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

Abstract

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-East-Pacific region, 2. with geographic region within species and, 3. among populations within geographic region. For R. mangle, longer chain alkanes were more important in the semi-arid regions of north-west Africa and the Pacific coast of north-central Mexico. Mantel tests showed that inter-population taxonomic distances for the major alkanes were correlated with taxonomic distances for annual rainfall and mean maximum temperature, but not with weighted geographic distance. Since alkane carbon chain length should affect the biophysical properties of waxes, with longer chain lengths increasing crystallinity and impermeability, our data provides support for the hypothesis that observed differentiation is due to natural selection, rather than stochastic processes in this species. The same pattern was not observed for R. racemosa or R. harrisonii. Since these two species occupy less saline conditions and are more restricted in their latitudinal range, selection pressure may be less important than other evolutionary forces such as genetic drift. There was some evidence that alkane composition was more closely correlated with mean minimum temperatures in R. racemosa, that might set the latitudinal limits in this species.