There are many laws in science (e.g., the laws of thermodynamics, Mendel's laws of heredity, Newton's inverse square law, the Hardy-Weinberg law). Laws are extremely useful empirical generalizations: they state what will happen under certain conditions. During cell division, under Mendel's law of independent assortment, we expect genes to act like particles and separate independently of one another. Under conditions found in most places on Earth's surface, masses will attract one another in inverse proportion to the square of the distance between them, following the inverse square law. If a population of organisms is larger than a certain size, is not undergoing natural selection, and has random mating, the frequency of genotypes of a two-gene system will be in the proportion p2 + 2pq + q2. This relationship is called the Hardy-Weinberg law.

Outside of science, we also use the term law. It is the law that everyone must stop for a stoplight. Laws are uniform and, in that they apply to everyone in the society, universal. We don't usually think of laws changing, but of course they do: the legal system has a history, and we can see that the legal code used in the United States has evolved over several centuries primarily from legal codes in England. Still, laws must be relatively stable or people would not be able to conduct business or know which practices or behaviors will get them in trouble. One will not anticipate that if today everyone drives on the right side of the street, tomorrow everyone will begin driving on the left. Perhaps because of the stability of societal laws, we tend to think of scientific laws as also stable and unchanging.

However, scientific laws can change or not hold under some conditions. Mendel's law of independent assortment tells us that the hereditary particles will behave independently as they are passed down from generation to generation. For example, the color of a pea flower is passed on independently from the trait for stem length. But after more study, geneticists found that the law of independent assortment can be "broken" if the genes are very closely associated on the same chromosome. So minimally, this law had to be modified in terms of new information—which is standard behavior in science. Some laws will not hold if certain conditions are changed. Laws, then, can change just as facts can.

Laws are important, but as descriptive generalizations, they rarely explain natural phenomena. That is the role of the final stage in the hierarchy of explanation: theory. Theories explain laws and facts. Theories therefore are more important than laws and facts, and thus scientists place them at the top of the hierarchy of explanation.

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