Evolutionary Biology - Atavism or reversion - Reappearance of ancestral characteristics скачать в хорошем качестве

Evolutionary Biology - Atavism or reversion - Reappearance of ancestral characteristics

-------------------- ©EXCEL_SCIENCE    / @excel_science   -------------------- ATAVISM OR REVERSION Atavism or reversion is the reappearance of those ancestral characteristics in an organism or in the organisms of a group, which do not occur normally or which represent the reminiscent of normal structures possessed by the individuals of other groups. Such abnormal structures are known as atavistic characters. Examples of Atavism Atavism, of course not very common, is well illustrated by numerous examples: 1. Cervical Fistula in Man: In fishes there are five pharyngeal pouches which open to the exterior. In man, in the normal course, only one pharyngeal pouch perforates to form an opening from the pharynx to the exterior in the form of external ear canal and eustachian tube. But rarely, the neck may possess an additional opening through which throat or nasal cavity communicates with the exterior. This represents the opening of an additional pharyngeal pouch to the exterior and is known as cervical fistula. 2. Tail: Tail is absent in man, but occasionally a human baby is born with a short fleshy tail. It is devoid of vertebrae and is removed by surgeon with no trouble. 3. Mammary Glands: Humans possess just one pair of mammary glands in the pectoral region. The same condition is noted in all the primates but in pigs, these occur in two rows one along either side of the chest and abdomen. Sometimes, extra-mammary glands or their nipples appear in man. 4. Hair on the Body and Face in Irish Dogman: Man is characterised by scanty hair on the body and no hair on face but in the relatives of man (apes) hair are present profusely. A man was born in Russia with profuse development of hair on the face and body (Irish Dogman). 5. Disappearance of Phalanges of 2nd and 4th Digits in Horse: In modem horse the third digit is most prominent and its metacarpal bears a hoof, whereas the 2nd and 4th digits are absent and their metacarpals are represented as splint bones. Occasionally a horse possesses one of the two splint bones (2nd and 4th) with phalanges and rarely with a reduced hoof. This represents the ancestral character since the prehistoric horses possessed three toes in each foot. 6. Homodont Dentition in Piscivorous Cetaceans: Cetaceans are mammals, charactersied by heterodont dentition. But piscivorous cetaceans possess simple, conical homodont teeth somewhat similar to predaceous reptiles or some fishes. Their ancestors are known to possess heterodont teeth of mammals. This could be considered to be a true reversal of evolution.   Types of Atavism 1. Family Atavism: It includes sudden reappearance of a character or characters in the offspring after remaining latent in the family for several generations. This phenomenon is controlled at gene level and can be explained by simple Mendelian laws of inheritance. For example, appearance of red hair in a child, whose parents and grand - parents all possess black hair, but the red hair were seen in some members of the family several generations ago. It means the gene or genes controlling red hair character could not gain expression for several generations due to some specific reason and were transmitted unnoticed or in the latent condition but appeared suddenly. 2. Race Atavism: It includes those cases of reversal where one or more characters of one race appear in the individuals of another race. For example, the profuse growth of hair on the body and face of Irish Dogman and the presence of additional mammary glands or their rudiments in man are race atavistic characters. 3. Atavism of Teratology: This includes the appearance in a race of such abnormal characters which were normal in other supposedly ancestral races. The appearance of cervical fistula in man, which actually corresponds to the gill-slit, or the appearance of external hindlimbs in a humpback whale or the homodont dentition in piscivorous cetaceans are examples of teratology. These changes could not be accounted by the changes in the gene pools of the populations but can be explained by the change in developmental fields. There are developmental fields for each and every structure in the developing embryo. For example, in mammals there are distinct developmental fields for incisors, canines, molars and premolars. Some physical factors such as temperature, pH, or chemical factors may suppress or change the development and differentiation introducing some change in the teeth structures. It means the environment can directly affect the development of a trait. The principle of reversibility was advocated by L. Dollo in 1893 and is now known as Dollo's law. [CSIR NET, GATE, NEET, UPSC, CGPSC, PCS Exams, Science, Evolution] -------------------- Educational Content. This Video (any images) / Channel is for Education / Information / Representation purpose only. Any images are for representation purpose only. -------------------- ©EXCEL_SCIENCE    / @excel_science   --------------------