Hi Christy:
Evolutionists insist that it is a fact, beyond all reasonable doubt, that the species “evolved.” It turns out that this term “evolved” can mean many different things. For example, it can involve natural selection, gradual change, and a common descent pattern. But it can also be the opposite. Depending on the particular data under analysis, evolutionary theory can say natural selection was not at work, that change was not gradual, that the data are not a consequence of common descent. Philosophers, such as Imre Lakatos, have long since pointed out that scientific theories can have a core theoretic, which is not forfeitable, and they can have surrounding sub hypotheses which are loosely generated by the core theoretic, but in fact need not be true. They are easily forfeitable.
So with evolution, most of the ideas that are typically taught in biology class, such as that evolution involves natural selection, gradual change, and a common descent pattern, are in fact sub hypotheses that are forfeitable. They are not at the heart of evolutionary theory.
So what is the core theoretic of evolutionary theory? It is that the species arose strictly by a combination of what appear to be chance events and naturalistic laws and processes. Or as Jacques Monod put it, Chance and Necessity. You may remember from your chemistry class that the scientific term for the evolution of a system from one state to another is “spontaneous”. In science, spontaneous does not mean fast, it merely means that the change occurs naturally, to a lower free energy state.
So the core theoretic of evolutionary theory is that the species arose spontaneously, from an earlier state where there were no species. From a scientific perspective, that would be highly unlikely, but evolutionists say it is a fact.
But evolutionary thinking is not at all restricted to the origin of species. It is a much broader system of thought that is not restricted to any one field. Evolutionary thought engages psychology, geology, abiogenesis (i.e., the origin of life), astronomy, cosmology, and so forth. Simply put, all of existence is thought to have arisen spontaneously. For instance, the famous physicist Steven Hawking has written about how the beginning of the universe—the Big Bang—was a spontaneous event. Evolutionists also talk about how natural laws themselves spontaneously arose.
So evolutionary thought is not restricted and really a program that seeks to provide a strictly naturalistic account of origins, in general. So while individual workers, and particular fields of discipline, may be concerned only with one specific aspect of evolutionary theory, evolutionary thought has no such restrictions.
And so, as you say, individual evolutionists do not always agree on how to view the big picture, across all these different disciplines. Some evolutionists, for example, say that the evolution of the species is a fact, but abiogenesis, or a spontaneous Big Bang, etc., are not necessarily facts. The one area that has pretty much 100% consensus on is the evolution of the species.