Question

What is proof that evolution is true?

Answer 1

While this is a very large topic, and one that has had a LOT written about it, but I will try to address your question succinctly.

Explanation:

Let me begin by trying to clarify a few points. First, that scientists rarely use the term "proof". Proofs can be logical and mathematical, but in science it is very difficult to be 100% certain we are 100% correct. We can be 99.9% sure that we are 99% correct, but are always on the lookout for information which will help refine our understanding further. So we talk about strong evidence, rather than "proof".
Second, the term "Evolution", covers two things: Evolution, and the theory of evolution.

Evolution is the change in living things over time. While biologists might define it as, at its simplest, a change in allele (gene) frequencies in a population, it is more generally thought of as the change in appearance of a species over time, as well as the appearance of new species.

The theory of evolution is an explanation of how evolution occurred. It should be noted that in science, the term "theory" has a different meaning than in everyday conversation. To non-scientists, "theory" means a notion or idea. In science, however, a theory is a mechanism that has stood up to multiple avenues of investigation. Atomic theory, gravitational theory, and quantum theory are all examples of well-tested scientific theories.

In terms of how we know that Evolution occurred, we have multiple lines of evidence:

1. The fossil record: We can see from the fossil record that the variety of animals and plants that have lived on Earth has changed over time.
2. Homology: It is striking that the similarity in body plan is so similar among a wide range of species. For example, the arrangement of bones in the skeletons of all terrestrial vertebrates (reptiles, birds, mammals and amphibians) is much the same. It is easy to see how one body plan can be adapted to perform so many different functions, such as the wing of a bat, which is made from skin stretched across a hand.
3. Embryonic development: the pattern of embryo development, similar to Homology above, shows very similar patterns over a wide range of animals.

There are also lines of evidence in:

1. Biogeography: the distribution of animals and plants.
2. Genetics: the study of how genes are similar and how they are different between different animals and plants.

The theory of evolution is a set of mechanisms that drive the process of evolution. The most widely known of these is the process of natural selection, but there is also genetic drift and a few other effects that contribute. All of the mechanisms have been tested thoroughly over the last century and a half, and the current theory of evolution which builds heavily on the work of Darwin and Wallace is the best explanation we have for the processes of evolution.