[The problem of archetype and current biology].
Two ideas of homology--transformational and taxic--are used in biology. The first one deals with homology of different structures from morphological point of view, the second one--with the homology of characters. The main question of taxic homology is: in what cases the same character is not identical in two different species? Transformational homologies are determined according the archaetype, taxic ones--according inheritance from the common ancestor of comparing taxa. Archaetype is an idea of an organism from the position of its components. Archaetype should be distinguished from the type character, i.e. the description of an organism combining general and special features. The main idea of archaetype is an idea of coherence of characters describing morphological organization. Archaetype was considered by Owen as mechanical construction. As a matter of fact, the organism is a dynamic system Its dynamic nature can be demonstrated by the conception of module organization of living systems. In the framework of this conception archaetype is a description of an organism from constructive position, focused on the characters of the parts reflecting ontogenetic and evolutionary autonomy. The progress in developmental genetics in understanding of genetic mechanisms of spatial structure formation during the last years opens the wide perspectives in interpretation of archaetype idea. Homeobox family of genes of Hox complex is especially interesting from this point of view. They are characterized by colinearity: spatial and temporal sequence of their expression is corresponding to their order in chromosome. As it was shown by several experiments, changes in the level and sequence of expression of Hox genes result in the changes of archaetype. The discovery of homological genes determining non homological morphological structures in non related groups is a new challenge to morphologists studying the problem of homologies. The disagreements on this subject are connected with non critical use of transformational (archaetypical) and taxic approximations. From transformational positions, eyes of vertebrates and invertebrates are homological, although they have different structure. At the same time, if we specify what type of eyes we are considered, the results will change. Thus, compound eyes of insects and bivalve mollusk Arca are not homological because they originated independently from forms without compound eyes.
Shatalkin
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Zoological Museum, M.V. Lomonosov Moscow University, Bolshaya Nikitskaya ul. 6, Moscow 103009, Russia.
D000828:Animal Population Groups / Q000145:classification* / Q000235:genetics; D000818:Animals; D005075:Biological Evolution; D005801:Genes, Homeobox