Evolution of the giant senecios and giant lobelias in eastern Africa.

Abstract

The mountains of eastern Africa provide a unique opportunity to study the mode and tempo of evolutionary events. The mountains are sufficiently numerous, tall, and isolated that, given the altitudinal zonation of vegetation, they form a stratified system of habitat-islands which is amenable to analysis and the formulation of testable predictions concerning the adaptive radiation of plants. Two unrelated groups, the giant lobelias (Lobeliaceae, Lobelia subgenus Tupa section Rhynchopetalum) and the giant senecios (Asteraceae, Senecioneae, Dendrosenecio), have diversified on these mountains. Both groups have independently evolved a giant-rosette growth-form, with similar modifications in a variety of habitats. This provides strong a priori evidence that these traits are adaptations to proximal environmental factors. Phylogenetic analysis of chloroplast DNA restriction-site variation among species (or smaller isolated populations) provided a framework of descent that was used to investigate the pattern of morphological modification with minimal circular reasoning. The insular habitats serve to limit the universe of possible evolutionary histories to a manageable set of plausible comparisons. The giant senecios are endemic to eastern Africa and are not closely related to any of the putative relatives suggested in the literature. They represent an old lineage that has recently diversified. They appear to have originated at high elevations on Mt. Kenya, the Aberdares, or Mt. Kilimanjaro, and most speciation resulted from dispersal between mountains, primarily at high altitudes. Subsequent diversification primarily involved invasion of habitats at lower altitudes. The giant lobelias in eastern Africa are part of a pantropical tetraploid group. They appear to have originated at mid-elevations in eastern Tanzania where they diversified upward, forming a group with branched inflorescences. One early lineage gave rise to a Brazilian species and two disparate east African species that may have been reintroduced from Brazil. Another early lineage spread to southern Tanzania, where it founded a group with unbranched inflorescences, and a relictual ancestor remains. It gave rise to endemic groups of the Western Rift, the Eastern Rift, and the Ethiopian mountains, as well as a widespread species that has independently reached these sites.