Multilocus genetic structure of ancestral Spanish and colonial Californian populations of Avena barbata.

We have applied a multivariate log-linear technique to the analysis of interlocus allelic associations among 14 allozyme loci in a sample of 4011 plants from 42 Spanish populations of Avena barbata. The loci fell into three natural groups of five, five, and four loci. The five loci of the first group are invariant, or nearly so, throughout the range of the species. The genetic organization of the loci of this set is defined by a single five-locus genotype; each allele of this predominant genotype is a "wild-type" allele that contributes favorably to adaptedness in all single-locus and multilocus configurations regardless of environment. Although allelic diversity is high in Spain for the nine loci of the second and third sets, log-linear analyses showed that these loci are tied together in Spanish populations through complex networks of overlapping lower-order interlocus interactions. The ancestral Spanish and colonial Californian gene pools are closely similar in allelic composition on a locus-by-locus basis; however, Spanish allelic configurations at two-locus and higher-order levels are usually different from and much less tightly organized than in Californian populations. We conclude that the major force involved in the evolution of the colonial populations was selection that led to reorganization, at the interlocus level, of the ancestral Spanish allelic ingredients into different multilocus genotypes adapted to Californian habitats.