How to taxonomically structure comparisons

For a recent grant, we proposed to measure aspects of the nitrogen and water economy of 30 species at Konza. The novelty of the proposed research was in measuring both water- and N-related traits for a wide variety of species, and then test how well they explain the abundance of the species in a native grassland.

One point that came up was how to frame the research. Part of our framing was that the results should help us understand the evolution of plant strategies and selection forces on species. Reviewers seemed to disagree.

One reviewer said, “The problem here is that because of the close evolutionary relationships of many of the selected plants, traits and responses will be co-correlated through evolutionary relationship and will therefore give an inflated estimate of independence.”

Another said, “It seems to me that the work in this project will yield much in the way of an understanding of the influence of resource availability in the evolution of land plants, since gaining such insights would really require a more extensive phylogenetic and perhaps phylobiogeographic sort of approach.”

This is something I still do not understand. How many species does one have to measure to be able to infer selection pressures and evolutionary tradeoffs? Ironically, we had initially proposed to measure 100 species, but were encouraged to measured fewer species. 30 is not enough? Shouldn’t 2 well-contrasted species be sufficient to provide some inference? Most of the initial work on C4 photosynthesis compared 4 species. Granted the work is still being refined, but isn’t 30 a good start? Also, why would 30 species be enough to test ecological processes, but not evolutionary?

I think the standards here have less to do with the science, than the scientist.

The current review is immaterial—the panel summarized that “The placement of this research in evolutionary context was undeveloped but will not affect the quality and novelty of the project outcome.” Yet, the gap in our scientific process is clear in the lack of anabolic comments being paired with the catabolic ones. Experimental designs to test for evolutionary patterns seem to require I-know-it-when-I-see-it tests. Constructively, we need some resolution on standardizing designs. I’ve pushed before for a standard species set, but we also need resolution on some key questions outside of any standardized set.

If there is one question I'd like to see answered, it's "how many species need to be measured and how should they be related?"

I don't expect one answer to this, but if we are serious about wanting to understand the evolution of ecological traits, we have to make the bar visible, rather than always place it just above our leaps.