One of the characteristic features of life on Earth is its extreme diversity, which is manifested in the existence of a great many very distinct species (diversity in the narrow sense of the word) and the differences between these species (disparity) It is obvious, and this also follows from the character of paleontological findings from the Proterozoic, that the biodiversity of organisms was incomparably less at the beginning of evolution and that its increase, including the increase in the number of individual species, occurred only gradually during evolution. The process, during which one or more new species are formed from a single old species, is called speciation. A number of kinds of speciation are known at the present time, which differ substantially in their mechanisms. It is assumed that some occur very frequently, while others are quite rare and there can be serious doubts about their very existence. This chapter will be concerned with the individual types of speciation as they can be encountered in the contemporary evolutionary literature. Particular attention will be devoted to the mechanisms of speciation in species with sexual reproduction. There are two reasons for this: to begin with, these species greatly predominate in nature (although not in the number of individuals and possibly not even in the total amount of biomass) and also because only for them does speciation also require the formation of reproductive isolation barriers between the old and new species. And it is the mechanisms forming reproductive isolation barriers that constitute a complicated and very interesting aspect of evolution.
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