8.2.2. Living insect distributions as evidence for antiquity
Evidence from the current distribution (biogeography) confirms the antiquity of many insect lineages. The disjunct distribution, specific ecological requirements, and restricted vagility of insects in a number of genera suggest that their constituent species were derived from ancestors that existed prior to the continental movements of the Jurassic and Cretaceous periods (commencing some 155 mya). For example, the occurrence of several closely related species from several lineages of chironomid midges (Diptera) only in southern Africa and Australia suggests that the ancestral taxon ranges were fragmented by separation of the continental masses during the breakup of the supercontinent Gondwana, giving a minimum age of 120 myo. Such estimates are substantiated by related Cretaceous amber fossil specimens, dating from only slightly later than commencement of the southern continental breakup.
The aphid subtribe Melaphidina, all species of which have complex life cycles involving gall induction on sumac (Rhus) species, has a distribution disjunct between Asia and North America. This biogeographic evidence provides an estimated minimum age for this aphid—sumac association of 48 million years, based on the date of climatically driven vicariance of the Asian and American plant lineages. The possibility of recent dispersal is refuted by appropriately aged fossils of melaphane aphids from both continents.
A similarly intimate relationship shown by the fig—fig wasp association (Box 11.4) has been subjected to molecular phylogenetic analysis for host figs and wasp pollinators. The radiations of both show episodes of colonization and radiation that largely track each other (co-speciation). The origin of the mutualism is dated to c. 90 mya (after Africa separated from Gondwana) with subsequent evolutionary radiations associated with continental fragmentation including the northward movement of India. Disjunctions in mutualistic relationships, such as the above two cases, strongly suggest concerted vicariant distributions, since both partners in the relationship must relocate simultaneously — which is highly unlikely under a dispersal interpretation.
The woodroaches (Cryptocercus) have a disjunct distribution in Eurasia, western USA, and the Appalachians of south-eastern USA where there is cryptic diversity. The species of Cryptocercus are near indistinguishable morphologically, but are distinctive in their chromosome number, mitochondrial and nuclear sequences, and in their endosymbionts. Cryptocercus harbor endo-symbiont bacteria in bacteriocytes of their fat bodies (see section 3.6.5). Phylogenetic analysis of the bacterial RNA sequences shows that they follow faithfully the branching pattern of their host cockroaches. Using an existing estimate of a clock-like model of molecular evolution, dates have been reconstructed for the major disjunctions in woodroach evolution. The earliest branch, the North American/east Asian separation, was dated at 70–115 mya, and the separation of western from eastern North American clades at 53–88 mya. Though doubts always will exist concerning reconstructions based on assumed clock-like change in nucleotides, these patterns evidently are rather old, and did not result from Pleistocene glaciations with recent dispersal. Morphological stasis is evident in the lack of obvious differentiation over this long time period.
Such morphological conservatism and yet great antiquity of many insect species needs to be reconciled with the obvious species and genetic diversity discussed in Chapter 1. The occurrence of species assemblages in Pleistocene deposits that resemble those seen today (although not necessarily at the same geographical location) suggests considerable physiological, ecological, and morphological constancy of species. In comparative terms, insects display slower rates of morphological evolution than is apparent in many larger animals such as mammals. For example, Homo sapiens is a mere 100,000 years old; and if we classify (correctly) humans as morphologically highly derived chimpanzees, then any grouping of humans and the two chimpanzee species is some 5 myo. Perhaps, therefore, the difference from insects lies in mammals having undergone a recent radiation and yet already suffered major extinctions including significant losses in the Pleistocene. In contrast, insects underwent early and many subsequent radiations, each followed by relative stasis and persistence of lineages (see section 8.6).