Basic Concepts of Speciation* Speciation (Origin of Species) is formation of new species from parental species. ® Though Charles Darwin in his book, The origin of Species, did not propose complete mechanism of speciation, Ernst Mayr recognized that speciation depends on 2 factors: Isolation of gene pool of populations. Genetic divergence of their gene pool.Isolation of gene pools of related populations (reproductive isolation): ° The central event in speciation is the separation of gene pool of ancestral species in to 2 or more isolated gene pools. * When isolated gene pool become so much different that interbreeding and exchange of genes between them is almost impossible, the populations with these gene pools become independent species establishing reproductive isolation. » If there is free gene flow in the gene pools of these populations, the 2 gene pools exchange genetic changes also and the 2 gene pools tend to be similar.2. Genetic divergence of their gene pool: © The isolated gene pools need to accumulate large no. of genetic differences so that their gene pools do not permit free gene flow. ° If isolated populations are reunited, they either fail to interbreed or do not produce fertile offsprings. * Their gene pools diverge due to different environmental pressure and natural selection or due to genetic drifts.Genetic drift, also known as random genetic drift, allelic drift ord effect, is the change in the he Wright I frequency of an existing gene variant in a Population due to random cha c nce. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. A gene variant is a permanent change in the DNA sequence that makes upa gene. This type of genetic change used to be known as a gene mutation, but because changes in DNA do not always cause disease, it is thought that gene variant is a more accurate term. Environmental pressures are forces such as the weather, the predators or the food source that affect an individual living in that environment. An individual has to have the phenotypic means to counteract and overcome these pressures if it is to survive and reproduce. Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.Modes of Speciation: Several possible mechanisms have been proposed to account for the occurrence of speciation. Based on different modes, following types of speciation have been recognized: Phyletic transformation or autogenous speciation Speciation through fusion of species True speciationI. Phyletic transformation or autogenous speciation » Isgradual transformation of one species A into another species B over a long period by accumulation of slow changes in the gene pool. » Neither there is splitting of the parental gene pool nor evolution of reproductive isolation. © The gene pool of parental species modifies due to changes in gene frequencies, chromosomal inversion, other structural changes in the chromosomes. « These changes accumulate in response to: » Adaptation toa shifting environment. >. Increasing specialization for a particular environment. >. Improved adaptation in a constant environment. * In phyletic speciation, evolving species present a line of succession in which one species is replaced by other.II. Speciation through fusion of species: » A new species may arise by fusion of 2 already existing species through the breakdown of their reproductive isolation. e Fusion of gene pool hybridization. F © The new species formed has the combination of characters from both parent species but is reproductively isolated from either of them. ® Several new plant species have arisen in nature by hybridization. is possible throughIll. True speciation: © Is the origin of 2 or more species from one parental species. © It requires splitting of parental gene pool or origin of 2 or more new gene pools from the gene pool of single species and establishment of reproductive isolation between them. * True speciation may be of 2 types: Sudden speciation or Quantum speciation 2. Gradual speciationZz = » Sudden speciation or Quantum speciation: *In this type of speciation, chromosomal rearrangement produces reproductive isolation and results in speciation. © These evolutionary events occur suddenly and intermittently. ® It may occur due to chromosomal aberrations (inversion, translocation) or change in chromosome’s number (polyploidy, autopolyploidy and amphidiploidy). ‘Quantum Model Popul ’ t 25 | FIG. 118: Quantum model o A Reduced 0 Isolation a popu ve 1 Pores jan 0 ot adas a genome abnormality in which a ttaches to a different Jocation is define of it real A chromosome trans! d either the whole or a portion chromosome breaks an chromosome possessing more than two complete sets of ‘as among certain groups of d leeches are polyploids. Condition OF common among plants, as well some salamanders, frogs, an polyploidy i the heritable chromosomes. Polyploids are fish and amphibians. For instance, habits Autopolypleidy? Autopolyploidy occurs when organisms have mor” than two eet chromosomes from the same species. Autopolyploids can originate from issues Jetarise during mitosis, such as doubling of the chromosomes without cell division. This wuld result ina tetraploid organism from a diploid organism. he presence of more than two copies of each ‘opolyploidy 13 taxonomically defined as t ere the genomes present must all originate ‘ome within an organism or species, wh lin the same species. iidiploid an individual that is a hybrid of two different species and that posses: ec ses.Gradual speciation It is a microevolutionary event. The new species originate from dau species by gradual accumulation differences over a long period of tim natural selection. In this process one of the species gives rise to one or more species. ghter population of same of many minute gene e under the influence of new May be of following types Allopatric speciation Parapatric speciation Alloparapatric speciation Sympatric speciationAllopatric speciation: Is the divergence of physically separated populations of a species, occupying separate geographical areas into new species. This is also called speciation by geographic isolation. Peripatric sp Origin of new sp from peripatric populatons that bud off from a periphery of a parental population. Such populations are also called marginal isolates.Allopatric Speciation: s in geographic Allopatric speciation occurs when the new species evolve: isolation from the parent species. The species range, becomes subdivided by a barrier such as anew mountain range or the change in the course of a river. Gene flow between the two subpopulations becomes impossible allowing evolution to proceed independently in each. Natural selection may favour different genotypes on either side of the barrier and random genetic drift and mutation could contribute to divergence. Over time, divergence may proceed to the point that were the two populations to meet again, they would not be able to interbreed and speciation would be complete.. adily in small This form of speciation may take place most re populations at the extreme edge of a species range. The peripheral population could become isolated, for example, during contraction of the main species range in response to changing climate. The isolated population would be subject to the founded effect and could be genetically different from the parent population. The combined effect of a small atypical population and extreme environmental conditions can cause rapid and extensive genetic reorganization through random genetic drift and strong natural selection, or, in other words a genetic revolution.[Jee fly ae oft SD Ucn, rae Leeipnte A> ‘An example of allopatric speciation is the Darwin's finches. The finches varied from each other mainly in shape and sizeof beak and colour of the feathers or plumage. According to Darwin, the species in the South American mainland were the original species from which different forms migrated to different islands of the Galapagos and became adapted to the environmental conditions of these islands. The adapted forms eventually became the new species (Fig. 34). In the case of the finches, geographical isolation led to the development of reproductive isolation and thereby to the origin of new species.Large ground finch ANCESTOR, [Ls yo seg ns gn Soh List > Sea fin D> ‘maaland) Sort > MeN ar wen - rom Se ae asian | |] Like cake ee cos Mg Site OD ee foc Fig 4 Divergence of the Galpagys iches frm snes clonirs fom she South American milan, An example of allopatric speciation is the Darwin's finches. ~ 4 The finches varied from each other mainly in shape and size of beak and colour of the feathers or plumage. According to Darwin, the species in the South American mainland were the original species from which different forms migrated to different islands of the Galapagos and became adapted to the environmental conditions of these islands. The adapted forms eventually became the new species (Fig. 34). 7 In the case of the finches, geographical isolation led to the development of 4 reproductive isolation and thereby to the origin of new species.Type #1. Allopatric Speciatior Allopatric speciation is the evolution of species in a population that occupy different geographical areas. Geographic isolation is often the first step in allopatric speciation. Other isolating mechanism may also operate that further restrict reproduction between’ populations. ‘An example of allopatric speciation is the Darwin’s finches. The finches varied from each other mainly in shape and size of beak and colour of the. feathers or plumage. According to Darwin, the species in the South American mainland were the original species from which different forms migrated to different istands of the Galapagos and. became adapted to the environmental conditions of these islands. The adapted forms eventually became the new species (Fig. 34). In the case of the finches, geographical isolation fed to the development of reproductive isolation and thereby to the origin of new species.Parapatric Speciation: Parapatric (para, near) speciation is the development of reproductive isolation among members of a continuous population or spatially distinct populations, between which there is some gene flow in the absence of geographical barrier. It means parapatric populations occupy adjacent regions with different selective pressure. : They coexist only in one or more overlapping regions at the peripheries of their geographical distributions and are not completely isolated.DF rae Speciation: (para, near) © This form of speciation occurs where the speciating populations are contiguous and hence only partially geographically isolated. * They are able to across a common boundary during the speciation process. = Where a species occupies a large geographical range it may become adapted to different environmental (e.g. climatic) conditions in different parts of that range. = Intermediate or hybrids, will be found but the large distances involved prevent the two types from merging completely. 5 © For example, the herring gull Larus argentatus is a ring species whose distribution covers a large geographical area. * Westwards from Britain toward North America its appearance changes gradually, but itis still recognizable herring gull. © Further west in Siberia it begins to look more like the lesser black-backed gull Larus fuscus. » From Siberia to Russia and into northern Europe it becomes progressively more like the lesser black-backed gull. * The ends of the ring meet in Europe and the two geographical extremes appear to be two good biological species.farapatric Speciation: (para, near) This form of speciation occurs where the speciating populations are contiguous and hence only partially geographically isolated. They are able to across a common boundary during the speciation process. Where a species occupies a large geographical range it may become adapted to different environmental (e.g. climatic) conditions in different parts of that range. Intermediate or hybrids, will be found but the large distances involved prevent the two types from merging completely. t For example, the herring gull Larus argentatus is a ring species whose distribution covers a large geographical area. Westwards from Britain toward North America its appearance changes gradually, but itis still recognizable herring gull. Further west in Siberia it begins to look more like the lesser black-backed gull Larus fuscus. From Siberia to Russia and into northern Europe it becomes progressively more like the lesser black-backed gull. The ends of the ring meet in Europe and the two geographical extremes appear to be two good biological species.Alloparapatric Speciation: Endler (1977) proposed the term alloparapatric speciation for a modified allopatric speciation. Initially, speciation begins in allopatric population which are geographically separated. In later stages these populations become parapatric.Category # 4. Alloparapatric Speciation: Itis specialised kind of speciation where differentiation in isolation takes place through barrier breakdown processes, as influenced by gradual environmental variation.Sympatric Speciation: sympatric (same country) speciation refers to the origin of new species due to the appearance of some biological barrier in the individuals of an initially randomly mating popula that live in the same geographic area.Sympatric Speciation: = Sympatric speciation describes a situation where there is no geo- graphical separation between the speciating populations. © All individuals are, in theory, able to meet each other during the speciation process. © This model usually requires a change in host preference, food preference or habitat preference in order to prevent the new species being swamped by gene flow. * Whether sympatric speciation happens at all is a contentious issue. © In theory it can occur where there is a polymorphism in the population conferring adaptation to two different habitats or niches. * Reproductive isolation could then arise if the two morphs had a preference for ‘their’ habitat. * There is some evidence for this in natural populations. * For example, caterpillars of the ermine moth, Yoonomeuta padellus, feed on apple and haw- thorn trees, Females prefer to lay their eggs on the species on which they were raised. © Caterpillars also prefer to feed on the plant on which their mothers were raised and adult moths prefer to mate with individuals from the same plant. * The apple and hawthorn types are not completely isolated, but may represent an intermediate point in on-going sympatric speciation.—a} Sympatric Speciation: * Sympatric speciation describes a situation where there is no geo- graphical separation between the speciating populations. * Allindividuals are, in theory, able to meet each other during the speciation process. * This model usually requires a change in host preference, food preference or habitat. preference in order to prevent the new species being swamped by gene flow. * Whether sympatric speciation happens at all is a contentious issue. * Intheory it can occur where there is a polymorphism in the population conferring adaptation’ to two different habitats or niches. * Reproductive isolation could then arise if the two morphs had a preference for ‘their’ habitat. * There is some evidence for this in natural populations. * For example, caterpillars of the ermine moth, Yponomeuta padellus, feed on apple and haw- thorn trees, Females prefer to lay their eggs on the species on which they were raised. * Caterpillars also prefer to feed on the plant on which their mothers were raised and adult moths prefer to mate with individuals from the same plant. » The apple and hawthorn types are not completely isolated, but may represent an intermediate point in on-going sympatric speciation.er ‘An-un-contentious example of sympatric speciation occurs in plants through polyploidy. Polyploidy is the spontaneous duplication of the entire genome resulting in an individual with a multiple of the original chromosome number. Polyploidy is common in plants, where it often results in larger, more vigorous forms. Itis usually fatal in animals, although some amphibians are polyploids. The polyploid plant is no longer sexually compatible with the parent population but is able to establish a distinct population which may occupy a different habitat. The sand dune grass, Spartina townsendii, is a polyploid derived from the original S. anglica. It is more vigorous than the parent and has colonized large areas of sand dune in Britain.