Gradual Adaptation Toward a Range-Expansion Phenotype Initiated the Global Radiation of Toads

tistical analysis on normalized compiled datafrom all the experiments reveals that both thedifference between low pH and median pH andthat between low pH and high pH are significant(P =0.044andP =0.005,respectively;t test,two-tailed distribution). Because we know thatpH variations in this range have no effect on theFe uptake capabilities of T. weissflogii ,theob-served decrease in Fe uptake rate is presumablydue to a change in the chemical speciation of Fewith pH. On average, the Fe uptake rate de-creased by about 10 to 20% from the high-pH(~8.4) to the low-pH (~7.8) conditions in the ex-periments we conducted, reflecting presumablythe acid-base chemistry of the mixture of func-tional groups responsible for Fe chelation in thevarious water samples used for the experiments.It would appear from the modest effect of acid-ification on Fe uptake rates that little of the ironwas bound to carboxylic acid moieties in ourfield samples. The only published experimentsontheeffect ofpHonironspeciationinseawatershow that, in a sample of surface water from theNorth Sea, Fe′ decreased with decreasing pH(27), which is consistent with our observations.Thoseresultsalsoimplythattheeffectivenessofnatural ligands in maintaining Fe in solutionmight be increased at low pHand may resultina slower Fe loss via the formation of Fe oxy-hydroxide precipitates. The very low ambientbiomass in Sargasso Sea samples and the highconcentrations of suspended particulate materialin unfiltered New Jersey samples precluded Feuptake experiments with the ambient phyto-plankton populations.The ongoing increase in atmospheric CO