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Essay on “Phylogenetic analysis of trophic associations” (Ives & Godfray 2006)
Essay on “Phylogenetic analysis of trophic associations” (Ives & Godfray 2006)
Closely related species may share traits simply because they have inherited them from a common ancestor. This idea underlies the concept of “independent contrast” (Felsenstein 1985), which has been widely used in studying correlated evolution and adaptive significance (Price 1997).
Most previous studies focus on estimating the extent of such phylogenetic signal on one trophic level, while little effort has been made on multiple trophic levels, which however, is important in studying species association and coevolution. For example, when two host species share a common trait and large proportion of their parasitic species, this common trait is always believed to cause the high parasite burden. However, after taking phylogeny into consideration, we might find that the two species are closely related, and the correlation between the common trait and parasite burden is just an artifact due to their common phylogenetic history.
Ives and Godfray’s work made a contribution in solving such problem. They aimed at estimating the extent of phylogenetic signal that determines the pattern of association between different trophic levels. Generally, they treated the degree of association as a continuous trait between the phylogenies of two different trophic levels and extracted the covariance matrix of association (the outer product of covariance matrix of each phylogeny) as error term to obtain a phylogenetically independent degree of association. By having the phylogenetically independent degree of association, they were able to test its correlation with other variates to look for real traits or factors that may cause the association between different trophic levels. For instance, in Ives and Godfray’s work, they obtained three phylogenies, one of leaf-mining moths, one of their parasitoid wasps, and one of their tree habitats. By estimating pairwise phylogenetically independent degree of association, they found a strong phylogenetic signal from the host moth phylogeny, but only a weak signal from the parasitoid phylogeny. Most surprisingly, signal from the tree species attacked by the leaf miners was even stronger than that of the host phylogeny itself. They interpreted this result as the evidence that the observed host-parasitoid association is actually the result of a strong association between tree and moth species and between tree and wasp species. They further confirmed the interpretation by strong correlations between the observed degree of association for wasps and the predicted degree of association estimated by the phylogenetic relatedness of moth’s host plants.
Ives and Godfray’s work is valuable in the applications surrounding invasive species and biological control because their method can be used to assess the risk of native species to invasive species and the effectiveness of biological control. However, the biggest contribution of their work is to develop a theoretical method to test hypotheses about general patterns of species trophic association while accounting for phylogenetic relationships. One of such hypotheses, for example, is whether the observed trophic association is the result of coevolutionary processes. Coevolution or cocladogenesis postulates that two interacting species speciate in synchrony and thus should predict strong phylogenetic signals for both interacting species (Hafner & Nadler 1988). Results like those in Ivens and Godfray’s work imply that other external factors are needed to maintain the observed association between leaf-mining moths and their parasitoid wasps. To look for these external factors, phylogenetically independent degree of association can be again used as a continuous trait in the generalized least square (GLS) method (Pagel 1997) that is well developed to test trait correlation in the context of phylogenetic relationships.
The tested trait that is supposed to correlate with phylogenetically independent degree of associations can also be a phylogenetic characteristic. For example, when species richness or diversification rate of a clade is used as such a trait, one can test the well-known Ehrlich and Raven’s (1964) hypothesis that the endless evolutionary arms race in insect-plant interactions result in high diversity of each taxa. A fluctuating degree of association along phylogenetic history and a negative relationship between diversity and degree of association would be good evidence for escape-and-radiation model (Thompson 1989) that was suggested to be an underlying process for Ehrlich and Raven’s hypothesis.
One can also test some long-standing ecological hypotheses by using ecological characteristics of a clade as the tested traits. As an example, the hypothesis that species at higher trophic levels are usually less feeding specific than those at lower trophic levels is widely supported as a trophic principle. By comparing phylogenetically independent degree of association among different trophic levels, however, we might distinguish the real cause of this energy flow constraints from the artifect of different phylogenetic structures among different trophic levels.
As one of the few tries to combine phylogenies of more than one trophic level, Ives and Godfray’s work is outstanding in its possible future application of the phylogenetically independent and quantitatively defined association index. This index can be used to test different evolutionary and ecological hypotheses, especially in community ecology and phylogenetics.
Reference:
Ehrlich, P. and P. Raven. 1964. Butterflies and plants: A study in coevolution. Evolution 18:586−608.
Felsenstein, J. 1985. Phylogenies and the comparative method. Am. Nat. 125:1−15.
Hafner, M. S. and S. A. Nadler. 1988. Phylogenetic trees support the coevoluition of parasites and their hosts. Nature 332:258−259.
Ives, A. R. and H. C. J. Godfray. 2006. Phylogenetic analysis of trophic associations. Am. Nat. 168:E1−E14.
Pagel, M. 1997. Inferring evolutionary processes from phylogenies. Zool. Scr. 26:331−348.
Price, T. 1997. Correlated evolution and independent contrasts. Phil. Trans. R. Soc. Lond. B 352:519−529.
Thompson, J. N. 1989. Concepts of coevolution. Trends Ecol. Evol. 4:179−183.