The synthetic theory. Origin of species

A new species arises when isolation appears between populations, their gene composition diverges and they eventually can no longer interbreed. According to Darwin there are three ways: phyletic (one species gradually turns into another, the number of species does not increase), hybridisation (a third species from the crossing of two) and divergence (separation of traits – several species from one). Two main types: allopatric (geographic) – the range is split by a river, mountain or sea and the populations change independently (Galápagos finches, the many species in Lake Baikal, spread of cotton-genus species over continents); sympatric (ecological) – the species stays in its range but isolation arises through differences in breeding time, habitat or behaviour. In plants polyploidy – a multiple increase of the chromosome set – can give a new species in one or two generations; many crop plants are polyploid: potato, cotton (Gossypium hirsutum, 2n = 52 – it arose when two species hybridised and the chromosomes doubled), bread wheat – hexaploid (2n = 42). G. Karpechenko (1928) crossed radish and cabbage (each 2n = 18) and obtained a fertile hybrid with 2n = 36, an example of a new species reproductively isolated from both parents. Speciation is usually slow, so it is hard to observe experimentally; yet polyploidy, and fast-breeding groups such as bacteria and insects, allow the process to be studied directly.