Please give definitive examples that by evolution, a species has changed into something else.
Some creationists cite what they say is an incomplete fossil record as evidence for the failure of evolutionary theory. I asume that you want a biological explanation. Natural selection has a lot to do with being in the wrong place at the wrong time, so let's ignore it for now and stick with solid molecular biology facts. The unifying principle of common descent that emerges from all the foregoing lines of evidence is being reinforced by the discoveries of modern biochemistry and molecular biology.
Regardless of the origin of life, evolution is a concept that has not yet been disproved. Evolution is the rate of change of every species' DNA, which is measurable. Two examples are: 'genetic dift' (that occur independently of selection) and 'spontaneous genetic mutation'. The average rate at which a particular kind of gene or protein evolves gives rise to the concept of a "molecular clock."
The more closely related two organisms are, the less different their DNA will be. Because there are tens of thousands of genes in humans and other organisms, DNA contains a tremendous amount of information about the evolutionary history of each organism. Evolutionary theory explains that biological diversity results from the descendants of local or migrant predecessors becoming adapted to their diverse environments. This explanation can be tested by examining present species and local fossils to see whether they have similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local ancestry had migrated into the locality. Wherever such tests have been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and South America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately 3 million years ago.
Some examples:
- The split of one species into two often starts because a group of individuals becomes geographically separated from the rest. This is particularly evident distant remote islands, such as the Galapagos. The ancestors of these finches emigrated from the South American mainland to the Galapagos. Today the different species of finches on the island have distinct beak shape, habitats, diets, and behaviors.
- The Hawaiian Islands are not less hospitable than other parts of the world for the absent species. For example, pigs and goats have multiplied in the wild in Hawaii, and other domestic animals also thrive there. The scientific explanation for the absence of many kinds of organisms, and the great multiplication of a few kinds, is that many sorts of organisms never reached the islands, because of their geographic isolation. Those that did reach the islands diversified over time because of the absence of related organisms that would compete for resources.
- In 1959, scientists at Cambridge University determined the three-dimensional structures of two proteins that are found in almost every multicelled animal: hemoglobin and myoglobin. During the next two decades, myoglobin and hemoglobin sequences were determined for dozens of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. All of these sequences were so obviously related that they could be compared with confidence; The differences between sequences from different organisms could be used to construct a family tree of hemoglobin and myoglobin variation among organisms. This tree agreed completely with observations derived from paleontology and anatomy about the common descent of the corresponding organisms.
- The evidence for evolution from molecular biology is overwhelming and is growing quickly. In some cases, this molecular evidence makes it possible to go beyond the paleontological evidence. For example, it has long been postulated that whales descended from land mammals that had returned to the sea. From anatomical and paleontological evidence, the whales' closest living land relatives seemed to be the even-toed hoofed mammals (modern cattle, sheep, camels, goats, etc.). Recent comparisons of some milk protein genes (beta-casein and kappa-casein) have confirmed this relationship and have suggested that the closest land-bound living relative of whales may be the hippopotamus. In this case, molecular biology has added on to the fossil record.
- Simple organisms like bacteria show us examples of evolution in a very short space of time as they reproduce fast. By doubling their numbers every 20 minutes they can produce several generations in a very few hours and therefore evolve in a relatively short time. One example is the bacterium E.coli. During replication their DNA can be damaged or changed. Most of the time this results in death, but occasionally the mutation is beneficial and may change a single protein to confer immunity against antibiotics. When antibiotics are poured onto the agar plate, all bacteria will die except ones with the beneficial mutation. If present, the mutation will give rise to an antibiotic-resistant strain (colony) of E.coli.
My favorite example: dog breeds.
In real life, evolution is not a ladder. It is a branching bush.