Extinction predispositions

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.
            The individual species differ in the width of their ecological valence. Eurytopic species are capable of utilizing a wider range of resources and successfully surviving in various types of habitats; in contrast, stenotopic species specialize on a narrow range of resources and are capable of surviving only in a narrow range of conditions. Comparative studies have shown that stenotopic species have a far greater tendency to become extinct than eurytopic species (Purvis, Jones, & Mace 2000). This is undoubtedly a result of the fact that eurytopic species are capable, when necessary, of reorienting themselves to a different type of resource, are capable of tolerating quite drastic climatic changes and generally occur over a broader area than stenotopic species. As will be mentioned below (XXII.6.3), the size of the geographic range tends to be the decisive factor from the viewpoint of survival of the species.
            At least in some cases, physical proportions affect the probability of survival of a species; specifically they are negatively correlated with the length of existence of a species (Raup 1994). However it is probable that the lower rate of reproduction of large species is important here. This dependence was quite apparent in the Late Pleistocene, although the causes were rather atypical. At that time, primarily large mammals and birds became extinct, specifically species weighing more than 44 kg (Gittleman & Gompper 2001). These species became extinct almost all around the planet over a very short period of time. Simultaneously, this extinction occurred at different times in the individual parts of the world. For example, in North America, this extinction event lasted approximately 200 years, in a period about 10,800 to 11,000 years ago, during which 72% of large animal species and only 10% of small animal species became extinct. This selective extinction was greatest in South and North America, Australia and Madagascar, while Africa and Asia were affected far less. The most probable cause of this extinction, which is also termed Blitzkrieg extinction, was hunting of large animals by humans. The time of disappearance of animals at the individual places correlated very well with the arrival of humans in this territory. The extinction was not so marked in places where settlement occurred gradually or where humans had lived from the very beginning, as the animals apparently had time to adapt to this dangerous predator; in contrast, the extinction was greater in territories that were settled rapidly.
            Greater sensitivity of large animals to extinction was also manifested at other times. It is highly probable that it could be connected with the relatively smaller sizes of the populations of larger organisms. The resistance to extinction is positively correlated with the size of the population and is thus negatively correlated with the size of the members of a particular species.

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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