Thumbs up...and heads down. More on 'selectionism'

Last week we used my glorious thumb and its several properties to illustrate the difficult problem of defining what a biological trait is, in relation to the widely held assumption that our traits are here strictly because of natural selection.  Our point was that it is quite tricky to define a trait, or specify just how precise, and in what ways, our assumed selection has worked, and is working.

A thumb is seemingly so simple that it can be surprising how difficult this is.  But here is another 'trait' that is clearly an evolved trait with what even I would agree is of fundamental importance to our existence, and in that sense, 'adaptive': the skull.  It, if it is an 'it', clearly has evolved and using evolutionary theory and comparative anatomy, we can identify what we assume are evolutionarily corresponding 'landmarks' (the dots in the figures below).  These are canonical anatomical locations shared by members of a species, but the distance between them varies within and among species.  These measures can be indicative of genetic and/or life experience variation.  But is the skull, or a landmark, the product of natural selection?  To answer that question, we must understand what 'it' is.

Landmarks: human, baboon, and mouse (Figure by Joan Richtsmeier)

This example is courtesy of work with our long-time collaborator (and good friend) Joan Richtsmeier, an expert on imaging and analyzing craniofacial dimensions and their pattern of variation.  Indeed, the figure shows various standard measures that people in this field use.  We and Joan, and Tim Ryan also in Anthropology here, along with Jim Cheverud of Loyola in Chicago, Heather Lawson at Washington University Medical School, Jeff Rogers at Baylor College of Medicine, and others, have been looking at the genetic contributions to craniometric traits in laboratory mice, and in a big baboon genealogy housed in Texas, and to compare that with what is known about human skull-shape variation.

We have CT scanned the skulls and made careful identification of standard 'landmark' points, the dots in the figure (the Figure just shows whole skulls, but in a CT scan the landmarks can be in 3-Dimensional locations and we can look at any slice through the object).  Then, using hundreds of individual skull-scans, we have been relating inter-landmark distances (lines that can be drawn between any two landmarks) to genetic variation, to 'map' the genomic locations that affect each distance's (or correlated distances') variation.  Yes, skeptical though we are of its over-interpretation, we are using GWAS-like methods (QTL mapping) to do this.  (We've described the difficulties with this work a few times on MT, including here and here.  The gene mapping issues we discussed in these posts remain unresolved.)

We chose the landmarks and distances based on what has been done before, or reflecting obvious locations like where sutures join or peaks in curvature and so on.  The statistical methods in the genomic analysis and so on are also well-tested (there are some controversies but they're beside the point, or comparable to the point that we're making here).  For most of the landmarks in one species, homologous locations can be found in the others (in our work, humans, baboons, and mice).  In this sense, the distances are very clearly 'objective,'  replicable measures of shape and size in the skulls.

But in what sense are they 'traits'?  Is the fact that we can identify and measure them, for reasons we decide on, a sufficient basis for the definition of a trait?  Is a measure's genomic 'control', regions that we identify by mapping, actually 'for' that measure?  If so it implies that other aspects of skulls, such as the bone thickness, dental cusp or shape patterns, and so on, are unrelated and irrelevant to the measure.   That seems hardly to be justified.

We know these traits evolved and that what we observe today ancestrally had to be consistent with successful survival and reproduction--passing the screening of natural selection or whatever other types of functional trials were imposed.  And they survived chance events, too.

But just as with the thumb example from Friday's post, it is we the  investigators who decide what a 'trait' is, and if we wish to infer adaptive scenarios it is we who decide what is adapting.  That leaves us the freedom to decide with equal subjectivity what it was adapting for.  Of course we know that other aspects than what we're testing had to have evolved consistently with what we're testing, or else a single, functionally viable and cohering skull would not have developed in the ancestral individuals as they and their various craniofacial 'traits' (whatever they be) were evolving.  But of the countless genes expressed in, and hence clearly contributing to, skull development, how is it that we can decide if the gene is 'for' some particular distance measure?  A gene may contribute to that strictly via some other effect than what we think our measurement is about (indeed, what 'strictly' means is itself questionable).  Indeed, most genes contribute to many different tissues and structures and developmental stages in the head, not just one.

Here, we're not just playing possibly-trivial thumb games, but are considering the nature and evolution of one of our most vital structures.  Yet the same question: what is a 'trait'? arises in regard to essentially every distance we assessed and to the very act of choosing landmarks and distances to measure.   So what we are really considering is the nature of evolution itself.

Chance is a chancy subject because it, and associated 'probability' are very hard to define.  But even if chance has, and always had, nothing to do with our 'traits', and if selection really were, as so often asserted, the be-all and end-all of life (except, of course, for love and lunch), and even if we've had Darwinian ideas for more than 150 years, and even if they were wholly correct, then even then, natural selection and the nature of evolution are still far from completely understood.

Somehow, however, many of people in evolutionary biology don't seem to realize this, or don't want to acknowledge it.  Or don't they care about truth, so long as they can tell a good story?