definition of Orthologues, Homologues, etc

James McInerney james.o.mcinerney at may.ie
Thu Oct 12 15:39:03 EST 2000

> Molecular evolutionists accept parology as an example of homolgy. This
> fact has overwhelmed Owen's definition of homology.  There are just
> too many different proteins that clearly evolved from a common ancestral
> form.  The term orthology is in the process of replacing Owen's homology.

I'm not sure that is quite true.  I would be very slow to accept that
just orthologs should be considered homologs, with paralogs being in
some other category.  Owen's definition still stands, but you just have
to pay attention to the second half of the definition "[...] under every
variety of form and function".  Paralogs are homologs and orthologs are
homologs (both would, in the majority of cases, display a significant
amount of sequence similarity that is due to common ancestry).

> This dispute arises from an old problem biologists have been debating since
> Darwin:  Can systematists, whose goal is classification and evolutionary
> biologists, whose goal is reconstructing evolutionary histories, share the
> same
> vocabulary?   I am not sure they can.
> Mike Syvanen

I'll have to be pedantic, I'm afraid.  Systematics is a discipline with
three divisions, as I see it. One is taxonomy (the naming of things),
another is phylogeny (evolutionary biology of sorts, certainly taken to
mean those people that are intrerested in reconstructing phylogenetic
relationships) and also there is identification.  I think your statement
above should replace the word "systematists" with the word "taxonomists".

I'm happy though to stick my neck out and say that we (taxonomists and
phylogeneticists) don't speak different languages and that homology
still means what it did in 1843 (we just know a bit more about biology now).




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