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28 de mayo de 2008

Literatura Reciente, 4

Cladistic analysis of continuous modularized traits provides phylogenetic signals in Homo evolution.
Rolando González-José, Ignacio Escapa, Walter A. Neves, Rubén Cúneo & Héctor M. Pucciarelli
Nature , | doi:10.1038/nature06891; Published online 4 May 2008.

Evolutionary novelties in the skeleton are usually expressed as changes in the timing of growth of features intrinsically integrated at different hierarchical levels of development1. As a consequence, most of the shape-traits observed across species do vary quantitatively rather than qualitatively2, in a multivariate space3 and in a modularized way4, 5. Because most phylogenetic analyses normally use discrete, hypothetically independent characters6, previous attempts have disregarded the phylogenetic signals potentially enclosed in the shape of morphological structures. When analysing low taxonomic levels, where most variation is quantitative in nature, solving basic requirements like the choice of characters and the capacity of using continuous, integrated traits is of crucial importance in recovering wider phylogenetic information. This is particularly relevant when analysing extinct lineages, where available data are limited to fossilized structures. Here we show that when continuous, multivariant and modularized characters are treated as such, cladistic analysis successfully solves relationships among main Homo taxa. Our attempt is based on a combination of cladistics, evolutionary-development-derived selection of characters, and geometric morphometrics methods. In contrast with previous cladistic analyses of hominid phylogeny, our method accounts for the quantitative nature of the traits, and respects their morphological integration patterns. Because complex phenotypes are observable across different taxonomic groups and are potentially informative about phylogenetic relationships, future analyses should point strongly to the incorporation of these types of trait.

The theoretical implications of the approach presented here are broad, but point in three main directions. First, reconstruction of phylogenetic relationships can be done by taking into account the modular development and evolution of complex phenotypes. In fact, this type of critical study on character independence should be the first phase of any cladistic analysis when no previous information is available5. Second, there is no reason to discard or force discretization of continuous multivariant traits. Conversely, our analysis shows that it is possible to recover relevant phylogenetic signals in characters previously ignored or arbitrarily discretized. Note, however, that even though there are relatively few truly discrete traits, many of them exhibit qualitatively different states that are very distinct in different groups of taxa, and thus are reasonably discrete. In this context, future work should emphasize the combined use of quantitative and qualitative traits. Finally, when geometric morphometrics methods are used to depict shape changes across a multivariate space, reconstruction of ancestral states in combination with the visualization of shape changes across phyletic lineages should be considered as a straightforward and common-sense procedure. In summary, valuable phylogenetic information is recovered from data sets that consider independence on a developmentally and functionally basis, and which preserve the multivariant and continuous nature of complex phenotypes.

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