primate evolution, including dispersals at different times from Asia to Africa, Africa to Eurasia, and North America to Eurasia.
Populations need not cross oceans or move among continents to become isolated from each other. Isolating barriers may arise for many reasons, including when high water levels separate islands from the mainland, when a drier climate spreads grasslands and divides areas of forest, and when an area in the midst of a population's range undergoes geologic uplift, forming hills or mountains. Isolation caused by physical separation between populations leads to allopatric speciation, meaning that the populations live in different geographic areas from each other.Peripatric speciation, on the other hand, occurs between populations that are not physically isolated from each other. A large population may be spread across a wide range of local environments that slightly differ from each other. On the edge of such a range, small local groups may be strongly affected by natural selection, tending to adapt them to their distinct environments. If survival at the edge of the range is difficult, such groups may not persist for very long, or may be mostly composed of migrants from the main population from the center. Such a population structure leads to the "center-and-edge effect," in which variation is greater at the center of the species range and limited by genetic drift at the peripheries of the range.
But if new adaptations in the edge subpopulations allow them to survive and proliferate, and the amount of gene flow from the rest of the population is small, then the force of natural selection may overcome the tendency of gene flow to limit divergence. As a subpopulation at the edge of the range becomes more adapted to its local conditions, reproductive contacts with the rest of the population may become less important. Individuals with genes from the center may be at a selective disadvantage, or reproductive contact may be lost entirely. This process can result in speciation, if the edge subpopulation succeeds.
The process of speciation involves an interplay of all the evolutionary forces that affect populations. The key factor is the cessation of gene flow between the populations involved. Speciation requires more than simply isolation, however. If two subpopulations once separated by geography are brought back into contact, without any other evolutionary changes to impose continued reproductive isolation, they will resume interbreeding with each other. In such a case, no speciation has occurred.
Such circumstances have continually happened to human populations. Often when a human population colonizes a new area, it becomes isolated from other populations for a period of time, sometimes hundreds of generations or more. For example, the initial colonization of the Americas across the Bering land bridge, at least 12,000 years ago, was followed by the isolation of peoples on these continents from the rest of the world after the post-glacial rise in sea levels cut off any further land access. But when the invention of ocean travel reestablished transoceanic contact, people resumed interbreeding, as attested by the large numbers of their common descendants today. This kind of isolation affected other areas, including Australia, where only intermittent immigration from Southeast Asia occurred after the initial habitation of the continent over 50,000 years ago. Even over such long periods of time, these populations experienced no changes in their reproductive functions, and therefore no speciation occurred.
Isolation is initiated by a physical fact: the loss of gene flow between populations. But it is made permanent by the subsequent evolution of reproductive functions in one or both populations. Such reproductive changes occur either (1) as a side effect of adaptive changes in other biological aspects of the population or (2) as part of a process of continual reproductive change that causes populations to diverge after physical or geographic isolation is established.
Processes of continual reproductive change are expressions of sexual selection. In many groups of animals, mating involves a complicated interplay of physical traits and behaviors that animals use to advertise their qualities to potential mates. In every group, the ability of an animal to recognize potential mates depends on the presence of these signals, called the "mate recognition system". In some groups, the mate recognition system depends on the presence and form of specialized anatomical features, such as the antlers of elk or the special facial coloration of mandrills. In other groups, an animal must perform special behaviors for mate recognition to occur. Like any other phenotypic characteristic, these features may evolve over time, either because of changes in the selective balance between these reproductive features and other features leading to reproductive success or because new behaviors become associated with mate desirability. Many of the more complex mating behaviors may be mostly transmitted by learning or observation, only weakly heritable, allowing mate recognition systems to change easily over short periods of time. Two populations that lose genetic contact will be very unlikely to evolve in the same way. If separated for any period of time, members of one population will not recognize the members of the other as potential mates, resulting in speciation.
The ability to reproduce may also be lost for other reasons. Many times, evolutionary change in chromosomal structure or genetic function may occur that makes reproduction impossible even if mating occurs between members of two populations. The chromosomal rearrangement during human evolution that gave us 46 chromosomes instead of the 48 in chimpanzees would by itself make chimpanzee-human hybrids impossible. In other cases, mating and the birth of hybrid offspring between two populations may occur, but genetic changes may make these offspring sterile. This is the case for horses and donkeys, which interbreed with each other, but produce sterile mules as offspring.
Speciation as a hypothesis
It is important to bear in mind that speciation is a concept that must be tested as a hypothesis of population relationships. In particular, when biologists say that a speciation has occurred, they hold the hypothesis that the resulting populations will never again exchange genes in a way that significantly shapes their evolution. The hypothesis of speciation can be tested only by the presence of absence of gene flow.
Often, reproductive isolation is maintained primarily by the continued geographic isolation of the populations involved, even if other substantial evolutionary changes have occurred. Prominent examples are found among zoo animals. Sometimes animals that never have the opportunity to encounter each other in nature are found to interbreed freely when placed in close proximity to each other in zoos. For example, common chimpanzees and bonobos are separated by geographic barriers in the wild: bonobos live only south of the Congo River and common chimps only to the north. These two groups practice distinctive mating behaviors and have some anatomical differences. However, when given the opportunity in captivity, they do interbreed and produce hybrid offspring. Likewise, Sumatran and Bornean orangutans are presently isolated in the wild, and their genetic differences indicate that gene flow between the populations was very limited or nonexistent even during periods when these islands were connected to each other, either because their forested habitat did not extend across the now-submerged land bridge or because intermediate populations died out (Warren et al. 2001). However, there are only slight apparent anatomical differences between the two populations, and they interbreed quite freely in captivity.
In these instances, it is not obvious whether speciation can be said to have occurred. Clearly the populations involved have been geographically separate for a long time, with no immediate prospect of rejoining each other in their natural ranges. If they were to contact each other in the wild, it is impossible to predict whether gene flow would be reestablished. Future events may or may not reestablish gene flow, so that neither the hypothesis of speciation nor the alternative hypothesis of no speciation can be refuted. Both hypotheses are untestable, because we cannot foretell the future relationships among these organisms.
Sometimes nature permits stronger tests, even when interfertility is possible. For example, lions and tigers are closely related to each other and occurred historically in overlapping geographic ranges in Asia. While the skeletal differences between them are slight, these two kinds of great cats obviously differ in coloration and external anatomy, and in the wild they are successful only in different environments. Tigers are adapted to hunting in densely vegetated areas and lions are adapted to grassy plains. In captivity these two cats can and do produce fertile offspring, called "ligers" or "tigons" (More on hybrid big cats). Even so, apparently mate recognition between these cats does not occur in the wild, even if they are in close contact, which by itself would tend to support the hypothesis of speciation between them. However, a speciation between the two forms is further supported by their reactions to habitat changes that have occurred in their Asian ranges. Lions and tigers remain limited to their environments of greatest adaptation, without moving into each other's ranges, and even under habitat pressure and reduced numbers they do not seek out each other as mates. Thus, gene flow will play no part in their future, even if their human managers allowed it.
In some other cases gene flow extends across a considerable range but does not connect one end of the range to another. It sometimes happens in nature that a long string of interbreeding populations all exchange mates with their immediate neighbors but not over long distances. Genes may flow across the entire length of the chain by continued exchange from one population to the next, and by this mechanism mutation, selection, and genetic drift may cause changes in the entire population. However, when individuals from either end of the chain are brought together, they may be intersterile. Such populations are sometimes called "ring species", especially when they occur around the edge of a large isolating barrier, such as a body of water. Such instances may be difficult to resolve with the concept of speciation, although any large population may contain pairs of individuals who cannot reproduce with each other, but might each reproduce with others. In the case of ring species, speciation may follow quickly if any of the intervening populations lose genetic contact.
Speciation and evolution
The branching process that describes how different kinds of organisms arise is at a different level than the reproduction of organisms within a population. Each speciation is the birth of a new species, and species may disappear by extinction, when none of their members survive. Speciations and extinctions follow a different pattern than the births and deaths of individuals within a population, and although both speciations and extinctions are the result of evolution within populations, their pattern cannot necessarily be predicted from the forces of evolution alone. The study of the factors influencing speciation, extinction, and the resultant diversification of different species of organisms is called macroevolution, while the regular forces of evolution within populations are often called microevolution in contrast.
Macroevolutionary and microevolutionary patterns can reinforce each other during speciation. The initial speciation of a new species may accompanied by strong selection to a new environment. One change may trigger others, and the species may rapidly change in many respects. New anatomical configurations may evolve, along with new behaviors that define a new adaptation with respect to other species in the environment. The new advantages of this species may allow it to spread beyond the range of its parent species, or it may spread back into the parent's range, replacing it. Such broad evolutionary changes might have been impossible without the action of selection and drift in the small isolated population created by speciation.
Because of this possibility of great evolutionary change at the time of speciation, some scientists believe that most evolutionary change happens during these events, and not at other times. This hypothesis is most notably expressed by the model of punctuated equilibrium proposed by paleontologists Niles Eldredge and Stephen Jay Gould (1972). This model predicts that most populations stay mostly the same for long periods of time, a state called evolutionary stasis. Only occasionally, driven by environmental events, do large populations sizes with selective optimizations in a particular environmental niche break down, resulting in the rapid adaptation of small populations to new adaptive niches and new combinations of genetic material. Thus, in punctuated equilibrium, evolution consists of long periods of stasis interrupted by bursts of change that include speciations.
This model may explain the fossil records of different animals that remained relatively unchanged for long periods and then changed greatly in a short time. For example, . Such examples might involve speciations and subsequent rapid adaptations. On the other hand, selection even in the absence of speciation is a rapid evolutionary process, so that the appearance and rapid selection of new variants might lead to the appearance of punctuated evolutionary change within a single species.
Indeed, given the speed of natural selection, slow evolutionary changes may be less common than rapid ones. Nevertheless, there are many examples of slow changes in the fossil record, a pattern called gradualism.
Sometimes, large numbers of speciations in a single group of animals happen at once. The new populations generated by this burst of speciations often are variable in their adaptations to the environmental niches they occupy, as well as in their anatomical and behavioral makeup. Such a burst of speciation is called an adaptive radiation.
Adaptive radiations can happen whenever the reproductive success of a population brings it into contact with new environments in which its members are able to adapt and succeed. For example, the evolution of the four-chambered stomach in the ancestors of antelopes and cattle allowed them to use grasses that were not well used by other herbivores. This adaptation allowed their descendants to radiate into many different niches based on their ability to digest this material, leading to their extensive diversity today, which ranges from tiny Thompson's gazelles to large bison. No one species could occupy all these specialized niches. But in competition with each other, as well as with other species, populations of the original bovid quickly diverged to encompass these adaptations and others. This original adaptive radiation was therefore enabled by the evolution of the digestive system, which provided new energetic and dietary opportunities for bovid species.
Although the great apes today include only a small number of species, apes in the past were much more numerous. One of the reasons underlying this great collapse of ape diversity is the success of the Old World monkeys, which compete with apes for resources in forest environments. The Old World monkeys themselves underwent a substantial adaptive radiation during the Middle Miocene, capitalizing on their great dietary breadth, partly marked by their effective shearing molars, as well as their relatively short birth intervals compared to the apes. As climatic shifts became more rapid and frequent during the Late Miocene, the apes lost further ground to the monkeys, who underwent at least one more adaptive radiation based on the gut specializations toward leaf eating in the colobine monkeys. The evolution of important adaptations--especially those that enable exploitation of new food resources--is an important reason for the success of some lineages.
Sometimes, an adaptive radiation may result from an old adaptation that leads to new opportunities. For example, animals may colonize new areas, exposing many new niches in which they can out-compete other animals. The arrival of monkeys in South America is one such case, in which the combination of placental birth and arboreal adaptation caused these primates to succeed and diversify over many indigenous marsupials and birds. In other cases, an adaptive radiation may occur when an adaptation becomes useful in a broad range of environments. Bipedal locomotion in hominids is an example. Bipedalism may have evolved to allow efficient above-branch walking or walking short distances between groups of trees, but proved very effective in the more open woodlands of the early Pliocene, possibly leading to a radiation of different bipedal species.
One of the patterns of macroevolution is competitive exclusion, which governs the outcome of competition between species for a limiting resource (Hardin 1960). Many natural populations coexist without dire consequences, because they depend on different resources, or are not limited in numbers by the resources that they share. But a limiting resource, the supply of which directly limits the number of individuals that can exist in a population, cannot be shared by two populations without competition. If two populations depend on the same rare food source, or if the activities of one species place the requirements of another in jeopardy, the populations can react in three possible ways:
- One of the populations moves away.
- One of the populations changes its adaptation to reduce its dependence on the resource.
- One of the populations becomes extinct.
The principle of competitive exclusion, described by these three predictions, implies that at most one of the populations can remain unchanged.
Often competitive exclusion occurs when exotic animals move into a new area, competing with native groups. For example, the introduction of dingoes into Australia caused them to come into competition with native marsupial carnivores for prey species. The outcome is a product of the long evolutionary histories of both kinds of mammals, which left the dingo with certain advantages over the native carnivores, leading to their extinction. Such extinctions are a regular pattern of macroevolution.
Eldredge N, Gould SJ. 1972. Punctuated equilibria: an alternative to phyletic gradualism. In: Schopf J, editor, Models in paleobiology. San Francisco: Freeman and Cooper. p 82-115.
Flynn JJ, Wyss AR. 1998. Recent advances in South American mammalian paleontology. Trends Ecol Evol 13:449-454.
Hardin G. 1960. The competitive exclusion principle. Science 131:1292-1297.
Warren KS, Verschoor EJ, Langenhuijzen S, Heriyanto, Swan RA, Vigilant L, Heeney JL. 2001. Speciation and intrasubspecific variation of Bornean orangutans, Pongo pygmaeus pygmaeus. Mol Biol Evol 18:472-480.