Eyes drive diversification

Just saw a cool paper by Martin Aberhahn, Sabine Nürnberg, and Wolfgang Kiessling that came out in Paleobiology recently (Paleobiology 38(2):187-204. 2012). I have several marine mammal papers lined up that I would like to talk about, but the Aberhahn et al.’s paper is so interesting that I didn’t want to wait blogging about it. So why is this paper so interesting? Well, it simply combines two of my favorite research areas: eyes and the origins of diversity.

In the last 10 years or so there has been quite an increase in the number of papers that attempt to explain patterns of diversity across the tree of life. Papers in this area usually fall into two distinct groups (unfortunately): the ‘paleo’ group, taking a wealth of historical data directly from the fossil record, and the ‘neo’ group which utilizes the quickly growing treasure of phylogenetic inferences from present-day species. I think one could gain an even better understanding by using all available data and methods, i.e., combining both approaches (in fact, Sam Price, Graham Slater, and I are actively working on establishing the necessary networks to bridge the two fields; more on this some other time).

Aberhahn et al. used a huge paleontological dataset (extracted from the Paleobiology Database) to tackle the question whether the presence of image-forming eyes promotes species diversification. They tested this question in shallow-marine invertebrates (excluding the ones that burrow through the mud of the sea floor).

Here are two of their main and compelling results.

1) The proportion of sighted genera is highest from the middle Cambrian to early Ordovician, coinciding roughly with the increase of ecological complexity in marine ecosystems. Since the Ordovician the proportion of sighted genera has remained stable at a lower level. The peak of sighted genera in the middle Cambrian to early Ordovician is largely driven by trilobites — most trilobites are equipped with eyes (see photo above). Very diverse but eyeless clades such as brachiopods and corals did not diversify until after the Cambrian.

2) Within trilobites, epifaunal bivalves, pectinoid bivalves, gastropods, and echinoderms the proportion of sighted genera increases over geologic time, probably due to higher extinction rates in genera in blind genera. This finding is seemingly at odds with de Queiroz work on present-day species (Evolution, 53:1654-1664. 1999). De Queiroz compared the species richness between clades that have eyes and their sister clades.

As mentioned above, I think a combination of paleontological and neontological approaches may improve the understanding of the origins of diversity. Obviously, for a study with such a large scope like Aberhahn et al. it will be very difficult to combine all paleontological with all neontological evidence mainly because of the challenge to develop an essentially all-encompassing phylogeny. But for studies with narrower taxonomic scope, good understanding of phylogeny, and an excellent fossil record, the synthesis of paleontological and neontological data and methods should yield some interesting new results.

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