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The Modal Status of Biological Laws
The second point I want to make about the implications for explanation of the contemporary
study of complex and evolved systems has to do with the role of contingencies. In both
philosophical accounts and scientific writing, one can find concepts of explanation that
require an explanation of a particular phenomenon to appeal to the laws governing the
phenomenon. There has been a long tradition of trying to characterize what it is that counts as
a law of nature or a law of science. Philosophers have typically described laws of nature as
universal and exceptionless. Laws carry with them natural necessity.
By way of example, consider a case from physics. There is something naturally
necessary, universal, and exceptionless about the relationship between falling bodies and the
gravitational constant of the earth. In some instances there may be
ceteris paribus
that must be met, but the gravitational law is necessary and holds both universally and
exceptionlessly. This kind of example gives rise to the view that there are universal
generalizations that are exceptionlessly true and that these laws explain why there are
connections between cause and effect. This is an old story, part of which is that this
understanding of laws fails to apply to most of biology.
Consider Mendel’s Law of Segregation. This holds that in a sexually reproducing
organism, the gametes that are produced in eggs and sperm have a 50–50 distribution of
parental genetic information. It turns out that Mendel’s Law is not universal as there are
exceptions, such as meiotic drive or segregation distortion, and these are generated by a
couple of different mechanisms.
Not only is Mendel’s Law not universally true, it does not seem to be necessary even
of all current sexually reproducing organisms. It seems that things could have evolved in a
very different way such that Mendel’s Law was neither lawful nor true. There could have
been a different distribution of gametes, say 30–70; there might not have been sexually
reproducing organisms; there might have been three or ten parents, or even one, rather than
two. All of the features of evolved organisms display this evolutionary contingency in a way
that some generalizations, for instance about the ideal gases, do not.
Why does the knowledge of biological systems fail to fit the standard account of
laws? Is it that biology is still in its infancy? Maybe it has had its Galileo but not yet its
Newton. Or is there something else responsible for this failure to fit the standard account of
There are different responses to this problem. Some people have argued that biology
simply does not have laws, such as John Beatty (1995) with his defense of an evolutionary
contingency thesis. He attempts to explain why biology does not have laws by arguing that
biology studies contingent structures; therefore, the causal relations that describe and explain
these structures are contingent. Others, including Robert Brandon (1997), have agreed with
Beatty that biology studies contingent, ephemeral, not necessary structures, so it does not
have laws. And indeed, others like Nancy Cartwright (1994) and Jim Woodward (2001) have
argued that science does not actually need this kind of law to explain. It is possible to explain