Wednesday, February 26, 2014

On Molecules, Mycota, and Metazoa

Ctenophore (Image: John Wollwerth)
A recent discovery in animal evolution underscores the need to look at the findings of molecular phylogenetics with a critical eye. A couple months ago, a paper by Ryan, et al. was published in Science with well-supported molecular phylogenetic evidence that the group of animals to branch first from the common animal ancestor are the Ctenophora (comb jellies) (left). (A very good summary in structured abstract form can be found here.) Other molecular studies from the last few years, also with fairly strong support, support the traditional morphologically-based view that Porifera (sea sponges) branched first. A news article in Science summarizes the findings here. (Note: the link should be accessible by non-subscribers to Science and can be paged through. The article is 3 pages long.)

The confounding problem in animal evolution is that the many branches of the Metazoa evolved during a relatively short period prior to the Cambrian explosion, and then underwent a great deal of evolution and change since then. Hence, getting "phylogentic signal" for the early branching of animals can be difficult. On the other hand, there are some pretty clear morphological affinities in animals that have been understood fairly well since Darwin and Haeckel's time, many of which has held up when looked at molecularly.

There are many analogies here to the current understanding of fungal relationships, and relationships within groups of fungi, some of which are pretty confounding. The relationship of the microsporidia to the other fungi is something that's still being worked out for example, as are the relationships among the organisms that used be lumped in to the Zygomycota. For several years, the Glomeromycota (arbuscular mycorrhizal fungi) have come up in phylogenetic trees as a sister group to the Asco- and Basidiomycota line (supporting the hypothesis of a grand fungal radiation diverging with early plants), but on the other hand at least one phylogeny published in the last few years has grouped the Glomeromycota with the Mucoromycota and other core "zygos", supporting the classification that was largely abandoned at least a decade ago (and supporting alternate hypotheses of mycorrhizal evolution).

What to make of all this? Basically that while molecular phylogeny is wonderful and revolutionary, and has really deepened our understanding of the relationships and evolution of organisms, the information content can be as contradictory and unclear as any other method of understanding evolutionary relationships among organisms, and hence can't necessarily be treated as the final word on the matter. And this is why morphological, developmental, and ecological data, in other words, the stuff of old-fashioned natural history, is still critical to understanding evolutionary relationships and much else about the living world.

Addendum: I didn't think to have a look at Jerry Coyne's take on the Ryan, et al. paper until after I'd written the article, but probably should have - his perspective on these kind of discoveries is always valuable, and of course, I defer to his knowledge on anything to do with the subject of evolution, speciation, etc. That said, I'm not sure I entirely agree with him on this. His take is that the quality of molecular phylogenetic work in this paper is the best to date for the deep phylogeny of Metazoa, and that while the implications about evolutionary morphology in animals has some odd implications (notably, that the earliest ancestor living animal groups already had a simple nervous system, possibly some degree of bilateral symmetry, etc) but these should be accepted and studied within the evolutionary framework that this study suggests. I'm less sure given that so many prior studies have supported the idea of Porifera as the earliest metazoan branch, and morphological evidence, notably the strongly choanoflagellate-like collar cells in sponges, lends support to the sponge-like common ancestor hypothesis as well. Also, I've seen papers before that seemed to strongly support some kind of odd relationship between organisms, only to be falsified by the next paper that comes along. I reserve judgement on this matter until, as is bound to happen within a few years, one of these hypotheses becomes settled science.

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