THE aim of the conservation of plant genetic resources is to collect and preserve representative samples of genetic variation from the primitive ‘landraces’ and the related wild species of crop plants on which man depends for use in future breeding programmes1,2. Counter proposals3 that breeders obtain specific alleles by mutagenesis at will are defensible when one is considering single loci. But groups of alleles at many loci which have jointly been subjected to natural selection, are unlikely to be assembled by mutation breeding. We report here the widespread occurrence of co-adapted combinations of alleles at the four esterase loci in populations of wild barley (Hordeum spontaneum) in Israel, thus reinforcing the case for genetic conservation.