id	author	title	date	pages	extension	mime	words	sentences	flesch	summary	cache	txt
cord-346267-l08ld2cy	Wertheim, Joel O.	Purifying Selection Can Obscure the Ancient Age of Viral Lineages	2011-06-24		.txt	text/plain	6414	327	47	Over longer periods of evolutionary time (i.e., along long internal branches on a phylogenetic tree), evidence of evolution at the nucleotide level could be lost, which could bias tMRCA estimates towards younger dates (i.e., underestimate the length of these branches). The results presented here demonstrate that not accounting for purifying selection may bias tMRCA estimation in RNA viruses towards more recent dates, and a degree of correction can be realized by employing more realistic codon-based substitution models, capable of partially accounting for the biasing effect of purifying selection. Using BMCMC analysis, we inferred the substitution rate and root age under a standard nucleotide substitution model (GTR + Γ 4 ) for each of our three viral data sets: MeV/RPV/PPRV nucleoprotein, EBOV glycoprotein, and AIV neuraminidase (table 1) . The nucleotide substitution model (GTR + Γ 4 ) underestimated the length of branches simulated under empirical (IFEL) selection regimes inferred from the three viral data sets ( fig.	./cache/cord-346267-l08ld2cy.txt	./txt/cord-346267-l08ld2cy.txt