The closest form of cooperation and the closest interconnection  exists between intracellular endosymbionts, called endocytobionts, and their host organisms or, to be more exact, their cells. When the intracellular endosymbiont begins to be transferred from one host to another solely through the sex cells of the host, its fitness becomes completely dependent on the fitness of that host. This basically ends any further “arms race” between the host species and the endosymbiont, as the selection pressure on the creation of traits permitting an increase in its own fitness at the expense of the other organism completely disappears. From that moment on, it is advantageous for both species to only cooperate and the coevolution of the two species ends with the two species dividing up the individual biological functions so that the chimeric cell formed can work as effectively as possible (see XIX.5.5.1). This division of functions sometimes goes so far that both species eliminate, from their genomes, the genes whose function would  be doubled or even replace their own genes in the genome by the genes of their partner – symbiont. Important organelles of eucaryotic cells, including mitochondria and plastids, evolved through this mechanism, (Margulis 1981). However, extensive results indicate that the endosymbiotic formation of mitochondria and plastids was probably not the first symbiogenetic event in the history of eucaryotic cells. Comparison of phylogenetic trees formed on the basis of various proteins has shown that the actual nucleus of the eucaryotic cell was most probably formed as a chimera through the combination of genomes, probably from two symbionts, one of which was a related gram-negative eubacterium and the other an archaebacterium (Golding & Gupta 1995; Gupta 1998). Study of the metabolic pathways of contemporary organisms and their organelles leads to similar conclusions (Martin & Müler 1998)(Fig. XVIII.3). It could be speculated that it was an increase in the size of pre-eucaryotic cells (by a volume 3-4 orders of magnitude greater than the procaryotic cells) that became a pre-adaptation for the emergence, first of the ability to phagocytose larger particles of food and, in direct connection with this ability, also pre-adaptation for the emergence of endosymbiosis. A series of subsequent endosymbiotic events could finally have led to the formation of modern eucaryotic cells and subsequently of multicellular organisms (Flegr 1990). Such an increase in the size of a eukaryotic cell could be made possible, for example, by the formation of a mechanism that permits overcoming the limitation of the rate of biochemical reactions by the slow rate of diffusion of the individual reactants in the cytoplasm (Flegr 1990).

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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