Fig. 2.—

Birth-death rates of the SP1-binding motif in mammals. Birth rates (α) denote the probability (per year) that an unoccupied position will gain a binding site; death rates (β) give the probability (per year) that an existing binding site is lost. Branches in the mammalian phylogeny were partitioned into three groups: early eutherian mammals (red), late eutherian mammals (black), and GA box-preferring noneutherian mammals (blue). Birth and death rates of each group were estimated for the GC box (GGGCGG), GA box (GGGAGG), and the nonfunctional motif GGGTGG (Letovsky and Dynan 1989; Wierstra 2008).