 Many of these issues—such as how to understand stabilizing mechanisms and selection, what biological individuals are, whether species are individuals, and whether species-level selection is reducible to individual-level selection—are ongoing. In the decades since Eldredge and Gould first outlined the theory of punctuated equilibria, various biological theorists have attempted to provide definitive and irreducible examples of species-level selection—ones that cannot be recreated at lower levels such as that of the organism. Species-level traits like range size (where a species may be wide-ranging even though its organisms are not; see Jablonski 1987) and rate of speciation (where a species may be slow to “reproduce” although its organisms are not; see Vrba 1987) are especially promising candidates. In addition to the issue of providing definitive examples of irreducible species selection, there is also the issue of providing a satisfactory definition of the concept itself. The biologist Elisabeth Vrba (1984) posits that all proposed definitions of species selection ought to identify causes, be restricted enough for testing, and employ a concept of selection that is consistent with how selection is understood to act at other levels as well. See Vrba (1984), Cracraft (1985), Grantham (1995), Jablonski (2008), and Turner (2011b: chapters 4 and 5) for further discussion of species selection.