Convergence in Biology

CONVERGENCE IN BIOLOGY. In biology, the term 'convergence' is most frequently applied in comparative anatomy, embryology, and paleontology. The concept of C. was introduced by zoologist Oskar Schmidt. It coincides with the concept of homoplasy (homoplasy) of Ray Lankester and parallelism of most phylogeneticists. Some scientists (Abel, Severzev) artificially draw a boundary between C. and parallelism.--In zoology and botany, C. is understood as the convergence or parallelism in individual adaptive traits or a sum of traits (for taxonomic categories of any volume), observed in animals and plants, phylogenetically in relation to the compared traits not related to each other. The incorrect frequent confusion of C. with analogy is often encountered. The polarity of thinking in terms of concepts requires the opposition of C.-divergence, analogy-homology. Moreover, C. occurs both in relation to analogous and homologous organs (the same with divergence). An example of homologous organs is the specific form, structure, and function of the anterior pair of limbs of vertebrates living in the same environment (e.g., fish, plesiosaurs, ichthyosaurs, sea turtles, penguins, and whales), or the reduced pelvis in whales and sirenians. A remarkable example of C. of an entire series of organs, external habitus and behavior on the basis of complete homology of the compared elements is the striking coincidence in the structure of the skeleton (skull, ribs, limbs, vertebrae, etc.), musculature (absence of certain limb muscles), sense organs (rudiments of eyes hidden under the skin) and habitus (absence of external ears, silky fur, etc.) of two in the system far distant from each other burrowing underground mammals: namely-the South African golden mole (Chrysochloris) and the Australian marsupial mole (Notoryctes). In general, the order of marsupials is saturated with convergent forms (marsupial mole, marsupial flying squirrel, marsupial wolf, marsupial ferret, marsupial rat, etc.). Another example of saturation with convergent forms: on the island of Madeira in the Atlantic Ocean, on a small group of volcanic islands in Antarctica and other islands, there is a huge number of species of usually winged insects, either completely having lost wings or having only rudiments thereof (wingless flies, wingless butterflies, etc.); winglessness in these different groups of insects is typical convergence. This also includes the similar yellow coloration of desert animals, the white coloration of many Arctic mammals and birds; absence of clavicle in running ungulates, some running rodents and carnivores, absence of eyes in cave or underground vertebrates. An example of convergence based on analogous organs can be the wings of insects and the wings of birds, the gills of mollusks and fish, the eyes of cephalopods and vertebrates, etc.--From the field of botany, we mention: the structure of stems of succulent or fleshy plants of deserts in different families, expressed in the absence of leaves, stems are swollen and abundantly covered with spines or thorns; another example: the formation by unrelated plants in the Arctic belt and in high mountains (as an adaptation to low temperatures) of loose or dense cushions from numerous closely adjacent shoots, thanks to which the plant retains daytime heat for longer. In relation to plants, it is often very difficult to establish whether we are dealing with true C. (coincidence in genotypic traits), or only with modification (see), or finally with the phenotypic reaction of each individual separately to the given local conditions. In the latter two cases, convergence should be sought not simply in the revealed trait, but in the norm of reaction (example: aquatic and aerial leaves in the same and the same species depending on whether ontogenesis proceeds in water or on land; often both types of leaves are found even in the same individual). All convergent phenomena should be considered secondary formations, arising in the process of phylogenetic development as new formations independently and independently in this phylogenetic series in the order of adaptation to new specific conditions of existence, different from those in which the ancestors of these forms lived.--The phenomenon of C. is particularly characteristic of numerous marine invertebrates leading a sedentary lifestyle, as well as for endoparasites (reduction of locomotor organs, muscular system, sense organs, nervous system and even the intestinal tract).--Of course, the convergence of positive traits is never complete, i.e., such that the coinciding traits or a group of traits in different organisms would be identical. The coincidence is always partial and relative. The impossibility of identity in the coincidence of positive traits is the result of the fact that the history of specific species, their phylogeny, is an irreversible process, since both the organism itself and the environment surrounding it, and the possible interactions between the organism and the environment in all their diversity cannot repeat already existing combinations. The dialectics of the development of nature is the progressive movement of specific matter in the direction of constant qualitative new formations, excluding identity of processes even in the phenomena of the cycle of substances, metabolism and ontogenesis. Every circular, reversible process in actual reality is therefore a process proceeding in a spiral. That is why in cases of C. even within the same systematic group, convergent traits are never absolutely similar, an exact repetition of the traits of other organisms. On the contrary, the convergence of negative traits in the limit of their complete disappearance is repeated exactly, while all degrees of reduction give only relative convergence. The relativity and limitation of C. is especially clearly manifested where convergent formations arise in the form of an analogous organ in the same phylogenetic series of a relatively small systematic category after the disappearance of a certain organ. An excellent example of such a case and at the same time proof of the irreversibility of the phylogenetic process (the so-called 'Dollo's law') is given by the history of leatherback turtles (Dermochelyidae). The marine leatherback turtle has a shell of mosaic bone plates, unlike other turtles that have a complete, typical for turtles bone shell; early ancestors of leatherback turtles were littoral forms (lived on the seashore) and had a complete bone shell; from these ancestors pelagic forms (living in the open sea) originated, almost completely having lost the bone shell; these pelagic forms again passed to a coastal lifestyle, and then a new hard shell was formed, but not a complete bone one, but a mosaic one. With this shell, the modern leatherback turtle (Dermochelys) again passed to life in the open sea. The convergent organs here are the shells (mosaic and complete bone). This example also proves the incorrectness of the usual definition of C. by comparative anatomists and paleontologists, introducing into the definition of convergence the absence of close relationship of the compared groups and not taking into account the possibility of the emergence of analogous organs (complete bone and mosaic shells are analogous, not homologous formations) in a group of morphologically and phylogenetically very close to each other and even forming one phylogenetic series organisms. The phenomenon of C. follows from the general limitation of possible transformations of all real bodies having their own history. The organism develops on the basis of certain limits of variability, conditioned by the stages already passed and creating a certain limitation of genotypic variation. Each new expansion and narrowing of the limits of variation is the result of the interaction of the mutational process and natural selection.--The metaphysical interpretation of C. as a result of the immanent directionality or reversibility of evolution leads to idealistic theories of the 'autonomy' of the evolutionary process, to theories of autogenesis, orthogenesis, and nomogenesis, metaphysically isolating endogenous factors from exogenous and the self-movement of a part from the self-movement of the whole. Such an idealistic understanding of convergence also leads to metaphysical views on the polyphyletic origin of the entire diversity of animal and plant forms. These include the well-known idealistic theories of Lamarck, Nägeli, Cope, Friedmann, Steinmann, Fleischmann, in the USSR Berg, Sobolev and others. Lamarckians and orthogeneticists usually speak of the 'law' of C., although no 'law' of C. exists. C. is not a law or a factor of evolution, but a result of evolution, the interaction of specific organisms with a specific environment. The assertion of the immanence of C. is scientifically untenable. The same conditions, acting on different organisms, cause different transformations both in the direction of C. and divergence.

It is impossible to predict the necessity of a certain direction in the development of organs, functions, or behavior of organisms toward convergence (K.) even with maximum knowledge of external conditions and maximum knowledge of the laws of self-movement of idioplasm, since the interaction of internal and external factors in the phylogenetic aspect is not unambiguous, but multivalent. Consequently, convergence by itself not only explains nothing, but, on the contrary, itself requires explanation. Convergence, like divergence, are problems that are resolved exclusively by the selective theory of C. Darwin.