How can you best explain the evidence behind evolution in a simple manner?
The main evidence for evolution is:Organisms resemble their parents and siblings, but vary slightlyThe information that determines how organisms will look and behave is largely held in their genesGenes of different organisms within a species generally resemble one another closely (and closely resembling genes are necessary for organisms to reproduce)Genes of organisms in different species resemble one another less closely, but the closer they are, the more similar the species tend to lookWe can form a ‘family tree’ of genes in different species that matches up closely to similarities between the species.That family tree shows clear patterns of inheritance: organisms can be arranged into families and sub-families, and traits that appear in one branch of the tree are never adopted in other branches. (*)All apparent exceptions to this rule turn out to be independent, based on different anatomical and genetic changes. For example, many species in different branches have wings, but the wings of bats, birds, pterodactyls and insects are fundamentally different and built on different principlesWe can find the remains of ancient organisms which also fit this pattern of inheritance. Fossils whose existence is predicted by the theory, such as fish with primitive legs, are often found in exactly the locations they were expected to be found.The fossils can be dated using techniques based on measuring quantities of different radioactive isotopes. Fossils of particular species are always found in rocks of a particular age, again matching the family tree predicted by anatomy and genes.It’s notable that if any one of these were found not to be true, it would present a serious challenge to the theory. If a fossil rabbit were found in ancient rocks from the time before vertebrates evolved, the entire theory would be overturned.(*) This one isn’t 100% accurate. It’s mostly true for complex organisms like us but not for bacteria, which exchange genes with one another quite freely. Even among complex organisms, retroviruses can sometimes cause genes from one species to hop into another, and of course humans have now begun to do this deliberately as well. But it’s close enough to being true for our purposes.