Pittsburgh, part I

First update, Friday morning:

I will keep this brief (because I want to go to the next session) and general, since I'd like people to get a flavor of what philosophers discuss when they get together, but obviously this isn't a technical blog aimed at professional philosophers.

So, Roughgarden -- a biologist -- gave a nice overview of the different kinds of models that biologists use in ecology and evolution, which set the tone for the following talks, given by philosophers. She explained how models are built, what kind of work they are supposed to do for biologists, and how mathematical and graphical models can be perfectly equivalent in the predictions they make about a given system.

Griesemer talked about what models in evolutionary biology mean and, more importantly, don't mean. He drew the standard distinction between statistical and causal models, and argued that measuring natural selection (which is done by statistical modeling of field data) is only a preliminary step, which does not amount to analyzing the causes of selection. The latter requires actual ecological information as well as manipulation of field conditions, to tease potential causes apart. While it is true that biologists of course know this, it is also fair to say that far too often they talk about studying selection as if that only entailed the preliminary statistical step (which is much easier to carry out).

Bouchard posed the question of what counts as an individual as opposed to an "ensemble" (a concept that includes what biologists call populations, but is in fact broader than that). He provided the example of a bioluminescent squid, which acquires its luminescence by incorporating millions of bacterial cells in its mantle. The association is usually beneficial to the squid (which paradoxically avoids predation that way, since at night predators swimming below confuse the squid for part of the starred background!). However, occasionally the bacteria take over and make the squid sick. This system is evolving either toward better integration of the two components (bacteria will become permanently associated with the squid) or toward parasitism (more sick squids). Either way, one can reasonably ask how many individuals make up the system at any given time during evolution: one (the squid, bacteria are part of the environment); a million (the bacteria, the squid is part of the environment); a million and one (bacteria + squid); or something else? Depending on the answer, different mathematical models of the evolution of the system can be set up.

Finally, Millstein took on that old biological and philosophical chestnut of what, exactly, is genetic drift. I have to premise that Roberta and I disagree on this issue, so you need to take my comments here as certainly influenced by my own views. She directed her criticisms toward a view that claims that drift is a purely mathematical construct, with no physical counterpart. Drift, according to this view, is statistical sampling error, period. I do not actually know of anyone who holds strictly to that view: if drift had no physical consequence than biologists simply would not be interested (and, frankly, neither would philosophers of science). Rather, what some people (including me) claim, is that there is ambiguity in the way biologists think of drift. On the one hand, drift is the stochastic fluctuation in gene frequencies that one expects in finite populations simply because of random variation. Think of flipping a fair coin: if you do it a small number of times you might get a deviation from the expected 50-50 ratio, not because the coin is loaded on one side, but by chance. (Roberta does not accept the exact equivalency between drift and the coin example, which many biologists use; in a discussion after the talk I think we agreed that the analogy is good enough to hold, despite some differences in the statistical properties of coin drift vs. genetic drift.) On the other hand, biologists also think of drift as "indiscriminate sampling" of genetic variants within a population: some individuals will survive and reproduce (and their genes will pass on) not because they are more or less fit than others, but just by luck. There is a huge technical literature that is pertinent here, and I don't have the time to get into further details, but suffice to say that I think Roberta and I agreed that at the very least biologists should be careful to state what they mean by "drift" in particular contexts, and whether they are talking about physical events at the individual level or statistical fluctuations at the population level. This is important because drift plays a major role in several aspects of population genetic theory in evolution.