Serial, Sexual, and Genetic Homology.
While the
evolutionary and phenotypic senses of homology are
the
most fundamental, several derivative senses of the
term are commonly
utilized. Serial, sexual, and genetic
homology apply to entire organs;
amino acid sequences
in proteins and base sequences in DNA allow the
application of homology at the molecular level.
Serial homology refers to more or less identical,
repeating structures in an
organism, for example, the
vertebrae or teeth. The criteria for serial
homology
include phenotypic homology of different structures
in the
same adult, phenotypic homology of structures
in the embryo, similarity of
connections of the repeat-
ing parts, and
phylogenetic arguments. The phenotypic
similarity of teeth, vertebrae,
ribs, etc. of an adult
mammal are obvious. Even if vertebrae in the
adult
were grossly different, however, their claim to serial
homology
might still be made on the basis of the serial
homology of the embryonic
somites from which they
arose. Thus, ontogenetic data can be utilized to
dem-
onstrate serial homology.
Similarity of connections of
the humerus and femur help establish them as
serially
homologous structures. Even if structures were very
different, if they could be shown to have evolved from
structures which
were themselves serially homologous,
then the derived structures might also
be said to exhibit
serial homology.
The occurrence of serially homologous structures
provides an important clue
about both evolution and
ontogeny. It appears that it is relatively easy to
evolve
by changing the number of repeating units which occur
in an
organism. Unfortunately, very little is known
about how repeating
structures are generated, or how
their number is controlled.
Sexual homology refers to structures which differ
between the two sexes of
the same species, but which
derive from a common embryonic rudiment. For
ex-
ample, the mammalian penis and clitoris
are homolo-
gous structures deriving from the
same region of the
genital ridge. The criteria for sexual homology are
therefore the phenotypic homology of embryonic parts
and careful
comparative anatomy of the stages of em-
bryonic development in the two sexes.
In its original sense, genetic homology meant that
if structures in two
organisms were the consequence
of the action of the
“same” gene in both organisms,
then those structures
were homologous. The homology
of the structures was a consequence of the
homology
of the gene(s). Unfortunately, there is no simple corre-
spondence between the genotype and
phenotype of an
organism. Alteration of a single gene may have effects
on many phenotypic traits, and alteration of each of
many genes may have
the same effect on a given
phenotypic character. A consequence of this
com-
plexity is the great difficulty in
trying to prove that
a given phenotypic character in two organisms is
due
to the action of the “same” gene(s) in both. It
is the
common experience of geneticists that if two initial
populations undergo an identical selection regimen
which successfully
maximizes some trait, then the gene
modifications which underlie that
change in the two
populations can differ, and usually do differ
strikingly.
Application of the notion of genetic homology in such
instances is useless. Because of these difficulties, genetic
homology is
not widely utilized.
Ontogenetic criteria of homology had wide appli-
cation when the Recapitulation Theory was accepted.
If ontogeny
recapitulates phylogeny, then to establish
the evolutionary sense of
homology between two or-
gans—that
they were derived from the same part in
a common ancestor—it was
sufficient to show they had
the same origin in ontogeny. In 1870, Karl
Gegenbaur
stated that special homology is the relationship be-
tween two organs which have had a common evolu-
tionary origin, and which, as a
corollary, have arisen
from the same embryonic Anlage. However, 
E 
. B.
Wilson (1895, p. 101) and De Beer (1958) have shown
cases of homologous
adult structures derived from
different embryological origins. Such a
straightforward
application of ontogenetic criteria no longer suffices
to prove that two adult structures are homologous in
the evolutionary
sense.
