MWEvolveList[{"ABA""ABBAB","AB""","ABABBB""AAAAABA"},{"ABABAB"},40];
StringProbabilities[hist_]:=Function[a,{First[#],Length[#]/Length[a]}&/@Split[Sort[a]]]/@Map[StringLength,hist,{2}]
SPGraphics[hist_]:=Graphics[MapIndexed[{GrayLevel[1-#1[[2]]^(1/4)],Rectangle[{#1[[1]],-#2[[1]]}]}&,StringProbabilities[hist],{2}],AspectRatioAutomatic]
Show[SPGraphics[%199]];
Show[SPGraphics[MWEvolveList[{"A""AB","B""A"},{"A"},7]]];
Show[SPGraphics[MWEvolveList[{"A""A","A""AA","AA""A"},{"A"},7]]];
Show[SPGraphics[MWEvolveList[{"A""AA","AA""A"},{"A"},7]]];
Show[SPGraphics[MWEvolveList[{"AA""BABBBBA","BAB""A"},{"ABAAB"},60]]];
Show[SPGraphics[MWEvolveList[{"ABA""BBAA","BAA""AAB"},{"BABBAAB"},60]]];
Show[SPGraphics[MWEvolveList[{"ABA""ABBAB","AB""","ABABBB""AAAAABA"},{"ABABAB"},200]]];