Introduction Speciation is a burning issue in evolutionary biology, but it is both fascinating and frustrating. Defining speciation depends on one’s species concept viz., typological, biological, evolutionary, recognition etc. In its simplest form, speciation is lineage splitting (ancestor-descendent sequence of populations); the resulting lineages are genetically isolated and ecologically distinct. Speciation is the process of evolutionary mechanism by which new biological species (or taxa) arise. There are two ways of new species (or taxa) origin from the pre-existing one:- i. by splitting of the parent species into two or more species (by the splitting of phylogenetic lineage) and ii. by transformation of the old species into a new one in due course of time. The Biologist O.F. Cook (1906) seems to have been the first to coin the term ‘speciation’ for the splitting of lineages (cladogenesis).The process of evolutionary mechanism by which new biological plant species (or taxa) arise, is known as plant speciation. General Mechanism of Speciation operating in nature: The mechanism of speciation is a two- staged process in which reproductive isolating mechanisms (RIM's) arise between groups of populations. Stage 1 • gene flow is interrupted between two populations. • absence of gene flow allows two populations to become genetically distinct as a result of their adaptation to different local conditions (genetic drift plays an important role here). • as populations differentiate, RIMs appear because different gene pools are not mutually coadapted. • reproductive isolation appears primarily in the form of postzygotic RIMs: hybrid failure. • these early RIMs are a byproduct of genetic differentiation, not directly promoted by natural selection. Stage 2 • completion of genetic isolation • reproductive isolation develops mostly in the forms of prezygotic RIMs. ● development of prezygotic RIMs is directly promoted by natural selection: Formation of new species. There are four geographic modes of speciation operate in nature, based on the extent to which speciating populations are geographically isolated from one another, viz., Allopatric, Peripatric, Parapatric and Sympatric speciations which are briefly discussed here. Fig 1. Four Geographic modes of Speciation models (Source:http://www/en.wikipedia.org). Allopatric speciation (figs 2 & 5): Greek allos means ‘other’ and patra means ‘fatherland’. It is also often called as Geographic speciation. It occurs when biological populations of the same species become isolated (i.e. vicariant) from each other to an extent that prevents or interferes with genetic interchange, for example, by habitat fragmentation due to geographical change such as mountain building. The isolated (vicariant) populations then undergo genotypic or phenotypic changes as: (a) they become subjected to dissimilar selective pressures, (b) they independently undergo genetic drift, and (c) different mutations may arise in the gene pools of two isolated populations. Two separate populations over time may evolve distinctly different characteristics. If the geographical barriers are later removed, members of the two populations may be unable to successfully mate with each other, at which point, the genetically isolated groups have emerged as different species. Allopatric isolation is a key factor in speciation and a common process by which new species arise. Examples observed: Darwin’s Finches (birds) in different islands of Galapagos originated due to allopatric isolation. Fig 2. Explanation of allopatric speciation (Source:http://www.geo.arizona.edu) Peripatric speciation or Mayr’s Peripheral isolate model: When allopatric speciation occurs in the peripheral populations, then it is called as Peripatric speciation. Ernst Mayr (1940) showed this speciation model based on his work on birds. Mayr (1940) argued that peripheral populations have greater divergences and differences than central populations of a species range. It is a subform of allopatric speciation. In this speciation, new species are originated in isolated smaller peripheral populations which are prevented from exchanging genes with the main population. It occurs in a few members of the population, which exhibit a different appearance than the majority. Genetic drift plays a major role in this speciation. It is also similar to the concept of Founder Effect, as small populations undergo bottlenecks. Portions of a populations that exist along the edges of the parent population's geographic territory have higher likelihood of developing reproductive isolation. Such peripheral populations are likely to possess genes that are different from the parental population. After isolation, the founding population is less likely to represent the gene pool of the parent population. In addition, peripheral isolates are likely to represent a small number of individuals, meaning their gene pool is more susceptible to the effects of genetic drift (random chance). Furthermore, it is likely that the peripheral population will inhabit an environment different from its ancestral gene pool, likely causing it to be subjected to different selective pressures as it colonizes new areas. Example observed: Origin of the Australian bird, Petroica multicolour Fig 3. Successive stages in the process of peripatric speciation. A small 'daughter' population on the periphery of a more widespread 'parental' population evolves reproductive isolation. Both species remain distinct if the peripherally isolated species invades the geographic range of the parental species. Under this model, the peripherally isolated population diverges from the parental population such that the latter remains unchanged (Source: http://www.trinitygreenconsultancy.com/population dynamics-2/variants-on-the-basic- model.html-2012). Mayr’s explanation: Mayr (1940) hypothesized that founder populations, as they are small, may have reduced genetic variation and low fitness due to genetic drift. Drift may increase the frequency of alleles that were rare in the ancestral population. In such a situation, selection for new combinations of alleles that are compatible with the newly fixed alleles may occur and allow increased fitness in the new conditions. A possible result is a reorganization of the genome that makes it incompatible with the ancestral population. Parapatric speciation (Partially geographically isolated populations): This speciation is an example of continuous variation within a single connected habitat acting as a source of natural selection rather than the effects of isolation of habitats found in peripatric and allopatric speciations. This type of speciation occurs when the speciating populations are contiguous and only partially geographically isolated. Individuals of each different population of the same species are able to meet across a common boundary during the speciation process, but due to reduced fitness of the heterozygote leading to the selection for behaviours that prevent their interbreeding. Ecologists refer to Parapatric and Peripatric speciations in terms of ecological niches. A niche must be available in order for a new species to be successful. Example observed: i. Ring Species formation by the Herring Sea Gull (Larus argentatus) around the North Arctic Ocean. ii. The Grass, Anthoxanthum odoratum has been known to undergo parapatric speciation in mine-contaminated areas. Fig 4. The first steps of parapatric speciation observed in the grass species, Anthoxanthum odoratum. Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around the mines have experienced natural selection for genotypes that are tolerant of heavy metals. Meanwhile, neighboring plants that don't live in polluted soil have not undergone selection for this trait. The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each other — so they seem to meet the first requirement of parapatric speciation, that of a continuous population. However, the two types of plants have evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups (Source:http://www.berkeley.edu/evosite/evo101/VC1dParapatric.shtml). Sympatric speciation (fig 1 & fig 5): In this type, the formation of two or more descendant species happened from a single ancestral species all occupying the same geographical area. In this type, species diverge while inhabiting the same place.This speciation requires a change in host, food and habitat preferences in order to prevent the new species being swamped by gene flow. In theory, it may occur where there is a polymorphism in the population conferring adaptations to two different habitats/niches. Reproductive isolation could then arise if the two morphs had a preference for their habitat. A common example of sympatric speciation occurs in plants through polyploidy. For example, Sand Dune Grass (Spartina townsendii) is a sympatric polyploidy originated from its parent, S. anglica due to polymorphism. More Examples observed: i. Diploid (2n=14), Tetraploid (2n=28) and Hexaploid (2n=42) wheat plants were originated by sympatric speciation through polyploidy. ii. Drosophila melanogaster (2n=8) and D. virilis (2n=12) were originated by sympatric speciation. Fig 5. Allopatric (left) and sympatric (right) models on speciation (Source: http://www.globalchange.umich.edu/speciation.html) Brief idea about other modes of speciation operating in nature: Phyletic speciation: