~.:ea Zoologica (Stockholm), Vol. 73, No. 5, pp. 327-334, 1992
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i992 The Royal Swedish Academy of Sciences
A Homage to Homology: Patterns of Copepod Evolution
Geoffrey A. Boxshall* and Rony Huyst
"Dc:partment of Zoology. The Natural History Museum. Cromwell Road. London SW7 5BD. U.K.
71\farinc Biology Section. fnsticuce of Zoology, Scace Universitv of Ghent. K.L.Ledeganckstraat 35. B-9000 Ghent. Belgium and Delta
lnstitutc for Hy.drobiological Research. Vier;iraac 28. -1-101 E.~ Yerseke. The Netheriands
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Abstract
Boxshall, G. A. & Huys. R. 1992. A homage to homology: patterns of copepod evolution.Acw Zuologica (Stockholm) 73: 327-33-1.
Lack of attention to determining the homology of character states is recognized as being
responsible for the ever increasing numbers of phylogenetic schemes for the Crustacea that
appear and disappear so rapidly. Detailed study of musculature. segmentation and setacion of
the limbs of all 10 orders of copepods revealed numerous phylogenetically informative characters. based on segmental fusion patterns and the presence of individually identified setation
elements. Simple counts of limb segments (or of setae) were found to be virtually useless for
constructing phylogenies in the copepods. This conclusion can probably be extended to other
crustacean groups.
Geoffrey A. Boxsha/l, Deparrment of Zoology, The Natural Hisrory Museum, Cromwell Road,
London SW7 5BD, U.K.
Introduction
The cladistic method provides a powerful tool for the
analysis of phylogenetic relationships. Used unwisely, like
any powerful tool, it can have unfortunate consequences.
In the past decade or more there has been an explosion
in the number of publications considering crustacean evolution. This is due in part to the discovery of new taxa,
such as the Remipedia (Yager 1981) and T<intulocarida
(Boxshall & Lincoln 1983). It is also due, in part, to the
iterative nature of cladistics, so that each new character
or interpretation can be incorporated into the data matrix
and the analysis can be performed again and again. The
widespread use of computer programs for generating phylogenies has made a major contribution to the explosion.
Concentration on the production of phylogenies and
on obtaining the most parsimonious tree has made us lose
sight of the relative importance of the various aspects of
any phylogenetic study. Computer analysis, the construction of phylogenies and the discussion of their various
merits are all secondary processes-the primary process
being the production of the character matrix. The key
concept here is not parsimony but h:omology. Only homologous derived character states (synapomorphies) can be
used to construct phylogenies and it is probable that lack
of attention to homology lies at the root of many of the
novel phylogenetic schemes that come and go with great
rapidity. The identification of homologous characters and
character states is the first and most difficult task in any
phylogenetic study. Ideally it requires an understanding
of their functional significance in order to define the
boundaries between a series of linked characters or states,
as well as similar experience of the out-group. At present
it appears that in practice the rule regarding homology
has been to assume that similar structures in related
organisms are homologous unless there is good evidence
otherwise. It is vital to improve the quality of the character sets that are used in phylogenetic analysis and this can
only be achieved by adopting a new attitude towards
homology. The ultimate goal must be to make a positive
statement on the homology of every single character in
the data matrix.
Homology
The original concept of homology, as espoused by comparative anatomists in the nineteenth century, had two
central components: it describes the relationship between
structures that correspond in relative position, and that
arise from similar precursors in embryonic development;
It is interesting to note that even this definition of homology -includes consideration of features of development.
Boxshall & Ruys (1989) argued that a simple
positional-embryological definition of homology was
inadequate because apparently homologous end products
could arise by non-homologous developmental processes.
Ferrari (1988) also produced evidence of developmental
convergence in which apparently homologous adult segmentation patterns in copepod swimming legs resulted
from non-homologous developmental processes. An
example of this is provided by Fit;!rs (1991) who examined
the development of a new genus-of'harpacticoid copepod
in the family Cancrincolidae. Adult females of the cancrincolid Abscondicola possess separate genital and first
abdominal somites. Its immediate ancestors within the
Harpacticoida have these somites fused to form the typical
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