Speciation gradual

Most speciations take quite a long time and these are called gradual speciations. Allopatric speciation is usually an example of this kind of speciation (see XXI.3). Genetic differences gradually accumulate between two populations of a single species occurring in two spatially separated territories, leading in time to phenotype differentiation of the members of the two populations and simultaneously to the formation of reproductive isolation barriers in sexually reproducing species. If the genetic differences accumulate only through the effect of genetic drift, the formation of sufficiently effective reproductive isolation barriers can take quite a long time. For example, in drosophila, the time required for accumulation of a sufficient number of genetic changes was estimated using the molecular clock at 1.5 – 3.5 million years (Coyne & Orr 1989). However, a great many examples of flora and fauna are known in which speciation does not occur even in situations where their American and Asian areas of occurrence were geographically separated for more than 20 million years. It has been stated that mammals lose the ability to cross after 8 million years of divergence, while birds and frogs retain it for 55-60 million years {9992}. If natural selection also contributes to speciation, then the progress of speciation can be substantially faster. For example, several hundred species of cichlids in Lake Victoria probably evolved from a single ancestor over 100 thousand years {9905} but, according to some ideas only over12 thousand years (Johnson et al. 1996).
            However, there are types of speciation that can occur almost in an instant. Polyploid speciation is an example of instant speciation; as a consequence of a cell division disorder, a tetraploid individual is formed from a diploid, usually flora, species. Because of their polyploid genome, the members of the new tetraploid species are simultaneously phenotypically different from their diploid predecessors and, because of their different phenotypes, also have different ecological requirements. The old diploid and new tetraploid species thus need not compete and can coexist permanently in a single territory.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more