(A) A model for how gene body methylation might be a consequence of evolutionary processes. Any gene could aberrantly acquire CG methylation, but in an inducible or tissue-specific gene, selection to maintain the gene is weaker so the increased mutation rate associated with 5mC leads to loss of gene function, whilst purifying selection preserves non housekeeping genes. (B) Recurrent acquisition of gene body methylation and loss of TE methylation characterises eukaryotic DNA methylation patterns. The guide tree is for illustration purposes only and branch lengths are not accurate. Only lineages in which some form of DNA methylation is retained are shown.