Homology and Homoplasy 443Box 1
Latent HomologyLatent homology [de Beer, 1971] is used when a feature in an ancestral taxon can
be shown to have given rise to a structurally different but homologous feature in
a descendant taxon. Rudiments that occasionally give rise to atavisms could be
considered as latent homologues of those atavisms. The implication is that both
features arise through developmental mechanisms that have been conserved but
modified; the developmental processes have shown descent with modification.
This was clear to Fritz M ̈uller, author ofF ̈ur Darwin[1864], the first published
test of Darwin’s theory of descent through modification of embryological devel-
opment: “From the beginning of all things the Creator knew, that one day the
inquisitive children of men would grope about after analogies and homologies,
and that Christian naturalists would busy themselves with thinking out his Cre-
ative ideas; at any rate, in order to facilitate the discernment by the former that
the opercular peduncle of theSerpulaeis homologous with a branchial filament,
He allowed it to make ad ́etourin its development, and pass through the form
of a barbate branchial filament” ([F. M ̈uller, 1869, 114], English translation of
[M ̈ uller, 1864]).
Mammalian middle ear ossicles are a classic example of latent homology, these
ossicles being homologues of bones of the lower jaws of mammal-like reptiles
(therapsids), a homology that would go unrecognized were it not for the fact
that the transformation is documented in the fossil record and in the ontogeny
of extant marsupials. Visceral arches in agnathan vertebrates/jaws in gnathos-
tomes, and anterior appendages in early arthropods/ mouth parts in crustaceans
are two other oft-cited examples. A more recently studied example is the search
among uro- and cephalochordates for the precursors (latent homologues) of the
vertebrate neural crest and neural crest cells.a
Henry Fairfield Osborn [1902] treated homoplasy as a law of latent homology,
a view that only a few continue to recognize: “Homoplasy is an alternative
perspective on homology, and when we can identify a phenomenon as latent
homology we begin to approach an understanding of how homoplasy relates to
homology on the one hand and to the production of diversity on the other” [D. B.
Wake, 1999, 45]. In practice, in cladistic analyses, the ancestral and descendant
features are identified as homoplastic because they are coded as independent
or apomorphic characters. Coding the developmental processes and resulting
features as multiple character states reveals the latent homologue.b
aSee Gregory [1913], Hopson [1966], Thomson [1966], Lombard and Bolt [1979],
Maier [1990] and Hall [2005a, especially Box 13.1] for the middle ear ossicles, Hall
[1999a; 2002a] and Abzhanov and Kaufman [2000] for visceral arches and arthro-
pod appendages, respectively, and Hall [1998; 1999b; 2000b; 2005b], Holland and
Holland [2001] and Stone and Hall [2004] for the neural crest.