250 years of hybridization between two biennial herb species without speciation

In 1759, Linnaeus convinced his followers that plants could hybridise by crossing flowers in the daisy family and producing intermediate offspring. These hybrids, between Tragopogon pratensis and T. porrifolius, exist naturally today in London, to all appearances the same. We find that most of the London hybrids are in their first generation, though we provide chromosomal evidence that one is a little older. These hybrids do not seem to have given rise to a new species, even though both parents have produced new hybrid species in the last century when crossed with T. dubius. Abstract Hybridization between plant species can generate novel morphological diversity and lead to speciation at homoploid or polyploid levels. Hybrids between biennial herbs Tragopogon pratensis and T. porrifolius have been studied in experimental and natural populations for over 250 years. Here we examine their current status in natural populations in southeast England. All hybrids found were diploid; they tended to grow taller and with more buds than their parental species; many showed partial fertility; a few showed evidence of backcrossing. However, we found no evidence to suggest that the hybrids are establishing as a new species, nor can we find literature documenting speciation of these hybrids elsewhere. This lack of speciation despite at least 250 years of hybridization contrasts with the fact that both parental species have formed new allopolyploid species through hybridization with another diploid, T. dubius. Understanding why hybrids often do not speciate, despite repeated opportunities, would enhance our understanding of both the evolutionary process and risk assessments of invasive species.