Trends in Genetics
ReviewThe genomics of organismal diversification illuminated by adaptive radiations
Section snippets
Outbursts of life
In adaptive radiation and in every part of the whole, wonderful history of life, all the modes and all the factors of evolution are inextricably interwoven. The total process cannot be made simple, but it can be analyzed in part. (G.G. Simpson, 1953 [1])
The history of life is a cumulative story of evolutionary radiations (see Glossary); such outbursts of unprecedented forms are thought to be responsible for much of the extant and extinct organismal diversity on Earth 1, 2, 3, 4. For example,
Key elements of ARs
AR is defined as the rapid diversification of an organismal lineage into an array of closely related species as a consequence of adaptation to different ecological niches 1, 2, 3 (a more comprehensive characterization is provided in Box 1). Closer inspection of such bouts of diversification reveals two important elements. The first is the generation of unique, distinct phenotypes specialized to different ecological niches – divergent evolution. This aspect of diversification is epitomized by
Genomic determinants of AR
It has long been recognized that the question why some groups of organisms diversify more extensively than others is tightly connected to the pivotal concept of ecological opportunity: ARs are triggered by access to novel or hitherto under-utilized ecological niches 1, 2, 12, sometimes in combination with sexual selection [22]. The answer to the question about the determinants of organismal diversification is thus partly ecological and also includes historical contingency 14, 23, but will
The genetic basis of phenotypic divergence in ARs
Ever since G.G. Simpson [1] emphasized the importance of ‘distinctive new adaptive types’ in triggering adaptive radiation, the identification of such evolutionary (key) innovations has been a major focus of adaptive radiation research. With progress in molecular genetic tools, emphasis has partly shifted to investigations of the molecular basis of adaptive divergence – an endeavor of course not restricted to ARs. The promise is to gain a general understanding of several longstanding issues in
Concluding remarks
In this review we have described the potential of molecular investigations in ARs to improve our understanding of the mechanisms underlying biological diversification. One of the major insights that has emerged is that rapid and extensive diversification and convergent evolution is facilitated by the diffusion of ancient adaptive variation, thus blurring the distinction between incomplete lineage sorting, standing genetic variation, and gene flow via introgressive hybridization. Certainly, a
Acknowledgments
We thank the Editor R. Macrae for the invitation and for comments; C. Jiggins plus an anonymous reviewer and the members of the laboratory of W.S. for discussion and comments on the manuscript; and J. Losos for providing photographs. This work was supported, in part, from grants from the Swiss National Science Foundation (SNF) to D.B. and W.S., and from the European Research Council (ERC, CoG ‘CICHLID∼X’) to W.S.
Glossary
- Divergence mapping
- marker-based search for genome regions exhibiting exceptionally strong differentiation (typically quantified by the fixation index FST) between ecologically-different populations as a consequence of divergent natural selection on specific loci.
- dN/dS ratio
- index combining the frequency of non-synonymous (i.e., amino acid changing) and synonymous (i.e., not amino acid changing) nucleotide substitutions across a gene to explore the type of selection that has acted on the gene.
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