440 Brian K. Hall
1963]. Reversals are regarded in the literature as homoplastic because the most re-
cent ancestor or closely related extant taxa lack the feature, the implication being
that the reversal is the result of independent evolution.
Dollo [1893], Meyrick [1927] and others claimed that reversal of evolution is not
possible — hence Dollo’s law [Hall, 2002b] — but they do not mean reversals in the
sense outlined here. Dollo did not deny reversibility entirely, only that complex
structures could not be recreated in their original form. But, if the developmental
programme is retained, and/or if a vestige of the structure remains (as, for exam-
ple, limb buds in snake embryos), the entire developmental programme does not
have to re-evolve; it was never lost completely.^23
11 RUDIMENTS AND VESTIGESAlthough rudiment or vestige (vestigial feature/structure) are often used as syn-
onyms, they can and should be distinguished on the basis of whether they occur
in embryos or adults.
Rudimentsare partly formed features that exist more fully (or fully) formed
in ancestral or related taxa. Rudiments are found in embryos, are usually non-
functional, but may serve a different function than did the ancestral feature. Ex-
amples include limb buds in snake embryos, hind limb buds in whale embryos, and
tooth buds in toothless (baleen) whales. Atavisms arise from rudiments.
Avestigeis a remnant in an adult of a feature that is more fully formed in ances-
tral or related taxa. Examples include the reduced eyes of blind cave dwelling fish
and invertebrates, the reduced wings of flightless birds, and the reduced clavicles
(collar bones) of mammals such as sheep and whales. The notochord, a vestige
of the ancestral chordate dorsal skeleton that existed before the cartilaginous ver-
tebral column evolved, is retained because of its major role in the induction of
the embryonic dorsal nervous system. Other vestiges have different roles than the
feature performed in the ancestral taxa. Some changes of role are subtle — the re-
duced wings of flightless birds are used in balance, display and courtship; penguins
and cormorants use their wings to steer and propel themselves underwater.^24
We are fortunate in having taxa in which the pattern (and perhaps the process)
of vestigilization can be observed. Limb and/or digit reduction in lizards provides
a particularly nice data set [Greer, 1987]. Hemiergis peronii, a lizard found in
Western Australia, is polymorphic for digit number, individual lizards having two,
three, four or five digits. Individuals with two or three digits also possess a vestigial
fifth digit [Shapiro and Carl, 2001; Shapiro, 2002; Shapiroet al., 2003].
(^23) See Gregory [1936], Gould [1970], Hall [1984; 2002b; 2003a], Marshallet al.[1994], Lee and
Shine [1998] and Teot ́onio and Rose [2001] for Dollo’s Law and the reversibility of evolution.
(^24) See Riedl [1978] and Hall [1983; 2005a] for the notochord as a vestige. For discussion and
examples of rudiments and vestiges, see Berzin [1972], Yablokov [1974], Klima [1990], Hall [1984;
1999a; 2001b; 2002; 2003a] and Bejder and Hall [2002]. See Hall [2003a,b] for a discussion
of the problems of character assessment when dealing with rudiments and vestiges, and Hall
[2003a,b; 2005b] for the possibility that the feature in question may be a neomorph that mimics
an ancestral feature.