Why is pollen yellow? And why are there so many species in the tropical rain forest?

Pollen appears to need protection from UV-B insolation, and some protection is provided by yellow flavonoids and some other compounds. UV-B insolation is mutagenic and could thus provide the mutations needed for speciation. Tropical montane vegetation experiences the highest UV-B insolation of any vegetation in the world. This will be enhanced by volcanic eruptions releasing aerosols. There is evidence of strong volcanicity and mutation in Permian times, when world vegetation changed dramatically. Palynological richness, used as a measure of palaeo-biodiversity, shows rapid increases in the Palaeo-Eocene and Early Miocene, both times of peak temperature. DNA evidence suggests increasing diversity at these times. Milankovitch cycles at these times would have caused vertical migrations of tree taxa, with magnitudes of c. 800 m. These migrations could have led to isolation of populations on mountain peaks, allowing allopatric speciation, especially in the montane elevated UV-B environment. This process, when repeated, could have led to a 'species pump', and thus to higher biodiversity.