[This blog post was prompted by this Scitable discussion. Unfortunately comments were closed before I could contribute.]
Laws are bits of language that describe regularities in nature. If the laws are true, the regularities are real. Laws are general claims, but they are more than accidental generalisations such as “everyone in this room is over five feet tall”. Laws are more like hyper-generalisations in that they don’t just describe what has actually been the case so far — they describe what would be the case, even if the states of affairs that would make them true have not yet come to pass.
There aren’t any laws about the heights of people who happen to be in a room together, but we’d be moving in that direction if we arranged some sort of screening mechanism that only allowed admittance to that room on the basis of height. Genuinely scientific laws rely on such mechanisms when they describe such things as the electric charge of fermions in an atomic nucleus.
Many fundamental laws of physics like Pauli’s Exclusion Principle do not admit of exceptions. Exceptionless laws like that are quite common in physics and chemistry. What about biology?
The question whether there are laws in biology is too often understood as asking whether there are exceptionless laws in biology. I’d guess there probably aren’t any such laws, because the categories of biology (species, etc.) are not like the categories of physics.
But it does not follow that biology has no laws. The salient feature of laws is not that they admit of no exceptions but that the links they express (between categories, concepts, etc.) are non-accidental.
Examples: animals with high male parental investment tend to be monogamous; mammalian mothers tend to be protective of their young. The biological functions of parental investment and pair-bonding are linked; and so are the functions of producing milk and caring for young.
Those links entitle us to draw inferences: if we hear that animals of species X exhibit high male parental investment, we can guess that they are monogamous, although there is always the possibility that we are dealing with an exception. If we hear that Y is a female mammal, we can guess that she is protective of her young, even though there is always the possibility that this particular individual’s behaviour is “aberrant”.
I hope it’s clear that biology does critically rely on and describe non-accidental links between categories — links that entitle us to make inferences between claims containing the corresponding concepts. It is that warrant to infer that makes for genuine scientific laws, not their exceptionlessness.
Biological laws have exceptions because many biological categories are “functional” (as exemplified above). In describing, explaining, predicting (etc.) things biologically, we adopt what Dennett calls the “design stance”. We assume that things have functions (purposes, goals, tasks, etc.) and that they perform those functions more or less well “as they were designed” to. “Working properly” shades into “less-than-optimal performance”, which in turn shades into out-and-out “malfunction”. Thus biological categories have fuzzy edges, in other words, these categories have grey areas where there are exceptions.
(Warning: of course nothing in biology is literally designed by a designer. The main point of evolutionary theory is to show how no such design is required. Talk of design, purposes, goals etc. in biology is just shorthand for past contribution to survival and reproduction.)