Atavism

ATAVISM (from the Latin atavus - great-great-grandfather), the appearance of traits that were not present in the immediate parents of a given individual but existed in its distant ancestors. Phenomena of atavism played a significant role in Darwin's theory as evidence for the phyletic origin of different animal species. Modern data from genetics and experimental embryology have narrowed the concept of atavism, which now does not extend to all traits that unexpectedly appear in offspring. Therefore, the concept of atavism is now encountered much more often in popular scientific literature than in scientific literature. Comparing the phenomena of atavism with genetic data, Plate defines atavism as "individual variations that in one or many traits correspond to or approximately resemble traits of distant ancestors, with which there is obvious or probable genetic affinity." Careful study of atavistic phenomena has shown that they can arise in various ways. Cases that best fit the concept of 'strict atavism' are unexpected (spontaneous, in Plate's terminology) atavisms when traits appear that do not exist in a given species but existed in its ancestors belonging to other systematic categories. Such atavisms include: horses with three toes, of which two are rudimentary (a characteristic trait of Hipparion, an extinct ancestor of horses); the existence of horses with striped coloring, like zebras; the appearance in humans of a tail-like appendage, horns in hornless cows (Suffolk and Galway-ways); pigment on the side of flounders facing the bottom, five-fingered limbs in newts; the appearance of chickens with the coloring of wild chickens (Gallus bankiva); a claw on the first toe of the hind legs of cats and dogs, etc. Sometimes this kind of atavism is explained by abnormalities in embryonic development; for example, cervical fistulas are essentially unresorbed gill slits; in humans, these abnormalities also include polymastia, hypertrichosis, and cryptorchidism. When castrated, a modern bull acquires some features of the primordial bull (Bos primigenius). The most common cases of atavism result from crossing. Darwin saw in it one of the main causes of atavism and in his book 'The Variation of Animals and Plants under Domestication' dedicated one chapter to the question of 'Crossing as a Direct Cause of Atavism.' According to Darwin, atavism in crossing appears because the inherited rudiments (gemmules) of various traits can remain in a latent state for generations, but through hybridization they can come into an active state and thus be revealed in offspring. Modern geneticists tend to the opinion that some traits are the result of the combined action of several factors, which in crossing can be separated or, by entering new combinations, give new traits. This property forms the basis of the artificial production of new breeds through hybridization. An example of the separation of factors can be seen in crossing chickens with pea-shaped combs and chickens with rose-shaped combs, the appearance of chickens in the second generation (F₂) with simple combs. The same can explain that when crossing a black Langshan with a guinea fowl (Numida meleagris), offspring are obtained with a simple head, without helmet and comb, without spurs, with dark reddish coloring resembling wild chickens, and with white transverse bands on the feathers, like in pheasants. From a genetic point of view, atavism can manifest in the first (F₁) or second (F₂) generation. In F₁, traits related to coloring are very often revealed, which are mostly the result of the combination of several factors, as, for example, when crossing different breeds of mice, rabbits, etc. The coloring of wild forms often appears. This also includes the appearance of transverse stripes on the legs of mules, while their parents had no striping at all, the appearance of stripes in a hybrid of a female quagga, etc. In the second generation (F₂), atavistic traits are revealed, mostly determined by recessive genes; thus, when crossing silky chickens with black Minorcas, we have in F₂ a return to the coloring of wild chickens. The same type of atavism includes the well-known Darwinian example of obtaining a wild blue pigeon from crossing domestic breeds of pigeons [P (parents) black pigeon × white peacock gives in the first generation (F₁) black pigeons with some whiteness, and in the second generation (F₂) bluish-gray feathers with stripes on the tail, i.e., a return to the wild pigeon (Columba livia)]. In domestic breeds, a return to the type of wild relatives can be observed when they are freed from the conditions of artificial culture created for them by humans. Thanks to natural selection, those traits that are harmful to them under natural conditions quickly disappear from their phenotype, and conversely, the useful traits of the wild ancestor are restored. Phenomena of atavism are often encountered in the processes of regeneration: thus, instead of an eye in a crab, a claw can grow; during the regeneration of claws in different species of crabs belonging to a later phylogenetic type, for example, Astacus fluviatilis, the restoration of claws of a phylogenetically more ancient type (Astacus leptodactylus) is observed. During the regeneration of legs in orthopterans, feet are restored that resemble the feet of lower forms (Thysanura). In popular literature, phenomena that have nothing in common with atavism are often called atavistic, for example, various pathological phenomena - microcephaly, the appearance of a hare's lip, caused by the cessation of growth of one half of its rudiment, etc. Errors in the definitions of atavism often occur from ignorance of exactly which traits the ancestors of these forms possessed. Many errors in interpreting phenomena as atavistic were also made in the social sciences, when many criminal and antisocial acts were explained as a return to a primitive wild state.