The problem for you is that the definition of the theory of evolution has not significantly changed since our understanding of genetics was incorporated into it, thus giving us the modern definition of "a change in gene frequency in a population over time". What usually happens is that creationists insist on conflating evolution like this:
quote:
A more appropriate definition for evolution is, a continuous naturalistic, mechanistic process by which all living things have arisen from a single living source which itself arose by a similar process from a non-living, inanimate world. This definition requires evolutionists to justify their claims of simplicity to complexity, life from nonlife, and common ancestry.
When we try to correct this, to inform you what science actually says about evolution, you complain that we are somehow changing the definition.
The change in gene frequency can be measured in different populations of a species or tested in the lab. We can also compare the genomes of extant species to identify relatedness and understand how a mutation to a gene influences the phenotype of an individual.
Look at that 2002 study on finches you cited in science magazine. What we see is how a single species over an extended territory can show differences in populations over such a short time, in response to selective pressures of their local environment. It's easy from this to suggest that given longer time and if gene flow was impeded that speciation could occur.
This is more than a guess, since we can look at the independent findings of other fields of science. For example we can look at geology which can identify through physics the age of the various strata and the overall age of the earth. Through geology, palaeontology can identify when different species existed, which then supports what comparative genomics tells us about when related groups shared a common ancestor.
It is all related, but whats important to remember is that each of those scientists, the physicist, geology, palaeontologist and geneticist, are focussed on their own small niche in science.