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| Speciation (origin of new species) |
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| A species is a group of similar organisms, which can breed among themselves producing fertile offsprings. The members of a species not only share a common gene pool, but also share a common ancestry. |
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| Origin of a new species from the existing one, is called speciation. It is an important aspect of evolutionary process. |
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| Members of a species generally do not live together as a single large population. Instead, they form small inbreeding groups that are called demes. The demes may occupy adjacent areas or may be distributed in far-off regions. The widely separated demes generally come across varied environmental conditions. As such in these isolated populations mutation and natural selection easily occur resulting in the formation of genotypic variations giving scope for speciation. |
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| Multiplicative Speciation |
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| It is the splitting of a given species into two or more new species. When it spans over several generations it is called 'gradual speciation'. When gradual speciation occurs with the population drifting apart, overlapping in their geographical distribution, it is called as 'allopatric speciation'. When it occurs due to other mechanisms of isolation, it is called 'sympatric speciation'. |
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| When speciation occurs through individuals and in a single generation by hybridisation, it is called 'instant speciation'. |
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| fig. 5.18 - Multiplicative and Phyletic Speciation |
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| Phyletic Speciation |
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| It is the replacement of one species by another, without any increase in the number of species. |
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| Speciation by Fusion |
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| Very rarely two different species may merge into one new species by a breakdown of reproductive isolation that existed earlier. This process brings about a reduction in the number of species. |
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| fig. 5.19 - Speciation |
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| It is known that members belonging to closely related species do not interbreed and even if they do so they do not produce fertile young. However, very rarely, fertile offspring may result from the interbreeding of two closely related species. In such cases, the resulting offspring represents a new species, since they are reproductively isolated from their parent. Such offsprings cannot mate with the parental types. This process, called hybridisation may combine the best traits of both the parent species. As a result the offspring has a better chance of survival. On the contrary if the new species combined the worst characters of both the parents, the offspring is unlikely to survive. The reason for this is unlike chromosomes cannot pair during meiosis and the gametes do not receive the full chromosome complement one of each kind. The hybrid species get established and breed only when the chromosome number becomes double by a type of chromosomal mutation called allopolyploidy. |
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| A classical example for this situation is the creation of Rhaphanobrassica, a new species, from the hybridisation radish (Raphonus) and cabbage (Brassica), each having a chromosome number of 2n = 18. The new species Raphanobrassica has a chromosome number of 2n=36. |
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| fig. 5.20 - Creation of Raphanobrassica |
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