12 October N+Vs layout ok

Biomass was almost always greater in the first-generation offspring of crosses involving foreign material, but declined in the second generation. Mortality was higher in both generations. Seed weight, like biomass, was generally higher in the first generation and lower in the second generation of foreign crosses. So it seems that, in the long term, the introduction of foreign genes into these weed populations is likely to make them less fit — that is, less able to survive and reproduce effectively. Relative reductions in fitness were estimated to be between 8% and 23%. The overall message is that the introduction of genes from distant populations is likely to do lasting harm to the native weed flora of an area. These findings have broader implications. The introduction of fresh breeding stock into fragmented and isolated populations is often seen as a way of increasing genetic diversity, and has been attempted in organisms ranging from butterflies to birds. The merits of such a policy can vary with the geographical distance from which introduced material is obtained and with the spatial variability in the genetic constitution of the species concerned. Geographical distance might not always correspond to genetic distance: it will also vary with the type of organism involved. The effects of genetic distance have been tested on a Californian shrub, Lotus scoparius, the deerweed, using degrees of enzyme variation as the genetic measure. Deerweed is a variable shrub of the coastal sage community of the west coast of North America, being found in both arid and well-watered habitats. In an experiment involving 12 populations, there was only weak correlation between geographical and genetic distances. But in transplant experiments the cumulative fitness of plants showed an inverse relationship to genetic distance. The closer the relationship is, the more likely it is that the individual will survive well in similar situations. So the introduction of this shrub should be determined by an analysis of genetic or ecological similarity (or both): geographical proximity of a seed source will not necessarily provide the most appropriate material for translocation. A further complication with flowering plants arises from their different breeding strategies. The corncockle, for example, often self-pollinates, so its genetic constitution is likely to be patchy on a local scale. Deerweed is pollinated by insects, which might also restrict the distances over which outbreeding is possible. How does this relate to wind-pollinated, outbreeding plants such as birch, poplar, alder and elm? A modelling exercise, based on data concerning the timing of flowering and its response to climate, has shown that there is little evidence of local genetic news and views