Sympatric Speciation: Norm or Exception?

Host-specific herbivorous insects have inspired speculation about sympatric speciation at least since the 1860s, when the now-famous host races of the apple maggot were described for the first time. Even Ernst Mayr admitted that “host races [of phytophagous insects] are a challenging biological phenomenon, and constitute the only known case indicating the possible occurrence of incipient sympatric speciation.” Any hypothesis of speciation by natural selection must consider how divergently selected (for example, ecologically adaptive) traits are genetically coupled to (and therefore result in) reproductive isolation. Speciation in allopatry is almost inevitable, given enough time, because all the possible causes of speciation can drive divergence of separated populations, unimpeded by gene flow. Furthermore, allopatric speciation caused by divergent ecological or sexual selection can be as rapid as sympatric or parapatric speciation. This chapter explores Guy L. Bush's sympatric speciation model and how often host-specific insects meet its assumptions. It considers the evidence on the genetics of host utilization, trade-offs, and colonization of novel host plants, and shows that it is meager.