ISSN 1062-3590, Biology Bulletin, 2008, Vol. 35, No. 3, pp. 318–326. © Pleiades Publishing, Inc., 2008. Original Russian Text © Z.S. Kaufman, 2008, published in Izvestiya Akademii Nauk, Seriya Biologicheskaya, 2008, No. 3, pp. 369–378.
DISCUSSIONS
Some Problems of Regressive Evolution Z. S. Kaufman Institute of Water Problems of the North, Karelian Research Center, Russian Academy of Sciences, pr. Aleksandra Nevskogo 50, Petrozavodsk, 185030 Republic of Karelia, Russia e-mail:
[email protected] Received June 6, 2006
Abstract—The concept of morphophysiological regress as one of the main ways to biological progress, as well as its major factors (the sedentary and parasitic modes of life), are discussed. Some notions of regressive evolution are critically reviewed. Special attention is paid to evolutionary transformations of the nervous system, one of the main integrating factors in the body. All theories of evolutionary progress based on sedentary organisms are demonstrated to be untenable. The entire progressive evolution of Metazoa has been related to mobile life. Since regressive trends are common in the evolution, the phylogenetic tree of Metazoa requires serious revision. DOI: 10.1134/S106235900803014X
One of the main evolutionary goals of speciation is biological progress, including the maximum possible increase in the species numbers, extension of the ecological potential, and either complication or simplification of organization. Severtsov (1939) and Schmalhausen (1968a) demonstrated that the main ways of biological progress are morphophysiological progress and regress. The choice between these ways is determined by the environment and mode of life of the species. Progressive evolution occurs in species characterized by an active, mobile mode of life under complex, changing environmental conditions. An evolving species becomes increasingly integrated, its information capacity increases, the degree of dependence on the environment decreases, and, hence, its ecological niche widens. Most free-living Metazoa have evolved in this way. Conversely, regressive evolution is accompanied by narrowing the ecological niche in a monotonous and extremely simplified environment and ceasing the mobile life. In this case, some organs and systems become maladaptive and are completely or partly reduced. The general structure of the body is simplified and disintegrated, with previously formed correlations being broken. Therefore, an organism becomes more dependent on the environment. This is the main evolutionary pathway of parasitic and sedentary Metazoa. Morphophysiological progress has been studied in sufficient detail, and its interpretations do not raise serious doubt (Sukhodolets, 1992); however, the notion of morphophysiological regress remains a matter of heated debates (Kuperman, 1988; Krasnoshchekov, 1990; Yastrebov, 1992; Krylov, 1993; Pozdnyakov, 1994; etc.).
Apparently, the sedentary and parasitic modes of life first appeared at the very dawn of biological evolution and have appeared many times afterwards. Organisms that have become sedentary or parasitic are subjected to permanent degeneration and disintegration. The longer they remain sedentary or parasitic and the higher the organization level of their ancestral forms, the more profound the changes (Polyanskii, 1964; Kaufmann, 2000). This improves their adaptation to these peculiar conditions. Therefore, the morphological series of these organisms should be constructed as a series of simplifications from more highly organized ancestral forms that did not yet acquire the corresponding regressive changes to forms that have long since become sedentary or parasitic and have accumulated profound degenerative changes in their entire organization. These modes of life create the conditions for extreme manifestations of regress. Sponges (Porifera) were apparently the first group of recent Metazoa to make the transition to sedentary life in the Ordovician. They have changed the most profoundly. It is generally known that their organizational level and organismal integrity are the lowest. However, their ancestors are likely to have had a more complex organization. This is evidenced by the regular and determinate cleavage of eggs, which is not characteristic of animals with a low organizational level. Their larvae are motile and have pigment spots regarded as simple eyes, and they display photopreference. It is assumed that they also have a kind of the nervous system. This may play a recapitulation role. In addition, many of the most ancient fossil sponges exhibit a distinct radial symmetry. These data indicate that the evolution of sponges occurred via simplification of organization. Modern forms are degraded, compared to ancient forms. Sponges are not primarily primitive;
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they have undergone secondary simplification due to a long period of sedentary life. Their evolutionary pathway is regress. Therefore, the interpretation of the morphological series of sponges regarding it as a series with a distinctly progressive trend, rather than a series of regressive transformations (which would have been logical), is hardly justifiable. Nevertheless, their evolution is considered progressive (Koltun, 1988). This postulate incorrectly reflects the vector of the evolution of Porifera; it would have been correct if the series were considered in the opposite direction. The same tendency was found in the morphological series of Brachiopoda, another phylum of sedentary animals. The most highly organized forms are dated to the Early and Middle Paleozoic (the Silurian and Devonian periods). Afterwards, their structure was simplified; i.e., evolutionarily earlier brachiopods also had a higher organizational level than later brachiopods. All their evolution was also a series of simplifications. The evolution of brachiopods, as well as that of sponges, exhibits a distinct regressive trend. Davitashvili (1972) noted this phenomenon but could not explain it. It upset the author’s evolutionary theory, which, strange as it may seem, did not include regression evolution at all. In addition, when analyzing a series, Davitashvili took into account changes in individual organs but not the entire body. Thus, morphological series of sedentary organisms should be directed towards simplification of organization, from more highly organized ancient forms to evolutionarily young forms with simplified organization. Degenerative changes are characteristic of all sedentary metazoans. This explains the specificity of their organization and their unclear taxonomic position (Boguta, 1984; Dewel, 2000; etc.). In parasitic organisms, regressive transformations are even deeper, because food is always abundant (they practically live in their food), and, hence, the problem of procuring it—the key problem for free-living organisms—lacks importance for them. In addition, the host is responsible for a considerable proportion of adaptations, relieving the parasite from the necessity to possess them. Especially profound transformations have occurred in parasitic mollusks and crustaceans, whose ancestors had a high organizational level (Iwanow, 1937a, 1937b, 1947, 1952, etc.; Vagin, 1949, 1951; Gruzov, 1963, 1965; Ginetsinskaya and Dobrovol’skii, 1978; etc.). Representatives of the family Enteroconchidae parasitizing marine invertebrates exhibit the highest degree of reduction among gastropods. For example, the respiratory, circulatory, excretory, and nervous systems and sense organs have entirely disappeared in Parenteroxenos dogieli, and its digestive system has been considerably reduced. Among crustaceans, Sacculina and Ascothoracidae (Rhizocephala) are the most reduced forms: their bodies are branched, saclike structures entirely consisting of the surface epithelium and gonads. Parasitic Copepoda have underBIOLOGY BULLETIN
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gone profound changes. Their dwarf males have reduced to mere testes (in Lernaeoidea and other families) (Makarevich, 1956). Morphological series of parasitic organisms, as well as those of sedentary organisms, should begin with more ancient forms, which have retained, to a certain degree, the original, higher organizational level, and proceed towards modern forms, which have been simplified during the evolutionarily long period of parasitic life. Notwithstanding the extremely low organizational level, some organs responsible for the main vital functions may be strikingly well-developed. Such are, e.g., the attachment apparatus, reproductive system, and, especially, covering epithelium of forms lacking the intestine. In this case, the epithelium fulfils the digestive, absorption, osmoregulatory, transport, secretory, excretory, and protective functions; i.e., it has become a multifunctional organ. Multifunctional organs should also be regarded as a sign of regress. In the ancestral forms of Diploblastica, two organs, the ectoderm and endoderm, fulfilled all functions. Progressive evolution is always directed towards higher differentiation and specialization of organs and functions. Individual organs of many sedentary organisms are also extremely well-developed. Note, e.g., their extraordinarily complicated and astonishingly effective filtration apparatuses. For example, the sponge Ephydatia fluviatilis can, during one day, pump through its body a volume of water that is 1200 times greater than its body volume (Kilian, 1952); another sponge, Halichoindria melanodovia, filters water at a rate of 1.42 l/h (Sorokin, 1972). The filtration apparatus of bivalves retains 90–100% of suspended particles no smaller than 1–2 µm (Kondrat’ev, 1970). However, their metabolic rate is still extremely low, which is related to their immobility (Tsikhon-Lukanina, 1987). Moreover, transition to filter feeding is accompanied by return to the primitive intracellular digestion. Ascidia retaining cavitary digestion are an exception. An example of other adaptations compensating for the functions of degraded organs is the ability to extract specific ions and molecules from water, even if their concentrations are negligibly small. Some forms have developed obligate symbiotic relationships with algae or bacteria. More than 100 genera of sedentary Cnidaria, especially hermatypic corals, as well as the mollusk Tridacna gigas, have symbiotic algae. Sulfoxidizing bacteria are symbionts of Pogonophora and some Bivalvia lacking the alimentary canal (Solemia, Petrasma, Achrax, etc.) (Kuznetsov and Shileiko, 1984). In essence, they have become autotrophic. Some authors regard the high degree of development of individual organs or functions in parasitic and sedentary organisms as evidence for their progressive evolution (Vagin, 1951; Krasnoshchekov, 1990; Yastrebov, 1992; etc.). However, we cannot agree with this conclusion. These authors consider the degree of development of individual organs but not the organism as a
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whole, whereas precisely the organism is the object of selection (Schmalhausen, 1968b, 1983, etc.). Not only do not sedentary animals, which live under very simple conditions, need a high degree of organization, but it is counterindicated for them, because it poses an excessive energy load. Only the maximum possible simplification of the general organization and mode of life can ensure their existence and biological progress (Kaufman, 2000). A decrease in integration, which is inevitable in the cases of parasitic and sedentary modes of life, substantially increases the range of regeneration. Note that, among Cnidaria, the mobile medusae almost completely lack the capacity for regeneration, whereas their sedentary forms (polyps and scyphistomas) astound with the diversity of regenerative processes. The medusae that have become sedentary (Lucernaria) have recovered the entire set of these capacities. On the basis of well-developed regeneration capacity, sexual reproduction is supplemented with asexual one. Sexual reproduction, which was once a vitally important function, has been gradually replaced by asexual reproduction as the duration of sedentary life increased in the course of evolution. In other words, the highest form of reproduction has been reduced to a primitive vegetative reproduction. This is true for almost all Mollicuta and some other sedentary organisms. In higher plants, it has become considerably more widespread, having entirely replaced sexual reproduction in some species (Kaufman, 1993). The sedentary mode of life became the decisive factor of the appearance and evolution of Metaphyta; therefore, all their adaptations to sedentary life take extreme forms. The general evolutionary pathway of Metazoa is an increase in the internal surface area and a decrease in the external surface, the maximum possible integration of the body, and a trend towards oligomerization. In contrast, the evolution of Metaphyta is accompanied by an increase in the external (chlorophyll-bearing) surface, which requires a growing connection with the substrate, polymerization of all organs except for the flower, and a very low integration. The plants that have remained mobile (i.e., metaphytes) cannot become multicellular. They are destined for an evolutionary dead end (e.g., Volvocaceae) (Kaufman, 2000). Vegetative reproduction is also characteristic of many endoparasites, including trematodes (the sporocyst of Leukochloridium paradoxum), cestodes (strobilation at the adult and larval stages of Cyclophyllidea), the crustaceans Rhizocephala (Sacculina carcini and Tompsonia japonica), and even the insects ichneumons (polyembryony), although it is generally untypical of arthropods (Ivanova-Kazas, 1977; etc.). Asexual reproduction not only serves as auxiliary to sexual one, but it also dramatically increases the total fecundity, which is very important for parasites and animals with a low
organizational level: a tremendous quantity compensates for a low quality. A decrease in the degree of organization, polymerization of organs, general disintegration, and the resultant increase in the separatism of individual parts of the body eventually lead an organism to a state of a colony. Colonies are rare among mobile forms but are characteristic of sedentary forms (hydroid polyps, hermatypic corals, bryozoans, ascidians, etc.). They should be considered to be evolutionarily secondary formations. In this relation, cestodes are the most interesting among endoparasites. It is known that the body of most cestodes is segmented into metameres, or proglottids, and is regarded as a colony (Beklemishev, 1964a; Iwanow, 1979; Oshmarin, 1981; Oshmarin and Stepanov, 1986; Ivanova-Kazas, 1995; etc.). Other endoparasitic flatworms have no colonial state because they are evolutionarily young. Cestodes are the oldest endoparasites (Fedotov, 1966; Ginetsinskaya, 1968; etc.). The evolutionarily long period of parasitic life reduced the level of their integration to the state of a colony. Cestodes vary in the degree of coloniality. They are subdivided into anapolytic (monozoic) species, which are the most integrated and lack metamery (many Pseudophyllidea, e.g., Ligula and Triaenophorus; Caryophyllaeidea; etc.) and apolytic (polyzoic) species, whose body is divided into metameres. The metamery of cestodes is interpreted in terms of two main theories: the monozoic or metameric theory and the polyzoic or strobilar theory. According to the former, a whole individual (such as Ligula) is the original form, proglottids having resulted from polymerization in the course of its evolution. According to the latter, conversely, metameric cestodes considered as a colony are the original forms, whereas cestodes lacking metamery are secondary forms, having resulted from progressive integration of proglottids to form a single individual. However, this theory indicates neither the cause of this integration nor its mechanisms (Beklemishev, 1964a; Iwanow, 1979; Dogel’, 1981; etc.). Among Neodermata, monogenean-like worms are most likely to have been direct predecessors of cestodes. Having adapted to the intestine of invertebrates, they acquired features characteristic of cestodes. Thus, cestodes were originally monozoic (Timofeeva, 2005). These forms are also considered to be the oldest. For example, Caryophyllidae, apparently, segregated from ancestral forms in the Early Paleozoic or even Proterozoic (Kulikovskaya, 1976; Kulikovskaya and Demshin, 1978). The long period of parasitic life accompanied by organizational simplification and disintegration has eventually made these animals polyzoic, led them to the state of colonies, which is logical to consider a secondary phenomenon. Segmentation of the anterior body and multiple anlagen of gonads in some Ligulidae and Diphyllobotriidae (Dubinina, 1966 etc.) should be regarded as iniBIOLOGY BULLETIN
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tial stages of transformation into colonies, rather than vestiges of former segmentation and signs of the developing integration, as the polyzoic theory postulates. This segmentation appears late in ontogeny (close to its end), which also confirms that it is evolutionarily late. It is impossible to agree that the monozoic structure is secondary; i.e., it has resulted from integration of proglottids into a single individual, which implies a morphologically progressive evolution of cestodes (Ginetsinskaya and Dobrovol’skii, 1978; Iwanow, 1979; etc.). It is difficult to imagine what factors and how could have caused this integration. Transformation of colonies into single, integrated organisms have never been found in Metazoa. This contradicts the law of evolution irreversibility. As noted above, disintegration is one of the most important elements of changes caused by parasitic and sedentary modes of life. It concerns not only definitive stages, but is also extended to embryonic development and even the system of cleavage. Cleavage of almost all sedentary organisms exhibits elements of disintegration and is variable and asynchronous (Ascidia are an exception to this rule too: their cleavage is bilateral and strictly determinate, which is probably because they belong to chordates). This is especially pronounced in the Cnidaria whose life cycle contains sedentary stages. Their cleavage lacks any signs of integration and is irregular or “anarchic” (Kaufman, 1988, 1990, 2004, etc.). In contrast, the Cnidaria that have no sedentary stages are characterized by regular radial cleavage; this indicates that irregular cleavage is evolutionarily secondary and has resulted precisely from sedentary life, rather than any other factors. In the case of anarchic cleavage, blastomeres may take any positions, even those seemingly corresponding to various other types of cleavage. This prompted some authors to derive all the remaining types of cleavage from it (Zhinkin, 1951; Ivanova-Kazas, 1995). However, we cannot agree with this concept. It is unlikely that a random arrangement of blastomeres, even if it apparently resembles another type of cleavage, may have been genetically fixed and become a characteristic element of the ontogeny of a group of animals. As to the cleavage pattern of parasitic flatworms, it is also anarchic (Rubicka, 1966; Ivanova-Kazas, 1975; Bazidov, 1976; Bazidov et al., 1978; Bazidov and Lyapkalo, 1980; Dobrovol’skii and Mukhamedov, 1983; etc.). The disintegration of the cleavage pattern of Neodermata is undoubtedly secondary and is determined by the general disintegration of the body caused by parasitism. Yolk cells also could make cleavage chaotic; however, they are too few in trematodes and cestodes to destabilize cleavage (Ivanova-Kazas, 1959). Their free-living ancestors had a spiral cleavage. Its traces are still discernible in the ontogeny of cestodes. Embryonization of development is undoubtedly the most important indicator of progressive evolution and prerequisite for occupying new ecological niches. The BIOLOGY BULLETIN
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protection of postembryonic stages with the envelope(s) of the egg is a higher step in the evolution of ontogeny. However, the opposite process, so-called deembryonization, when the embryonic period is shortened and hatching occurs at increasingly earlier developmental stages, has also been observed. De-embryonization is extremely rare in metazoic organisms, where it is mainly found in parasitic flatworms (IvanovaKazas, 1975; Gulyaev, 1996; Timofeeva, 2005; etc.) and parasitic insects (Ivanova-Kazas, 1948, 1961). The degree of de-embryonization increases in parallel with adaptation to parasitism, i.e., from monogeneans to cestodes. It is not only an expression of the general regressive trend of the evolution of parasitic organisms, but an important condition of their existence ensuring a considerable increase in fecundity. De-embryonization is well-developed precisely in the polyzoic cestodes, the evolutionarily oldest forms with the most disintegrated body structure (Timofeeva, 2005). The ontogeny of Echinodermata is one of the most primitive and de-embryonized among Metazoa. Their embryos become free-living as early at the blastula stage, which may be a consequence of a long period of sedentary life that they have passed in the course of evolution. Since the nervous system plays a special role in integration, the degree of its development is one of the most important indicators of the level of progressive evolution (Zavarzin, 1950; Beklemishev, 1964a, 1964b; etc.). Its structural characteristics and the degree of its centralizing function in sedentary and parasitic organisms are particularly interesting in terms of understanding the trends of their evolution. The nervous system of Cnidaria substantially differs in structure from those of all other animals. Its characteristic features are accounted for by the facts that cnidarians are a very old group and that a considerable proportion of their life cycle, or the entire cycle, is sedentary. For example, this nervous system consists of the ectodermal and endodermal diffuse neuronal networks, the cells being already well differentiated. Apparently, two nervous systems develop because the two embryonic cell layers of cnidarians are still little differentiated from each other. They are practically under equal conditions, both being washed with water and the internal and external environments only slightly differing from each other (Kaufman, 1990). However, it cannot be excluded that the existence of several nervous systems is related to sedentary life. Indeed, echinoderms have three nervous systems. Regarding centralization, the nervous systems of radial animals (cnidarians and echinoderms) remains at the level of primitive aggregations of neurons that cannot yet be regarded as ganglia. This is mainly determined by the sedentary life and, as a consequence, radial symmetry. The radial structure merely constructively precludes centralization of the nervous system
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and its progressive development and functioning (Boguta, 1984). Medusae lack the aboral organ, a single coordinating nervous center whose importance is obvious, because their life cycle includes a sedentary period, with free-living planulae metamorphosing into sedentary polyps or scyphistomas. Planulae settle on the substrate and attach to it with the anterior end of the body, where most nervous cells are aggregated. These cells die upon attachment. The opposite end, which gives rise to medusae or ephyrae, does contain enough nervous cells to form a nervous center. The same occurs in larvae of other sedentary animals (Kamptozoa, Phoronida, Brachiopoda, Bryozoa, Hemichordata, Pogonophora and Chordata); they also attach to the substrate at the aboral part of the body and lose their well-developed aboral nervous center. Their definitive nervous centers are poorly developed and their origin is not related to the aboral organ. They are termed secondarily cerebral centers (Malakhov, 1991). In Ctenophora, which live under the same conditions as medusae but lack a sedentary stage in their life cycle, the aboral organ is well developed. Radial symmetry is known to develop as a result of an evolutionarily long period of sedentary life (sometimes, it develops in planktonic forms) (Gilyarov, 1944; Dogel’, 1981; etc.). It appears not only in animals but also in the sporophores of higher fungi and in higher plants. Its pronounced development in cestodes is also secondary and is largely determined by endoparasitism and immobility (Beklemishev, 1964a). Development of radial symmetry increases the number of elements and enhances the repetition of body parts; i.e., a kind of structural polymerization takes place. This weakens integrative connections and decreases the integrity of the body. In contrast, bilateral symmetry developing during the evolution of mobile organisms is poorer in elements (oligomerized) and ensures a higher level of integration. Thus, the structural similarity of the nervous systems of different groups of animals with radially symmetrical bodies should be regarded as convergence caused by a similar (sedentary) mode of life, and as a secondary simplification. Apparently, their free-living ancestors were bilateral animals and had another, more advanced type of the nervous system. This is evidenced by the combination of well-developed neurons and a poorly developed nervous system in Radialia. The absence of radial symmetry in some other sedentary forms is explained by an evolutionarily shorter period of their sedentary life. Substantial changes in the nervous apparatus are also found in sedentary Polychaeta. In most species that lose parapodia, the pedal ganglion also disappears (or only its rudiments remain), and sense organs degrade. The subdivision of the brain into lobes disappears in representatives of some families; in some forms, it is
reduced to a transverse strip. The stalked bodies are also reduced (Beklemishev, 1964a, 1964b; etc.). Comparing the nervous system of the sedentary polychaete Owenia fusiformis with those of other polychaetes, Bubko and Minichev (1972) note its marked primitiveness determined by the sedentary mode of life. However, they consider this state to be primary and original for the nervous systems of all other polychaetes. This concept is hardly acceptable. The low degree of organization of Oweniidae is secondary, caused by sedentary life, and for this reason alone cannot be considered original. One should search for the original type of the nervous system only among Erantia (errant polychaetes). A typical nervous system of flatworms is orthogonal and has a well-developed endon (cerebral ganglion) located within the mass of parenchyma, three or four pairs of posterior and several pairs of anterior longitudinal nerve trunks. However, this structure has substantially changed in cestodes. They have lost all sense organs, including the statocyst, which are factors of the development of the nervous system. The endon is absent or underdeveloped, being reduced to bulges of the lateral nerve trunks of the orthogon; however, the number of longitudinal trunks is increased to 60 pairs (Kotikova, 1976, 1977, 1979; Kuperman, 1988; etc.). Despite this morphological complexity, its functional capacity is still low, compared to that of the nervous system of the ancestral turbellarians of the order Rhabdocoela. Larvae of cestodes have only three pairs of nerve trunks, as most of other flatworms have, but their development is retarded. This confirms that the increased number of nerve trunks is evolutionarily secondary. Ioffe (1990) believes that the increase in the number of nerve trunks in cestodes was caused by the considerable increase in their body length, which made it necessary to enhance integrative mechanisms. If so, we should have expected the number of longitudinal trunks to be considerably increased in Baikaloplana, the giant Triclada from Lake Baikal as long as 40 cm, but this is not actually the case. Like all planarians, they have three pairs of nerve trunks, but the pattern of their nervous network has changed. It has become cellulate and has subdivided into the ventral and dorsal networks. Large Temnocephalida have the same structure of the nervous system. Interestingly, the number of longitudinal nerve trunks is increased in the turbellarian Gaffilla; however, this has occurred because it has become a parasite. In general, an increase in body size does not lead to complication of organization. The increase in the number of nerve trunks (their polymerization) decentralizes the nervous system, decreasing its integrative capacity. Forms with large numbers of nerve trunks are likely to have long since become endoparasites; they should be considered more primitive forms that have the lowest organismal integrity and, hence, are better adapted to their mode of life. BIOLOGY BULLETIN
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It is noteworthy that the fine structure of neurons is also substantially changed. It is considerably simplified. The neurons of these organisms are characterized by a poorly developed endoplasmic reticulum, an increased number of free ribosomes, a decreased number of mitochondria, and an altered mitochondrial internal structure. Comparison of ultrastructural organization of the nervous system has shown its simplification in the order Turbellaria–Monogenea–Trematoda–Cestoda (Golubev, 1976, 1978, 1982). All this makes nerve cells of cestodes the most primitive evolutionary models of neurons. In the light of the above, the notion concerning an integrative role of the increased number of nerve trunks in cestodes and a progressive nature of this phenomenon, put forward by Kotikova (1976, 1977, 1979) and Ioffe (1990), seems ill-grounded. It is well known (Zavarzin, 1950; Beklemishev, 1964a, 1964b; Boguta, 1984; etc.) that progressive evolution of the trunk nervous system of Bilateralia proceeds in the opposite direction, towards oligomerization, a decrease in the number of nerve trunks and formation of a single, major medial trunk (the monoaxial state). Only this state can enhance the centralizing role of the nervous system. In cestodes, the main trend of the evolution of the nervous system is disintegrating rather than integrating. In the sedentary polychaetes Oweniidae, the brain has already undergone some degenerative changes, but the longitudinal nerve trunk is still at the high degree of development. The reason is that these polychaetes only recently became sedentary. Such changes typically begin with the cranial division of the nervous system, gradually spreading caudally. In summary, biological progress can be achieved both via progressive evolution (mobile, free-living forms) and via morphophysiological regress (parasitic and sedentary forms). The greater the simplification, the better the organism is adapted to its specific habitat. Although some organs and functions of these simplified organisms may be very well-developed, the integration of the body as a whole remains low, and precisely this integration determines the degree of organization (Schmalhausen, 1968b; Malevich, 1972; Kuperman, 1988; Kaufman et al., 2000; etc.) Regressive evolution affects not only the entire morphological and physiological organization of an animal, but also its genetic system. Some of the hereditary information stored in genes is lost, the haploid chromosome number is decreased, and the chromosome sizes are reduced (Stebbins, 1950, 1971), which makes evolution irreversible. Parasitic forms, in which these changes are more profound, cannot return to the former mode of life. All evolutionary pathways except regressive are closed for them. The return is not excluded, however, for some sedentary forms in which degenerative changes are not so deep (echinoderms). Nevertheless, even if they again become mobile, they will never BIOLOGY BULLETIN
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be absolutely free of the changes caused by their previous, sedentary state; they cannot resume evolutionary progress and are destined to remain an evolutionary dead end. In this connection, it should be emphasized that all theories of evolutionary progress in which sedentary organisms are assumed to be original, founder forms (Zakhvatkin’s synzoospore theory, Jegerstren’s bilatogastrea theory, Hadzi’s theory of polyenergid origin of Metazoa, Remane’s spiral theory, etc.) are a lost cause. Evolutionary progress is related exclusively to mobile forms (Kaufman, 2000). The only exception is Protozoa, in which sedentary and parasitic modes of life are the factors of progress (except for intracellular parasites) (Shul’man, 1968; Polyanskii, 1964; etc.). Recent molecular biological data put to question the notion that the common ancestor of Metazoa had radial symmetry; i.e., that it was a sedentary form; therefore, the problem of the origin of bilateral symmetry from the primary radial one was ill-posed from the very beginning (Martindale and Henry, 1999; Dewel, 2000; Aleshin and Petrov, 2002; etc.). The origin of Turbellaria, particularly Acoela, is undoubtedly the key problem of evolutionary morphology. Its solution will determine the understanding of the origin of multicellularity, bilaterality, metamery, spiral cleavage, etc. However, this question remains unanswered thus far. Some researchers consider the simple structure of Acoela to be primary and original for all Bilateralia. Others believe that it has resulted from secondary simplification of hypothetical ancestors that had a coelom and were segmented; i.e., their evolutionary pathway was regressive (Iwanow and Mamkaev, 1973). We think that the latter hypothesis is implausible. As repeatedly noted above, regressive evolution is related to sedentary or parasitic life. Neither has been found in Acoela. Conversely, these turbellarians are active, freeliving hunters, and there is no reason for their regressive evolution. The simple organization of Acoela should be considered to be primary. Recent data on molecular phylogeny do not contradict this assumption (Littlewood et al., 2001; Aleshin and Petrov, 2002). If they had undergone regress, progressive evolution would have been impossible for them. Since sedentary and parasitic organisms are secondarily simplified, and their ancestors had a higher degree of organization, the current position of these groups in the general phylogenetic tree should be considered conventional. In conclusion, it should be emphasized that evidence in favor of some or another trend of evolution is mostly based on the analysis of morphological series and some other data of classical evolutionary morphology. However, their trends sometimes have no reliable markers, which allows for different interpretations. Molecular phylogeny, which has been thriving in recent years, employs fundamentally different criteria of phylogenetic relationship. They are based on the analysis of the sequence of monomers in small riboso-
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