Homology is still a relatively fuzzy concept in biology. Generally, biologists consider homology to be the similarity of two things (behaviors, structures, genes) due to common descent. In other words, the common ancestor of these two species should have had this thing, which was passed down to those two (modern day) species. Homology in this sense in contrasted with multiple independent origins. For example, the wings of insects and the wings of birds are typically taken to be not homologous, because the common ancestor of insects and birds did not have wings. This was all fine and well until developmental genetics came along...

Developmental genetics has shown us that while wings in these two groups were derived independently, many of the same genes are used to build these two structures. With this new understanding, we are left in a quandary over whether the wings in these species are really derived independently or not- it seems that they are partially homologous (ie partially similar due to common descent).

The reverse case is also a problem: how similar must two things be to be homologous? For example, genes can take on new functions in new species. This gene might be doing different things in two species, but it is present in both of them due to common descent. How different can the two genes be and still be homologous? These are open questions in biology, and are sure to be active areas of research and debate in the near future. Let us know if you have any thoughts...

Finally, this term is often used by molecular biologists when comparing how similar to DNA sequences are. For example, they might say that two genes are "40% homologous", meaning that they have identical bases at 40% of their sites. While it is possible for two genes to be only partially identical by descent (say due to horizontal gene transfer), this is usually not what has occurred in the typical molecular biology use.

Ho*mol"o*gy (?), n. [Gr. agreement. See Homologous.]

1.

The quality of being homologous; correspondence; relation; as, the homology of similar polygons.

2. Biol.

Correspondence or relation in type of structure in contradistinction to similarity of function; as, the relation in structure between the leg and arm of a man; or that between the arm of a man, the fore leg of a horse, the wing of a bird, and the fin of a fish, all these organs being modifications of one type of structure.

Homology indicates genetic relationship, and according to Haeckel special homology should be defined in terms of identity of embryonic origin. See Homotypy, and Homogeny.

3. Chem.

The correspondence or resemblance of substances belonging to the same type or series; a similarity of composition varying by a small, regular difference, and usually attended by a regular variation in physical properties; as, there is an homology between methane, CH4, ethane, C2H6, propane, C3H8, etc., all members of the paraffin series. In an extended sense, the term is applied to the relation between chemical elements of the same group; as, chlorine, bromine, and iodine are said to be in homology with each other. Cf. Heterology.

General homology Biol., the higher relation which a series of parts, or a single part, bears to the fundamental or general type on which the group is constituted. Owen. -- Serial homology Biol., representative or repetitive relation in the segments of the same organism, -- as in the lobster, where the parts follow each other in a straight line or series. Owen. See Homotypy. -- Special homology Biol., the correspondence of a part or organ with those of a different animal, as determined by relative position and connection. Owen.

 

© Webster 1913.

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