It is striking that translocation and, in fact, other chromosomal restructuring rarely affect the genes on the sex X-chromosomes of mammals (termed Ohno’s rule (Ohno 1967).)Apparently, such restructuring, and especially translocation of genes between sex chromosomes and autosomes could interfere in the mechanism of determining sex based on the gene dose (number of copies of the given gene in the diploid cell) or in the mechanism of compensation of the gene dose (Kelley & Kuroda 1995).
Theory of neutral evolution
see Evolutionary constraints
Ontogenesis (individual development) is a process, in which originally single-cell zygotes develop into the body of a multicellular organism through complicated developmental processes. In addition to genetic processes, epigenetic processes are also of substantial importance in the development of a multicellular organism.
Another step in the evolution of behavior-controlling mechanisms is creating useful behavioral patterns through operant conditioning based on inner motivation. The organism’s motivation should be seen as a particular physiological state of the organism, not as an abstract term describing heading towards a goal. The basis for a new behavioral pattern is not the development of one of the many existing specific behavioral patterns, whose trigger stimulus would be connected with other outer time or locally associated stimuli. It consists in strengthening of those behavioral patterns that the organism has found to be connected to a specific pleasant inner stimulus (Lorenz et al. 1974). Specifically, this entails behavioral patterns that evoke a feeling of pleasure or inhibit an unpleasant feeling of distress. Different stimuli coming through the organism’s senses are continuously transformed into a common pleasure-distress “currency”; this simplifies and makes more effective the creation and strengthening of momentarily useful behavioral patterns necessary for the survival of the organism. Transformation of the outer stimuli into the inner common currency enables the organism to free itself from the constraints of its material world. If – from the point of view of the fitness of the individual – it is advantageous to seek a particular objectively unpleasant stimulus, e.g. one that is usually followed by an injury, biological evolution can “program” the members of the species to a certain form of “masochism”; the objectively unpleasant stimulus will be perceived as pleasant in the particular situation (see examples of passive cannibalism in some kinds of arthropod males during mating) (Fedorka & Mousseau 2002).
Behavioral regulation through the above-described pleasure-distress mechanism can be compared to regulation by a proportional regulator, as the intensity of the output signal (e.g. the feeling of delight) is proportional to the intensity of the input signal – stimulus coming from the surroundings. In behavior control, a similar effect can be achieved through regulation by a derivation regulator (the intensity of the output signal is proportional to the fall or rise of the intensity of the input signal) and integration regulator (the intensity of output signal is proportional to the duration (and usually the intensity too) of the input signal) (Fig. XVI.2). Integration regulators can be used to control the spontaneous activity of organisms. If there is a prolonged lack of incoming stimuli, a phenomenon we can call “charging the boredom condenser” may occur. If the unpleasant feeling of boredom is too strong, the animal will try to discharge the “boredom condenser”, for example by playing. Play is – amongst other things – a highly effective way of testing new behavioral patterns. The patterns that have been shown to be effective for an individual with a particular phenotype in its usual environment can later be included into a behavioral repertoire of the individual.
Two genetic codes are required for proteosynthesis (deDuve 1988): the universal genetic code, which determines the key according to which the sequence of triplets in mRNA will be translated into an aminoacid sequence in the protein, and the operational RNA-code, which determines which aminoacid will be “charged” by a particular tRNA. While the universal genetic code is so regular that it tends to recall the more perfect Morse code alphabet, the operational genetic code is apparently far less regular and also less “universal”, especially in the structure according to which the enzymes of the aminoacyl-tRNA-synthetase differentiates the individual tRNA (Schimmel 1989; de Pouplana & Schimmel 2001) (Fig. X.7). From this point of view, this code recalls any other product of biological evolution. As a greater problem is presented by the evolution and development of the universal genetic code than the evolution and development of the operational genetic code, we will further consider only the evolution of the universal code (in the spirit of the “best” traditions of evolutionary biology).
Evolution is capable of creating useful structures and patterns of behaviour. However, in contrast to human beings, it is not capable of predicting and planning ahead. This inadequacy of biological evolution, its certain “short-sightedness” and opportunism is manifested in various ways. While a human being can estimate how a product that he intends to make should look and is capable of modifying his approach for this purpose, evolution works completely mechanically, entirely on the basis of the momentary conditions. Thus, it sometimes gets into blind alleys or creates quite strange, not very useful structures.
A classical example of the consequent “evolutionary tinkering” consists in the eyes of vertebrates. In the eyes of vertebrates, the nerve fibres leaving the light-sensitive cells lead to the brain through the retina, so that the light impinging on the retina must first pass through a layer of these fibres. In addition, the fibres themselves must pass through the retina to the other side at a certain point, leading to the existence of blind spots – sites on the retina that are incapable of receiving optical signals. The eyes of cephalopods don’t have such a structural drawback; the optical fibres are located behind the retina and can thus achieve better optical parameters. It is probable that, for the originally imperfect eye containing only a few light-sensitive cells, it made no difference from a functional standpoint whether the nerve fibres were in front of the retina or behind it. Only after the retina became larger and the density of the light-sensitive cells increased did the disadvantages of the original structural design become apparent. However, at that time, a change in the anatomy of the vertebrate eye would require such fundamental changes in its ontogenesis that it was practically impossible.
Organisms adjust of the frequency of mutations through a number of mechanisms, mostly connected with the intensity of repair processes, some of which are even capable of effectively reacting to changes in the environment (Echols & Goodman 1991). For example, if a population of bacteria finds itself in a stressing situation and is in danger of extinction, for example after transfer to an environment with an abnormal temperature, the bacteria undergo an SOS reaction. The individual bacteria begin to mutate faster, increasing the probability that a mutant that will be resistant to this stress factor will occur in the population (Taddei et al. 1997). It happens very frequently that turning off the process of repair of unpaired bases (the mismatch repair system) contributes to increasing the frequency of mutations. Turning off this process in bacteria increases the probability of interspecies recombination by more than an order of magnitude (Velkov 2002).
Experimental results have shown that evolution has “tuned” the ratio of the repair and replication activities of DNA-polymerase in the individual groups of organisms to achieve the optimum frequency of mutations from an evolutionary and functional standpoint (Cox 1976).The consequent frequency of mutations is not very high, so that the organisms and populations are not exposed to an excessive mutation burden, but not too low, so that species do not stagnate evolutionarily and can adapt to changing conditions.Experimentally, it is possible to select a line of bacteria with a much lower mutation rate (Radman, Taddei, & Matic 2000).
It is interesting and very important from a theoretical standpoint that the number of nonsynonymous substitutions/genomes related to the generation time is very similar for the most varied groups of organisms (Drost & Lee 1995)and is not related to the size or metabolic activity of the particular species.As the number of synonymous substitutions/genomes related to the generation time or the number of nonsynonymous substitutions/genomes related to the number of divisions or to the time, to the contrary, differs substantially for various types of organisms (Bromham, Rambaut, & Harvey 1996), it is probable that the present-day frequency of mutations was optimized from the standpoint of the rate of evolution and not from the standpoint of (energetic) “costs” and “profits” (Sniegowski et al. 2000). It is most certainly not determined only by physical or chemical laws valid for DNA replication, for example, the number of tautomeric transitions.
Comparative studies performed at the intraspecific and interspecific level often demonstrate that substantial development of a certain organ is frequently accompanied by the reduction of other organs. In some cases, the reasons for this organ competition are quite obvious. For purely spatial reasons, the males of certain species of beetles of the Scarabaeidae family cannot simultaneously have enormous protuberances on their heads, which they use in battles for females, and eyes of the same size as the females or as males with small protuberances (Emlen 2001). In other cases, the reasons for application of the “trade-off” principle are less apparent and organ competition can occur during allocation of resources during individual development (Wolf et al. 2001). The trade-off principle, applicable in inter-organ competition, can substantially contribute to overall diversity in nature. The individual species necessarily differ in a greater number of traits, where it is probable that species with certain more advanced organs will, on the other hand, have other organs that are poorly developed and suboptimal from a functional viewpoint. This reduces the probability that a certain species would force out all its competitors in a particular environment through its attained level of anagenesis.
see Baldwin effect
Organisms are usually being characterized as the systems with irritability andmetabolism. However, theirritability, i.e. the ability to receive signalsfrom the external environment, is exhibited by a great many nonliving systems, such as the security devices in cars or a central heating regulator. Again, the ability of metabolism, material conversion, is exhibited by a great many chemical dissipation systems. The only actually unique property of living systems, i.e. organisms (including the viruses), remains the capability of biological evolution.At the same time, it is quite probable that this is simultaneously a necessary and sufficient condition. It can be assumed that any system capable of undergoing biological evolution, whatever its physical nature, will sooner or later develop into a living system, i.e. also acquire the other types of traits encountered in contemporary organisms.
The capability of “biological evolution” must be defined in a manner so that we will avoid unacceptable circular definitions. Actually, the ability to undergo biological evolution overlaps to a certain degree with the ability to undergo natural selection (the source of usefulness). The ability to undergo biological evolution is a property consisting of a number of individual components. Only sufficiently complex systems, capable of undergoing natural selection, i.e. containing mutually competing elements capable of reproduction, variability and inheritance, can (and probably will) become a subject of biological evolution.
The Rh-blood group system in humans is also a typical case. Rh-positive people carry the immunodominant protein RhD with D-antigen (combination of molecular sections recognized by anti-RhD-antibodies) on their red blood cells. However, a substantial part of the European population is Rh-negative, i.e. both alleles of the relevant RHD-gene are nonfunctional or altered in Rh-negative people, so that this protein does not appear on their surfaces or the D-antigen (epitope) is missing on it. The function of the protein is not known; however, its structure suggests that it functions as a membrane transporter or rather co-transporter of ammonia or CO2 ions (Kustu & Inwood, 2006; Biver et al., 2006). With the exception of haemolytic diseases of Rh-positive babies born to Rh-negative mothers, until 2008, no effect of Rh-positivity or negativity on the health or any other properties of human beings has been described. The results of three independent studies performed on blood donors, soldiers undergoing compulsory military service and university students indicated, however, that there are very substantial differences between Rh-positive and Rh-negative persons in the rate of reaction to simple stimuli and especially that their reaction rate changes following infection by the parasite Toxoplasma gondii (Novotná et al. 2008, Flegr et al. 2008). It was found that, amongst uninfected men, Rh-negative individuals react much faster than Rh-positive individuals. However, the ability to react rapidly to a stimulus decreases in Rh-positive men only minimally following infection by T. gondii, while this decrease is very substantial in Rh-negative men and their reaction times are finally much worse than those of Rh-positive men (Fig. VIII.8). Approximately 30% of the people in Europe are infected by T. gondii. A study performed on blood donors showed that, amongst infected persons, the performance of Rh-positive heterozygotes Rh +/– is best, that of Rh-negative Rh -/- homozygotes is worst (and worsens almost immediately after infection) and the performance of Rh-positive homozygotes Rh +/+ is only slightly better than that of Rh-negative homozygotes (but worsens more slowly). Thus, it is highly probable that the current occurrence of both alleles of the RHD-gene in the population is maintained in the long term by selection for heterozygotes. Selection for heterozygotes apparently played a great role particularly in the past when an individual’s reaction time could play an important role in the survival and reproduction success of an individual. Selection pressure for Rh-positive heterozygotes, however, apparently still plays a certain role in modern society. When 3900 military drivers were examined for toxoplasmosis and Rh phenotype on entering 1.5-year compulsory military service and the records of the military police were subsequently examined, it was found that Rh-negative persons infected by toxoplasmosis had more than twice the probability of being involved in a traffic accident than uninfected persons or Rh-positive persons. Amongst Rh-negative persons recently infected by toxoplasmosis (i.e. persons with high anti-toxoplasmosis antibody titres) the probability of an accident was as much as 5x higher than amongst other persons.
The high proportion of Rh-negative persons in the European population could be connected with the fact that, until recently, big cats (the definitive host of Toxoplasma gondii) were practically not present here and thus toxoplasmosis was rare (and Rh-negative persons were at an advantage compared to the rest of the population). The low percentage of Rh-negative persons in Africa (less than 1%) could be related to the high prevalence of toxoplasmosis there, which often approaches 100%.
The primary cause of the emergence of sexuality, i.e. differentiation of individuals of a single species into males and females, was apparently morphological differentiation of gametes into two types, microgametes and macrogametes, i.e. the emergence of morphological anisogamy. This differentiation is a phenomenon that is very old in evolution; however, it was preceded by functional differentiation, i.e. functional anisogamy. The formation of two or more mating types of cells that cannot reproduce sexually within the group and can reproduce only with the members of another mating type occurs in organisms in which specialized sex cells, gametes, are not formed and where their function is fulfilled by a relatively unspecialized somatic cell. This situation is encountered, e.g., yeasts and ciliates.
see Evolutionary constraints