The crucifer Barbarea vulgaris grows naturally in Europe and Western Asia. In Denmark and neighbouring countries, the subspecies arcuata exists in two types that differ in insect and pathogen resistance, as well as in several other traits. The types have been named after the degree of leaf pubescence; one has glabrous leaves and is therefore called G-type while the other has pubescent leaves and is called the P-type. The G-type is resistant to most genotypes of the flea beetle Phyllotreta nemorum as well as some other Brassicales specialists, and this resistance is conferred by saponins. The P-type is susceptible to flea beetles, but remains mostly symptomless when exposed to the oomycete Albugo sp. which causes white blister rust. The G-type on the other hand is highly susceptible to this pathogen. The two Barbarea types also differ in glucosinolate composition and are genetically divergent. The types grow mostly in separate populations and only few hybrids have been discovered.

The existence of these different forms of B. vulgaris offer unique opportunities to observe the effects of selection pressure that vary over time and space and involve tradeoffs, geographical differences and the potential for hybridization.

The results presented in this PhD thesis show that the P and G-type maintain their distinctive associated traits over a large geographical range where they occupy different areas and that Fennoscandia constitutes a meeting zone where both types are found. The two types appear to have evolved in allopatry, possibly in different refugia during the last ice age.

Hybrid individuals are able to exhibit dual resistances to both Albugo and flea beetles, and a combined resistance can in some respects be advantageous. However, hybridisation between types in nature appears to be limited. Our results show that the distinctiveness of the two types is most likely maintained by a considerable reproductive barrier, and that selection against hybridisation occurs both before and after fertilisation.