Letters to Nature

Nature 421, 259-264 (16 January 2003) | doi:10.1038/nature01274; Received 12 July 2002; Accepted 18 October 2002

Speciation along environmental gradients

Michael Doebeli1 & Ulf Dieckmann2

  1. Departments of Zoology and Mathematics, University of British Columbia, Vancouver, Canada V6T 1Z4
  2. Adaptive Dynamics Network, International Institute for Applied Systems Analysis, A-2361 Laxenburg, Austria

Correspondence to: Michael Doebeli1 Correspondence and requests for materials should be addressed to M.D. (e-mail: Email: doebeli@zoology.ubc.ca).

Traditional discussions of speciation are based on geographical patterns of species ranges1, 2. In allopatric speciation, long-term geographical isolation generates reproductively isolated and spatially segregated descendant species1, 3. In the absence of geographical barriers, diversification is hindered by gene flow1, 3, 4. Yet a growing body of phylogenetic and experimental data suggests that closely related species often occur in sympatry or have adjacent ranges in regions over which environmental changes are gradual and do not prevent gene flow5, 6, 7, 8, 9, 10, 11, 12, 13, 14. Theory has identified a variety of evolutionary processes that can result in speciation under sympatric conditions15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, with some recent advances concentrating on the phenomenon of evolutionary branching18, 23, 24, 25. Here we establish a link between geographical patterns and ecological processes of speciation by studying evolutionary branching in spatially structured populations. We show that along an environmental gradient, evolutionary branching can occur much more easily than in non-spatial models. This facilitation is most pronounced for gradients of intermediate slope. Moreover, spatial evolutionary branching readily generates patterns of spatial segregation and abutment between the emerging species. Our results highlight the importance of local processes of adaptive divergence for geographical patterns of speciation, and caution against pitfalls of inferring past speciation processes from present biogeographical patterns.