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From Wikipedia, the free encyclopedia

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages.[1][2][3] Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species.[4] He also identified sexual selection as a likely mechanism, but found it problematic.

There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry, agriculture, or laboratory experiments. Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion.

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  • Speciation: Of Ligers & Men - Crash Course Biology #15
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  • Allopatric and sympatric speciation | Biology | Khan Academy
  • Speciation


You and me? We've got some stuff in common. More in common than, say, you and my dogs Lemon and Abby here. For starters, you and I are probably the same species. And Lemon and Abby are dogs, which is a different species. As you may have guessed by now, this video is going to be about species! But at the very end, we're going to talk about dogs. So hang in there, because the puppies are coming. Before we bust out the puppies, let's talk about people. Our species, Homo Sapiens, is the single remaining member of the genus Homo. Our buddies Homo Erectus and Homo Habilis and Homo Neanderthalensis bought the farm a long time ago. So these days, all us Homo sapiens are pretty different from even our closest living relatives in the animal Kingdom, the chimps and bonobos. Humans are a species, a specific type of organism that's different from all the other types of organisms out there. But what is it that makes us human? Well, we're a specific type of animal called a primate. Monkeys, apes, lemurs, and tarsiers are also primates. Unless you're Sacha Baron Cohen or something, most of us are lacking significant body hair. We're bipedal, meaning we stand on two feet, and we've got these huge-normous brains, that allow us to do all kinds of stuff like talk real good, solve complicated problems, write bad poetry during adolescence, and figure out how little we can get away with tipping a mediocre waiter at a restaurant without seeming like a total prick. THAT, my friends, is something that giraffes rarely have to deal with. But being a species is more than having a bunch of stuff in common. Instead we describe a species as a group of organisms that can interbreed and produce fertile offspring. Seems pretty simple, right? Two of the same species that can produce blah blah blah... HEY! Pay attention! That last part is important! The two organisms need to be able to produce fertile offspring. It seems like it would be enough for organisms of the same species to be able to make babies, but those babies need to be able to make babies, too. Now it turns out, two animals of a different species can sometimes technically have a baby. Take, for instance, the noble liger, Napoleon Dynamite's favorite animal, which I know because I had the very best Napoleon Dynamite costume in the United States for Halloween in 2005. But, I didn't just bring ligers up to brag. A liger is what happens when a male lion and a female tiger have a little cub. Only, it's not very little because a liger is generally larger than both of its parents. And ligers are sterile. Which leads us to our understanding of what makes a species: lions and tigers are different species because they don't produce fertile offspring together. We call animals like ligers hybrids, the offspring resulting from the cross-breeding of two distinct species. And even though hybridization between two animals is a dead end when it comes to creating a new species, we know that through evolution, or the change in the heritable characteristics of a species across generations, new species have formed in the past, and they continue to develop all the time. It's tough to nail down every single way this process we call speciation can happen, but we know of at least a couple ways that species evolve into other species. And they both involve one requirement: reproductive isolation, meaning two populations of the same species can no longer mate together successfully. Note that I said successfully. One way populations can become isolated from each other is that they can mate, but their offspring aren't fertile or viable. Ligers are a good example of this. So are mules, they're the product of a male donkey and a female horse. Unlike lions and tigers, donkeys and horses don't even have the same number of chromosomes, so even though the donkey sperm can fertilize the horse egg, the mule won't have the genetic instructions it needs to produce its own sex cells. This kind of isolation is call post-zygotic, because the parents can form a zygote together, but after that it's all over for their lineage. Other examples of post-zygotic isolation include pairings of species that always lead to miscarriage or no development of the embryo at all, or things like big fetuses that kill the mother at birth. The other type of isolation is pre-zygotic, meaning the isolation happened between groups of the same species before an egg even thought about getting fertilized. This can include stuff like behavioral changes within a species, like when birds of the same species start singing two different songs to attract mates. Or when one group of a species that does all its business in the daytime gradually becomes nocturnal, so the two groups never end up hanging out at the same time. Pre-zygotic isolation can also be geographic, meaning simply that the populations are separated by great distances or physical barriers, so that they can no longer get together to bump uglies. When one species diverges into two new species because of geographic isolation, it's called allopatric speciation, allopatric coming from the Greek for "different countries." The two populations of a species end up evolving differently because conditions are different on each side of this river here. It might be colder on one side of the river, so the animals on this side grow thicker, more luxurious coats because those guys just do better over there. They probably also put on thicker layers of fat, and change their behavior, and accumulate a bunch of other possibly random changes. Meanwhile, on the warm side of the river, these animals also accumulate changes, and lose some fur and add a bunch of sweat glands. Given enough time, and given a complete lack of gene flow between the two populations, thick-coated animals will eventually only be able to breed with other thick-coated animals, and sweaty animals with sweaty animals. This propagation of specific traits based on how kick-ass those traits make the animal that has them is called natural selection. And a guy named Charles Darwin or Chuck Darwin, or Chucky D to his friends... was the one who let us know what was up with natural selection and how it can lead to allopatric speciation. Stop me if you've heard this one before, but Darwin visited the Galapagos Islands in the 1830s. So Darwin was obsessed with barnacles, but that didn't keep him from noticing the finches, which were actually misidentified by him as grosbeaks, on each island were all pretty similar to the finches on the other islands AND very similar to the ones on the mainland of South America BUT they were also obviously their own species. Darwin believed that the process that led to these finches becoming separate species was incredibly slow, so slow that we couldn't actually witness the process, we just had to take his word for it. Now, for a long time after Darwin made these observations, allopatric speciation was the main explanation for how species diverge. But today we know that's not the whole truth. Now, we've got lots of new-fangled DNA testing and other special gadgetry that tells us that one species can diverge into two without being geographically separated, but instead, when they're reproductively isolated in some other way. This is called sympatric speciation, meaning "same country," and it also means that it's time for a trip to the chair! So, here's a little biological love story for all you romantics out there. Peter and Rosemary Grant, two British evolutionary biologists (they are, in fact, a married couple) have, since the early 1970s, been spending 6 months of each year together on a secluded island in the Galapagos studying Darwin's finches, trying to catch them in the act of evolving. These are, mind you, the same animals that Darwin studied, and the ones that he said were evolving at an imperceptibly slow pace. The island in the Galapagos that the Grants hang out on is called Daphne Major, and when they started their research in 1973, it was occupied by two different finch species: the medium ground finch and the cactus finch. But in 1981, another finch arrived on Daphne Major from a nearby island. It was a ground finch-cactus finch hybrid, and it was a whole lot bigger than either of the local finches. Its beak was also extra wide, and its song was like a mashup of the jams ground finches sang on its home island and the ones sung on Daphne Major. The newcomer set to work crooning to the local ground finch ladies, and eventually landed one. The Grants followed the descendants of these two birds for the next 28 years. But after about 4 generations, Daphne Major experienced a severe drought which killed many of the finches. There were only TWO surviving descendants of that one immigrant finch, sort of like cousins of each other, basically and they mated with each other, and that seems to have set the stage for speciation to occur. The descendants of these two survivors sang a very distinctive song that was learned from their parents and which was different from the other Daphne Major finches. Gradually as the finch population on the island rebounded, the hybrid finches, the great-great- great-great-great grandchicks of that one bird, began mating exclusively with each other. In December 2009, the Grants announced that, since the drought, the lineage of that one immigrant ground finch has been genetically isolated from the other local finches on the island. So, that, my friends, is both super romantic and also an example of super-quick sympatric speciation in action. Okay, so I promised you puppies, so I'm gonna give you puppies. You've probably noticed that, you know, a corgi looks pretty different from a greyhound. They were bred to be different. Corgis were bred to herd animals and guard farm houses, while greyhounds were bred mostly to run. Dog breeding kind of takes the "natural" out of natural selection, in fact, it's what we call artificial selection, but it's still a kind of selection. You've probably wondered what it would be like if a corgi and a greyhound had puppies together? Because they CAN have puppies together. Even though that's really weird. What's that, Lemon? You're both girls? Oh, well- anyways.. My point here is that they're the same species, meaning that these dogs, even differenter dogs, like an Irish wolfhound and a chihuahua, could have fertile offspring together. Like, how? How... How? Would- HOW!? Various dog breeds are similar enough that post-zygotic isolation isn't an issue. But in a natural setting, a chihuahua-wolfhound pairing would be extremely rare because of the difficulties involved in the gettin' it on process. Or "pre-zygotic obstacles." So, think about it like this, if you were to put a bunch of chihuahuas and a bunch of wolfhounds on an island somewhere they probably wouldn't breed together and if they did, the birthing process, at least for the chihuahua mommies would be... Gah! Oh god. But what this means is that the gene flow between the two groups would stop, and they would become reproductively isolated. Over time, they could become different enough that they could no longer successfully breed together at all, and thus become different species. Thank you for watching this episode of Crash Course. If you missed anything don't forget to go back and review. If you have any questions, please ask them in the comments or of Facebook or Twitter. We will endeavor to answer them. Thank you to everyone who helped put this episode together. We'll see you next time.


Historical background

In addressing the question of the origin of species, there are two key issues: (1) what are the evolutionary mechanisms of speciation, and (2) what accounts for the separateness and individuality of species in the biota? Since Charles Darwin's time, efforts to understand the nature of species have primarily focused on the first aspect, and it is now widely agreed that the critical factor behind the origin of new species is reproductive isolation.[5] Next we focus on the second aspect of the origin of species.

Darwin's dilemma: Why do species exist?

In On the Origin of Species (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species.[6] Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the absence or rarity of transitional varieties in habitat space.[7]

Another dilemma,[8] related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species.[6]

The effect of sexual reproduction on species formation

It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity.[9][10][11][12][13] The cost of rarity arises as follows. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species’ population size happens, by chance, to increase (at the expense of one or other of its neighboring species, if the environment is saturated), this will immediately make it easier for its members to find sexual partners. The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, and therefore form pairs less frequently than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to extinction. Eventually, only a few species remain, each distinctly different from the other.[9][10][12] The cost of rarity not only involves the costs of failure to find a mate, but also indirect costs such as the cost of communication in seeking out a partner at low population densities.

 African pygmy kingfisher, showing coloration shared by all adults of that species to a high degree of fidelity.[14]
African pygmy kingfisher, showing coloration shared by all adults of that species to a high degree of fidelity.[14]

Rarity brings with it other costs. Rare and unusual features are very seldom advantageous. In most instances, they indicate a (non-silent) mutation, which is almost certain to be deleterious. It therefore behooves sexual creatures to avoid mates sporting rare or unusual features.[15][16] Sexual populations therefore rapidly shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta. This remarkable uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, mammals, reptiles, insects, and many other taxa, in which a species can be described with a single illustration (or two, in the case of sexual dimorphism). Once a population has become as homogeneous in appearance as is typical of most species (and is illustrated in the photograph of the African pygmy kingfisher), its members will avoid mating with members of other populations that look different from themselves.[17] Thus, the avoidance of mates displaying rare and unusual phenotypic features inevitably leads to reproductive isolation, one of the hallmarks of speciation.[18][19][20][21]

In the contrasting case of organisms that reproduce asexually, there is no cost of rarity; consequently, there are only benefits to fine-scale adaptation. Thus, asexual organisms very frequently show the continuous variation in form (often in many different directions) that Darwin expected evolution to produce, making their classification into "species" (more correctly, morphospecies) very difficult.[9][15][16][22][23][24]


All forms of natural speciation have taken place over the course of evolution; however, debate persists as to the relative importance of each mechanism in driving biodiversity.[25]

One example of natural speciation is the diversity of the three-spined stickleback, a marine fish that, after the last glacial period, has undergone speciation into new freshwater colonies in isolated lakes and streams. Over an estimated 10,000 generations, the sticklebacks show structural differences that are greater than those seen between different genera of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.[26]


During allopatric (from the ancient Greek allos, "other" + Greek patrā, "fatherland") speciation, a population splits into two geographically isolated populations (for example, by habitat fragmentation due to geographical change such as mountain formation). The isolated populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to dissimilar selective pressures; (b) they independently undergo genetic drift; (c) different mutations arise in the two populations. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes. Island genetics is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include insular dwarfism and the radical changes among certain famous island chains, for example on Komodo. The Galápagos Islands are particularly famous for their influence on Charles Darwin. During his five weeks there he heard that Galápagos tortoises could be identified by island, and noticed that finches differed from one island to another, but it was only nine months later that he reflected that such facts could show that species were changeable. When he returned to England, his speculation on evolution deepened after experts informed him that these were separate species, not just varieties, and famously that other differing Galápagos birds were all species of finches. Though the finches were less important for Darwin, more recent research has shown the birds now known as Darwin's finches to be a classic case of adaptive evolutionary radiation.[27]


In peripatric speciation, a subform of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation.

Case Studies:


In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography; individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms that prevent their interbreeding. Parapatric speciation is modelled on continuous variation within a "single," connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation.

Parapatric speciation may be associated with differential landscape-dependent selection. Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop.[29] Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards Darevskia rudis, D. valentini and D. portschinskii all hybridize with each other in their hybrid zone; however, hybridization is stronger between D. portschinskii and D. rudis, which separated earlier but live in similar habitats than between D. valentini and two other species, which separated later but live in climatically different habitats.[30]

Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. A niche must be available in order for a new species to be successful. Ring species such as Larus gulls have been claimed to illustrate speciation in progress, though the situation may be more complex.[31] The grass Anthoxanthum odoratum may be starting parapatric speciation in areas of mine contamination.[32]


 Freshwater angelfish, a cichlid
Freshwater angelfish, a cichlid

Sympatric speciation is the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

Often-cited examples of sympatric speciation are found in insects that become dependent on different host plants in the same area.[33][34] However, the existence of sympatric speciation as a mechanism of speciation remains highly debated.[35]

The best illustrated example of sympatric speciation is that of the cichlids of East Africa inhabiting the Rift Valley lakes, particularly Lake Victoria, Lake Malawi and Lake Tanganyika. There are over 800 described species, and according to estimates, there could be well over 1,600 species in the region. Their evolution is cited as an example of both natural and sexual selection.[36][37] A 2008 study suggests that sympatric speciation has occurred in Tennessee cave salamanders.[38] Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal.[39]

Budding speciation has been proposed as a particular form of sympatric speciation, whereby small groups of individuals become progressively more isolated from the ancestral stock by breeding preferentially with one another. This type of speciation would be driven by the conjunction of various advantages of inbreeding such as the expression of advantageous recessive phenotypes, reducing the recombination load, and reducing the cost of sex [40]

The hawthorn fly (Rhagoletis pomonella), also known as the apple maggot fly, appears to be undergoing sympatric speciation.[41] Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apples, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as that six out of thirteen allozyme loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4-6% hybridization rate) suggests that sympatric speciation is occurring.[42]

Methods of selection


Reinforcement, sometimes referred to as the Wallace effect, is the process by which natural selection increases reproductive isolation.[18] It may occur after two populations of the same species are separated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two separate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys.)[43]

The reasoning behind this is that if the parents of the hybrid offspring each have naturally selected traits for their own certain environments, the hybrid offspring will bear traits from both, therefore would not fit either ecological niche as well as either parent. The low fitness of the hybrids would cause selection to favor assortative mating, which would control hybridization. This is sometimes called the Wallace effect after the evolutionary biologist Alfred Russel Wallace who suggested in the late 19th century that it might be an important factor in speciation.[44]
Conversely, if the hybrid offspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact.

Reinforcement favoring reproductive isolation is required for both parapatric and sympatric speciation. Without reinforcement, the geographic area of contact between different forms of the same species, called their "hybrid zone," will not develop into a boundary between the different species. Hybrid zones are regions where diverged populations meet and interbreed. Hybrid offspring are very common in these regions, which are usually created by diverged species coming into secondary contact. Without reinforcement, the two species would have uncontrollable inbreeding. Reinforcement may be induced in artificial selection experiments as described below.


Ecological selection is "the interaction of individuals with their environment during resource acquisition".[45] Natural selection is inherently involved in the process of speciation, whereby, "under ecological speciation, populations in different environments, or populations exploiting different resources, experience contrasting natural selection pressures on the traits that directly or indirectly bring about the evolution of reproductive isolation".[46] Evidence for the role ecology plays in the process of speciation exists. Studies of stickleback populations support ecologically-linked speciation arising as a by-product,[47] alongside numerous studies of parallel speciation, where isolation evolves between independent populations of species adapting to contrasting environments than between independent populations adapting to similar environments.[48] Ecological speciation occurs with much of the evidence, "...accumulated from top-down studies of adaptation and reproductive isolation".[48]

Sexual selection

It is widely appreciated that sexual selection could drive speciation in many clades, independently of natural selection.[49] However the term “speciation”, in this context, tends to be used in two different, but not mutually exclusive senses. The first and most commonly used sense refers to the “birth” of new species. That is, the splitting of an existing species into two separate species, or the budding off of a new species from a parent species, both driven by a biological "fashion fad" (a preference for a feature, or features, in one or both sexes, that do not necessarily have any adaptive qualities).[49][50][51][52] In the second sense, "speciation" refers to the wide-spread tendency of sexual creatures to be grouped into clearly defined species,[53][19] rather than forming a continuum of phenotypes both in time and space - which would be the more obvious or logical consequence of natural selection. This was indeed recognized by Darwin as problematic, and included in his On the Origin of Species (1859), under the heading "Difficulties with the Theory".[6] There are several suggestions as to how mate choice might play a significant role in resolving Darwin’s dilemma.[19][9][15][16][17][54]

Artificial speciation

 Gaur (Indian bison) can interbreed with domestic cattle.
Gaur (Indian bison) can interbreed with domestic cattle.

New species have been created by animal husbandry, but the dates and methods of the initiation of such species are not clear. Often, the domestic counterpart of the wild ancestor can still interbreed and produce fertile offspring as in the case of domestic cattle, that can be considered the same species as several varieties of wild ox, gaur, yak, etc., or domestic sheep that can interbreed with the mouflon.[55][56]

The best-documented creations of new species in the laboratory were performed in the late 1980s. William R. Rice and George W. Salt bred Drosophila melanogaster fruit flies using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.[57] The history of such attempts is described by Rice and Elen E. Hostert (1993).[58][59] Diane Dodd used a laboratory experiment to show how reproductive isolation can evolve in Drosophila pseudoobscura fruit flies after several generations by placing them in different media, starch- and maltose-based media.[60]

Drosophila speciation experiment.svg

Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods.[61] Research in 2005 has shown that this rapid evolution of reproductive isolation may in fact be a relic of infection by Wolbachia bacteria.[62]

Alternatively, these observations are consistent with the notion that sexual creatures are inherently reluctant to mate with individuals whose appearance or behavior is different from the norm. The risk that such deviations are due to heritable maladaptations is very high. Thus, if a sexual creature, unable to predict natural selection's future direction, is conditioned to produce the fittest offspring possible, it will avoid mates with unusual habits or features.[63][64][15][16][17] Sexual creatures will then inevitably tend to group themselves into reproductively isolated species.[16]


Few speciation genes have been found. They usually involve the reinforcement process of late stages of speciation. In 2008, a speciation gene causing reproductive isolation was reported.[65] It causes hybrid sterility between related subspecies. The order of speciation of three groups from a common ancestor may be unclear or unknown; a collection of three such species is referred to as a "trichotomy."

Speciation via polyploidization

Polyploidy is a mechanism that has caused many rapid speciation events in sympatry because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny.[66] However, not all polyploids are reproductively isolated from their parental plants, and gene flow may still occur for example through triploid hybrid x diploid matings that produce tetraploids, or matings between meiotically unreduced gametes from diploids and gametes from tetraploids (see also hybrid speciation).

It has been suggested that many of the existing plant and most animal species have undergone an event of polyploidization in their evolutionary history.[67][68] Reproduction of successful polyploid species is sometimes asexual, by parthenogenesis or apomixis, as for unknown reasons many asexual organisms are polyploid. Rare instances of polyploid mammals are known, but most often result in prenatal death.

Hybrid speciation

Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The Mariana mallard is thought to have arisen from hybrid speciation.

Hybridization is an important means of speciation in plants, since polyploidy (having more than two copies of each chromosome) is tolerated in plants more readily than in animals.[69][70] Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.[68] Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations.[71]

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heliconius butterflies [72] and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species.

Gene transposition

Theodosius Dobzhansky, who studied fruit flies in the early days of genetic research in 1930s, speculated that parts of chromosomes that switch from one location to another might cause a species to split into two different species. He mapped out how it might be possible for sections of chromosomes to relocate themselves in a genome. Those mobile sections can cause sterility in inter-species hybrids, which can act as a speciation pressure. In theory, his idea was sound, but scientists long debated whether it actually happened in nature. Eventually a competing theory involving the gradual accumulation of mutations was shown to occur in nature so often that geneticists largely dismissed the moving gene hypothesis.[73]

However, 2006 research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species.[74] This validates the reproductive isolation mechanism, a key component of speciation.[75]


 Phyletic gradualism, above, consists of relatively slow change over geological time. Punctuated equilibrium, bottom, consists of morphological stability and rare, relatively rapid bursts of evolutionary change.
Phyletic gradualism, above, consists of relatively slow change over geological time. Punctuated equilibrium, bottom, consists of morphological stability and rare, relatively rapid bursts of evolutionary change.

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant and gradual over time (known as "Phyletic gradualism" - see diagram), some palaeontologists such as Niles Eldredge and Stephen Jay Gould[76] have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium. (See diagram, and Darwin's dilemma.)

Punctuated evolution

Evolution can be extremely rapid, as shown in the creation of domesticated animals and plants in a very short geological space of time, spanning only a few tens of thousands of years. Maize (Zea mays), for instance, was created in Mexico in only a few thousand years, starting about 7,000 to 12,000 years ago.[77] This raises the question of why the long term rate of evolution is far slower than is theoretically possible.[78][79][80][81]

Plants and domestic animals can differ markedly from their wild ancestors
Top: wild teosinte; middle: maize-teosinte hybrid; bottom: maize

Evolution is imposed on species or groups. It is not planned or striven for in some Lamarckist way.[82] The mutations on which the process depends are random events, and, except for the "silent mutations" which do not affect the functionality or appearance of the carrier, are thus usually disadvantageous, and their chance of proving to be useful in the future is vanishingly small. Therefore, while a species or group might benefit from being able to adapt to a new environment by accumulating a wide range of genetic variation, this is to the detriment of the individuals who have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the capability to evolve would require group selection, a concept discredited by (for example) George C. Williams,[83] John Maynard Smith[84] and Richard Dawkins[85][86][87][88] as selectively disadvantageous to the individual.

The resolution to Darwin's second dilemma might thus come about as follows:

If sexual individuals are disadvantaged by passing mutations on to their offspring, they will avoid mutant mates with strange or unusual characteristics.[64][15][16][54] Mutations that affect the external appearance of their carriers will then rarely be passed on to the next and subsequent generations. They would therefore seldom be tested by natural selection. Evolution is, therefore, effectively halted or slowed down considerably. The only mutations that can accumulate in a population, on this punctuated equilibrium view, are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "neutral mutations," which can be, and are, used to trace the relatedness and age of populations and species.[15][89]) This argument implies that evolution can only occur if mutant mates cannot be avoided, as a result of a severe scarcity of potential mates. This is most likely to occur in small, isolated communities. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves,[90] or during the aftermath of a mass extinction.[89] Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition.[90] Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. If an isolated population such as this survives its genetic upheavals, and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come in being. In geological terms this will be an abrupt event. A resumption of avoiding mutant mates will thereafter result, once again, in evolutionary stagnation.[76][79]

In apparent confirmation of this punctuated equilibrium view of evolution, the fossil record of an evolutionary progression typically consists of species that suddenly appear, and ultimately disappear, hundreds of thousands or millions of years later, without any change in external appearance.[76][89][91] Graphically, these fossil species are represented by horizontal lines, whose lengths depict how long each of them existed. The horizontality of the lines illustrates the unchanging appearance of each of the fossil species depicted on the graph. During each species' existence new species appear at random intervals, each also lasting many hundreds of thousands of years before disappearing without a change in appearance. The exact relatedness of these concurrent species is generally impossible to determine. This is illustrated in the diagram depicting the distribution of hominin species through time since the hominins separated from the line that led to the evolution of our closest living primate relatives, the chimpanzees.[91]

For similar evolutionary time lines see, for instance, the paleontological list of African dinosaurs, Asian dinosaurs, the Lampriformes and Amiiformes.

See also


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