Figure 3

Thresholds for changes in the number of stop codons. Contour plots showing the length of coding sequence, in triplets, at which the transition from (a) one to two stop codons; (b) two to three stop codons; (c) three to four stop codons; (d) four to five stop codons; and (e) five to six stop codons becomes possible, for values of R from 1 to 100 and values of k from 0.1 to 1 (see Equations 1 and 2). Contour lines separate lengths of coding sequence from 0-25 triplets (darkest areas) to 200-225 triplets (lightest areas), in increments of 25 triplets. Transitions to greater numbers of stop codons become increasingly difficult as the number of stop codons increases; the transition from four to five stops is favoured by selection only when the mean coding sequence length is very low (very dark shading over most of the plotted area).