The suitability of a particular gene for addressing phylogenetic (or
process-related) questions at some level of organization depends in part
upon the rate of evolution of that gene. Ideally, there should be a close
correspondence between the ages of the alleles under study and the ages of
divergence of the higher-level taxa. For very old evolutionary events,
such as the diversification of land plants, very slowly evolving, highly
constrained genes are useful. A common such gene in plants is rbcL, the
large subunit of ribulose biphospate carboxylase (an key enzyme in
photosynthesis). Other common choices for deep splits are ribosomal RNA
(rRNA) genes. For more recent splits, rapidly-evolving regions such as
the mitochondrial D-loop are used
. One (rather contentious) application of short-term
divergences is the attempt to reconstruct patterns of early human history
and evolution from present-day genetic patterns. Several genes have been
used for this purpose (most giving conflicting results). A recent addition
to the group is the Alu class of mobile elements.