Comparative Parasitology 67(1) 2000 - Peru State College

Eucestoda) coincided with the divergence of the
common ancestor of the Chondrichthyes and the
common ancestor of the rest of the gnathostomous
vertebrates (Brooks, 1985b). Complementary
independent assessments within the eucestodes
(Hoberg, Gardner, and Campbell, 1999;
Hoberg, Jones, and Bray, 1999; Hoberg, Mariaux,
and Brooks, 2000) and monogenoideans
(Boeger and Kritsky, 1997) suggest origins extending
to the Devonian from 350 million to 420
million or more years ago.
Other studies (Brooks et al., 1985, 1989) indicated
that parasite ontogenies evolve as coherent
units and that larval and adult morphological
traits are phylogenetically congruent.
The degree of adaptive response by each life cycle
stage is thus constrained by common evolutionary
history. Finally, although molecular
systematics is in its infancy within parasitology,
studies to date show that there is a high degree
of concordance between phylogenies based on
molecular and morphological traits, when proper
phylogenetic methods are used and careful character
analysis is performed (e.g., for ordinal-level
relationships among the eucestodes, Hoberg
et al., 1997; Hoberg, Mariaux, and Brooks,
2000; Mariaux, 1998). The merits of studies
based on "total evidence" that combine morphological
and molecular databases (Kluge,
1989, 1997, 1998a, 1998b; de Queiroz et al.,
1995; Huelsenbeck et al., 1996; Sanderson et al.,
1998) are also apparent (Hoberg, Mariaux, and
Brooks, 2000). Leon-Regagnon et al. (1999)
have recently emphasized this point, showing
that a combination of molecular and morphological
data could help resolve outstanding specieslevel
taxonomic problems within a group of frog
digeneans.
Parasites and the evolution of life history
traits: The extent to which the individual components
of reproductive biology, development,
and ecology, as well as their complex interactions,
can be highlighted and examined in parasite-host
systems is impressive. Phylogenetic
analysis also allows us to examine phylogenetically
associated changes in reproductive and
nonreproductive male and female characters. We
can then ask questions, such as what are the
costs and benefits of different reproductive strategies?
Do male and female characters covary in
either their origin or their loss (digeneans, monogenoideans)?
What is the relationship between
BROOKS AND HOBERG—PARASITE BIODIVERSITY 15
reproductive conservatism and reproductive
flexibility in male or female characters (digeneans,
monogenoideans)? What is the relationship
between sexual reproduction and the appearance
of character novelty (eucestodes)? And if asexual
reproduction is good and sex is better, is sex
combined with asexual reproduction the best (digeneans)?
How does dioecy evolve in monoecious
lineages (Platt and Brooks, 1997)? Recent
studies (Morand, 1996a, 1996b; Poulin, 1992,
1995a, 1995b, 1997a, 1997b; Sasal et al., 1997,
1998; Sasal and Morand, 1998) confirm the suitability
of parasite systems for studies of the evolution
of life history strategies. Their results
confirmed the assertion by Brooks and Mc-
Lennan (1993a) that parasites show the same
kinds of life history evolution as their closest
free-living relatives.
Parasites as model systems for studying adaptive
radiations: Parasites have not experienced
unusually high degrees of adaptive radiation but
do show interesting patterns. Within the parasitic
flatworms, the monogenoideans appear to have
undergone adaptive radiation, whereas the digeneans
and the eucestodes appear to have experienced
evolutionary radiation that may or
may not have been adaptive (Brooks and Mc-
Lennan, 1993b, 1993c). It is important to realize,
however, that a relatively species-poor sister
group balances each species-rich group, so it is
inaccurate to speak of parasites in general as
having experienced high levels of adaptive radiations.
The question of the relative extent of
parasite adaptive radiations cannot be answered
until we have comparable databases for free-living
groups. At the moment, we can say that the
monogenoideans, digeneans, and eucestodes, not
unlike ostariophysan and percomorph fishes and
passerine birds, provide a wealth of information
about radiations, adaptive or not. This information,
in turn, supports the hypothesis that such
radiations were primarily a function of diversification
of life cycle components. For example,
4 putative key innovations were identified during
examination of the database for the parasitic
flatworms (Brooks and McLennan, 1993a,
1993c): (1) the evolution of a direct life cycle (a
developmental change, possibly caused by peramoiphosis,
with an ecological outcome); (2)
the appearance of additional larval stages (a developmental
change); (3) the appearance of
asexual amplification of larval stages (a devel-
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