However, if this effect was indeed present, then speciation was not sympatric but parapatric at least according to some definitions ( Gavrilets & Vose, 2007). The authors emphasized the importance of an environmentally induced shift in flowering for facilitating genetic divergence. Another example is a recent case of proposed sympatric speciation of palms on an oceanic island ( Savolainen et al., 2006). 140–141) argued that the strong selection levied against females that switched habitats made the scenario effectively allopatric by reducing gene flow. For example, regarding Rice & Salt’s (1990) classic experimental demonstration of speciation by habitat selection in Drosophila, Coyne & Orr (2004, pp. The definition of sympatric speciation is important in the interpretation of case studies. 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). Such situations were dubbed ‘microallopatric’ by Smith (1955, 1965), who was dissatisfied with the simple dichotomy between allopatry and sympatry advocated by Mayr (1942, 1963). 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. The problem is most severe in host-specific parasites, phytophagous insects and other situations where ecological differentiation necessarily involves spatial structure. 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, 1963). In fact, some of the disagreement over the prevalence and importance of sympatric speciation rests on disagreement over what sympatric speciation is.
However, sympatric speciation is not always clearly defined, and not all clear definitions describe the same set of phenomena. 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. Sympatric speciation is of great interest to evolutionary biologists, in part because it has been consistently controversial since the inception of our field ( Sulloway, 1979 Mayr, 1982 Bush, 1998 Feder, 1998 Berlocher & Feder, 2002 Coyne & Orr, 2004 Bolnick & Fitzpatrick, 2007), and in part because it challenges us to synthesize ecology, genetics and behaviour when attempting to understand how it might occur in nature. Rather, we believe this context can be better understood by modelling and measuring quantities, such as gene flow and selection, rather than assigning cases to discrete categories like sympatric and allopatric speciation.
We do not deny the importance of geographical context for understanding divergence. 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 than evaluating the biological processes affecting divergence. The most precise definitions make sympatric speciation an infinitesimal end point of a continuum. Definitions based on biogeography do not always produce the same conclusions as definitions based on population genetics. Here, we explore some of the definitions that have been used in empirical and theoretical studies. However, different investigators have used different definitions of sympatric speciation and different criteria for diagnosing cases of sympatric speciation. Sympatric speciation has always fascinated evolutionary biologists, and for good reason it pits diversifying selection directly against the tendency of sexual reproduction to homogenize populations.