Genetical variation in two related annual Senecio species occurring on the same habitat

Isoenzyme analysis of 14 different enzymes of 4 Netherlands populations of Senecio sylvaticus and 3 populations of S. viscosus, two closely related, partly co-occurring annual pioneer species on clearings, revealed that no genetical variation exists within nor between populations of these species. Differences between the species are used to estimate genetic identity and genetic distance.The possible cause of this genetical invariability is discussed in view of the constant confrontation with the founder situation on clearings, together with the pollination mechanism and the niche breadth of both species.