Senecioneae (Asteraceae) of the Succulent Karoo and “geography of macroevolution of higher plants”: a chorological essay

Abstract

The geographic characteristic of plant macroevolution is manifested in general in neither even, nor random distribution of the archaic and advanced representatives of a higher taxon in its range. The explanatory concepts proposed are still contradictory. Besides, they are poorly testable, because they concern too long-time intervals during which multiple major changes in both the environment and the ranges of taxa can have occurred; these changes usually continue untraceable in fossil records. The tribe Senecioneae in the Succulent Karoo is best suited for studying the geographic patterns of plant macroevolution for the following reasons: i) the environment of the Succulent Karoo has resulted from unidirectional climate change over 10 million years (accordingly, large fluctuations of the taxon ranges there are highly unlikely); ii) the phylogeny of the tribe Senecioneae is quite fully recognized (and it is not distorted by extinctions, at least at the level of genera); iii) Senecioneae are completely allochthonous in the Succulent Karoo (accordingly, interpretation of results becomes easier and simpler). The archaic genera of Senecioneae are as numerous in the Succulent Karoo as the highly advanced ones, whereas the mid-advanced genera are completely absent there. Such a genus composition of the tribe in the area concerned cannot be explained by the cradle and museum concept, since this area is outside of center of origin/diversification of Senecioneae. The zonal stratification concept is inapplicable to this case, since the climate of the Succulent Karoo was changing unidirectional all the time without noticeable fluctuations. All other concepts proposed are also inapplicable, as they treat the emerging of archaism gradient(s) in the taxon range, but not a deficiency/absence of mid-advanced representatives of a higher taxon in a territory occupied by its archaic and highly advanced members. The absence of mid-advanced members of Senecioneae in the Succulent Karoo could be explained as follows. Any higher taxon very rarely and at long time intervals acquires traits that enable it to spread to areas which greatly differ in their environments from the center of origin/diversification of this taxon. All new subordinate taxa that originate during these time intervals remain confined in the center of origin/diversification of the higher taxon. Accordingly, only archaic and most advanced representatives of this higher taxon would be found faraway its center of origin/diversification.