Michael L. Arnold – författare

This study includes data from sources which support the paradigm of natural hybridization as an important evolutionary process. The review of these data results in a challenge of the dogma that is the explicit or implicit framework used by a large proportion of evolutionary biologists - that the process of natural hybridization is maladaptive and it is because it represents a violation of divergent evolution. In contrast, this book presents evidence of a significant role for natural hybridization in furthering adaptive evolution and evolutionary diversification in both plants and animals.

The study of genetic exchange resulting from natural hybridization, horizontal gene transfer, and viral recombination has long been marked by controversy between researchers holding different conceptual frameworks. Those subscribing to a doctrine of 'species purity' have traditionally been reluctant to recognise inferences suggesting anything other than a marginal role for non-allopatric divergence leading to gene transfer between different lineages. However, an increasing number of evolutionary biologists now accept that there is a growing body of evidence indicating the existence of non-allopatric diversification across many lineages and all domains of biological diversity. Divergence with Genetic Exchange investigates the mechanisms associated with evolutionary divergence and diversification, focussing on the role played by the exchange of genes between divergent lineages, a process recently termed 'divergence-with-gene-flow'. Although the mechanisms by which such divergent forms of life exchange genomic material may differ widely, the outcomes of interest - adaptive evolution and the formation of new hybrid lineages - do not. Successive chapters cover the history of the field, detection methodologies, outcomes, implications for conservation programs, and the effects on the human lineage associated with the process of genetic transfer between divergent lineages.This research level text is suitable for senior undergraduate and graduate level students taking related courses in departments of genetics, ecology and evolution. It will also be of relevance and use to professional evolutionary biologists and systematists seeking a comprehensive and authoritative overview of this rapidly expanding field.

Even before the publication of Darwin's Origin of Species, the perception of evolutionary change has been a tree-like pattern of diversification - with divergent branches spreading further and further from the trunk. In the only illustration of Darwin's treatise, branches large and small never reconnect. However, it is now evident that this view does not adequately encompass the richness of evolutionary pattern and process. Instead, the evolution of species from microbes to mammals builds like a web that crosses and re-crosses through genetic exchange, even as it grows outward from a point of origin. Some of the avenues for genetic exchange, for example introgression through sexual recombination versus lateral gene transfer mediated by transposable elements, are based on definably different molecular mechanisms. However, even such widely different genetic processes may result in similar effects on adaptations (either new or transferred), genome evolution, population genetics, and the evolutionary/ecological trajectory of organisms. For example, the evolution of novel adaptations (resulting from lateral gene transfer) leading to the flea-borne, deadly, causative agent of plague from a rarely-fatal, orally-transmitted, bacterial species is quite similar to the adaptations accrued from natural hybridization between annual sunflower species resulting in the formation of several new species. Thus, more and more data indicate that evolution has resulted in lineages consisting of mosaics of genes derived from different ancestors. It is therefore becoming increasingly clear that the tree is an inadequate metaphor of evolutionary change. In this book, Arnold promotes the 'web-of-life' metaphor as a more appropriate representation of evolutionary change in all lifeforms.