Evolution of the Insects

CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 646 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: All the past these mountains saw All the years of toil and strife Lives unknown that went before They gave us this: Our present life. –T. D. A. Cockerell, 1927 15EEpiloguepilogue ton also allows insects to defend themselves with potent WHY SO MANY INSECT SPECIES? toxins without poisoning themselves. This question has occupied entomologists for centuries, and 2. Metamerism and repetitive pairs of appendages allowed some of the explanations we offer are unfortunately little dif- specialization of some appendages while retaining the ferent from previous ones. New knowledge of phylogeny and original functions of others. Much the way gene duplica- discoveries of fossils over the past several decades, however, tion has allowed the divergence of redundant elements, have improved our understanding. Simply put: there is no redundant appendages led to divergent adaptations. The single answer to the evolutionary and ecological success of forelegs of various insects, for example, have become rap- insects, but is due instead to various intrinsic features of torial or fossorial while the mid and hind legs retained the insects. ability for walking. Even more significant are the mouthparts. Virtually every element within the major sets of paired mouthpart appendages – the mandibles, maxillae, and labium – have been modified in some form to suit AGE diverse diets optimally (Figures 8.2, 12.24, 13.9). Thin adult Insects are among the earliest terrestrial animals, and the mouthparts that siphon fluids, for example, evolved in recent discovery that Rhyniognatha from the Early Devonian Palaeodictyopterida, Thysanoptera, Hemiptera, anoplu- was an insect (perhaps even a pterygote!) indicates that they ran lice, Lepidoptera, many Diptera, bees, a few extinct evolved in the Silurian. As terrestrial ecosystems evolved, Mecoptera, and even a few Coleoptera and Trichoptera. insects were on the scene ready to exploit the new The mouthpart elements in each kind of proboscis resources. Major radiations of insects can be related in the evolved differently, illustrating the unique capacity in fossil record, for example, to the evolution of vascular plants, insects to develop different solutions to the same prob- to angiosperms, and (for ectoparasitic and blood-feeding lem. The evolution of mouthparts probably facilitated the groups) to birds and mammals. evolution of phytophagous diets in insects (Labandeira, 1997). 3. Flight greatly improves the ability to escape from preda- DESIGN tors and allows aerial reconnaissance of food and mates. Consider, for example, an early apterygote and pterygote Design is traditionally the main explanation for the evolu- feeding on the sporangia at the tips of a Carboniferous tionary success of insects. No single innovation accounts for plant. The apterygote must climb among the branches the success of insects, but rather there has been a cascade of and down the plant, the pterygote can flit among innovations, each of which was refined later in the evolution branches and plants, more easily finding sporangia and of insects. better selecting the most productive and nutritive ones. 1. The first of these was clearly the jointed, arthropod The ability to fold wings over the abdomen – neoptery – exoskeleton, which provided insects with the physical pro- allowed insects to invade small, cryptic spaces while pro- tection and skeletomuscular strength to live on land. tecting their best means of dispersal. Associated with this is the tracheal system of respiration, 4. The origin of the larva allowed holometabolans to acceler- which with an exoskeleton allowed animals with such ate the development of immatures, to facilitate diapause large ratios of surface area to volume to reduce desicca- during periods of stress, and to exploit diets different from tion in the terrestrial environment. A protective exoskele- the adult. A separation of adult and larval diets may have 646 CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 647 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: EPILOGUE 647 allowed the less mobile and more vulnerable immatures LOW RATES OF NATURAL EXTINCTION to invade cryptic spaces and also escape predators and Evidence for this comes primarily from the fossil record. parasitoids that attack the adults. Insects show minor to negligible effects on mass extinctions It must be stressed that, again, no one of these innovations that decimated major groups of organisms, the best evidence are responsible for the success of insects. Flight probably for which is from the Cretaceous-Tertiary extinctions. The would not have evolved if metamerism, serial appendages, only significant example for the extinction of major mono- and an arthropod exoskeleton did not evolve first. Neoptery phyletic insect groups are the Palaeodictyopterida, Caloneu- then further improved wings. rodea, and Miomoptera, presumably at the End Permian Event, which was a crisis that was unmatched in evolutionary history. The combination of low extinction rates and high specia- CAPACITY FOR HIGH SPECIATION RATES tion rates, beginning well before 400 million years ago, resulted in a present day accrual of insect species that is Evidence from the fossil record indicates that speciation rates unprecedented in the 3 billion years of life on earth. The low in insects can be considerably greater than what is known in extinction rate itself is related to several highly adaptive another terrestrial group of animals, the mammals. For designs of the insect body plan, each of which evolved in example, two major lineages of phytophagous insects, the response to changes that occurred over thousands to millions Lepidoptera and phytophagan beetles, comprise approxi- of years. The environmental changes that have taken place mately 250,000 Recent species. Because the great proportion over merely the past century, however, are proving to be the of these feed on angiosperms, it is reasonable to estimate that greatest evolutionary challenge to all organisms, even to ones over the past 100 million years there has arisen on average as adaptable as insects. two or three species every 1,000 years in these groups. This is a gross underestimate because it does not take into account the total diversity of extinct lepidopterans and phytophagans over the last 100 million years, which is impossible at present THE FUTURE to estimate, nor the higher numbers of total Recent species (described and undescribed). Note that this is not the actual People commonly think that because no species of mosquito, time it takes two populations to diverge into separate species roach, or other pest has been eradicated, that all insects will (evidence from the Hawaiian fauna shows that speciation easily thrive amidst the environmental destruction we are probably takes on the order of 100,000 to 300,000 years, pos- inflicting on our planet. There is probably no single greater sibly shorter), but it does reveal the remarkable rate of prolif- misconception about insects. Some insects are indeed tena- eration of insects. cious pests, but more than 99.99% of insect species are highly Traditional explanations as to why insects are intrinsically sensitive to the typical threats of pollution, pesticides, inva- capable of such high speciation rates have typically cited sive species, and habitat loss (Collins and Thomas, 1991; their short generation times, the reasoning being that with Samways, 1994; Pullin, 1995; Deyrup, 2001). A small percent- short generation times more mutations accrue and allow age of insects are endangered, and some have become faster genetic divergence. However, the excellent fossil record extinct, though the number is without doubt a gross underes- of proboscideans shows that these mammals have a high timate. The International Union for the Conservation of speciation rate, and their generation times are on the order of Nature (IUCN) has listed that over the past 100 years 600 about 50 years. Rather, it is the stunning rate of increase in plant and 491 animal species have become extinct as a result insects that probably provides the genetic grist for their evo- of humans, only 72 of which are insects. Forty red-listed lutionary mill; this is a quotient of the reproductive capacity insect species alone are from Hawaii and other remote of a species and average generation time. It has been deter- islands, faunas of which can collapse under the effects of mined, for example, that if Drosophila melanogaster lays 100 introduced rats, pigs, goats, mongoose, ants, and vespid viable eggs per female, and that half of these produce females wasps. Bermuda, for example, is a relatively depauperate that do the same, the exponential rate of increase is so great island, and an intensive study of its fauna found 258 species (taking into account the deaths of each generation) that by of Diptera, 17 of which are probably endemics and 7 of these the end of the twenty-fifth generation the ensuing mass of are possibly extinct because they have not been seen in years flies would be much larger than the earth itself. Obviously, (Woodley and Hilburn, 1994). None of those seven species there is a tremendous amount of mortality, but the actual was IUCN listed. Documenting the hemorrhaging of insect populations of fruitflies vastly outnumber (and probably out- biodiversity is fraught with difficulties. The large, flightless weigh) terrestrial vertebrates, and thus generate higher Lowd Howe Island stick insect, Dryococelus australis, for genetic variation. example, was recently discovered alive, some 80 years after it CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 648 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files: 648 EVOLUTION OF THE INSECTS was thought to be extinct. If a creature as conspicuous as this evades notice, what about the great majority of insect species, many of which have not even been named yet? The death of the last passenger pigeon at the Cincinnati Zoo in 1914, for example, is well known, and with it were purport- edly of two species of lice, Columbicola extinctus and Cam- panulotes defectus (these lice were later found on other species of pigeons).

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