It's important in any discussion of biological evolution to distinguish between microevolution and macroevolution. Creationists readily concede the existence of microevolution - changes in characteristics within a species over time. They realized that it would be next to impossible to explain how farmers were able to breed more useful plants and animals over thousands of years without the existence of microevolution.
But Creationists absolutely refuse to give the same consideration to macroevolution - enormous numbers of changes in characteristics in organisms occurring over a very long time. Macroevolution includes speciation, or endless branchings in the Tree of Life ever since the first forms of life established themselves billions of years ago.
Creationists are most adamant in opposing the idea that Homo sapiens sapiens are a part of this process. Macroevolutionary theory states that we humans share this planet with millions of other species, all of which are our distant cousins.
The theory of biological evolution incorporates millions of different but consistent findings in the lab and in the field that buttress the idea that macroevolution does indeed occur. These findings can be grouped into dozens of generalized observations, evidence for macroevolutionary change. Here are a few examples:
(1) This is a particularly good example of hot-off-the-press, cutting-edge science. A few days ago (in April 2009), scientists announced that 10 of the 20 amino acids found in all life on Earth form relatively easily throughout the universe. The same amino acids are found in interstellar clouds by astronomers running spectroscopic analyses, in oceanic vents, and in meteorites that survive the plunge through the atmosphere. The other 10 are more complex. They probably formed later in Earth's history.
By analyzing their thermodynamic characteristics, scientists have matched their own computer models for the amino acids' predictive occurrence with their real-life occurrence exactly. In other words, amino acids form exactly the way scientists think they form.
How does this tie in with evolutionary theory? Amino acids are the building blocks of life. If half of them form relatively easily (and now we know they do), that means the earliest forms of life formed relatively easily.
(2) Genetic similarity determines relatedness. Darwin and Wallace had no access to genetic knowledge, and could not use the insights modern evolutionary biologists have access to. Genes not only specify the proteins organisms are made of, they also preserve that information for future generation.
As such, genes serve as markers for evolutionary biologists. They've found genes that every organism share - plants and animals alike. They've also found genes that are reserved to a portion of the Tree of Life, and other genes that are similar, but not exactly the same in different, but closely related species. Such genes serve as genetic clocks for researchers attempting to determine when the parent species split. The match between genetic clock findings and evidence found in the fossil record for estimated speciation dates and the sequence of speciation events is extraordinarily close.
(3) Genetic drift. Some genes specify proteins for structures in the body that need to be specified exactly. Others specify for structures that merely need to do the job, such as protein scaffolding for DNA molecules. Changes in those proteins that don't affect its ability to hold the double helix together occur without consequence to the organism. And so they do. This is called genetic drift. It serves as another genetic clock for researchers. They can determine when species split from parent by checking the specific differences in genetic coding in non-critical genes and comparing them.
(4) The fossil record. As geologists carefully excavated layers of rock around England and Europe in the late 18th and early 19th centuries, they made an astonishing discovery: The types of fossils found in one specific layer of rock always matched everywhere that particular layer was found. The same thing was true for all the other layers. The types of organisms changed between layers, but remained the same within a layer. Darwin realized the true significance of these findings. Rock strata became a time machine for him. The rocks told him that life forms change drastically over long periods of time.
(5) Nested hierarchies. Instead of a ladder of ascent, as described in the old religious theory of "the Great Chain of Being," species are found to exist in predicted patterns of "groups within groups," or nested hierarchies. Linnaeus noted this pattern when he set up his system of classification in the 18th century. This pattern is confirmed by genetic comparisons between species, families, orders, and so on. Nested hierarchies confirm predictions made by evolutionary theory, specifically those describing descent with modifications and speciation as a continually branching process.
(6) The missing link. Intermediate and transitional forms have been found numerous times by paleontologists. These include intermediates between bird and reptiles, fish and reptiles, and birds and mammals. These findings match the forms intermediate and transitional organisms should have according to theory.
(7) Vestigial organs. Evolutionary theory predicts that descendant species should retain vestigial organs that no longer help them survive, but don't hurt them either. Why? Because evolution isn't neat and clean. Natural selection only operates against harmful variations by killing them off. These vestigial organs have been found. Examples include vestigial pelvises in snakes, vestigial legs in whales, rudimentary eyes in animals living in caves with no light for thousands of generations, and the occasional appearance of the true human tail. These organs serve as genetic reminders of the characteristics of a species ancient parent species.
(8) Biogeography. This scientific interdiscipline studies the distribution of species around the world, both living organisms and fossils. Evolutionary theory states that closely related species should live closer together than more distantly related species in the Tree of Life. This distribution pattern has been mapped out in the real world, not just in computer models.
This short survey of evidence for the existence of macroevolution is convincing. The fact that all of these predictions made by theory have been found to be true in nature shows us that biological evolution has been going on for a very, very long time. It also serves as circumstantial evidence for the assertion that evolution is not planned or guided, that instead it is a self-directing, emergent process.
For further examples of evidence for the reality of biological evolution, check out theTalkOrigins site.