Biology Reference
In-Depth Information
In conclusion of this section, both gene duplication and gene loss
must be understood in order to clarify the relationship between genome
evolution and developmental (and hence, morphological) evolution.
3. Developing Bioinformatic Tools
for Evo-Devo
3.1. Defining Homology for Bioinformatics
Homology is one of the most fundamental concepts in biology. It has
also generated abundant terminological and conceptual discussions
(e.g. Hall
69
). It was originally defined based on the similarity of anatom-
ical structures, with emphasis on their relations, thus clarifying that a bird
wing and a mammalian forelimb are homologous. The term later
acquired a historical dimension: homologous structures are assumed to
derive from the same ancestral structure, to which they owe their simi-
larity, whereas analogous structures have converged to similarity from
different ancestral starting points. This definition can create complex sit-
uations, since the bat wing and the bird wing are analogous as wings,
having converged from an ancestral walking limb; but they are homolo-
gous as forelimbs, since they derive from the forelimb of an ancestral
tetrapode. Moreover, different fields of research have developed different
operational definitions
70
: historical homology, defined by phylogenetic
continuity; morphological homology, defined by structural similarity; or
biological homology, defined by similar developmental constraints.
Morphologists also define serial homology between organs repeated
along the axis of the same organism, such as vertebrae (discussed in
McKitrick
71
). Since the original evolutionary formulations of homology,
two fields of research have hugely influenced our view of biological diver-
sity and evolution: molecular evolution and evo-devo.
At the molecular level, homologs between species have been discov-
ered whose origin predates the divergence of bacteria and eukaryotes;
while inside each species, genomes include many families of homolo-
gous genes. To clarify this situation, three main types of molecular
homology are distinguished: orthology, or divergence by speciation;
paralogy, or divergence by sequence duplication; and xenology, or
