The rich diversity within each of the five histone families (H1, H2A, H2B, H3, 
and H4) can hardly be reconciled with the notion of homogenizing evolution. The 
prevalence of birth-and-death long-term evolution over concerted evolution has 
already been demonstrated in the linker histone H1 family as well as for the 
H2A, H3, and H4 core histone families. However, information about histone H2B is 
lacking. In the present work, we have analyzed the diversity of the members of 
this histone family across different eukaryotic genomes and have characterized 
the mechanisms involved in their long-term evolution. Our results reveal that, 
quite in contrast with other histones, H2B variants are subject to a very rapid 
process of diversification that primarily affects the male germinal cell lineage 
and involves their functional specialization probably as a consequence of 
neofunctionalization and subfunctionalization events after gene duplication. The 
overall parallelism observed between the molecular phylogenies and the 
relationships among the electrostatic potentials of the different variants 
suggests that the latter may have played a major structural selective constraint 
during H2B evolution. It thus seems that the reorganization of chromatin 
structure during spermiogenesis might have affected the evolutionary constraints 
driving histone H2B evolution, leading to an increase in diversity.