A case of natural selection in Atlantic-East-Pacific Rhizophora

We have examined possible adaptation in cuticular alkane composition in the halophytic mangrove genus Rhizophora. Relative composition of the dominant alkanes varied: 1. among the three sympatric species from the Atlantic-Eas t-Pacific region, 2. with geographic region within species and, 3. among po pulations within geographic region. For R. mangle, longer chain alkanes wer e more important in the semi-arid regions of north-west Africa and the Paci fic coast of north-central Mexico. Mantel tests showed that inter-populatio n taxonomic distances for the major alkanes were correlated with taxonomic distances for annual rainfall and mean maximum temperature, but not with we ighted geographic distance. Since alkane carbon chain length should affect the biophysical properties of waxes, with longer chain lengths increasing c rystallinity and impermeability, our data provides support for the hypothes is that observed differentiation is due to natural selection, rather than s tochastic processes in this species. The same pattern was not observed for R. racemosa or R. harrisonii. Since these two species occupy less saline co nditions and are more restricted in their latitudinal range, selection pres sure may be less important than other evolutionary forces such as genetic d rift. There was some evidence that alkane composition was more closely corr elated with mean minimum temperatures in R. racemosa, that might set the la titudinal limits in this species.