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MARGINALIA

The Biology of What Is Not There

Is it only natural selection that guides the shapes seen in nature?

Robert L. Dorit

Rediscovering History

Although the constructionist perspective enhances our ability to account for the distribution of biological objects in shape space, it lacks an acknowledgment of the role that history plays in the filling of shape space. At its most basic, history matters because evolution often works by composing variants on a theme. As a result, once a functional shape evolves, the adjacent shape space is most likely the first to be fully explored. This tendency to remain in the shape neighborhood of one’s ancestors is further enforced if the existing shape is surrounded by less-functional alternatives. Evolution will not cross through less-fit alternatives even if better-functioning alternatives lie within view.

The extent to which history controls the distribution of realized shapes also depends on just how easily one shape can be transformed to the next. In our shell example, we can safely claim that cone shells, in terms of transformation, are conservative. They give rise to other cone shells, both from one generation to the next and over evolutionary time. The corner of the tree of life represented by the genus Conus is bushy indeed, with over 3,000 named species, each of them crowded into a distinctive zone in shell space. But for shapes at other scales—the shape of folded RNAs, for instance, or the shape of proteins—a few changes in the sequence may suffice to cause a flip from one location in shape space to a completely new location.

Perhaps if every lineage had world enough and time, if the tree of life had an infinity to explore shape space, it would eventually discover every successful form that could be built given constructionist constraints. But history dictates that because the evolutionary life of most lineages is brief, many paths and shapes remain unexplored.

Having defined the universe of the possible, we will have to populate it with the actual. We will need statistical approaches to determine whether there are over- and under-populated zones beyond what we would expect from sampling alone. We will need to flesh out the consequences of the ways in which living form is built and the constraints imposed by the materials of life. We will need to elucidate the role of time and history in shaping shape. And only then will we be able to invoke the sculpting power of natural selection. Yet daunting as the occupancy problem is, the nonisotropic distribution of realized forms appears both universal and independent of scale. Such regularity in biology is strangely beguiling.

When you next look out at the stars, notice how some parts of the night sky are teeming with stars, whereas others seem comparatively dark. This is no optical illusion. At all scales, and for all of the classes of objects that astronomers can identify—stars, galaxies and constellations—the universe, too, is nonisotropic. The variability of the heavens, like the variety of living things, underlies much of their beauty.





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