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.
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.
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.
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.
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 Monods 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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