Convergent Evolution: Limited Forms Most Beautiful

Chapter 7. Functional and Developmental Constraint in Convergent Evolution

7 Our results strongly support the hypothesis that the essential elements of organic structure are highly constrained by geometric rules, growth processes, and the properties of materials. This suggests that, given enough time and an extremely large number of evolutionary experiments, the discovery by organisms of “good” designs—those that are viable and that can be constructed with available materials —was inevitable and in principle predictable. —Thomas and Reif (1993, 342) Was mich eigentlich interessiert, ist, ob Gott die Welt hätte anders machen können. —Einstein (quoted in Seelig 1956, 72) Albert Einstein once mused, “What really interests me is whether God could have made the world in a different way.” To Einstein, God represented the laws of nature. He was asking whether the evolution of the universe was so constrained by the initial conditions of the Big Bang, by the observed constants of nature, that only the present universe could have evolved? Or were alternative universes possible, universes that would have evolved along physical pathways not followed by our present universe? This question can be framed with respect to biological evolution as well. Is the evolution of life so constrained by the geometry of the universe , by the physical constants of nature, that its outcome is predictable? Or are so many alternative evolutionary pathways possible that it would never be possible to predict the trajectory of the evolution of life? We can easily visualize a universe in which every species is morphologically different from every other species, and in which each species has its own unique ecological role, or niche, in nature. That universe does not exist. Instead, we live in a universe where convergence in evolution is rampant at every level, from the external forms of living organisms down to the Functional and Developmental Constraint in Convergent Evolution 246 Chapter 7 very molecules from which they are constructed, from their ecological roles in nature to the way in which their minds function. Since convergent evolution is so ubiquitous in nature, as we have seen in the previous five chapters, the total extent of convergent evolution might best be revealed by studying its opposite: unique evolution. That is, rather than compiling lists of convergences, we might compile lists of solitary evolutionary innovations in species that have not been independently discovered by other species in their evolutionary pathways. Vermeij (2006) set out to do just that, and compiled a list of evolutionary innovations said to be unique. He discovered that “purportedly unique innovations either arose from the union and integration of previously independent components or belong to classes of functionally similar innovations” and that “important ecological, functional, and directional aspects of the history of life are replicable and predictable” (Vermeij 2006, 1804). What are the possibilities for evolution in our universe? Can we even think about considering the total spectrum of what is possible and not possible in biological evolution? The answer is yes, by using the analytical techniques of theoretical morphology, in particular by the construction of theoretical morphospaces (McGhee 2001, 2007). The concept of the theoretical morphospace originated in evolutionary biology (McGhee 1999), but it has subsequently caught the attention of philosophers (Maclaurin 2003), linguists, cultural anthropologists, and neuroscientists (Hauser 2009) who are seeking to explore the spectrum of both possible and impossible languages and cultures. Here we will use the concept to analyze the phenomenon of convergent evolution with respect to the spectrum of existent, nonexistent, and impossible biological form. Convergent Evolution in Theoretical Morphospace The analytical techniques of theoretical morphology allow us to take a spatial approach to the concept of convergent evolution. Any given biological form, or f in abbreviation, may be described by a set of measurements taken from that form—how tall is it, how wide, how long? Each type of measurement (height, width, length, etc.) can be considered as a dimension of form. The total set of the possible dimensions of form can be used to construct a hyperdimensional morphospace of possible form coordinates (figure 7.1). Each point within this theoretical morphospace represents a specific combination of form measurements that will produce Functional and Developmental Constraint in Convergent Evolution 247 the form coordinate for a hypothetical form f. Convergence occurs when forms originally present in different regions of the morphospace evolve in such a way that they move to the same spatial region in the morphospace (figure 7.2). Returning to figure 7.1, we can begin...