XXI.6 The probability of extinction, and thus also the average length of existence of species, varies amongst groups of organisms, depending on their taxonomic affiliation and ecological requirements

As was already mentioned in the part dealing with mass extinction, the members of certain groups of organisms were affected more by extinction in the individual periods of the history of the Earth.The sensitivity of the individual groups differed in the various cases, apparently in dependence on the immediate cause of the particular mass extinction.However, irregardless of the cause of the particular mass extinction, certain long-term trends and long-term patterns can be seen in the sensitivities of the members of the individual groups of organisms to extinction.In general, it can be stated that the probable length of existence of a species differs substantially in dependence on its taxonomic affiliation and its life style.Marine species of mussels and snails exist for an average of 10 – 20 million years.In contrast, the average lengths of the lives of mammal species are far shorter, generally substantially less than 5 million years.Amongst marine invertebrates, plankton species have shorter lifetimes than benthic species.If we ignore the theoretical possibility that a great many extinctions actually correspond to pseudo-extinctions and that the rates of extinctions actually correspond to the differences in the rates of anagenesis or even the convention of taxonomists dealing with the particular group of organisms, the main reason for these differences lies in the unequal probability of extinction of species within the individual groups of organisms.As the most intense competition can be expected for mutually closely related species, it is probable that the frequency of extinction can also be increased by increased frequency of speciation.According to some authors, the very fact that a certain species splits off a daughter species substantially increases the probability of its extinction (Pearson 1998){5302}.In relation to the duration of existence of higher taxa, on the other hand, the species abundance of the taxon will reduce the probability of its extinction and taxa including a greater number of species will, on an average, exist for a longer time.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.