Ongoing debates: contingency versus convergence

The twin forces that shape Darwinian evolution are random variation and the relative fitness of those variants to the environment. As we have seen above, the process is most easily expressed in abstract terms, but until abstract models of evolution are parameterised they can lead to a wide range of predictions which may or may not correspond to the real world. An important step in parameterisation is to assign the weights of these two forces: chance, and selection to environment.

If the chance component is dominant we can conclude that natural selection will lead to any number of ends. This means that trends we see in the past are in reality arbitrary, and future changes are for all practical purposes arbitrary also. Palaeontologist Stephen Jay Gould and anthropologist Irven DeVore have advocated this view.

On the other hand, if we stress the role of the environment then natural selection will lead to more predictable ends because environments can remain relatively stable for long periods of time. Admittedly, if we focus in on the evolution of specific species we see the story of evolution being played out in an unpredictable sequence of environments and so leading to unpredictable ends. However, when viewed at larger scales, there are elements of the environment that are for all intents and purposes constant, and so natural selection leads towards a more definite range of ends. These pervasive requirements and constraints explain the phenomenon of ‘convergent evolution’ where different species have evolved similar adaptations independently. This is why many fish and dolphins share a similar shape; their form is in part constrained by the fluid dynamics of water.

While most evolutionary thinkers acknowledge that evolution is both contingent and convergent to some degree, there is strong disagreement on which is dominant. As mentioned above, Gould is most well known for stressing the contingency component. Writing on the convoluted path of human evolution in Wonderful Life: The Burgess Shale and the Nature of History Gould concludes that “we are the accidental result of an unplanned process .... The fragile result of an enormous concatenation of improbabilities, not the predictable product of any definite process.”

The resolution of this debate will have profound implications for the philosophy of biology and beyond. If evolution on Earth can be shown to be primarily a random walk, with natural selection merely ensuring adaptation to the environments along the way, then it would seem as though Monod’s position is correct; we should say that the ‘cause’ of the biosphere is blind chance, or at least chance when coupled with the algorithm of natural selection. However, if the convergence we see in species can be shown to be a significant factor, then an accounting of causes in evolutionary history needs to be expressed in different terms. There are four main possibilities:

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