What is it called when two theories ultimately become one theory in science?

Question

I am trying to think of an example of scientific explanation whose scope was in fact broader than we initially thought. The idea would be the following:

Initially, we used H (the explanation) to explain a certain phenomenon (call it x) and we took a range of phenomena to be relevantly similar to x in the sense that H would also apply to them. We latter discovered that the phenomena we took to be relevantly dissimilar to x (call it y) were not so and that the other explanation needed to explain them was in fact highly similar. We essentially discovered that H or H', a derivative of H, not only explained x but also y.

So, can anyone provide from the philosophy of science solid examples of the broadening of scope of scientific explanation and theory?

Answer 1

In the physics world, such an event is called unification. Here are some examples:

Maxwell's equations unified all the various experimentally-derived laws of electromagnetics into one set of four equations that concisely expressed all that was known in that field, and in the process allowed the speed of light to be calculated, and the laws of optics and electromagnetism to be unified by demonstrating that light was fundamentally electromagnetic in nature.

Electroweak unification revealed the underlying connections between the electromagnetic and the weak interaction, demonstrating that they were on a deep mathematical level different aspects of the same underlying physical phenomena.

Grand Unified Theories attempt to unify gravity with the electroweak mechanism. Over the years, many have been devised mathematically but so far none of them accurately represents the world we inhabit.

Answer 2

Short Answer

The subject you refer to is often known broadly as reductionism or theoretical reduction and among applied ontologists ontological alignment, and there are some well-known examples.

Long Answer

Sometimes two distinct phenomena are found to be reducible to some simpler and common set of principles. A classic example in the philosophy of science comes from organic and inorganic chemistry which today are both thoroughly understood in terms of atomic theory. This wasn't always the case.

Before modern atomic theory (in contradistinction to the atomic theory of the Pre-Socratics) united the understanding of organic and inorganic substances, it was thought there might be something inherently different about substances coming from living beings given the differences in how biological organisms and inert matter in the environment differ in their dispositions. Of course, we now understand how subatomic particles compose substances of both sorts, and that proteins, nucleic acids, lipids, and carbohydrates can be built from inert matter, but the idea that living beings have something fundamentally different to their composition survived into the 20th century in the form of the élan vital and the theory known as vitalism. This is a fantastic example of how scientific methods refine explanation over the ages to increase reliability and utility of knowledge.

It should be noted that sometimes a hypothesis or theory actually presumes it explains more than it does, and after time it becomes apparent that there are aspects of phenomena that aren't covered by the original theory. This includes Dalton's original theory of atomism and the conservation of mass. See PhilSE: Are there examples of the narrowing scope of scientific explanations?