The ecological niche plays a central role in the process of ‘ecological speciation’, in which divergent selection between niches drives the evolution of reproductive isolation (Muller 1942; Mayr 1947, 1963; Schluter & Nagel 1995; Funk 1998; Schluter 2000). Ecological by-product speciation occurs because ecological traits that have diverged between populations via divergent selection, or traits that are genetically correlated with such traits, incidentally affect reproductive isolation. This process can occur under any geographic arrangement of populations (e.g. allopatry, parapatry or sympatry). A central prediction of ecological speciation is that ecologically divergent pairs of populations will exhibit greater levels of reproductive isolation than ecologically similar pairs of populations of similar age. Another prediction is that traits under divergent selection, or those genetically correlated with them, should often incidentally affect reproductive isolation (e.g. mate preference, hybrid fitness). In recent years, these predictions have been supported in a range of taxa (see Feder et al. 1994; Funk 1998; Via 1999; Rundle et al. 2000; Jiggins et al. 2001; Funk et al. 2002, 2006; Bradshaw & Schemske 2003; Rundle & Nosil 2005; and Funk, this volume, for review), and processes such as resource competition and predation are now known to be involved (Mallet & Barton 1989; Schluter 1994; Rundle et al. 2003; Vamosi 2005; Nosil & Crespi 2006a).