Some researchers have hypothesized that this new habitat coupled with a different diet, including Acacia, may have exposed giraffe ancestors to toxins that caused higher mutation rates and a higher rate of evolution.
Furthermore, as seen in Figure 1 the genes in clade III have branches longer than those of the genes in the other clades, indicating a higher rate of evolution.
The divergence observed at the mtDNA locus was much higher (11.8%) while the diversity was lower than the nuclear average, in accordance with a higher rate of evolution and a smaller effective population size of the mitochondrial genome.
If either of these two scenarios is true, then Xist sequences might be expected to show a higher rate of evolution in O. palustris than in species that have retained L1 activity.
The higher rate of evolution of the GABAA receptor ε subunits may explain why these subunits have so far only been found in mammals while the receptor γ4 subunits are present in both birds and reptiles [20].
If interaction between L1s and Xist is direct, a higher rate of evolution of the Xist gene would be expected in order to ensure recognition of either way stations that are mutating to greater degeneracy or co-option of different repetitive sequences to serve as way stations.
