Saturday, February 27, 2010

Do I have to phylogenetically correct my grocery list?

The figure of Westoby et al. (1995) that summarizes their view of the tension between phylogeny and ecology in understanding trait relationships.

For some, a simple grocery list can pose a dilemma. Just yesterday, I went to the store with 21 items to buy. Others would look at my list and suggest I only bought 11 items. Fresh peas and frozen peas shouldn't really be counted as different items--they were both peas. Cauliflower, broccoli, and collard greens are the same species. Mustard part of the same genus as the previous three. Hot dogs and pork chops both from pigs (I hope). So although 21 items went into the cart, one could phylogenetically correct my list and arrive at the conclusion that I only bought 11 unique items.

It might seem silly to phylogenetically correct one's grocery list, but how to consider both phylogenetic and ecological data when examining species relationships lays bare the same fundamental tension as describing my last trip to the grocer.

In 1995, Westoby, Leishman, and Lord published a forum piece, “On misinterpreting the 'phylogenetic correction'”. The genesis for the forum piece came during the review process of a paper on seed mass in plants. Most likely, during the review of that paper, differences in opinions between reviewers and authors were laid bare. In the original paper, the authors showed that tall plants had large seeds. The reviewers likely insisted that the relationships between plant height and seed size could be due to phylogenetic relationship. The authors disagreed. Differences in opinions became forums, which by ecology standards unleashed a bit of a storm.

The questions associated with the topic of how to match ecological and phylogenetic data are ripe, but “phylogenetic correction” essentially adjusts relationships by weighting closely related species less than distantly related species. The fundamental differences of opinion pin whether closely related species hold similar traits because of phylogenetic constraint or ecological constraint. Closely related species might have similar traits because there has been little time for radiation, or because they are under similar ecological selection pressure. Distantly related species might have different traits because initial trait differences have long been conserved due to fundamental difficulties associated with character displacement or because they have been under the same ecological pressures for a long time.

The issues of how to identify adaptations or evolutionarily beneficial relationships cannot be covered here, but these fundamental issues have never been resolved, near as I can tell. The current détente that seems to exist is to examine ahistorical and “phylogenetically corrected” relationships among traits and hope that the patterns are the same. When setting to test relationships among species, choose congeneric species pairs from distantly related genera and hope the patterns work out consistently.

It’s currently an uneasy impasse. Both sides recognize that correlation does not necessarily imply causation. But outside of hoping that the evolutionary and ecological patterns parallel, there is still no resolution to the question of how to compare the traits of species.

I do know that if I want to shrink my grocery list, I'll start by not buying both cauliflower and broccoli rather than phylogenetically downweighting closely related taxa on my list.