8.6. Insect diversification
An estimated half of all insect species chew, suck, gall, or mine the living tissues of higher plants (phytophagy), yet only nine (of 30) extant insect orders are primarily phytophagous. This imbalance suggests that when a barrier to phytophagy (e.g. plant defenses) is breached, an asymmetry in species number occurs, with the phytophagous lineage being much more speciose than the lineage of its closest relative (the sister group) of different feeding mode. For example, the tremendous diversification of the almost universally phytophagous Lepidoptera can be compared with that of its sister group, the relatively species-poor, non-phytophagous Trichoptera. Likewise, the enormous phytophagous beetle group Phytophaga (Chrysomeloidea plus Curculionoidea) is overwhelmingly more diverse than the entire Cucujoidea, the whole or part of which forms the sister group to the Phytophaga. Clearly, the diversifications of insects and flowering plants are related in some way, which we explore further in Chapter 11. By analogy, the diversification of phytophagous insects should be accompanied by the diversification of their insect parasites or para- sitoids, as discussed in Chapter 13. Such parallel species diversifications clearly require that the phytophage or parasite be able to seek out and recognize its host(s). Indeed, the high level of host-specificity observed for insects is possible only because of their highly developed sensory and neuromotor systems.
An asymmetry, similar to that of phytophagy compared with non-phytophagy, is seen if flightedness is contrasted to aptery. The monophyletic Pterygota (winged or secondarily apterous insects) are vastly more speciose than their immediate sister group, the Zygentoma (silverfish), or the totality of primitively wingless apterygotes. The conclusion is unavoidable: the gain of flight correlates with a radiation under any definition of the term. Secondary aptery occurs in some pterygotes — amongst many isolated species, some genera, and in all members of the Phthiraptera (parasitic lice) and Siphonaptera (fleas), two small orders showing an ectoparasitic radiation. The Phthiraptera are less diverse than their flighted sister group (part or all of the Psocoptera), although the Siphonaptera are more speciose than their likely sister group (part or all of the Mecoptera). Many Phasmatodea are flightless, and there are indications that the order originated from unflighted ancestors. Radiation within the phasmids thus seems to have included sporadic regain of wings. This seemingly anomalous hypothesis deserves further study, not least regarding how developmental pathways regulating wing development function.
Flight allows insects the increased mobility necessary to use patchy food resources and habitats and to evade non-winged predators. These abilities may enhance species survival by reducing the threats of extinction, but wings also allow insects to reach novel habitats by dispersal across a barrier and/or by expansion of their range. Thus, vagile pterygotes may be more prone to species formation by the two modes of geographical (allopatric) speciation: small isolated populations formed by the vagaries of chance dispersal by winged adults may be the progenitors of new species, or the continuous range of widely distributed species may become fragmented into isolates by vicariance (range division) events such as vegetation fragmentation or geological changes.
New species arise as the genotypes of isolated populations diverge from those of parental populations. Such isolation may be phenological (temporal or behavioral) as sympatric speciation, as well as spatial or geographical, and host transfers or changes in breeding times are documented better for insects than for any other organisms.
The Endopterygota (see section 7.4.2) contains the numerically large orders Diptera, Lepidoptera, Hymenoptera, and Coleoptera (section 1.3). An explanation for their success lies in their metamorphosis, discussed in detail above, which allows the adult and larval stages to differ or overlap in phenology, depending upon timing of suitable conditions. Alternative food resources and/or habitats may be used by a sedentary larva and vagile adult, enhancing species survival by avoidance of intraspecific competition. Furthermore, deleterious conditions for some life-history stages, such as extreme temperatures, low water levels, or shortage of food, may be tolerated by a less susceptible life-history stage, for example a diapausing larva, non- feeding pupa, or migratory adult.
No single factor explains the astonishing diversification of the insects. An early origin and an elevated rate of species formation in association with the angiosperm radiation, combined with high species persistence through time, leave us with the great number of living species. We can obtain some ideas on the processes involved by study of selected cases of insect radiations in which the geological framework for their evolution is well known, as on some Pacific islands.