Contingency

However, surrounded by chaos, we are understandably led to temper the full horror of contingency by suggesting that certain things happen because they have to, thereby giving the mess of life a sustaining purposiveness and direction. Though the dice may roll a number of ways, we frantically draw up patterns of necessity, never more than when it is the inevitability that one day we will fall in love. We are forced to believe that this meeting with our redeemer, objectively haphazard and hence unlikely has been prewritten in a scroll slowly unwinding in the sky, and that time must therefore eventually [however reticent it has been till now] reveal to us the figure of our chosen. What lies behind this tendency to read things as destiny? Perhaps only its opposite, the anxiety of contingency, the fear that the little sense there is in our lives is merely created by ourselves, that there is no scroll [and hence no preordained face awaiting] and that what may or may not be happening to us [whom we may or may not be meeting on airplanes] has no sense beyond what we choose to attribute to it – in short, the anxiety that there is no God to tell our story and hence assure our loves. - from On Love: a novel by Alain de Botton

How much of our world does natural selection explain? Let us focus this question; What explains the distribution and abundance of living forms at a particular time and place? We begin by noticing that there are a nested set of worlds, the possible and the actual. Most interesting for this discussion is that there is a large gap between the possible and the actual. If you buy the notion that lineages facilitate and limit their descendants then history really matters. Strong constraints come from the requirement that every lineage comes from another and that lineages explore only the neighborhood in morpho-space that is accessible from where they start. So many more organisms are possible (allowed by physics and chemistry) than are realized; those whose lineages survived. When this view is coupled with the observation that the vast majority of lineages have gone extinct it suggests that the dynamics of overall tree of life goes a long way to explaining the distribution and abundance of forms. This way of describing the situation makes historical contingency primary, lineages responsible for carrying much of the load after that and natural selection finally explaining local modifications within lineages. This inverts the adaptationist position that makes a selectionist account central to explaining evolution.

Adaptationists may cry foul and insist that natural selection is meant to explain the wonderful fit of the organism with the environment, not the overall distribution and abundance of types. Fair enough, the inversion of factors that I recommend is going beyond simply fleshing out natural selection. Every trait we examine is an admixture of physical constraints, natural selection and chance with history. These features are constitutive, not optional. We do not need not pit these factors against each other. It is a step in the right direction to place a selectionist account (the four steps described above) at the center and then constraining it with history and chance. Still, we should not to conclude that the question of adaptations is the only or most important one for evolutionists. This question has never been the only one.