Hybrids have long been recognized as a potential pathway for gene flow between species that can have important consequences for evolution and conservation biology. However, few studies have demonstrated that genes from one species can introgress or invade another species over a broad geographic area. Using 35 genetically mapped restriction fragment length polymorphism (RFLP) markers of two species of cottonwoods (Populus fremontii x P. angustifolia) and their hybrids (n = 550 trees), we showed that the majority of the genome is prohibited from introgressing from one species into the other. However, this barrier was not absolute; Fremont cpDNA and mtDNA were found throughout the geographic range of narrowleaf cottonwood, and 20% of the nuclear markers of Fremont cottonwood introgressed varying distances (some over 100 km) into the recipient species' range. Rates of nuclear introgression were variable, but two nuclear markers introgressed as fast as the haploid, cytoplasmically inherited chloroplast and mitochondrial markers. Our genome-wide analysis provides evidence for positive, negative, and neutral effects of introgression. For example, we predict that DNA fragments that introgress through several generations of backcrossing will be small, because small fragments are less likely to contain deleterious genes. These results argue that recombination will be important, that introgression can be very selective, and that evolutionary forces within the hybrid population to effectively "filter" gene flow between species. A strong filter may make introgression adaptive, prevent genetic assimilation, lead to relaxed isolating mechanisms, and contribute to the stability of hybrid zones. Thus, rather than hybridization being a negative factor as is commonly argued, natural hybridization between native species may provide important genetic variation that impacts both ecological and evolutionary processes. Finally, we propose two hypotheses that contrast the likelihood of contemporary versus ancient introgression in this system.

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We define a genetic similarity rule that predicts how genetic variation in a dominant plant affects the structure of an arthropod community. This rule applies to hybridizing cottonwood species where plant genetic variation determines plant-animal interactions and structures a dependent community of leaf-modifying arthropods. Because the associated arthropod community is expected to respond to important plant traits, we also tested whether plant chemical composition is one potential intermediate link between plant genes and arthropod community composition. Two lines of evidence support our genetic similarity rule. First, in a common garden experiment we found that trees with similar genetic compositions had similar chemical compositions and similar arthropod compositions. Second, in a wild population, we found a similar relationship between genetic similarity in cottonwoods and the dependent arthropod community. Field data demonstrate that the relationship between genes and arthropods was also significant when the hybrids were analysed alone, i.e. the pattern is not dependent upon the inclusion of both parental species. Because plant-animal interactions and natural hybridization are common to diverse plant taxa, we suggest that a genetic similarity rule is potentially applicable, and may be extended, to other systems and ecological processes. For example, plants with similar genetic compositions may exhibit similar litter decomposition rates. A corollary to this genetic similarity rule predicts that in systems with low plant genetic variability, the environment will be a stronger factor structuring the dependent community. Our findings argue that the genetic composition of a dominant plant can structure higher order ecological processes, thus placing community and ecosystem ecology within a genetic and evolutionary framework. A genetic similarity rule also has important conservation implications because the loss of genetic diversity in one species, especially dominant or keystone species that define many communities, may cascade to negatively affect the rest of the dependent community.

We argue that the genetic diversity of a dominant plant is important to the associated dependent community because dependent species such as herbivores are restricted to a subset of genotypes in the hostplant population. For plants that function as habitat, we predicted that greater genetic diversity in the plant population would be associated with greater diversity in the dependent arthropod community. Using naturally hybridizing cottonwoods ( Populus spp.) in western North America as a model system, we tested the general hypothesis that arthropod alpha (within cross-type richness) and beta (among cross-type composition) diversities are correlated with cottonwood cross types from local to regional scales. In common garden experiments and field surveys, leaf-modifying arthropod richness was significantly greater on either the F 1 (1.54 times) or backcross (1.46 times) hybrid cross types than on the pure broadleaf cross type ( P. deltoides Marshall or P. fremontii Watson). Composition was significantly different among three cross types of cottonwoods at all scales. Within a river system, cottonwood hybrid zones had 1.49 times greater richness than the broadleaf zone, and community composition was significantly different between each parental zone and the hybrid zone, demonstrating a hierarchical concentration of diversity. Overall, the habitats with the highest cottonwood cross-type diversity also had the highest arthropod diversity. These data show that the genetics of habitat is an important conservation concept and should be a component of conservation theory.Resumen: Argumentamos que la diversidad genética de una planta dominantes es importante para la comunidad dependiente asociada porque las especies dependientes, como herbívoros, están restringidas a un subconjunto de genotipos en la población de plantas hospederas. Para plantas que funcionan como hábitat, predijimos que la mayor diversidad genética en la población de plantas estaría asociada con mayor diversidad en la comunidad de artrópodos dependiente. Utilizandoálamos (Populus spp.) naturalmente hibridizantes en Norteamérica occidental como sistema modelo, probamos la hipótesis general de que las diversidades alfa (riqueza intra tipo de cruza) y beta (riqueza inter tipo de cruza) de artrópodos están correlacionadas con los tipos de cruza deálamos desde escalas locales a regionales. En experimentos de jardín comunes y muestreos de campo, la riqueza de artrópodos modificadores de hojas fue significativamente mayor en la F 1 (1.54 veces) o en híbridos de retrocruza (1.46 veces) que en el tipo de cruza pura de hoja ancha (P. deltoides Marshall or P. fremontii Watson). La composición fue significativamente diferente en los tres tipos de cruza deálamos en todas las escalas. En un sistema ripario, las zonas deálamos híbridos tenían 1.49 veces más que la riqueza de la zona de hoja ancha, y la composición de la comunidad fue significativamente diferente entre cada † 380 Conservation of Habitat Genetic DiversityBangert et al.zona parental y la zona híbrida, lo qu...

We tested the hypothesis that leaf modifying arthropod communities are correlated with cottonwood host plant genetic variation from local to regional scales. Although recent studies found that host plant genetic composition can structure local dependent herbivore communities, the abiotic environment is a stronger factor than the genetic effect at increasingly larger spatial scales. In contrast to these studies we found that dependent arthropod community structure is correlated with both the cross type composition of cottonwoods and individual genotypes within local rivers up to the regional scale of 720,000 km(2) (Four Corner States region in the southwestern USA). Across this geographical extent comprising two naturally hybridizing cottonwood systems, the arthropod community follows a simple genetic similarity rule: genetically similar trees support more similar arthropod communities than trees that are genetically dissimilar. This relationship can be quantified with or without genetic data in Populus.