Homology and Homoplasy 44112 ATAVISMSRetention of developmental information is a means to maintain genetic diversity
and retain the potential to introduce morphological change. Anatavismis a fea-
ture that was present in an ancestral taxon and which reappears in low frequency
in one or more individuals of a derived taxon; elements representing portion of the
skeleton of the hind limbs seen in whale ancestors are found in 0.02 % of sperm
whales [Berzin, 1972]. Three toes in modern horses, dew claws in dogs, and ac-
cessory nipples in mammals are others. An atavism or character reversal could
arise in an organism with a rudiment of that feature if a chance mutation or an
environmental cue that released hidden genetic variation, prolonged and expanded
development of the rudiment, and if that new phenotype were selected for.^25
13 THE CONTINUUMHomology and homoplasy can be assigned at one level of the biological hierarchy,
for example, the phenotype, without implying or prejudging statements about ho-
mology or homoplasy at other levels, e.g., the developmental or genetic basis of the
feature. At the level of developmental processes, shared and divergent develop-
ment can be two classes of homology because homologous features can arise from
pathways that have diverged. Therefore we cannot distinguish homology from
homoplasy on the basis that homologous features share a common development
but homoplastic features do not. Parallelism, reversals, rudiments, vestiges and
atavisms are features that form using similar developmental processes. Conver-
gence is not based on shared development but reflects different processes producing
similar features.
As outlined at the outset of this entry, we set homology against homoplasy
in a dichotomy in which homoplasy includes (subsumes) convergence, parallelism,
reversals, rudiments, vestiges and atavisms. Hall [2007a] suggested that examining
nearness of relationships and degree of shared development reveals a continuum
within the expanded homology category. The more refined version of that analysis
set out by Hall [2003a] and as outlined in this entry, emphasizes a continuum from
homology to parallelism, with convergence as the sole class of homoplasy.
The combined developmental/phylogenetic approach to homology and homo-
plasy outlined here resolves into: (1) Homology, which reflects phylogenetic con-
servation or retention of features in organisms with common descent and which
subsumes parallelism, reversals, rudiments, vestiges and atavisms (i.e., all classes
except convergence) as similar (homology), and (2) Convergence, which reflects
(^25) See Struthers [1881], Evans [1955], Howell [1970], Raikow [1975], Riedl [1977], Lande [1978],
Hall [1984; 1995b; 1999a; 2002a; 2003a], McKitrick [1986], Stiassny [1986; 1992], Wake and
Larson [1987], Wyss [1988], Shubinet al.[1995], Tintant and Devillers [1995], Verhulst [1996],
Greene and Cundall [2000] and Bejder and Hall [2002] for discussion and examples of atavisms.
See Hallgr ́ımsson and Hall [2005] and Hallgr ́ımssonet al.[2005] for variation and the generation
of features of the phenotype.