Theory and Fact
Part of the problem is that the objectors are not using the word "theory" in the sense that scientists usually use it, and defenders don't call them on it. dictionary.com has as its first definition
A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.This is pretty close, but it's too much like physics and not enough like biology, especially the "predictions" part. The last definition is
An assumption based on limited information or knowledge; a conjecture,which is more what people usually mean by "just a theory", or "I have a theory about that", or "Evolution is just a theory".
It's more accurate to say that "evolution has a theory" (starting but not ending with Darwin), and that many of the conclusions to which the evidence has led biologists are so well supported that we are justified in considered them to be things that actually happened, i.e. fact.
So what does a theory look like?
Some simple theoretical statements are:
(1) A planet moves in an elliptical orbit with the sun at one focus. (Kepler's first law of planetary motion)
(2) If mating organisms are both heterozygous for a particular gene, then approximately 25% of their progeny will be homozygous for one allele, 25% will be homozygous for other allele, and 50% will be heterozygous (a modern version of Mendel' law of independent assortment)
Now consider two more statements:
(3) (a) The acceleration of a body is equal to the mass divided by the force (Newton's second law of motion)
(b) The sun attracts a planet with a force that decreases as the square of the distance (modified form of Newton's law of gravitation)
(4) When two organisms mate, each contributes one set of chromosomes to each of its progeny.
The thing to note here is that statement (1) can be derived from statements (3a) and (3b), and (2) can be derived from (4). In other words, if (2) and (4) are true, we know that (1) and (3) have to be true. (3) and (4)4 are more general than (1) and (2), so we say that (1) and (2) are reduced to (3) and (4). More about reductionism in a future post. Also note that (4) is a statement about biology, so it is not always as simple as it sounds. More later about that too.
I used the word "gene" just now. It is common in biology to say "Genes control growth". But what is a gene? The first definition of "gene" was that it is "the unit of heredity". This is some help, but not much. It turns out that the definition of "gene" has changed, as biologists's understanding of heredity and growth have changed. This vagueness in definition has been helpful, because it allowed understanding to grow underneath a very flexible blanket. (The definition of "evolution" itself is also somewhat vague. You guessed it: More about that in the future.)
The root problem is that you can't see a gene directly. If you want to investigate the motion of planets, at least you can see a planet. You may not understand what it is, but even if you call it a god, you can still observe its motion. But the only thing people knew about genes in 1900 was that something was transmitted from one organism to its progeny. Mendel called these units "factors"; biologists started calling them "genes", to denote that these were factors for inheritance. This vagueness allowed biologists to build a concept under the bare definition. This is a sort of bootstrapping: Build your definitions in the air, then put foundations under them. It's happening today in astronomy with dark energy. Nobody knows wht it is, but they know something about how it acts.
This is similar to the concept of operationalism, which says that if you can't observe a thing directly, you define it in terms of an operation that you use to measure it. If you are talking about temperature (which is of course not a "thing", but an attribute of a thing), operationalism defines temperature as the reading on a thermometer.
To talk about an electron, in this view, you talk about an experiment, or set of experiments, in which you measure the charge, mass, and other properties of something. Then you call that something an "electron". Biologists did the same thing with the "gene". As their understanding of heredity grew more sophisticated, so did the definition of "gene". More about genes in a later post.
The theoretical part of evolution starts with Darwin. There are four major assumptions:
(1) A new generation of organisms will vary from the parents. Different individuals will vary in different ways.
(2) Some variations are heritable. That is, if one individual is larger or smaller than its parent, the offspring of that individual will tend to be larger or smaller than the grandparent. Nevertheless, the members of that next generation will vary among themselves.
(3) Some variations will tend to help or hinder the survival chances of the individual, depending on the environment.
(4) Any organism can reproduce in greater numbers than the environment can support. For example, a finch might live five or six years. Two finches can produce three of four offspring for several years, for a total of maybe more than 20. If the population of finches remains constant, then most of these offspring have to die before they themselves reproduce.
(1), (2) and (4) are simple observations. (3) is natural selection, which is based on the idea of selective breeding.
The result of all these starting points is that the average member of a population will tend to have more or less optimum characteristics for survival in its particular lifestyle. That's overly simple; I'll try to amplify it in the future.
Darwin used these theoretical ideas to interpret his very keen observations of animals and plants, and concluded that many species have descended from other species. The number of original species is something that has to be determined by observation and analysis, not by pure thought. Biologists by and large have decided that life originated only once, or at least that if there were other originations, the others have become extinct. There is a great deal of evidence that points to this conclusion, so scientists feel justified in calling this a fact.
The third thing that is often confused is the path of evolution: What is descended from what? The Tree of Life project is an attempt to gather the available information. There is sometimes controversy, and things change from time to time. That's one of the things that makes science interesting.