Glacial refugia and postglacial migration are major factors responsible for the present patterns of genetic variation we see in natural populations. Traditionally postglacial history has been inferred from fossil data, but new molecular techniques permit historical information to be gleaned from present populations. The chloroplast tRNA(Leu1) intron contains regions which have been highly conserved over a billion years of chloroplast evolution. Surprisingly, in one of these regions which has remained invariant for all photosynthetic organisms so far studied, we have found intraspecific site polymorphism. This polymorphism occurs in two European oaks, Quercus robur and Q. petraea, indicating hybridisation and introgression between them. Two distinct chloroplast types occur and are distributed geographically as eastern and western forms suggesting that these oaks are each derived from at least two separate glacial refugia.

To study the evolution of this Pyrenean hybrid zone following postglacial secondary contact, a crossing programme was performed involving sequential mating of single females by males of both subspecies in both orders. Progeny were scored as embryos using Acridine orange and C-banding to identify their male parent. This revealed a number of significant departures from straightforward inheritance. There was an excess of pure over hybrid progeny. This homogamy is produced at fertilization but may have been determined before through sperm preference or competition. There was also sperm precedence with the second mating producing most of the progeny. This may be due to sperm ageing or differential storage and utilization by the female. The fundamental sex ratio in the progeny was biased towards males. Of particular interest is the finding that homogamy is markedly greater when C.p. parallelus is the mother, suggesting some form of differential biochemical recognition. This asymmetry should have important consequences for the dynamics of the hybrid zone. All these results are discussed in the light of the divergence of the two subspecies and the mixing of their genomes in the hybrid zone.

Refugial differentiation and routes of postglacial migration are major determinants of the patterns of geographical variation we see in natural populations today. We used patterns of chloroplast DNA (cpDNA) variation to investigate the postglacial colonization history of the European oak species Quercus robur and Q. petraea. By sequencing two cpDNA segments using universal primers, we revealed four polymorphic sites which identify four cytotypes with characteristic geographical distributions. Of these, the principal eastern, central and western cytotypes divide the range into three longitudinal zones, each extending from the south to the north of Europe. This corroborates the idea that the postglacial colonization started from three distinct southerly refugia. The fourth cytotype, restricted to East Anglia, was probably derived from the western type postglacially. As a special problem, we addressed the controversial origin of Q. robur at its northern limits in south-western Finland, where it currently occupies a narrow coastal zone disjunct from the remaining oak range. Using a PCR-RFLP assay that discriminates the eastern cytotype, a contact zone of two cytotypes was identified in the region of the Salpausselkä ridges. This suggests that the marginal northern occurrence was independently colonized both from the east and from the west, across the Baltic Sea.

Random amplified polymorphic DNA (RAPDs) and inter‐simple sequence repeats (inter‐SSRs) have been used to study clonal growth and hybridization in some non‐native, gynodioecious, invasive weeds from the genus Fallopia (Polygonaceae). At the study site (the River Kelvin, Glasgow, UK) a single genotype of Japanese knotweed (Fallopia japonica) was detected, consistent with all the individuals sampled being ramets of a single clone. Two genotypes of giant knotweed (F. sachalinensis) were detected, with one genotype accounting for all but one of the samples, again indicative of widespread clonal growth. Five genotypes of the hybrid between Japanese and giant knotweed (F.×bohemica) were recovered. F.×bohemica is the only male‐fertile taxon present at the site and it seems probable that at least some of this genetic variation is attributable to hybrid fertility. A single plant identified using morphological methods as a backcross between F. japonica and F.×bohemica was analysed, and the molecular data were consistent with this theory. A comparison of RAPDs and inter‐SSRs showed that the two techniques gave data that are broadly congruent, and both techniques showed a similar sensitivity in the number of genotypes detected.