XXVI.7 A similar course of anagenesis within a phylogenetic evolutionary line or within several independent lines is described as an evolutionary trend

Some anagenetic processes, for example, an increase in body dimensions or a reduction in the number of body segments, occur in evolution on a time scale exceeding the existence of the species and tending to approach the length of existence of a phylogenetic line (Simpson 1961).The paleontological record then contains younger and younger species in which the particular anagenetic change is increasingly developed.Such a phenomenon of gradual changes in certain phenotype traits in species of decreasing evolutionary age is termed an evolutionary trend.In some cases, this trend is manifested within a single phylogenetic line while, in other cases, similar changes appear simultaneously in species that are members of different phylogenetic lines.In the extreme case, this can be a general trend that is encountered in most phylogenetic lines, not only during a single time interval, but also in lines that developed during different time periods that did not overlap.The best known evolutionary trend of this type is the gradual increase in body dimensions in the species of a single phylogenetic line.The existence of this trend has been described as Cope’s rule:Within a single phylogenetic line, species gradually increase their body dimensions during their phylogenesis.The existence of this trend has been tested on a number of phylogenetic lines with greater or lesser success.In a great many cases, in which the trend towards an increase in body dimensions could be confirmed, its existence is apparently a result of the wall effect, i.e. the fact that the founder of the line tended to be a small species.In these cases, we sometimes speak of a passive trend.However, in other cases, it seems that this could actually be an active trend and that its existence could be the result of a special, so-far undescribed mechanism.In its original form, Cope’s rule was broader, as it included not only an increase in body size, but also the morphological nonspecialization of the founders of the line and the diversity and specialization of the final forms.Other described evolutionary trends include a gradual decrease in the number of body segments and their mutual differentiation; these trends are described by Williston’s rule.

Apparently some trends are operative only in a certain phase of the existence of a phylogenetic line.For example, a trend in spinization has been described – the formation of numerous long protuberances and barbs in species formed in a later phase of the evolution of a phylogenetic line.Apparently spinization precedes the extinction of a particular line (Rensch 1959).Most similar trends were described only ex post, on the basis of collected material, and are related to only selected parts of phylogenetic lines.Thus, it is not simple to statistically verify their existence.For example, if we discover that our sample contains a trend towards a gradual increase in the number of species with a certain trait that is statistically significant at a level of 1%, then there is still a great probability (approx. 10%) that this could be a random manifestation if we tested the existence of 10 different trends on our sample and only one of them was statistically significant.Of course, if the same trend is demonstrated on another independent set of species in the same line or if the same trend is manifested in different phylogenetic lines, the existence of this trend will be far more probable.

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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.