The Evolution And Extinction Of Biodiversity

Evolution, simply speaking, is change through time. In genetic terms, evolution is an alteration in the frequency with which different genes are represented in a population, and it results primarily from the processes of natural selection and random drift. Natural selection operates through differential survival and reproductive success of individuals in a population, which determines their contribution to the genetic composition of the next generation. Natural selection acts on individual phenotypes best suited to the environment.

There has been life on Earth for at least 3.5 billion of the 4.6 billion years that the planet has existed. Multicellular plants and animals have evolved in just the last 1.4 billion years. The earliest fossil insect, or insect relative, is a hexapod, the collembollan Rhyniellapraecursor, from the Lower Devonian (about 380 mya) from Scotland. It is unlikely that insects existed before the Devonian, and there was extensive radiation during the Carboniferous. There are fossils from 300 mya of several nonextinct groups, such as Paleodictyoptera, Meganisoptera, Megasecoptera, and Diaphanopterodea. The only extant orders represented by Carboniferous fossils are Ephemeroptera, Blattodea, and Orthoptera. Orders of modern insects, except Hymenoptera and Lepidoptera, appear to have been established by the Triassic (225 mya), and some of the early groups had disappeared by the late Permian. The massive explosion of insect diversity appears to coincide with that of the flowering plants (angiosperms) in the Cretaceous

Numerous studies have shown that there have been periods of rapid evolution of biodiversity and even more dramatic periods of extinction. Four of the five big episodes of extinction in the last 500 million years of the fossil record saw the removal of approximately 65 to 85% of the animal species in the ocean that are preserved as fossils, and the fifth resulted in the loss of 95% or more. In spite of these huge losses, it is now estimated that through subsequent rapid evolution, the present-day diversity of organisms, at both the species level and higher taxonomic levels, is greater than at any other time. Some suggest that present-day diversity may represent roughly 1% of all the species that have ever existed.

There have been many attempts to estimate the life span of species in the fossil record and these range from 0.5 to 13 million, although a few species present today appear to be unchanged in the fossil record for up to 50 million. Some data suggest that the average life span for species is 4 to 5 million.

The extinction of species, just like the evolution of species, is a natural process, and thus the extinction of existing species should occur at the same time as the evolution of new ones. The current list of all plants and animals that are recognized as having become extinct in the last few hundred years is relatively short. In total this amounts to just 600 plant and 491 animal species, and of these only 72 are insects. It is not surprising therefore that the fate of many thousands of threatened species of insects, other invertebrates, and fungi is almost completely overlooked. The death of the last passenger pigeon, "Martha," in 1914 is well known to many conservation biologists, yet the coextinction of two species of lice (Columbicola extinctus and Campanulotes defectus) that were host specific to this bird went unheralded. Some estimated extinction rates would indicate that most insect species are more likely to become extinct than to be named by taxonomists

Of the 72 species of insects listed on the IUCN's Red Data List as extinct, more than 40 are from Hawaii, and many of the others are from other islands. Proving that a species as small as an insect has become extinct can be very difficult, and indeed one of the largest species of insects that was thought to be extinct, the 15-cm-long Lord Howe Island stick insect (Phasmatodea), was discovered surviving in a remote part of this small island 80 years after its extinction had been declared. Of the insect species that no longer exist, most were driven to extinction by the introduction of other animals such as rats or invasive insects, whereas the demise of most extinct species of birds and mammals resulted from overhunting or loss of habitat.

It seems that there is a genetic or population threshold below which the survival of a species diminishes rapidly. For some species this "minimum viable population" may be 10 individuals and for others, hundreds or thousands. Such species with numbers of individuals below this threshold, the "living dead," although not presently extinct, appear to be doomed to extinction in the near future. A critical factor in the long-term survival of a single species or group of species is the maintenance of the intricate web of interacting species that are important in some way or other for each other's survival. For example, the Brazil nut tree, Bertholletia excelsa, relies on euglossine bees for pollination and seed setting, whereas the bees rely on the availability of other resources in the forest to complete their life cycle. Loss of these resources through forest fragmentation or disturbance could lead to the loss of the bees. The Brazil nut tree, however, might survive for many years before becoming extinct. This is just one example from the continuum of cosurvival of species, from those that are entirely dependent on the existence of one other species to those that are only in part dependent on one or a number of species. In this way, the survival or extinction of species or groups of species is linked to the survival of whole habitats or ecosystems.

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