The increasing maximal hierarchical complexity of organisms is one of the best-supported macroevolutionary trends. The nature and causes of this trend, as well as several accompanying macroevolutionary phenomena, are, however, still unclear. In this theoretical article, we propose that the cause of this trend might be the increasing pressure of species selection, which results from the gradual decrease of (macro)evolutionary potential (i.e. the probability of producing major evolutionary

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... ions). As follows from the Theory of Frozen Evolution, this process is an inevitable consequence of the sorting of genes, traits, and their integrated groups (modules) based on their contextually dependent stability, which causes effectively unchangeable elements of genetic architecture to accumulate during the existence of evolutionary lineages. Although (macro)evolutionary potential can be partially restored by several processes, a profound restoration of (macro)evolutionary potential is probably possible only by means of a transition to a higher level of hierarchical complexity. However, the accumulation of contextually more stable elements continues even on this higher level. This leads to the diminution of the modular character of composite organisms and a repeated pressure to increase the level of hierarchical complexity. Our model explains all components of McShea's "Evolutionary Syndrome," i.e. the trend of increasing the hierarchical complexity of organisms, the growth of variability among elements on the immediately lower level, and their gradual machinification. This pattern should be characteristic of sexual eukaryotes and especially their complex representatives. Our model also sheds new light on several related macroevolutionary phenomena, such as the gradual acceleration of the trend or the striking difference between pre-Neoproterozoic and Phanerozoic evolution.