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Speciation is the
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
ary process by which populations evolve to become distinct
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
. The biologist Orator F. Cook coined the term in 1906 for
cladogenesis Cladogenesis is an evolutionary splitting of a parent species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defi ...
, the splitting of lineages, as opposed to
anagenesis Anagenesis is the gradual evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes th ...
, phyletic evolution within lineages.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
was the first to describe the role of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
in speciation in his 1859 book ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
''. He also identified
sexual selection Sexual selection is a mode of natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrat ...
as a likely mechanism, but found it problematic. There are four geographic modes of speciation in nature, based on the extent to which speciating
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
s are isolated from one another:
allopatric Allopatric speciation (from Ancient Greek ἄλλος, ''allos'', meaning "other", and πατρίς, ''patris'', "fatherland"), also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model, is a mode of ...
, peripatric,
parapatric In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes In biology Biology is the natural science that studies life and living organisms, including th ...
, and
sympatric In biology, two related species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, ...
. Speciation may also be induced artificially, through
animal husbandry Animal husbandry is the branch of agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Ex ...
, agriculture, or laboratory experiments. Whether
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

genetic drift
is a minor or major contributor to speciation is the subject of much ongoing discussion. Rapid sympatric speciation can take place through
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y, such as by doubling of chromosome number; the result is progeny which are immediately
reproductively isolated The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
from the parent population. New species can also be created through hybridization followed—if the hybrid is favoured by natural selection—by reproductive isolation.


Historical background

In addressing the origin of species, there are two key issues: # the evolutionary mechanisms of speciation # how the separateness and individuality of species is maintained 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.


Darwin's dilemma: why species exist

In ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory". In discussing these "difficulties" he noted This dilemma can be described as the absence or rarity of transitional varieties in habitat space. Another dilemma, 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.


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 Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
has an intrinsic cost of rarity. 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 Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

extinction
. Eventually, only a few species remain, each distinctly different from the other. 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. Rarity brings with it other costs. Rare and unusual features are very seldom advantageous. In most instances, they indicate a ( non-silent)
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
, which is almost certain to be deleterious. It therefore behooves sexual creatures to avoid mates sporting rare or unusual features (
koinophilia peacock is unlikely to find a mate and reproduce in a natural setting, due to its unusual coloration. However, its striking colour is appreciated by humans, and may be included in artificial selective breeding File:Big and little dog 1.jpg, This ...
). 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 uniformity of all the adult members of a sexual species has stimulated the proliferation of
field guide A field guide is a book A book is a medium for recording information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of its ...
s on birds, mammals, reptiles, insects, and many other
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
, in which a species can be described with a single illustration (or two, in the case of
sexual dimorphism Sexual dimorphism is the condition where the sex Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in ...
). 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. Thus, the avoidance of mates displaying rare and unusual phenotypic features inevitably leads to reproductive isolation, one of the hallmarks of speciation. 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.


Modes

All forms of natural speciation have taken place over the course of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
; however, debate persists as to the relative importance of each mechanism in driving
biodiversity Biodiversity is the biological variety and Genetic variability, variability of life, life on Earth. Biodiversity is a measure of variation at the Genetics, genetic, species, and ecosystem level. Terrestrial biodiversity is usually greater near ...

biodiversity
. One example of natural speciation is the diversity of the
three-spined stickleback The three-spined stickleback (''Gasterosteus aculeatus'') is a fish native to most inland and coastal waters north of 30°N. It has long been a subject of scientific study for many reasons. It shows great morphological variation throughout its ra ...

three-spined stickleback
, a
marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
fish that, after the last glacial period, has undergone speciation into new
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...

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 Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.


Allopatric

During allopatric (from the ancient Greek ''allos'', "other" + ''patrā'', "fatherland") speciation, a population splits into two geographically isolated populations (for example, by
habitat fragmentation Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biop ...
due to geographical change such as
mountain formation Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (List of tectonic plates, tectonic plates). Fold (geology), Folding, ...
). The isolated populations then undergo genotypic or
phenotypic In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotypic
divergence as: (a) they become subjected to dissimilar
selective Selective may refer to: * Selective school, a school that admits students on the basis of some sort of selection criteria ** Selective school (New South Wales) Selective strength: the human body transitions between being weak and strong. This rang ...
pressures; (b) they independently undergo genetic drift; (c) different
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s 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
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s. Island genetics is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include
insular dwarfism Insular dwarfism, a form of phyletic dwarfism, is the process and condition of large animals evolving or having a reduced body size when their population's range is limited to a small environment, primarily islands. This natural process is distin ...
and the radical changes among certain famous island chains, for example on Komodo. The
Galápagos Islands The Galápagos Islands (official name: , other Spanish language, Spanish name: , , ), part of the Republic of Ecuador, are an archipelago of High island, volcanic islands. They are distributed on either side of the equator in the Pacific Ocean ...
are particularly famous for their influence on Charles Darwin. During his five weeks there he heard that
Galápagos tortoise The Galápagos tortoise complex or Galápagos giant tortoise complex is a species complex of 15 (13 Extant taxon, extant and 2 extinct) very large tortoise species in the genus ''Chelonoidis'' (which also contains three other species from mainl ...

Galápagos tortoise
s could be identified by island, and noticed that
finch The true finches are small to medium-sized passerine A passerine () is any bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular ...

finch
es 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 England is a Countries of the United Kingdom, country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest of England and the Celtic Sea to the southwest. E ...

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 Darwin's finches (also known as the Galápagos finches) are a group of about 18 species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiver ...

Darwin's finches
to be a classic case of adaptive evolutionary radiation.


Peripatric

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 In population genetics Population genetics is a subfield of genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed ...

founder effect
, since small populations often undergo
bottleneck Bottleneck literally refers to the narrowed portion (neck) of a bottle A bottle is a narrow-necked container made of an impermeable material (clay, glass Glass is a non- crystalline, often transparency and translucency, transparent amorpho ...

bottleneck
s. Genetic drift is often proposed to play a significant role in peripatric speciation. Case studies include Mayr's investigation of bird fauna; the Australian bird ''''; and reproductive isolation in populations of ''
Drosophila ''Drosophila'' () is a genus of fly, flies, belonging to the family (biology), family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the character ...

Drosophila
'' subject to population bottlenecking.


Parapatric

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 Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈ ...
leads to selection for behaviours or mechanisms that prevent their
interbreeding In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
. 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 Selection may refer to: In science: * Selection (biology) Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is ill ...
. Even if there is a
gene flow In population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredient ...

gene flow
between two populations, strong differential selection may impede assimilation and different species may eventually develop. 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
hybrid Hybrid may refer to: Economics and finance * Hybrid market, a system allowing stock trades to be completed either electronically or manually * Hybrid security, a type of economic instrument Technology Electrical power generation * Hybrid generato ...
ize with each other in their
hybrid zoneA hybrid zone exists where the ranges of two interbreeding species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often ...
; 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. Ecologists refer to parapatric and peripatric speciation in terms of
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
s. A niche must be available in order for a new species to be successful.
Ring species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

Ring species
such as ''
Larus ''Larus'' is a large genus of gulls with worldwide distribution (by far the greatest species diversity is in the Northern Hemisphere). Many of its species are abundant and well-known birds in their ranges. Until about 2005–2007, most gulls ...

Larus
'' gulls have been claimed to illustrate speciation in progress, though the situation may be more complex. The grass '''' may be starting parapatric speciation in areas of mine contamination.


Sympatric

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 A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (19 ...
plants in the same area. The best known example of sympatric speciation is that of the
cichlid Cichlids are fish from the family (biology), family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses (Labridae), in the order Perciformes, but molecular studies have co ...
s of
East Africa East Africa, Eastern Africa, or East of Africa is the eastern sub-region A subregion is a part of a larger region or continent and is usually based on location. Cardinal directions, such as south or southern, are commonly used to define a subr ...
inhabiting the
Rift Valley lakes The Rift Valley lakes are a series of lakes in the East African Rift File:East Africa Rift System GPS and stresses.png, Main rift faults, plates, plate boundaries, GPS plate velocities between adjacent blocks and minimum horizontal stress direc ...

Rift Valley lakes
, particularly
Lake Victoria Lake Victoria ( luo, Nam Lolwe; lg, 'Nnalubaale; rw, Nyanza; also ''Ukerewe'') is one of the African Great Lakes. Though having multiple local language names, the lake was renamed after Queen Victoria by the explorer John Hanning Speke, the ...

Lake Victoria
,
Lake Malawi Lake Malawi, also known as Lake Nyasa in Tanzania and Lago Niassa in Mozambique, is an African Great Lakes, African Great Lake and the southernmost lake in the East African Rift system, located between Malawi, Mozambique and Tanzania. It is th ...

Lake Malawi
and
Lake Tanganyika Lake Tanganyika is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal Lake Baikal (; russian: Oзеро Байкал, Ozero Ba ...

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 Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and ...
and
sexual selection Sexual selection is a mode of natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrat ...
. A 2008 study suggests that sympatric speciation has occurred in
Tennessee cave salamander The Tennessee cave salamander (''Gyrinophilus palleucus'') is a species of salamander in the family Plethodontidae, endemic to the Appalachian Mountains in the United States. Its natural habitats are streams in caves. It is threatened by habitat d ...
s. Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's
Lake Baikal Lake Baikal (; russian: Oзеро Байкал, Ozero Baykal ; bua, Байгал далай, Baigal dalai; mn, Байгал нуур, Baigal nuur) is a rift lake A rift lake is a lake A lake is an area filled with water, localized i ...

Lake Baikal
. 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. The hawthorn fly (''''), also known as the apple maggot fly, appears to be undergoing sympatric speciation. Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after
apple An apple is an edible fruit In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this fie ...

apple
s, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of . The current hawthorn feeding population does not normally feed on apples. Some evidence, such as that six out of thirteen
allozyme Alloenzymes (or also called allozymes) are variant forms of an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substr ...
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.


Methods of selection


Reinforcement

Reinforcement, also called the Wallace effect, is the process by which natural selection increases reproductive isolation. Adapted from ''Evolution'' (2004), 3rd edition (Malden, MA:
Blackwell Publishing Wiley-Blackwell is an international scientific, technical, medical, and scholarly publishing business of John Wiley & Sons John Wiley & Sons, Inc., commonly known as Wiley (), is an American multinational Multinational may refer to: * Multina ...
), .
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, as in
horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

horse
s and
donkey The donkey or ass is a domestic animal This page gives a list of domestic animals, also including a list of domestication of animals, animals which are or may be currently undergoing the process of domestication and animals that have an exten ...
s. One 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 (ecological speciation). The low fitness of the hybrids would cause selection to favor
assortative mating Assortative mating (also referred to as positive assortative mating or homogamy) is a mating pattern and a form of sexual selection Sexual selection is a mode of natural selection Natural selection is the differential survival and ...
, which would control hybridization. This is sometimes called the Wallace effect after the evolutionary biologist
Alfred Russel Wallace Alfred Russel Wallace (8 January 18237 November 1913) was a British natural history, naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution throug ...
who suggested in the late 19th century that it might be an important factor in speciation. 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. Another imporant theoretical mechanism is the arise of intrinsic genetic incompatibilities, addressed in the Bateson-Dobzhansky-Muller model. Genes from allopatric populations will have different evolutionary backgrounds and are never tested together until hybridization at secondary contact, when negative epistatic interactions will be exposed. In other words, new alleles will emerge in a population and only pass through selection if they work well together with other genes in the same population, but it may not be compatible with genes in an allopatric population, be those other newly derived alleles or retained ancestral alleles. This is only revealed through new hybridization. Such incompatibilities cause lower fitness in hybrids regardless of the ecological environment, and are thus intrinsic, although they can originate from the adaptation to different environments. The accumulation of such incompatibilities increases faster and faster with time, creating a "snowball" effect. There is a large amount of evidence supporting this thoery, primarily from laboratory populations such as ''Drosophila'' and ''Mus'', and some genes involved in incompatibilities have been identified. 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 common in these regions, which are usually created by diverged species coming into
secondary contact Secondary contact is the process in which two allopatricaly distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated The mechanism ...
. Without reinforcement, the two species would have uncontrollable
inbreeding Inbreeding is the production of offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspri ...
. Reinforcement may be induced in artificial selection experiments as described below.


Ecological

Ecological selection is "the interaction of individuals with their environment during resource acquisition". 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". 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, 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. Ecological speciation occurs with much of the evidence, "...accumulated from top-down studies of adaptation and reproductive isolation".


Sexual selection

Sexual selection Sexual selection is a mode of natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrat ...
can drive speciation in a clade, independently of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
. 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). In the second sense, "speciation" refers to the wide-spread tendency of sexual creatures to be grouped into clearly defined species, rather than forming a continuum of
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
s both in time and space – which would be the more obvious or logical consequence of natural selection. This was indeed recognized by
Darwin Darwin most often refers to: * Charles Darwin (1809–1882), English naturalist and writer, best known as the originator of the theory of biological evolution by natural selection * Darwin, Northern Territory, a capital city in Australia * Darwin ( ...

Darwin
as problematic, and included in his ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' (1859), under the heading "Difficulties with the Theory". There are several suggestions as to how
mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
might play a significant role in resolving Darwin's dilemma. — Commentary by Mazhuvancherry K. Unnikrishnan and H. S. Akhila If speciation takes place in the absence of natural selection, it might be referred to as nonecological speciation.


Artificial speciation

New species have been created by
animal husbandry Animal husbandry is the branch of agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Ex ...
, but the dates and methods of the initiation of such species are not clear. Often, the domestic counterpart can still interbreed and produce fertile offspring with its wild ancestor. This is the case with domestic
cattle Cattle, taurine cattle, Eurasian cattle, or European cattle (''Bos taurus'' or ''Bos primigenius taurus'') are large domestication, domesticated Cloven hoof, cloven-hooved herbivores. They are a prominent modern member of the subfamily Bovinae ...

cattle
, which can be considered the same species as several varieties of wild ,
gaur The gaur (''Bos gaurus''; ), also known as the Indian bison, is a bovine native to South South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east and west. Etymology The word ...

gaur
, and
yak The domestic yak (''Bos grunniens'') is a type of long-haired domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care o ...

yak
; and with domestic
sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), order Artiodactyla, the even-toed ungulates. Although the name ''sheep'' applies to many species ...

sheep
that can interbreed with the
mouflon The mouflon (''Ovis gmelini'') is a wild sheep ''Ovis'' is a genus of mammals, part of the Caprinae subfamily of the ruminant family (biology), family Bovidae. Its seven highly sociable species are known as sheep. Domestic sheep are members of ...

mouflon
. The best-documented creations of new species in the laboratory were performed in the late 1980s. William R. Rice and George W. Salt bred ''''
fruit flies Fruit fly may refer to: Organisms * Drosophilidae, a family of smaller flies, including: ** ''Drosophila'', the genus of small fruit flies and vinegar flies ** ''Drosophila melanogaster'' or common fruit fly, an important model organism in modern b ...
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. The history of such attempts is described by Rice and Elen E. Hostert (1993). Diane Dodd used a laboratory experiment to show how reproductive isolation can develop in ''
Drosophila pseudoobscura ''Drosophila'' () is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also ref ...
'' fruit flies after several generations by placing them in different media, starch- and maltose-based media. Dodd's experiment has been replicated many times, including with other kinds of fruit flies and foods. Such rapid evolution of reproductive isolation may sometimes be a relic of infection by ''
Wolbachia ''Wolbachia'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including their ...

Wolbachia
'' bacteria. An alternative explanation is that these observations are consistent with sexually-reproducing animals being 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 high. Thus, if an animal, 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. Sexual creatures then inevitably group themselves into reproductively isolated species.


Genetics

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. 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 polyploidy

Polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y is a mechanism that has caused many rapid speciation events in
sympatry In biology, two related species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of ...
because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny. However, among plants, not all polyploids are reproductively isolated from their parents, and gene flow may still occur, such as through triploid hybrid x diploid matings that produce tetraploids, or matings between gametes from diploids and gametes from tetraploids (see also
hybrid speciation Hybrid speciation is a form of speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the G ...
). It has been suggested that many of the existing plant and most animal species have undergone an event of polyploidization in their evolutionary history. Reproduction of successful polyploid species is sometimes asexual, by
parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction Asexual reproduction is a typ ...
or
apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal Plant reproduction#Asexual reproduction, asexual reproduction" o ...
, 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 In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

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 The Mariana mallard or Oustalet's duck (''Anas oustaleti'') is an extinct type of duck of the genus ''Anas'' that was endemism, endemic to the Mariana Islands. Its taxonomic status is disputed, and it has variously been treated as a full species, ...
is thought to have arisen from hybrid speciation. Hybridization is an important means of speciation in plants, since
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y (having more than two copies of each
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
) is tolerated in plants more readily than in animals. 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. Polyploids also have more genetic diversity, which allows them to avoid
inbreeding depression Inbreeding depression is the reduced biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
in small populations. Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in ''
Heliconius ''Heliconius'' comprises a colorful and widespread genus of Nymphalidae, brush-footed butterflies commonly known as the longwings or heliconians. This genus is distributed throughout the tropical and subtropical regions of the New World, from So ...

Heliconius
''
butterflies Butterflies are insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', m ...

butterflies
and
sunflower ''Helianthus'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), ...

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


Rates

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 Paleontology, palaeontologists such as Niles Eldredge and Stephen Jay Gould 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#Origin, Maize (''Zea mays''), for instance, was Zea (plant)#Origin of maize and interaction with teosintes, created in Mexico in only a few thousand years, starting about 7,000 to 12,000 years ago. This raises the question of why the long term rate of evolution is far slower than is theoretically possible. Evolution is imposed on species or groups. It is not planned or striven for in some Lamarckism, Lamarckist way. 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 (biologist), George C. Williams, John Maynard Smith and Richard Dawkins "Remarks on an earlier article by Elliott Sober, [Elliot] Sober [sic] and David Sloan Wilson, who made a more extended argument in their recent book ''Unto Others : The Evolution and Psychology of Unselfish Behavior''" "Commentary on Wilson & Sober: Group Selection." 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. 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 theory of molecular evolution, neutral mutations", which can be, and are, used to trace the history of molecular evolution, relatedness and age of populations and species.) 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 population size, small, isolated communities. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves, or during the aftermath of a Extinction event, mass extinction. 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. Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. If an isolated population such as this Toba catastrophe theory#Genetic bottleneck theory, 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 into being. In geological terms, this will be an abrupt event. A resumption of avoiding mutant mates will thereafter result, once again, in evolutionary stagnation. In apparent confirmation of this punctuated equilibrium view of evolution, the Fossil#Dating, 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. Graphically, these fossil species are represented by lines parallel with the time axis, whose lengths depict how long each of them existed. The fact that the lines remain parallel with the time axis 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 Punctuated equilibrium#The fossil record, distribution of hominin species through time since the Hominini, hominins separated from the line that led to the evolution of their closest living primate relatives, the chimpanzees. For similar evolutionary time lines see, for instance, the paleontological list of List of African dinosaurs#Timeline, African dinosaurs, List of Asian dinosaurs#Timeline, Asian dinosaurs, the Lampriformes#Timeline of genera, Lampriformes and Amiiformes#Timeline of genera, Amiiformes.


See also

* Bateson–Dobzhansky–Muller model * Chronospecies * Court jester hypothesis * Macroevolution * Selection (genetic algorithm) * Species problem


References


Bibliography

* * * * * * The book is available fro
The Complete Work of Charles Darwin Online
Retrieved 2015-09-12. * * Reprinted in * * * * * * * * * * * * * * * * * * *


Further reading

* * * * * * * *


External links

* * * {{Authority control Speciation, Ecology Evolutionary biology Sexual selection