A theory is a system of ideas but anyway what test's of evolution have u substantiated then?
In science, "theory" has a very specific definition. It is an idea or group of ideas that have been repeatedly verified by evidence. A theory begins as a hypothesis, which is a proposed explanation for an observed phenomenon.
Darwin observed the diversity of life, and proposed a hypothesis that living populations change over time. His hypothesis had these predictions.
1. more are born than can survive
2. every organism is somewhat different than it's parents
3. some of these differences affect the chances of the organism to survive long enough to reproduce.
4. favorable differences tend to increase and unfavorable differences tend to disappear
5. these differences accumulate and will eventually produce new species of organisms.
Over time, these predictions have been repeatedly confirmed by observation, and so his theory is confirmed. Notice that a hypothesis must be testable.
A theory is stronger than a law; laws merely predict what will happen under specific circumstances. Theories predict and explain what happens.
Hence, Kepler's laws describe how planets orbit the Sun, while Newton's theory of gravitation explains why they move that way, and extends the understanding to motion of moons, galaxies, the tides, and apples falling from trees.
Since the definition of evolution from Darwin was "descent with modification", his theory was confirmed by observing changes in populations over time. The modern definition,incorporating genetics, "change in allele frequencies in a population over time", is a restatement of Darwins definition. And yes, that too is confirmed.
Speciation has been repeatedly confirmed as well; most creationists now acknowledge speciation, arguing that it's "not really evolution." But as you see, they have to alter the scientific definition to reach that conclusion.
Usually, creationists say that they object to evolution, when it's really common descent that bothers them. And there is considerable evidence to support common descent. Would you like to talk about that?