Sometimes, we want to analyze selectivity during some rapid event, say recent climate change, in relation to a dataset of multiple taxa. We would be looking at whether these taxa have experienced some sort of selective event or not as a result: they might have undergone globally extinction, locally extinction, range shifts, decreased growth rate, etc. In our little example, we have reason to believe it is tied to some phenotypic variable (for example, body size). To measure the correlation between the selectivity and the phenotypic trait should the dataset be analyzed using independent contrasts or not? Is it necessary? Is it valid?
(Tangent: Is there such a thing as a use of independent contrasts that is not necessary but is valid? It seems all situations where independent contrasts could be applied are also situations in which they should be applied.)
I think that it is not a valid approach, because if we are interested in the instantaneous selection acting on the trait itself, then the actual determining factor is the actual phenotypic traits held at that time. But what do you guys think?
As I pointed out in class, this question is important for those interested in the fossil record because its the same exact situation when we want to understand selectivity during a mass extinction event. Analyses like Dave Jablonski's analysis of geographic range and extinction risk in K-T molluscs was performed on the raw variables. I've done similar analyses on morphological correlates of extinction risk for graptolites in the Late Ordovician, without taking into phylogenetic structure (not yet published). I'm not troubled by this, because I think the point of these analyses is to understand the causal relationship between the actual phenotypic trait and the occurrence of extinction, not the possible relationship between a potential unmeasured evolving trait that decides extinction risk and the phenotypic traits.
Matt Friedman thinks differently, and he gives a fairly good argument against my position in his paper, cited below. Note that PIC and raw values gave two very different results for his analysis: body size was a significant correlate until he corrected for phylogeny.
Friedman, M. 2009. Ecomorphological selectivity among marine teleost fishes during the end-Cretaceous extinction. Proceedings of the National Academy of Sciences 106(13):5218-5223.
http://www.pnas.org/content/106/13/5218.full
So, what do you guys think?
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I think that is a tough question. Since I have a viewpoint more similar to Friedman's, I thought I'd share it. On the one hand, I think that it is valid to say that we are interested in the influence of body size on extinction regardless of the phylogenetic history of the organisms in question. In the case of extinction, it may not matter how much larger something is than its sister taxon, but only if it is above a certain threshold size. However, there is the possibility that extinction is acting on some trait other than body size which is phylogenetically correlated. If body size also displays a strong phylogenetic signal, it may be difficult to tell what the relationship is, as Friedman discusses. For example, suppose a clade was characterized by both high body size relative to other fish and a certain kind of metabolic pathway that was made disadvantageous during the extinction event. Using raw data, selectivity against this metabolic pathway might also cause an apparent selectivity against large body size. It becomes even tricker because selectivity could also be correlated with some other factor, such as generation time, in a phylogenetically independent manner. I think this is less of an issue because I think Friedman and others realize that apparent selection on body size could in fact be selection on traits expected to correlate with body size. I think a related question this paper prompts is: despite the arguments of Freckleton 2009, is it really always inappropriate to include both PIC and traditional regression analysis of the same data set, or can they be useful in highlighting different aspects of phenomena such as extinction events? Friedman seems to want to use both, as he discusses vacant ecospace opened up due to the demonstrable extinction of large bodied forms (from the results of the traditional analysis), while emphasizing that body size itself may not have been the target of selection (using independent contrasts).
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