Especiación simpátrica

Review

TRENDS in Ecology & Evolution Vol.16 No.7 July 2001

381

Sympatric speciation in animals: the ugly duckling grows up
Sara Via
Sympatric speciation has become increasingly accepted in the past decade, as a result of new models substantiating its plausibility and new evidence that the conditions specified by the models are met in many natural populations. Retrospective phylogeneticand population genetic signatures of sympatric speciation have also been derived, and these are beginning to be tested. This new work has helped increase the acceptance of sympatric speciation as a plausible process, although it remains difficult to show conclusively that specific pairs of taxa have speciated through sympatric processes alone. It might be time for a re-evaluation of thegeographical classification of speciation modes in favor of one based primarily on evolutionary mechanisms.

Sara Via Dept of Biology and Dept of Entomology, University of Maryland, College Park, MD 20742, USA. e-mail: sv47@umail.umd.edu

Darwin believed that natural selection was responsible for producing the extravagant array of diverse species on ‘the tangled bank’1. However, in the mid-1900s, thefocus of SPECIATION (see Glossary) research shifted away from natural selection as the driving force and towards the role of geography in limiting gene flow and promoting GENETIC DRIFT. The dogma of this geographical view (championed by Mayr and reviewed in Ref. 2) is that populations found sympatrically (‘within cruising range of one another’2) can only escape the homogenizing effects of gene flowunder exceptional circumstances. By contrast, geographically isolated (allopatric) populations can diverge freely or, if they are small, be subject to strong genetic drift, which leads to REPRODUCTIVE ISOLATION. The work of Dobzhansky and Muller in the 1930s–1940s (reviews in Ref. 3 and Turelli et al.4, this issue) provided a ready genetic mechanism for ALLOPATRIC SPECIATION. In geographicallyisolated populations, reproductive isolation is thought to accumulate as a byproduct of independent evolution through the substitution of incompatible alleles, which is hampered by gene flow. This model, allopatric speciation through postzygotic genetic incompatibilities, has been the dominant view of speciation for the past six decades (see also Turelli et al.4, this issue). In this climate,SYMPATRIC SPECIATION has been extremely controversial: embraced and defended by a stalwart group of empiricists5,6, but thought by many to be implausible and of limited applicability7,8. Recently, however, a variety of approaches have been used to re-evaluate the mechanisms that can lead to species formation and to provide additional empirical evidence that sympatric speciation can occur. I reviewtheoretical work that supports the plausibility of sympatric speciation under a variety of conditions (see also Turelli et al.4, this issue). I then discuss evidence that crucial conditions specified by the models have been found in natural populations, and summarize some historical (phylogenetic and

population genetic) signatures of sympatric speciation that have been proposed (see also Barracloughand Nee9, this issue). I argue that the objections to sympatric speciation raised by key papers between 1966 (Ref. 10) and 1981 (Refs 7,8) have been addressed, that a variety of alternative routes to sympatric speciation have been identified, and that evidence is accumulating that conditions favorable to the process are found in a variety of taxa. This work strongly suggests that sympatricspeciation occurs under certain circumstances, vindicating the long-maligned proponents of the process. This recent work raises questions for the next phase of speciation research, and supports a new classification of speciation modes based primarily on mechanisms instead of on geography.
A brief life history of sympatric speciation The ugly duckling is born (1930s–1966) and cast out (1966–1981)...