Reference:
  Author: Ziheng Yang
  Title: "PAML 4: phylogenetic analysis by maximum likelihood"
  Journal: Molecular Biology and Evolution
  Year: 2007
  Volume: 24
  Number: 8
  Pages: 1586-91
  DOI: 10.1093/molbev/msm088 
  PMID: 17483113
  URL: http://mbe.oxfordjournals.org/content/24/8/1586
  eprint: http://mbe.oxfordjournals.org/content/24/8/1586.full.pdf+html
