Sympatric speciation, the evolution of reproductive isolation without geographic barriers, remains highly contentious. As a result of new empirical examples and theory, it is now generally accepted that sympatric speciation has occurred in at least a few instances, and is theoretically plausible. Instead, debate has shifted to whether sympatric speciation is common, and whether models’ assumptions are generally met in nature. The relative frequency of sympatric speciation will be difficult to resolve, because biogeographic changes have obscured geographical patterns underlying many past speciation events. In contrast, progress is being made on evaluating the empirical validity of key theoretical conditions for sympatric speciation. Disruptive selection and direct selection on mating traits, which should facilitate sympatric speciation, are biologically well supported. Conversely, costs to assortative mating are also widely documented, but inhibit speciation. Evaluating the joint incidence of these key factors may illuminate why sympatric speciation appears to be relatively uncommon.

The importance of geographic isolation in speciation has been debated since the 19th century. Since the beginning of the 20th century, the consensus has been that most speciation involves divergence in allopatry. This consensus was based largely on decades of observations by naturalists and verbal arguments against speciation without isolation. Recent attempts to quantify the importance of allopatric versus sympatric speciation using comparative methods called "age-range correlation" (ARC) suggest that allopatric speciation is more common than sympatric speciation. However, very few taxa have been studied and there are concerns about the adequacy of the methods. We propose methodological improvements including changes in the way overlap between clades is quantified and Monte Carlo methods to test the null hypothesis of no relationship between phylogenetic relatedness and geographic range overlap. We analyze 14 clades of mammals, chosen because of the availability of data and the consensus among mammalogists that speciation is routinely allopatric. Although data from a few clades clearly indicate allopatric speciation, divergence with gene flow is plausible in others and many results are inconclusive. The relative rarity of significant correlations between phylogenetic distance and range overlap may have three distinct causes: (1) post-speciation range changes, (2) relative rarity of range overlap, and (3) a mixture of geographic modes of speciation. Our results support skepticism about ARC's power for inferring the biogeography of speciation. Yet, even if few clades provide clear signals, meta-analytic approaches such as ARC may set bounds on the prevalence of alternative modes of speciation.

When introduced or cultivated plants or animals hybridize with their native relatives, the spread of invasive genes into native populations might have biological, aesthetic, and legal implications. Models suggest that the rate of displacement of native by invasive alleles can be rapid and inevitable if they are favored by natural selection. We document the spread of a few introduced genes 90 km into a threatened native species (the California Tiger Salamander) in 60 years. Meanwhile, a majority of genetic markers (65 of 68) show little evidence of spread beyond the region where introductions occurred. Using computer simulations, we found that such a pattern is unlikely to emerge by chance among selectively neutral markers. Therefore, our results imply that natural selection has favored both the movement and fixation of these exceptional invasive alleles. The legal status of introgressed populations (native populations that are slightly genetically modified) is unresolved by the US Endangered Species Act. Our results illustrate that genetic and ecological factors need to be carefully weighed when considering different criteria for protection, because different rules could result in dramatically different geographic areas and numbers of individuals being protected.conservation | California Tiger Salamander | genetics | hybridization | Ambystoma

in part because it challenges us to synthesize ecology, genetics and behaviour when attempting to understand how it might occur in nature. Both proponents and skeptics agree on the importance of understanding the contexts and processes influencing the likelihood of sympatric divergence, and of identifying the kinds of evidence necessary to diagnose individual case studies. However, sympatric speciation is not always clearly defined, and not all clear definitions describe the same set of phenomena. In fact, some of the disagreement over the prevalence and importance of sympatric speciation rests on disagreement over what sympatric speciation is. Such arguments do little to advance the study of evolution, and we advocate research aimed at understanding mechanisms of divergence, rather than classifying cases into a taxonomy of 'modes of speciation ' (sensu Mayr, 1942' (sensu Mayr, , 1963.The problem is most severe in host-specific parasites, phytophagous insects and other situations where ecological differentiation necessarily involves spatial structure. Differentiated populations, 'host races' or descendant species that occur in different, discrete habitat patches may have broadly overlapping geographical ranges and yet never encounter one another at the same time and place because of their distinct ecological niches. Such situations were dubbed 'microallopatric' by Smith (1955Smith ( , 1965, who was dissatisfied with the simple dichotomy between allopatry and sympatry advocated by Mayr (1942Mayr ( , 1963. Several recent authors have raised concerns over conceptual and evidential confusion between 'microallopatry' and 'sympatry ' (Berlocher & Feder, 2002;Dres & Mallet, 2002;Dieckmann & Doebeli, 2004;Provine, 2004;Mallet, 2005).The definition of sympatric speciation is important in the interpretation of case studies. For example, regarding Rice & Salt's (1990) classic experimental demonstration of speciation by habitat selection in Drosophila, Coyne & Orr (2004, pp. 140-141) argued that the strong selection levied against females that switched habitats made the allopatry; biogeography; migration; parapatry; population genetics; selection; speciation; sympatry. AbstractSympatric speciation has always fascinated evolutionary biologists, and for good reason; it pits diversifying selection directly against the tendency of sexual reproduction to homogenize populations. However, different investigators have used different definitions of sympatric speciation and different criteria for diagnosing cases of sympatric speciation. Here, we explore some of the definitions that have been used in empirical and theoretical studies. Definitions based on biogeography do not always produce the same conclusions as definitions based on population genetics. The most precise definitions make sympatric speciation an infinitesimal end point of a continuum. Because it is virtually impossible to demonstrate the occurrence of such a theoretical extreme, we argue that testing whether a case fits a particular definition is less informative...

Cave organisms occupy a special place in evolutionary biology because convergent morphologies of many species demonstrate repeatability in evolution even as they obscure phylogenetic relationships. The origin of specialized cave-dwelling species also raises the issue of the relative importance of isolation vs. natural selection in speciation. Two alternative hypotheses describe the origin of subterranean species. The 'climate-relict' model proposes allopatric speciation after populations of cold-adapted species become stranded in caves due to climate change. The 'adaptive-shift' model proposes parapatric speciation driven by divergent selection between subterranean and surface habitats. Our study of the Tennessee cave salamander complex shows that the three nominal forms (Gyrinophilus palleucus palleucus, G. p. necturoides, and G. gulolineatus) arose recently and are genealogically nested within the epigean (surface-dwelling) species, G. porphyriticus. Short branch lengths and discordant gene trees were consistent with a complex history involving gene flow between diverging forms. Results of coalescent-based analysis of the distribution of haplotypes among groups reject the allopatric speciation model and support continuous or recurrent genetic exchange during divergence. These results strongly favour the hypothesis that Tennessee cave salamanders originated from spring salamanders via divergence with gene flow.