Origin
The genus Oenothera may have originated in Mexico and Central America from which it spread into North and South America and, with the advent of international travel, species are now found in most temperate regions. During the Pleistocene era a succession of ice ages swept down across North America, with intervening warm periods. This was repeated for four ice ages, with four separate waves of colonization, each hybridizing with the remnants of the previous waves This generated a present-day group of species forming the subsection Euoenothera which is very rich in genetic diversity, spread right across the North American continent. These species are morphologically diverse and are largely interfertile and so the species boundaries have been a source of dispute amongst taxonomists.
This pattern of repeated colonizations resulted in a unique genetic conformation in the Euoenotherae whereby the chromosomes at meiosis can form into circles of varying size, rather than complete chromosome pairing as in common meiosis. This is the result of several reciprocal translocations between chromosomes so that the pairing occurs only at the tips. This phenomenon has some apparently non-Mendelian genetic consequences. By combining this mode of chromosome segregation with a system of balanced-lethal genes, genetic recombination is prevented and the plants enjoy the vigour of heterosis. This resulted in the evolution of a large number of sympatric races over North America, east of the Rocky Mountains. Analysis of the cytology of these races and of artificial hybrids between them allowed a detailed understanding of the genetic and geographic evolution of the Euoenotherae. This whole subject was a major area of genetic research during the first half of the 20th century.
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