| Comments |
Periodical cicadas (Magicicada), with their unusual life-history mix of long prime-numbered life cycles, synchronized multispecies emergences every 13 or 17 years, and dependence on predator satiation via high adult population densities, present an especially intriguing speciation problem (Williams and Simon 1995). Each of the seven species is most closely related to a geographically adjacent counterpart with the alternative life cycle, suggesting that life-cycle changes may contribute to speciation in the genus. Allochrony, or isolation in time, has been of interest in speciation theory as a means by which populations may become genetically isolated without geographic isolation. However, to be plausible, allochronic speciation scenarios for Magicicada must account for the apparently ancient dependence on predator satiation—emergences of periodical cicadas appear unlikely to survive avian predators unless they number many thousands per hectare (Lloyd and Dybas 1966a Karban 1982 Williams et al. 1993). Three 17-year species coexist in near-perfect sympatry and synchrony across the northeastern United States to the Great Plains. Two of these 17-year species (M. cassini and M. septendecula) each have one closely related 13-year counterpart (M. tredecassini and M. tredecula, respectively) (Fig. 1) these 13-year relatives are found in sympatry and synchrony across the southern and midwestern states. The remaining 17-year species (M. septendecim) has two close relatives (M. tredecim and M. neotredecim) which coexist in synchrony with the other 13-year forms, but in only partial sympatry with one another, overlapping along a 50–150-km zone reaching from southern Indiana to northern Arkansas. The 13- and 17-year species groups meet with almost no overlap along a wandering line reaching from coastal Maryland to eastern Oklahoma. Along this line, adjacent populations emerge in synchrony every 221 years. See BNID 101490 |