Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

History of evolutionism - neo-Darwinist period

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 W_opdenotes 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 (W_op, 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 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

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

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.

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.

see Gonochorism

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 x_i  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 Π_ij is the number of differences between the i-th and j-th sequence.

The emergence of genetics, a field of science whose theoretical foundations were established in the 19^th 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.

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).

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 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.3.2.2.2).
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.

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.

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.

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.

- 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 

see History of evolutionism - neo-Darwinist period

see Speciation hybridization

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.