Adaptive radiations, like that of the Galápagos finches, can lead to a wide diversity of plants and animals in a short evolutionary time. But not all radiations are adaptive. The Cape flora of southern Africa is among the richest and most diverse in the world, but its diversity appears to involve both adaptive and non-adaptive evolution. This project will examine a group in the plant family Proteaceae to determine whether its radiation has been primarily adaptive or non-adaptive. Plant traits will be measured in experimental gardens and wild populations to determine their relationship to growth and reproduction and to identify traits associated with species differences. Variation in these traits will be compared to variation in neutral genetic markers to determine trait differences are the result of natural selection or of random differentiation associated with geographic isolation.
Scientists from the South African National Biodiversity Institute (SANBI) will collaborate on the project, and the project will enhance SANBI''s bioregional planning initiatives. Field work will immerse faculty, students, and post-doctoral research associates in a rich multicultural environment. The project will engage both undergraduates and high school students, with special emphasis on high schools with large proportions of students from groups underrepresented in science. This award is co-funded by NSF''s Office of International Science and Engineering.
Southwestern South Africa has an enormous diversity of plant species, and many of them are found nowhere else in the world. Much of that diversity is concentrated in a few large groups, including heathers (Erica), ice plants (Mesembryanthemaceae), geraniums (Pelargonium), grass-like restios (Restionaceae), and the macadamia nut family (Proteaceae). Scientists have long wondered whether such diversity arises because of adaptation to different environments or because populations and species simply diverge when they become geographically isolated from one another. Our study of one group within the genus Protea, the white proteas, showed that some differences among the species and among the populations within species are the result of adaptation to differences in the amount and timing of rainfall and in how cold winter temperatures are. Natural selection led the populations to evolve traits that suit them to the environments in which they grow. At the same time, populations are genetically isolated from one another, but they remain similar to one another in markers not subject to natural selection. Some of the evolutionary differences among populations and species reflect the chance occurrence of random mutations and the random loss of genes that were once present. Thus, the evolutionary differences we see among populations and among species in this group represent a combination of adaptation to differing environments and random differentiation associated with geographical isolation. In addition, the associations we found between the environment and plant traits we measured sometime went in different directions. In Protea aurea, for example, we found that short, heavy roots were associated with areas having cold winters and highly seasonal rainfall. In P. mundii, on the other hand, long, light roots were associated with those same environmental conditions. The differences we measured between these two species probably indicate that there are other traits we didn't measure that also influence how plants respond to soil moisture and winter temperatures and that evolution in some combination of these traits determines whether plants will prosper in a particular environment. Our analysis of genetic variation within the group also revealed that there are two very distinct genetic groups withn what is currently regarded as a single species, Protea mundii. Populations in the western part of the range are separated by several hundred kilometers from those in the eastern part of the range and are genetically very distinct. Because of our findings, we recommend that conservation managers in South Africa treat the two sets of populations as different species. These analyses also confirmed that a population once thought to be part of Protea subvestita is a hybrid between two other members of the white protea group. Had our analyses shown that it was part of P. subvestita, it would have been of substantial conservation importance because it would have been the only member of this species in the Province of Western Cape. Instead, it is a hybrid between two species that are widespread in that province. Thus, its conservation importance, if any, will depend on the importance associated with its unusual hybrid origin, not on a large geographical disjunction.
