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SUBJECT: ZOOLOGY EVOLUTION SPECIATION TYPES CONTENT Speciation Introduction Species characteristics Formation of speciation Modes of speciation Types of speciation Allopatric speciation Peripatric speciation Parapatric speciation Sympatric speciation INTRODUCTION; The term speciation is coined by American botanist Orator F .cook(in 1906 )Speciation – it is an evolutionary process by which population Evolves to become distinct species.That are reproductively isolated from one another. Evolution is the successive modification in inherited traits over a huge span of time, usually over generation. Charles Darwin (1859) English biologist, proposed the theory of evolution he mentioned about evolution in his book the origin of species. Charles Darwin noted that living organisms change their physical and Anatomical structures over a long period of time for better adaptation to the changing environment. The change is by natural process and these organisms which do not adjust to it, find it difficult to survive. Please put forward the concept of natural selection And Darwin called it survival of the fittest. Species Definition: species is a group of organisms or individuals with similar characteristics, where they can interbreed to produce fertile offspring. All the organisms of a species share in the same gene pool. Characteristics of a species: 1. Members of a species exhibit distinctive features in common, in which they different from all other such groups. 2. Intermediate or transitional forms between the species are usually not found i.e., species are reproductively isolated. 3. Members of separate species do not usually interbreed, so that natural hybrids of the species are either totally absent or are very rare. Even if hybrids are produce artificially, these are usually sterile 4. Allied species usually have separate but adjoining territories. 5. Species occupies a specific ecological niche, unoccupied or unutilised by another closely related species. 6. Species interacts with the environment and with other species present in a given environment. 7. Species have fully effective reproductive isolating mechanisms. 8. A species is a genetic unit consisting of a large intercommunicating gene pool. Origin of species (speciation) Formation of new species from the parental species is called speciation or Origin of species. Ernst Mayr recognised speciation depends on two factors: 1.Isolation of gene pool of populations (Reproductive Isolation) 2. 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 into two or more isolated gene pools. When isolated gene pools 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 two gene pools exchange genetic changes also and the two gene pools tend to be similar. Genetic Divergence: The isolated gene pools need to accumulate large number 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 offspring. Their gene pools diverge due to different environmental Pressures and natural selection or due to genetic drifts. MODES OF SPECIATIONS Several possible mechanisms have been proposed to account for the occurrence of speciation. Based on different modes, following types of speciation have been recognised 1. phyletic Transformation or Autogenous speciation : phyletic speciation is gradual 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 inversions, other structural changes in the chromosomes. These changes accumulate in response to: adaptations to a shifting environment increasing specialisation for a particular environment improved adaptations in a constant environment In Phyletic speciation, evolving species present a line of succession in which one species is replaced by other 2 Speciation through Fusion of Species : A new species may arise by the fusion of two already existing species through the breakdown of their reproductive isolation. Fusion of gene pools is possible though hybridisation. The new species formed has combination of characters from both parent species but is reproductively isolated from either of them. Several new plant species have arisen in nature by hybridisation. 3.True Speciation :True speciation is the origin of two or more species from one parental Species, it requires splitting of parental gene pool or origin of two or more new gene pools from the gene pool of single species and establishment of reproductive isolation between them. The true speciation may be of two types: 1. Sudden speciation or Quantum speciation 2. Gradual speciation Sudden Speciation or Quantum Speciation or Instataneous speciation: In this type of speciation, chromosomal rearrangements produce reproductive isolation and result in speciation. These evolutionary events occur suddenly and intermittently. Quantum speciation may occur due to chromosomal aberrations (inversion and translocation) or change in chromosome number (polyploidy, autopolyploidy and amphidiploidy). According to Simpson, the quantum evolution represents accelerated pace of phyletic gradualism because of accumulation of genetic changes in relatively Rapid succession. in Quantum speciation biological barrier to gene exchange arise within an initial randomly mating population without any spatial segregation of the incipient species. it means quantum or sudden speciation due to disruptive selection. Gradual Speciation: Gradual speciation is a microevolutionary event. The new species originate from daughter populations of the same species by a Gradual accumulation of many minute gene differences over a long period of time under the influence of natural selection.in this process one of the species gives rise to one or more new species. The Gradual speciation may be of following types: — Allopatric speciation Peripatric speciation Parapatric speciation Sympatric speciation. ALLOPATRIC SPECIATION: (Greek: allos, "other"; patri, "Fatherland") Definition: Allopatric speciation is the divergence of physically separate populations of a species, occupying separate geographical areas into new species. This is also called Speciation by geographic isolation. Methods of Physical Isolation of Populations: Physical isolation of an original large population of a species may occur in three ways: by physical splitting of a large habitat with a widespread population by some new physical barrier into two or more large isolated areas (Vicariance) by Divergence of a few members of a widely distributed population to some new habitat (Founder effect) by the extinction of intermediate links in a chain of interconnected populations The physical splitting of habitat is called vicariance. Speciation that begins with Physical isolation of populations either by dispersal or vicariance is allopatric Speciation, Allopatric speciation by vicariance : volcanoes, earthquakes, storms, tidal waves, Glacier, floods and formation or destruction of mountains and forests forest and appearance or disappearance of water bodies acts as vicariance agents for speciation in terrestrial population. The geographically separated populations are exposed to variable forces of natura selection, genetic drift, mutations, random mating that cause their gene pools to diverge. Their gene pools become so different that the free gene flow between isolated populations is totally prevented and reproductive isolation is established. Finally, these reproductively isolated populations become independent species. In case, the geographic barrier is lifted, the members of these species meet but do not interbreed. Stages in Allopatric Speciation by Vicariance: Allomen (1992) divided the process of allopatric speciation into following three stages: Fragmentation of a widespread population due to geographical barriers and separation of its gene pool. Persistence of isolation of populations and their gene pools, so that their gene pools become different from that of parental population, i.e., genetic divergence. Establishment of reproductive isolation between new populations and formation of new species, i.e., speciation, Examples of Allopatric Speciation by Vicariance: Pupfish of Nevada, California: The blue- gray pupfish which inhabits a warm spring at the base of mountains near Death Valley, Nevada, has evolved by allopatric speciation. This spring got isolated from other water bodies about 50,000 years ago. The fish trapped in the spring, became geographically isolated from main fish population. In due course of time its gene pool became so different that it became a distinct species. PERIPATRIC SPECIATION OF MARGINAL POPULATION: (Greek: peri, "around"; patri, "Fatherland") Definition: It is origin of new species from peripatric populations that bud off from the Periphery of a parental population, Such populations are also Called Marginal isolaters. Mayr (1982) described founder effect speciation as peripatric speciation. His hypothesis was based on
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