from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The quality or condition of being homologous.
- n. A homologous relationship or correspondence.
- n. Chemistry The relation of the elements of a periodic family or group.
- n. Chemistry The relation of the organic compounds forming a homologous series.
- n. Mathematics A topological classification of configurations into distinct types that imposes an algebraic structure or hierarchy on families of geometric figures.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A homologous relationship.
- n. A correspondence of structures in two life forms with a common evolutionary origin, such as flippers and hands.
- n. The relationship between the elements in the same group of the periodic table, or between organic compounds in a homologous series.
- n. A theory associating a system of quotient groups to each topological space.
- n. A system of quotient groups associated to a topological space.
- n. The presence of the same series of bases in related genes.
from the GNU version of the Collaborative International Dictionary of English
- n. The quality of being homologous; correspondence; relation.
- n. 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.
- n. 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; , all members of the paraffin series. In an extended sense, the term is applied to the relation between chemical elements of the same group. Cf. Heterology.
from The Century Dictionary and Cyclopedia
- n. The state or character of being homologous; correspondence.
- n. In geometry, the relation between two corresponding figures lying in the same plane which are such that corresponding points are collinear With a fixed point called the center of homology, while corresponding lines intersect on a fixed line called the axis of homology.
- n. In chem., the relationship which the members of a homologous series bear to one another.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. the quality of being similar or corresponding in position or value or structure or function
a part or organ in a different animal; the determination of which homology indicates that such animals are constructed on a common type; when, for example, the correspondence of the basilar process of the human occipital bone with the distinct bone called 'basi-occipital' in a fish or crocodile is shown, the _special homology_ of that process is determined.
By the term homology he expresses the unity or identity of character between the bones so answering to one another in different animals.
Knowledge of homology is certainly very useful, can give us a good idea of the path of descent, and can constrain our hypotheses.
I may be interpreting you wrong, but are you saying that because of the tacit admission that the genetic code and ribosomes and the like evolved multiple times, pretty much anything can evolve that many times independently and therefore the logic used to infer molecular homology, namely that homology is the only logical way to explain the molecular uniformity when there are so many different ways in principle to evolve these structures, is wrong.
I replied that when molecular homology is decoupled from organismal (vertical) descent, as Woese and others have done for universally-shared characters such as the genetic code, unexpected consequences follow for inferences about the branches of the Tree of Life.
Analogy vs. homology is how biologists draw family trees.
Sequence homology is certainly good evidence in support of the HGT – eukaryote hypothesis, but even if there were a known transposon with sequence homology to RAG (and I think there is), an alternative hypothesis might be that the transposon arose through escape of a RAG gene rather than vice versa.
Assume for the sake of argument that everything Nick has written about sequence homology is accurate including his flagellar inferences.
Um, guys, sequence homology is a direct result of the well-understood process of gene duplication and sequence divergence.
However, homology is still a funny word: in the context of proteins and genes, it makes sense only if we don’t think about it too deeply.