XXVI.4.3 The rate of evolution of a species is not correlated with the length of the generation period

It is quite natural to expect that the rate of evolution should be greater for organisms with a shorter generation period than for organisms with a longer generation period.However, real data do not confirm this assumption and, in fact, very frequently show quite the opposite.For example, elephants are one of the lines with the greatest taxonomic rate; on the other hand, some lines of small fauna with short generation times evolve very slowly (Simpson 1961).

            Several possible explanations have been proposed for this paradox.One of them is that organisms with a long generation time are simultaneously organisms with large body dimensions and large organisms also have relatively small numbers of individuals in their populations.Because of the greater effectiveness of genetic drift and lower effectiveness of stabilizing selection in small populations, species in small populations can evolve faster (see  IX.2).Another explanation is based on the idea that most evolutionary changes constitute a response of the organisms to a change in the environment (Gillespie 2001).Thus, the rate of evolution is actually limited, not by the ability of the population to respond rapidly to a change in the environment, but by the rate of these changes.In organisms with a long generation time, greater changes occur in the environmental conditions during this generation time and thus the intensity of selection pressures acting on them is greater on an average than for organisms with a shorter generation 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.