Some epigenetic processes are evolutionarily very old and occur in phylogenetically quite distant groups of organisms. Through the effect of anagenesis undergone by the given species, the phenotype of the relevant adults can differ drastically; nonetheless, very similar or even identical epigenetic processes can occur in them during ontogenesis. One of the consequences of this fact is that organs or their basis can develop in the embryo during ontogenesis, which were present in the phylogenetic ancestor of the particular species, but are no longer present in the body structure of modern organisms or are present only in the form of rudimentary organs (Fig. XII.9, XXVII.5). These organs have thus lost their importance for the functioning of the adult organism, but can frequently be of irreplaceable importance for the functioning of epigenetic processes that control the formation of other organs during ontogenesis. This fact led to the formulation of Haeckel’s recapitulation theory. This theory, also called the recapitulation rule or biogenetic law, basically states that ontogenesis is repeated phylogenesis (see also XXIII.6.1). In this inadmissibly simplified form, this rule is, of course, not valid because the appearance of individual stages of the developing embryo can certainly not tell us anything about the appearance of the sequence of phylogenetic forebears of the particular species. On the other hand, information on the developmental stages of the embryo can certainly provide us with very important information on the phylogenesis of a certain taxonomic group. If the foundations of a certain organ are present in the embryo at some stage of development, it can be assumed with absolute certainty that this organ was actually present in a phylogenetic ancestor of the studied species. The opposite is, of course, not true. An organ that was present in phylogenetic ancestors and did not have any function in epigenetic developmental processes or its function was somehow replaced during evolution need not be formed at all during the ontogenesis of its progeny. Similarly, the order in time of establishment of the individual organs in embryogenesis need not exactly correspond to the order of formation of these organs during phylogenesis. If the individual epigenetic processes in which the foundations of certain organs play a role are functionally mutually unrelated, the order of formation of the individual organs can be different from the order in which they were formed in phylogenesis.
The body structure of an adult organism thus contains rudiments of organs that do not have any functional importance for the given species. The reason for the evolutionary survival of these rudiments could be their irreplaceable role in some epigenetic process functioning in the ontogenesis of the particular species. However, in some cases, the reasons for the existence of rudiments can be quite different; a role can be played here, for example, by the fact that the particular rudiment does not benefit its carrier, but also does not harm it and insufficient mutations have been accumulated by genetic drift to prevent its formation during ontogenesis. It will apparently also frequently occur that the particular rudiment does not fulfill the main function for which it emerged in evolution, but performs some auxiliary function that it began to fulfill only secondarily some time in evolutionary history. Digestion of food does not occur in the appendix of humans as in ruminants; however, the microflora of the appendix are apparently important for the synthesis of some vitamins.
History of evolutionism - neo-Darwinist period
see Rates of anagenetic changes
Most models of evolution in the area of population genetics consider only hard natural selection. Consequently, geneticists occasionally encounter apparently unsolvable paradoxes. An example is Haldane’s dilemma (Haldane 1957), describing the substitution cost accompanying the replacement of one allele in the population by some other, more advantageous allele. The substitution cost (L) is defined by the equation
where Wopdenotes the fitness of an individual with optimal genotype and W is the average fitness of individuals in the population i.e. a parameter expressing how many times the average fitness of individuals in the population is lower than would be the average fitness in a population formed exclusively of individuals with optimum genotype. In models describing hard selection, the magnitude of the substitution cost for the population is directly proportional to the number of genetic deaths, i.e. the number of organisms eliminated by natural selection in substituting a suboptimal allele by an optimal allele. If new alleles constantly appear in the population, increasing the fitness of their bearers, the relative fitness of all the bearers of the other alleles is reduced (Wop, i.e. the fitness of individuals with optimal phenotype is always set at 1), increasing the substitution cost for the population. Haldane pointed out that, with simultaneous selection in favour of a greater number of suitable alleles from various genes, the substitution cost for a particular population can attain unrealistically high values and the number of genetic deaths can easily exceed the reproduction potential of the population.
However, when we take into consideration that the individual alleles can be eliminated by soft selection, the situation looks rather different. The cost is constant in each generation, i.e. actually equal to zero. Natural selection always eliminates a constant percentage of individuals from the population without regard to the specific values of the average fitness of individuals in the population (Nunney 2003). On the other hand, it is apparent that selection can occur simultaneously in favour of only a limited number of traits; if selection occurs to the benefit of a great many traits, the viability of the population is not endangered (as it would be if hard selection were active), but the effectiveness of the selection of the individual traits would be proportionally reduced, and the Hill-Robertson effectwould be manifested. This effect is especially marked when there is a close genetic connection between the loci in which selection occurs, i.e., e.g., in asexually reproducing organisms or for loci in areas in which genetic recombination does not occur for some reason (Charlesworth & Charlesworth 2000). The reduced effectiveness of simultaneous selection for a greater number of traits can play a significant negative role in the evolutionary response of the population or species to a rapidly changing environment (Nunney 2003).
It very frequently occurs in interspecific hybridization that the members of one sex are affected more by reduced fertility or viability. Haldane’s rule, formulated in 1922 by J.B.S. Haldane, states that, in this case, the heterogametic sex, i.e. the sex whose cells contain both types of sex chromosomes, is affected far more frequently or to a far greater degree (Haldane 1922). In cases of the Drosophila type (e.g, in mammals), females with sex chromosomes XY are affected more than males with sex chromosomes XX; in cases of the Abraxas type (e.g. in birds), on the other hand, females with sex chromosomes ZW are affected more than males with sex chromosomes ZZ (Civetta & Singh 1999). The validity of Haldane's rule has been repeatedly demonstrated for a wide range of species belonging to various taxa. Of 223 known cases of sterility affecting only the members of one sex, 99% of them corresponded to Haldane’s rule. In relation to the inviability of hybrids, 90% of 115 described cases obeyed Haldane’s rule (Turelli 1998). This survey included only cases of complete sterility or complete inviability of one sex; if we were to also consider partial sterility and reduced viability of hybrids of one sex, the number of known cases would increase many fold; however the percentage of cases obeying Haldane’s rule would remain roughly the same.
It is apparent that several mechanisms are simultaneously valid here, of which the best known, i.e. the dominance hypothesis, faster male hypothesis, hypothesis of greater resistance of oocyte formation, recessive gene hypothesis, somatic mutation hypothesis and ultraselfish genetic element hypothesis – will be described below. This relatively unimportant phenomenon of lower viability and fertility of hybrid members of the heterogametic sex will be discussed in greater detail because this can be a key to understanding an extremely important phenomenon – speciation dependent on genetic incompatibility. In this connection it is recommended that the reader study more carefully the sections on the dominance hypothesis, the faster male hypothesis and the somatic mutation hypothesis. Unless explicitly stated otherwise, the relevant mechanisms are applicable similarly as for the Drosophila and Abraxas types. For simplification, where possible, the situation will be described for the Drosophila type.
It was repeatedly found that, in the natural population, the most common alleles are usually dominant and, on the other hand, minority alleles are frequently recessive. If, on the other hand, we isolate individuals in the laboratory that bear two newly formed mutated alleles, or if we obtain individuals bearing minority alleles in mutually isolated natural populations, then the relationship of partial dominance is mostly found between their alleles. The explanation suggested by Haldane says that suitable dominant alleles will be more readily spread in the population and will thus become majority alleles more readily than similarly advantageous recessive alleles. While the usefulness of dominant alleles is also manifested in a heterozygote, the usefulness of similar recessive alleles is manifested only in recessive homozygotes, in the outbred population, i.e. in a population in which random crossing occurs between its members, i.e. only when its frequency is substantially increased.
History of evolutionism – post-neo-Darwinist period
History of evolutionism – post-neo-Darwinist period
History of evolutionism – post-neo-Darwinist period
History of evolutionism – post-neo-Darwinist period
History of evolutionism – post-neo-Darwinist period
The handicap hypothesis, which was formulated in 1975 by A. Zahavi (Zahavi 1975), assumes that, under certain conditions, it may be advantageous for the female to choose, as the father of her offspring, a handicapped male, for example, a male with long tail feathers. The long feathers represent a substantial handicap for their bearer in the fight for survival. Thus, if a male with abnormally long tail feathers, i.e. with an abnormally large handicap, has survived to reproductive age, it is almost certain that this must be an abnormally fit individual.
Simultaneously, this handicap need not be only a secondary sexual trait. It could, for example, be a physical defect incurred through an injury or even the old age of the individual (Kokko & Lindstrom 1996; Sundberg & Dixon 1996). It has been observed, for example, for sparrows and finches, that, in extra-pair parentage (EPP), females prefer old males (Wetton et al. 1995; Sundberg & Dixon 1996). Simultaneously, the frequency of extra-pair copulation (EPC) with old and young individuals is the same. In perching birds (Passeriformes), the female determines whether copulation will leads to transfer of sperm or not. The fact that, for the same frequency of EPC, a greater number of offspring are fathered by older males suggests that the purpose of this “gerontophilia” lies in an attempt of the female to obtain the best genes for her offspring.
The handicap hypothesis on the origin of secondary sexual traits has been subject to fundamental criticism in the past. Mathematical analysis of the effect of a handicap of the father on the fitness of offspring has shown that the advantage represented by the greater fitness of the father is exactly compensated in the offspring by the existence of the handicap that the progeny also inherit. However, at the present time, it seems that the model could work in a number of situations (Pomiankowski 1987; Siller 1998; Hastings 1994). At the same time, it is important that the coefficient of heritability of the handicap, a factor reflecting the probability that the offspring will inherit the particular trait, for example long feathers, is less than the average coefficient of heritability of the other traits determining the fitness of the individual. If, in addition to genetic factors, the effect of the environment also has a substantial effect on feather length, the coefficient of heritability of this trait can be very low. Under these conditions, it is really advantageous for the female to prefer reproduction with a handicapped male.s
Haplotype denote a combination of alleles located in a single continuous section of one DNA molecule in the given individual, for example in a certain section of a pair of homologous chromosomes. The term haplotype can also be employed in the sense of the combination of all the alleles located in the genome of a haploid cell, for example in the genome of a sex cell (gamete). The genotype of a haploid individual thus contains two haplotypes of each DNA section; new haplotypes can be mixed from these two haplotypes through recombination during meiois.
The frequency of the individual genotypes in the equilibrium state can be readily calculated from the frequency of the alleles in a certain locus. This equilibrium state is called the Hardy-Weinberg equilibrium. In a large panmictic population, i.e. in a population whose members reproduce together quite at random, this equilibrium is established during a single generation. It follows from the laws of combinatorics that, at equilibrium, genotypes a1a1, a1a2 and a2a2 will be present in a ratio of ¦2: 2¦a¦a:¦2, where ¦aand ¦adenote the frequency of alleles a1 and a2 in the previous generation (Fig. II.14). Here, the ratio in which the individual genotypes were present in the previous generation is not in any way important. During a single generation, the same representation of the individual genotypes is established, regardless of whether the original population contained, for example, only homozygotes a1a1 and a2a2, or only heterozygotes a1a2.
If we study several genes, amongst which there is no genetic linkage, for example, if each of them is located on a different chromosome, equilibrium is again established within a single generation; the numbers of the individual genotypes can be calculated for this state according to the simple rules of combinatorics.
see Revealing hidden genetic variability in unfavourable conditions.
Ability to passed the individual characters between generations that is responsible for the resemblances between parents and offspring.Natural selection is effective, and the biological evolution can operate, only if individual differences between organisms are hereditary.Various degrees of heritability of properties can exist; in some systems, a characteristic property of a certain individual can be passed on to its progeny in unaltered form and degree, at other times to a lesser degree or may appear in progeny only with increased probability.
Amongst modern organisms, heredity is based on copying genetic information, instructions for creation of the body of organisms. Theoretically, completely different mechanisms could also exist, based, for example, on direct copying of the actual structure of the organism itself.
Trait heritability describes the share of the genetically determined variability of the trait in its total variability. Individual traits vary in their heritability. The trait heritability of qualitative traits expresses the probability that the traits will be transferred in unaltered form to the next generation while, for quantitative traits, this corresponds to the degree to which they are transferred from one generation to the next. The heritability of a trait is, however, defined in genetics as the fraction of genetically determined variability in the given trait in the total variability of this trait, i.e. also the environmentally determined variability of this trait. Some components of genetically determined variability are inherited from one generation to the next, while others are not. Consequently, attention is concentrated especially on heritability in the narrow sense of the word, i.e. the component of variability of a given trait determined by genes whose effects can be simply added, i.e. genes with additive effect. Other components of heritability include environmentally determined variability, variability determined by interactions between alleles in a single locus (the dominance component) and variability determined by interactions between alleles in various loci (the epistasis component) It is, of course, possible to also define other components of the total variability, with contributions, e.g., from interactions between the environment and dominance, interactions between dominance and epistasis, etc. The evolution of a certain trait through selection is fundamentally affected by the additive genetically determined variability; the other components of the variability mostly reduce the effectiveness of selection. See also Heredity.
One of the very important possibilities that has been applied many times in anagenesis consists in heterochrony (Wakahara 1996; Klingenberg 1998; Richardson 1995), the evolutionary modification of the rate of formation and development of the individual organs and organ systems (Tab. XII.2). A very small genetic change is sufficient for a certain organ to begin to be formed in the ontogenesis of an individual of a certain species sooner or, to the contrary, later and thus arrive at a different state during ontogenesis than in its phylogenetic ancestors. Consequently, a small change in the timing of the individual ontogenetic events can be of fundamental importance for the phenotype of the members of a particular species. An individual with altered phenotype can, again, fundamentally alter its life niche and this change in life niche again fundamentally changes the selection pressures to which the particular species is exposed. Thus, fundamental changes can occur in the body structure as a consequence of minimal genetic changes, for example changes in the regulation region of a single gene.
The heterozygosity index (H), which is basically the frequency of heterozygotes in the population, is another commonly employed measure of the degree of polymorphism in the population. For the individual genes, this index is usually calculated from the frequencies of the individual alleles:
where xi is the frequency of the i-th allele in the population. Thus, a population containing a large number of alleles with the same frequency has the largest H value. The average heterozygosity index for the given population can be calculated on the basis of the heterozygosity indices as the arithmetic mean for the individual genes. If the heterozygosity index is calculated on the basis of sequence data, it is also sometimes called the gene diversity index. The nucleotide (aminoacid) diversity index (B) can also be calculated on the basis of sequence data; this corresponds to the average number of nucleotide (aminoacid) differences between all the pairs of alleles in the sample divided by the length of the sequences of the relevant alleles. The average number of pair differences Π can be calculated for the whole population or, to be more precise, for the population sample, as
where n is the number of observed sequences (so that is the number of various pairs of sequences) and Πij is the number of differences between the i-th and j-th sequence.
see Selection hard and soft
The emergence of genetics, a field of science whose theoretical foundations were established in the 19th century by Johann Gregor Mendel (1822–1884) (Fig. XXVIII.9), greatly assisted in the development of the theory of evolution. Genetic knowledge especially assisted in eliminating the greatest conceptual inadequacies of Darwin’s model of the evolution of adaptive structures by natural selection. As long as biologists assumed that the predispositions derived from both parents are mixed together and “averaged” in sexual reproduction, Darwin’s model was incapable of functioning. The hereditability of traits, including evolutionary innovations was necessarily vanishing; a substantial trait in a parent appeared to a lesser degree in its progeny, even less in its grand-descendants and basically disappeared, i.e. was lost in the standard population, over a few subsequent generations. In order for Darwin’s model to be functional, it would require an unrealistically high rate of formation of new inheritable variability. The rediscovery of Mendel’s laws around 1900 demonstrated that the predispositions for the individual traits actually do not affect one another during sexual reproduction and, to the contrary, are transferred from one generation to the next in unaltered form. However, over time, the frequency of the individual predispositions (alleles) can change in the gene pool of the population, enabling Darwinistic evolution. As the proponents of the other concepts gradually died, the study of evolution slowly became identical with study of the changes in the numbers of the individual alleles in the gene pool of the population and all evolutionary phenomena taking place above the level of the species began to be considered to be simply evolutionary consequences and accompanying phenomena of processes occurring at the intraspecific level.
The geneticist Theodosius Dobzhansky (1900–1975), paleontologist George Gaylord Simpson (1902–1984), zoologist Ernst Mayr (1904-2005) and botanist George Ledyard Stebbins (1906–2000) are generally considered to be the main representatives of Neo-Darwinism responsible for evolutionary synthesis. Bernhard Rensch (1900–1990), Julian Sorell Huxley (1887–1975), Ronald Aylmer Fisher (1890–1962), Sewall Wright (1889–1988), John Burdon Sanderson Haldane (1892–1964) and a great many others also contributed substantially to the formulation of Neo-Darwinism. The Neo-Darwinist period is characterized by emphasizing and stressing the importance of selection in evolution. Other mechanisms, with the possible exception of genetic drift, are considered to be marginal. The main scientific efforts were concentrated on the study of evolutionary processes occurring at the intraspecific level or speciation processes. Study of macroevolutionary processes lies, to a substantial degree, outside of the main sphere of interest of a major portion of evolutionary biologists.
Traditionally the history of evolutionary biology is divided into three stages: the pre-Darwinist period, the classical Darwinist period and the Neo-Darwinist period, also called the period of evolutionary synthesis. The first phase basically lasted to 1859, i.e. the year that saw the publication of Darwin’s fundamental work “On the Origin of the Species by Natural Selection”. The beginning of Neo-Darwinism is much more difficult to date; the key works of the main representatives of Neo-Darwinism were, however, published in the 1930’s and 1940’s and the rediscovery of Mendel’s laws and the development of classical genetics in the first decades of the 20th century provided the main stimulus for their formulation. In relation to the character of the shift in the conception of evolutionary biology that has occurred over the past thirty years, it seems to be useful to define a fourth period that, for lack of greater inventiveness, I call the Post-Neo-Darwinist period (and simultaneously, I feel sorry for my successors who might, in the future, want to create a name for the next phase in development of the field). The Post-Neo-Darwinist period basically began at the middle of the 1960’s and beginning of the 1970’s; however, to this day, textbooks tend to be based globally on the concepts of Neo-Darwinism. Thus, we are still virtually living in the epoch of Neo-Darwinism, while in actual fact most important evolutionary biologists are representatives of Post-Neo-Darwinism.
Just as, in his work, Darwin admitted the importance of a number of independent evolutionary mechanisms acting on organisms together with natural selection, a similar plurality approach to the theory of evolution was prevalent in the entire professional community (Gould 2002). Amongst the non-biological public, Darwinism was almost universally identified with his theory of natural selection. This theory was attractive for a large part of society, because it agreed well with the general experience of the inhabitants of England and the rest of the developed world with the functioning of human society in the period of emerging capitalism. Competition amongst individual entities and the success and preservation of the strongest and best adapted to the prevailing conditions was generally recognized as a motor for social change and social, of course primarily material, progress. In contrast, professionals were frequently aware that Darwin’s theory of natural selection has a number of obstacles and that some of the conclusions following from this theory were even contradictory to the then-known empirical facts. For these reasons and also because of the absence of centralized supranational science, a great many various concepts of the theory of evolution, based on a number of fundamentally different mechanisms, coexisted for a period of at least 60 years. Various autogenetic concepts, which assumed that living organisms are characterized by a certain internal tendency towards gradual directed development (Galton, Chambers, von Nägeli, Eimer, Osborn) and frequently even towards gradual perfection (Teilhard de Chardin) always held a very strong position. Other theories assumed that Lamarckian mechanisms of strengthening of highly utilized structures and inheritance of acquired traits are of great importance in the evolution of adaptive structures and in mutual divergence of individual species of organisms (partially also supported by Darwin himself). Some theories considered the direct effect of environmental factors on the properties of organisms (St. Hilaire). Others assumed a fundamental effect of sudden jump-like changes in the properties of organisms (Bateson, de Vries, Goldschmidt, Schindelwolf). Of course, theories assuming the fundamental effect of natural selection also maintained a strong position (Wallace, Weismann).
It holds in general that it is not difficult to characterize a stream of thought, once it has ended. However, in the absence of a suitable temporal and personal distance, it is very difficult and perhaps impossible to describe a stream of thought that extends into the future or that is only forming at the present time. While I will attempt to do something of this sort in this chapter, I am aware of the danger that I could be completely mistaken. It is quite possible that future historians of science will consider the Post-Neo-Darwinist stream of thought to be a quite logical outcome of evolutionary synthesis and thus its integral component. Nonetheless, I am of the opinion that, at the very least for didactic reasons, it is useful to risk future loss of face and to attempt to define the currently emerging line of thought in respect to Neo-Darwinism.
Possibly with the exception of S. J. Gould, all the representatives of Post-Neo-Darwinism tended to consider or still consider themselves to be representatives of classical Neo-Darwinism. Nonetheless, some of their works have apparently exceeded the conceptual context of original evolutionary synthesis and have led or will in the future lead to a radical change in evolutionary paradigm. If we neglect the predecessors to which modern authors did not, at least consciously, refer in their thinking, i.e. the author of the model of shifting balances, Sewall Wright, and partly also the author of the concept of genetic revolution, Ernst Mayr, the first step in this direction was taken at the end of the 1960’s and beginning of the 1970’s by George C. Williams (1926-2010) (Williams 1966) and William D. Hamilton (1936-2000) (Hamilton 1964a; Hamilton 1964b), when they published their gene-centered concept of evolution. In their work, they implicitly assumed and clearly demonstrated on specific cases that, in studying a certain structure or a certain pattern of behavior, evolutionary biology must not ask how the particular traits provides an advantage for its bearer, but only how the particular trait provides an advantage for the allele that is responsible for its formation. Richard Dawkins (*1941) explicitly explained this idea and popularized it amongst the professional and lay public, originally in his popular-instructive book “The Selfish Gene” (Dawkins 1976) and subsequently in his professional work “The Extended Phenotype” (Dawkins 1982). He demonstrated that an individual cannot be an object of selection and that biological fitness cannot be a criterium of his success in sexually reproducing organisms. The object of selection must always be only a specific allele and the criterium of its evolutionary success is the increase in its frequency in comparison with the other alleles at the given locus. Works related to evolutionarily stable strategies, written jointly by John Maynard Smith (1920-2004) and George R. Price (1922-1975) (Maynard Smith & Price 1973), constituted another involuntary attack on the basic paradigm of Neo-Darwinism. These works, similar to the works of their successors, demonstrated that the criterium of success of a certain pattern of behavior and, in general, a certain biological trait is not how it increases or decreases the fitness of its bearer, but rather whether this corresponds to an evolutionarily stable strategy in the sense of game theory. They demonstrated that only a strategy that, once it predominates in the population, is capable of preventing the invasion of any other (even potentially more successful) minority strategy, has a chance from the long-term point of view. Taken to its logical conclusion, even the reproductive ability of the individual alleles is not the decisive criterium for their evolutionary success in a polymorphic population.
The work published by paleontologists Niles Eldredge (*1941) and Stephen Jay Gould (1941-2002) led to a further basic shift in the character of evolutionary biology. At the beginning of the 1970’s, these authors demonstrated that, contrary to expectations following from the Neo-Darwinist model of evolution, the evolution of species has a substantially discontinuous character (Eldredge & Gould 1972). Species mostly change only immediately after their formation and their existence is characterized by evolutionary stasis for a subsequent, incomparably longer time. Eldredge and Gould originally suggested that genetic homeostasis is basically responsible for evolutionary stasis and that genetic revolution (which occurs as a result of the founder effect (Mayr 1963)) is responsible for anagenetic changes, i.e. just those effects whose existence is directly connected with the competition of the individual alleles for an evolutionarily stable strategy. However, they later abandoned these ideas (see XXVI.5.3), possibly in connection with a certain tension present in the 1990’s between the representatives of American and British schools of evolutionary thought. Irregardless of the nature of the actual mechanism responsible for the punctuated character of evolution, this character in itself requires basic modification of the Neo-Darwinist concepts of the course of macroevolutionary events. As anagenesis is apparently closely coupled with cladogenesis amongst sexually reproducing organisms and selection can substantially affect the traits of organisms only at the moment of speciation, there has been a substantial increase in emphasis on other evolutionary mechanisms (species selection, interspecific competition, evolutionary trends driven by evolutionary constraints), whose effectiveness and importance in evolution (compared to selection) the Neo-Darwinists mostly doubted. Thus, evolutionary biology is, in a certain sense, returning to a plurality approach, which tended to be characteristic rather for the time of classical Darwinism and which was abandoned temporarily during the time of evolutionary synthesis (Gould 2002).
see Genetic draft
While phenograms are formed on the basis of all the traits that can be differentiated in the members of the studied species, a scheme of cladogenesis, which is intended to express not similarity but rather relatedness of species, must be expressed solely on the basis of certain subsets of these traits. For example, it is quite obvious that relationships between species cannot be derived on the basis of shared traits, formed in the individual species during evolution independently of one another, i.e. traits that were not present in the closest common ancestor of the compared species. Such a trait is termed homoplasy. Homology is the opposite of homoplasy; this is a common trait of two or more species that these species inherited from their closest common ancestor.
The definition of a homological trait is, in a certain sense, relative. On the one hand, the wings of birds and the wings of bats are homoplasies, as no exclusive common ancestor of birds and bats had wings. However, wings evolved only once in the line of birds and the line of bats. The presence of wings in birds and bats is thus homoplasy from the viewpoint of the two groups together and is a homological trait from the viewpoint of each group separately. The situation is further complicated by the fact that, in both birds and bats, wings were formed from the front limbs of vertebrates and the common ancestor of these two groups had front limbs. Thus, the presence of front limbs must also be considered to be a homological trait that birds and mammals inherited from their common ancestor.
see Concept of species structuralist
see Homological trait in phylogenetics
- If a parasite is transmitted horizontally through direct contact between an infected and uninfected individual, parasitosis is generally less virulent. From the standpoint of spreading of the parasite, it would be inexpedient if it were to substantially harm the state of health of the infected host and thus limit its number of contacts with healthy individuals. To the contrary, the virulence of parasites tends to be greater for some other means of spreading parasitosis. For example, parasites spread by vectors tend to have high virulence. This is true both for parasites spread by animal vectors (Ewald 1983), such as mosquitoes (Fig. XIX.10), as well as parasites spread by abiotic vectors, for example flowing water. (Ewald 1991) (Fig. XIX.11). The infected animal can be harmed by the parasite or even immobilized, as the vector is responsible for transfer of the infection to another individual in the population. If water acts as the vector, high virulence of the parasite is also enhanced by a large size of the infection inoculum, which generally enters a single host during infection, with the related high genetic variability within the infrapopulation (see XIX.4.2.2). If the vector is an animal, its manifestations of parasitosis are generally much milder than those in the actual host. It is the role of the vector in the life cycle of the parasite to spread the infection in space and any damage to its state of health would be detrimental to this function.
Parasites capable of independent active motion from one individual to another within the host population, for example, parasitic Hymenoptera or Diptera, also frequently exhibit high virulence. In this case, the parasite is very frequently converted to a parasitoid during evolution, i.e. a parasite that kills its host after a certain period of symbiosis with the infected individual.
Parasites spread alimentarily, specifically by predation, also exhibit high virulence (Ewald 1995). In these cases, the pathological manifestations of parasitosis can increase the chance that the infected individual will be caught by a predator, leading to transfer of the infection. Simultaneously, the predator is usually harmed substantially less than its prey. Otherwise, it would be a relatively easy matter for individuals to be selected in the population of predators that would learn to avoid infected individuals amongst prey when hunting. It could also be important that the definitive host often also acts as a vector and harming the vector would be detrimental to its function in the life cycle of the parasite – spreading infection in space. Simultaneously, the frequent combination of the function of a vector and definitive host need not be accidental and can have a certain importance. During spreading of parasites in space, offspring can frequently encounter conditions differing from those under which their parents lived; under these circumstances, even temporary variability, formed during multiplication through recombination and segregation, can be an advantage (see for example, XIII.220.127.116.11).
High virulence can be exhibited by sit-and-wait infections, i.e. infection transmitted through long-lived resistant stages, spores and cysts, which remain where the infected host died and then infect another host that comes some time in the future (Ewald 1995). Anthrax and smallpox are typical examples of sit-and-wait infections and their spores can remain in the environment for years or even decades. The high virulence of infection in these parasites is a result of the fact that the probability of infecting another host is proportional to the number of spores that remain at the site of death of the infected individual. Consequently, a parasite will attempt to convert as large a part of the body of its host into its own spores or cysts as fast as possible. This is, of course, mostly incompatible with the life of the host organism.
see Rates of anagenetic changes
The evolution of a parasite and its host often has the character of an “arms race”, in which the host develops more or less specific mechanisms of defense against parasitization and the parasite, on the other hand, develops mechanisms allowing it to avoid or overcome these defense mechanisms. However, the fact that a parasite adapts perfectly to a certain species of host means that it closes the route to parasitization on other species. Thus, “arms races” frequently lead to very narrow specialization of the parasitic species and often result in narrow host specificity of the parasite. Narrowing of the host spectrum frequently ends with a state where the parasite is capable of completing its life cycle only in the members of a single host species and does not attack even closely related species at all (Fig. XIX.3). This is in sharp contrast with the situation for a number of groups of predators, where narrow specialization of the predator on a single species is certainly not the rule.
For example, molecular mimicry is characteristic for some parasitic microorganisms (including viruses) (Moloo, Kutuza, & Boreham 1980). The parasite adapts the structure of its macromolecules to the structure of the relevant macromolecules of the host organism. If, for example, a certain virus were capable, through gradual accumulation of substitution mutations, of eliminating, from its proteins, all the peptides that are recognized as being foreign by the immune system of the host species, and thus adapt its “peptide vocabulary”, i.e. the set of peptides occurring in its proteins, to the vocabulary of its host, it would quite certainly escape from the reach of the immune system of the host, and could thus spread uncontrollably in the relevant host population. (The fact that it is not an easy matter for the parasite to achieve this final state and the role of MHC-antigens and sexuality in the defense of the host were described in Section VIII.4.3.1.) However, if the peptide vocabularies of two host organisms are very different, and this is highly probable as a result of the relevant selection pressure from parasites (see below), then a parasite cannot simultaneously adapt its vocabulary to two different vocabularies of two host species, except at a cost of drastic limitation of its own peptide vocabulary, which is incompatible with functionality of the proteins. Any form of utilization of the principle of molecular mimicry thus again creates selection pressure for gradual narrowing of the host spectrum of the parasite.
During evolution, the individual species adapt to the conditions in their environment and to changes in these conditions. This adaptation is reflected in the anagenesis of organisms and is manifested in adaptive changes in the morphological and functional structures of individual species. Changes in the external conditions, manifested simultaneously over a larger area or even on a global scale, mostly occur slowly, so that the individual species are usually able to gradually adapt to them evolutionarily. Of course, drastic and rapid changes in the quality of the environment also occur during development of life on Earth, amongst other things as a consequence of global catastrophes caused, for example, by the impact of large meteorites, comets or small planets on the surface of the Earth (see XXII.5.3.2). These drastic, but basically temporary changes, which frequently led to the extinction of a major portion of the species occurring on the Earth at the particular time, tend, however, to affect macroevolution rather than microevolution. During their existence (i.e. during a period of usually several million years), most species never encounter such rapid changes in their environment (and if they do encounter them, they mostly become extinct).
The above is true only for changes in the abiotic factors in the environment.
It can be justifiably assumed that interactions amongst various species of organisms and selection pressures following from these interactions constitute the main driving force for biological evolution. The phenomenon of parasitism is very widespread and a substantial portion of all the organisms on the Earth consists of various species of parasites (Price 1980). Simultaneously, the evolution of a parasite and its host are very closely connected and “arms races” between the two actors in co-evolution are generally very intense. It can thus be anticipated that a major portion of biological evolution and a large percentage of adaptive traits in the framework of biological evolution are in some way connected with the phenomenon of parasitism. According to some concepts, the two most conspicuous evolutionary phenomena, sexuality and speciation, could have emerged as a consequence of selection pressure on the part of parasites (see XIII.3.4.2, XIX.2.1 and XXI.5.4).
The biogeography, i.e. study of the distribution of the individual species provides an enormous amount of evidence for the evolution of species of fauna and flora through gradual branching off from a common ancestor. If the individual species of organisms were to be formed independently at the same time or one after another, the distribution of their occurrence over the surface of the Earth should also be independent or this distribution should reflect only the differences in natural conditions in various parts of the Earth. However, reality is quite different. Species belonging in a particular taxon very frequently occur in the individual areas of the Earth, although species belonging in other taxa could also be very successful there. The reason why, for example, there are no local species of big cats in Australia is certainly not that there were not suitable conditions there for them, but that they did not have any species from which they could evolve, there was no species of cat that they could gradually evolve from. On the other hand, it is clear why there are a great many local species of bats there – their ancestors could quite easily get there by air. This phenomenon is particularly obvious on islands. As soon as an island is far from the mainland, there is a lack of the members of taxa for which the ocean represents an obstacle to spreading. In contrast, these species are present on islands located at suitable distances from the mainland and they very frequently form separate species that are different from the species occurring on other islands or on the mainland.
Another obvious phenomenon that is encountered on islands and groups of islands consists in radiation of taxa, whose members have only a very narrow niche on the mainland. Darwin’s finches are mostly given as a typical example; in actual fact, these are a group of closely related buntings occurring on the individual islands of the Galapagos Islands. The individual species became morphologically differentiated in the local environment and divided up various ecological niches that are occupied on the mainland by birds of various taxa. Numerous species of Drepanididae on the Hawaiian Islands represent a similar case (Fig. XXVII.3). The theory of evolution predicts the formation of species with this character of distribution of occurrence the individual species. Local species of woodpeckers cannot exist in the Galapagos simply because no common ancestor got there. If the relevant niche were to be filled, a species of bird that got to the island in the past, in this case a bunting, would have to (at least imperfectly) adapt to it. In contrast, other theories of the origin of species would encounter substantial difficulties in explaining similar data. If the species were to have been formed independently, for example by autogenesis, or if they were to have been formed in a single instant, either in one place or in a great many places by a rational being, either there would be woodpeckers on the Galapagos, or they would not be there; however, they would apparently not be replaced by a local species of bunting and quite certainly closely related species of this species of bunting would not replace several other unrelated groups of birds in their very different ecological niches in the Galapagos.
Although a number of facts demonstrating the correctness of the evolutionary explanation of the origin of species exist, it must be emphasized that none of them proves the existence of evolution when taken alone. Let’s ignore for the moment the already mentioned fact that no scientific theory can be definitively proven and that, in the best case, all the momentarily known alternative theories can be, at most, shown to be false. We would most probably be capable of finding an explanation for any of the above-mentioned facts that did not encompass the fact of evolution. For example, all of Darwin’s finches could have emerged or have been formed in a single moment at a single place and could fly to the Galapagos together purely by chance. It is, of course, possible to calculate how great this chance would have to be; nonetheless, even if we were to obtain quite negligible probability, we could not completely exclude this scenario. The identity of the phylogenetic trees created on the basis of sequences of various genes could also be caused by the fact that the species were created by a Martian, by the “copy and paste” method, i.e. in each case he would create a species from another already existing species. Of course, life on Earth could just as easily have been created by an omnipotent God who, for some unknown reason, wanted us to think, on the basis of our data, that he had nothing to do with it and that the species were formed by natural biological evolution. (However, in this case, wouldn’t it be a good idea to do just this?) However, with the exception of the latter model, all the alternative explanations are post hoc explanations, sometimes not very probable and sometimes rather awkward and, in addition, frequently mutually incompatible or incompatible with the currently accepted explanations of other phenomena. In contrast, the model of biological evolution was established prior to accumulation of most of the data that now confirm its validity. It is a very good explanation for phenomena that we encounter at all levels and in a wide range of disciplines. Fundamentally, it is possible to state that, at the present time, no facts are known that would be contradictory to the model of evolution based on gradual splitting off of individual species from a common ancestor.
It was very soon found from study of the anatomy of various species of organisms, and especially in studying the ontogenesis of their individual organs, that the structural plans of the bodies of even very dissimilar organisms within the individual taxa are extremely similar in their individual details. For example, amongst vertebrates, the human hand, the digging limb of a mole and the wing of a bird contain the same groups of bones (Fig. XXVII.4). We say that the individual bones in various species can be homologized, i.e. that they are homologous structures, homologues. In addition, these bones were formed in the course of ontogenesis by basically the same mechanisms from the same basic forms. Simultaneously, if we compare the limbs of other groups of organisms, for example the limb of a mole and the limb of a mole cricket, it is immediately obvious that functionally identical types of limbs can be formed by completely different means, from completely different parts and through completely different ontogenetic mechanisms.
The occurrence of homologous structures within individual monophyletic taxa is now considered to be a consequence of the fact that the individual species were formed by divergence from a common ancestor. As biogeography has demonstrated that daughter species can be formed only from some parent species, comparative anatomy has also confirmed that a new anatomical structure can be formed only by modification of a different structure that already existed in the parent species.
The existence of rudiments and atavisms also confirm that the occurrence of homologous structures actually does not have a functional cause. Rudiments are residual organs that occur in all the individuals of a certain species, where they do not fulfill any function in this species. Thus, they are mostly much smaller and structurally simpler than where they fulfill some sort of function (Fig. XXVII.5). Some rudiments in the given species of organism are formed only during embryonic development and cannot be found in adult individuals. The appendix, a worm-like protuberance of the intestine, is considered to be a rudiment in humans. In a great many taxa, the appendix plays a very important role in digestion and is quite large (Fig. XXVII.6). It has been substantially reduced in humans and its removal apparently does not negatively affect the fitness of the individual.
Atavisms represent a similar case to rudiments. Atavisms are structures that, in contrast to rudiments, occur in only some individuals of a particular species, have no functional importance for their bearers and simultaneously occur in all the members of some other species, where they do have functional importance. For example, in humans, individuals occur very rarely that have an atavism in the form of a tail (Fig. XXVII.7). The occurrence of atavisms is once again very good evidence for the evolutionary mechanisms of the origin of species. The organisms can manage quite well without these structures, so that their presence has no functional basis. They are not formed in most individuals, so that their presence necessarily cannot follow from the properties of the components from which the body of the organism is formed or from the character of the processes of ontogenesis (see the structuralist explanation of evolutionary trends, XXVI.7.4). The only reasonable explanation that remains is the evolutionary explanation – the occurrence of rudiments and atavisms follows from the fact that these organs existed in functional form in an ancestor of the given species, so that the ability to form them is also borne by the members of successor species.
Molecular biology and molecular taxonomy has provided a great deal of evidence for the evolutionary theory of the origin of species. When we obtain sequences of a section of a certain gene from the members of several taxa, the difference between the sequences of these genes allows us to establish the probable order of branching off of the relevant taxa from the common ancestor. This is, in itself, still not evidence for the existence of biological evolution. The fact that the particular species branched off from a common ancestor in a certain order was already included amongst the assumptions forming the basis for the relevant method for construction of the phylogenetic tree. For example, if we were to take a collection of stamps, one from each country of Europe, measured all the possible characteristics of their patterns and words and were to feed these characteristics into a suitable computer program for molecular taxonomy, it would provide us with a phylogenetic tree without any hesitations, and this would show the order in which these stamps apparently branched off from a common ancestor. (I am exaggerating a bit; a good molecular-taxonomic program would also allow us to determine that our data set does not contain sufficient phylogenetically significant information and thus that the topology of the tree is not very credible.) However, evidence for the evolutionary theory could be obtained if we were to subsequently scan several other genes from the particular species and create a separate phylogenetic tree for each of these genes and were to discover that all the obtained trees would be identical, at least in their general characteristics. In addition, this topography would most probably be identical with the topography of the tree obtained on the basis of classical, for example morphological traits. In contrast, if we were to take our collection of stamps and measure first the chemical composition of the glue, secondly, for example, the structure of the paper, thirdly the distribution of colors in the pattern, etc. and again were to feed the obtained datasets into a computer, the resultant phylogenetic trees formed on the basis of the various sets of traits would most probably differ fundamentally This result is quite difficult to explain in any other way than that the particular organisms, in contrast to the stamps, were formed in a certain sequence, one from another. Of course, this still does not demonstrate that they were formed during a long period of natural evolution; however, it does exclude all theories based on the independent, natural or supernatural origin of the species.
- Paleontology, the study of fossils, provides us with a great deal of evidence for the simple fact of evolution. However, the existence of fossils is not, in itself, evidence for evolution. Nonetheless, because of the possibility of dating fossils using a number of physical methods, at the very least it reliably overturns the ideas of some opponents of evolution about the recent emergence of animals, plants and humans on the Earth approximately 6000 years ago. While there are not now a great many proponents of the “Young Earth” theory, relying on a literal interpretation of the Bible, they are disproportionately vociferous. However, the results of stratigraphic analysis document the evolution of organisms from a common ancestor, showing that the individual organisms do not appear in the paleontological record at random, but rather in the order that corresponds to their mutual similarity and thus assumed relatedness. On the basis of the similarity of the members of the main taxa of vertebrates – fish, amphibians, reptiles and mammals – it can be concluded that they were most probably formed one from another in this order. Theoretically, it would also be possible that they would be formed from one another in the opposite order, i.e. that first mammals would be formed, from them reptiles, then amphibians and finally fish. However, this possibility can be refuted by comparison of fish and mammals with invertebrate fauna, which certainly have far more common traits with fish than with mammals. Perusal of the paleontological record reveals that, of these groups, fish actually did appear first on the Earth, followed by amphibians, reptiles and finally mammals. The same analysis can be performed in any monophyletic taxon, i.e. within a group of organisms for which it can be assumed, on the basis of independent data, that they evolved from a common ancestor. In every case, we obtain results that are in full agreement with the predictions made on the basis of the theory of evolution. This indicates that the individual groups of organisms emerged gradually over time through divergence from a common ancestor
Evolution cannot plan ahead and always create structures that are suitable for an organism at a particular moment. The functional approaches that evolution “selected” at a certain moment can prove to be disadvantageous after some time when the particular organ completely develops in evolution. Consequently, organisms have a great many organs that are obviously suboptimal or even completely senseless in their design from the viewpoint of their present-day function. We have already mentioned the example of the eye in vertebrates with nerve fibers located in front of the retina in the optical pathway of beams and innervation of the pharynx in giraffes with a senseless many-meter-long loop reaching as far as the aorta. The frequent occurrence of these senseless structures indicates that the organisms were certainly not created according to a pre-established plan, but developed by the “blind” opportunist process of biological evolution.
see History of evolutionism - neo-Darwinist period
see Reproductive isolation postzygotic
see Speciation hybridization
– see Mutations at the level of the entire chromosome set
Amongst sexual organisms, species frequently evolve allopatrically or peripatrically (in geographically separate regions) from a common ancestor, so that their original areas of occurrence are not in contact or are in contact only at a place where, because of the presence of a certain barrier in the environment (mountain range, river), there is only very limited gene flow between the two species. The places, where the areas of occurrence of emerging species are in contact and where a certain degree of gene flow occurs between the species, are called primary hybrid zones. Primary hybrid zones are apparently rather rare (Jiggins & Mallet 2000). If this were not true and if substantial gene flow were to occur between emerging species, in most cases a new species could not even be formed or, after formation, would again merge with the original species. However, the areas of occurrence of the individual species are frequently not static and grow larger or smaller or are shifted in response to changes in the natural conditions. Thus, it can occur that, in time, two species, that were not originally in contact or were in only very limited contact, come into close contact or their areas of occurrence can even merge. Under these circumstances, there are three possible consequences. If internal (e.g. ethological or ecological) barriers have formed in the species to prevent crossing, the two species can live sympatrically, i.e. in the same territory, one next to the other, or the ecologically better adapted species can suppress the less well adapted species over a certain area. If no reproduction barriers have been formed in the species, the two species can fuse to form a single species, even if they were not originally sibling species – i.e. species that were formed by a single speciation from a common ancestor. If the species have already formed certain reproduction barriers, the contact of the areas of occurrence of two related sexual species usually leads to the formation of a secondary hybrid zone along the line of secondary contact of the two areas. Crossing of the members of two different species occurs at sites in the hybrid zone and thus their hybrids are encountered here. This zone is usually very narrow but may extend for hundreds of kilometers across a continent. The alleles of the individual genes characteristic for one or the other species penetrate through the hybrid zone into the area of occurrence of the other species to different depths (Fig. XX.7), so that detailed genetic studies generally demonstrate that the edges of the hybrid zone are fuzzy (the zone has a different width for each gene) and asymmetric from the viewpoint of the two species – the alleles of a certain gene of the first species penetrate deep into the area of the second species, while the relevant alleles of the second species do not penetrate at all or penetrate to only small distances into the area of the first species (Gündüz et al. 2001). The depth to which a particular allele penetrates depends on the degree to which this allele is compatible with the alleles occurring in the neighboring species, i.e. the degree to which it reduces the fitness of its carrier when combined with the alleles of the genes of the foreign species. However, other phenomena also contribute to the formation of asymmetry, for example differences in the migration activities of the two species and asymmetry in the reproduction barriers – the males of the first species might be able to successfully reproduce with the females of the second species, but the males of the second species are not capable of successful reproduction with the females of the first species (Tiffin, Olson, & Moyle 2001). Some zones are static and remain in the same place for a long time, while others are mobile and move at a certain rate in both directions. In this case, the area of occurrence of one species will gradually expand at the expense of the area of occurrence of the other species.
At a genetic level, a hybrid zone is typically characterized by unusually high occurrence of some otherwise very rare alleles (Schilthuizen, Hoekstra, & Gittenberger 1999; Schilthuizen, Hoekstra, & Gittenberger 2001). Their elevated occurrence is apparently a consequence of epistatic interactions between the alleles derived from one or the other species. Such a combination of alleles cannot occur outside of the hybrid zone, so that the alleles that function very well for these combinations, i.e. increase the fitness of the carriers of alleles derived from different species, are rare there.
The chapter dealing with speciation will discuss the formation of a new species through interspecific hybridization. However, species are formed in evolution not only through unique hybridization events but, rather, it sometimes occurs that they can exist in nature over long periods of time through interspecific hybridization, appearing anew in each generation, i.e. through hybridogenesis. It should be mentioned, however, that some authors, especially botanists, use the term hybridogenesis to denote any formation of a species through interspecific crossing.
Kleptospecies are an extreme example of real hybridogenesis. The green water frog (Rana esculenta) is a well-known example of such a species (Reyer et al. 2003). This locally very abundant species is formed by hybridization of two species, the marsh frog (R. ridibunda) and the pool frog (R. lessonae)(Fig. XX.8). The genes of both parent species participate in the formation of the somatic tissues of the green water frog; however, the chromosomes originally derived from the pool frog are eliminated in the germinal cell line in the Western part of Europe. In contrast, in the Eastern part of Europe, the green water frog eliminates the chromosomes derived from the marsh frog. Thus, if two green water frogs were to reproduce together (which does not occur very often because males and females rarely exist in the same population), the progeny would have the genotype of the marsh frog in the Western part of Europe. Similarly, if the green water frog were to reproduce with the marsh frog, all the progeny would have the genotype of the marsh frog. Crossing of the green water frog with the pool frog would again yield only the green water frog. A group of all three types of frogs together is sometimes called a synklepton. This situation is very advantageous from the viewpoint of the marsh frog (in the Western part of Europe) because it combines the advantages of both sexual and asexual reproduction. Green water frogs maintain the same amount of intraspecific variability as a sexually reproducing species (so that, for example, it does not so readily submit in the coevolutionary battle with parasites) and, simultaneously, a marsh frog that crosses with the pool frog need not pay the two-fold genetic cost of sex (the cost of meiosis, see XIII.2.3)) as its progeny will pass only their own genes on to the next generation and not the genes of their sexual partners. On the other hand, the pool frog can only lose in this situation as its genes that find themselves in the bodies of green water frogs cannot be passed on to further generations. From its point of view, the green water frogs (in actual fact primarily marsh frogs) “steal” its gametes and frequently also its ecological niche (from which is derived the name of the phenomenon – “klepto” – to steal).
Cases where one species steals gametes (microgametes) from another species are relatively common in nature, but rarely lead to the formation of kleptospecies. In most cases, the microgametes of one species only activate the development of the macrogametes of the other species and its genes do not participate in any way in formation of the bodies (and, of course, also the gametes). This type of parthenogenetic reproduction is called gynogenesis. In the greatest number of cases, parthenogenetic females of a polyploid species that cannot reproduce sexually because of their polyploidy, steal gametes in this way. This situation is, of course, not as advantageous for the female as the formation of kleptospecies. The females avoid the two-fold cost of meiosis and also the two-fold cost of males, but, in parthenogenetic reproduction the genetic polymorphism is constantly reduced in the given line which, in time, can lead to losing out in the co-evolutionary battle with parasites or with sexually reproducing competitors of the same or related species.
The hypercycle model, which was developed mainly by Nobel prize winner for chemistry, Manfred Eigen, is a theoretical model of a self-replicating system, whose elements are arranged in a cycle, in which every element (enzyme, protoenzyme?) somehow assists in the formation of one or more further elements (Eigen & Winkler-Oswatitsch 1992). Simultaneously, the elements of a cycle are mutually connected only functionally, i.e. through their functions in the cycle, and need not be spatially related, e.g. concentrated in structures of the coacervate type. Mathematical models have demonstrated that mutual competition may occur in a medium containing the components of several different hypercycles. Thus, biological evolution can occur under certain conditions.
The hypothesis is very interesting from the point of view of the theory of systems. Its chief importance for protobiology lies in the fact that it permitted study of the aspect of whether the formation of spatially delimited structures is a necessary condition for the functioning of natural selection and thus a necessary condition for the emergence of biological evolution. However, the hypothesis does not deal with how systems of the hypercycle type could be specifically formed in an abiotic medium, i.e. from which chemical components they could be formed. Moreover, the most frequently considered hypercycle model, consisting in a system of functionally interconnected molecules with enzymatic activity capable of self-reproduction as a whole (Fig. X.5), seems somewhat unrealistic from the viewpoint of modern protobiology. The study of mathematical models of competition between spatially undelimited hypercycles finally indicated that the stability of such a system is seriously endangered by the possibility of formation of parasitic hypercycles, i.e. hypercycles that take some components from other hypercycles, but do not return anything to the system in their place. These parasitic hypercycles can reproduce substantially faster than complete hypercycles and can thus relatively easily disrupt the entire system (Cronhjort 1995). This indicates that compartmentation of the individual proto-organisms is apparently an essential precondition for the emergence of biological evolution.