of molecular tools has revitalized systematic research. In recent
years this increased interest has brought attention to two points*.
Firstly, the comparison of several species gives information on adaptive
processes. Secondly, phylogenetic relationships can confound the interpretation
of observations and the validity of correlations. That is, two species
can share traits not because of a common adaptation, but due to inheritance
from a common ancestor. Comparative analysis must therefore take into
In the case of host-parasite relationships, numerous studies have
shown that host evolution is not independent of the history of its
parasites. Two types of historical relationship between hosts and
parasites have been established.
Firstly, most studies have addressed models of co-speciation which
are based on the hypothesis that each speciation event in the host
is accompanied by parasite speciation. This results in congruence
between the phylogenetic trees of the host and the parasite. Examples
of such co-speciation are models which potentially allow the study
of the co-evolution of host defense genes, and parasite counter-defense
Secondly, it has been observed that the constitution of parasite communities
is not independent of host phylogeny. It is therefore possible to
reconstruct the degree of relationship between hosts using parasites
as markers even in the absence of co-speciation. Hosts which are phylogenetically
close share common parasites because of a shared recent evolutionary
Phylogenetic reconstructions are necessary for both hosts and parasites.
Comparison of these phylogenies will be used to test hypotheses concerning
co-speciation and co-evolution in biogeographical contexts. Finally,
the hypothesis that parasites are a major force in evolution remains
to be tested. If it turns out to be true, it would prove the influence
of parasites on the diversification of host species.
* The Laboratory "Biologie des
populations d'helminthes parasites" (CNRS-Université de
Perpignan) does these researches.
further information :
- R. Poulin and S. Morand. (2000). The Diversity of Parasites. Quart.
Review of Biology (à paraître).
- Morand S. (2000). Wormy word : comparative tests of theoretical
hypotheses on parasite species richness. In: Evolutionary biology
of host-parasite relationships : reality meets models. Éds
Poulin R., S. Morand and A. Skorping. Elsevier (à paraître).