Philosophy of Biology

Natural Selection 125

withT 1 (the fitter trait) survives and the second organism (withT 2 ) dies. In the
second scenario, lightning kills the fitter organism withT 1 and the organism with
T 2 survives. It would seem incoherent to say that selection operates in the first
scenario while drift operates in the second.
This is correct, but defenders of the traditional view accept this consequence.
One cannot say how much drift and selection each contribute in an individual
case. The way to determine if drift has had an effect is to compare populations —
either two actual populations with different sizes or one actual population with a
hypothetical population of infinite size (i.e., where there is no drift). It is natural
to think of a population’s effective size as one of the causes of why traits in the
population evolve. If its size were much bigger or smaller, we wouldexpectdifferent
frequencies of genes or genotypes to evolve.
The debate about how to interpret natural selection, drift, and other factors will
doubtless continue, especially since the disagreements depend in part on disputes
about the nature of causation that lie in the background. We have seen that there
are a number of issues about natural selection that have interesting philosophical
dimensions, and doubtless there are many others.

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