Most people think of evolution in terms of descent with modification, to use Darwin’s term, of large organisms, and in particular animals. Of course, the process is the same for humans, HIV, or anything else, but let us take a look at the macroscopic outcome of evolutionary change. This is the realm of comparative anatomy and paleontology, and the evidence for the evolution of plants and animals comes from studies of their physiology, morphology and development. Additional evidence comes from the much-maligned fossil record, which is much less incomplete than creationists claim.
There are many great examples of morphological evolution in plants and animals, but perhaps one of the most spectacular is the one concerning the evolution of modern whales, which has largely been elucidated recently by a series of paleontological findings and molecular studies.
Biologists now think that the story began with terrestrial animals hunting in the proximity of rivers back 55 millions years ago (belonging to the group of Artiodactyls; i.e., to the same group as hippos, pigs, camels, and sheep), continued through a sleuth of intermediate forms of semi-aquatic and aquatic animals, to end with the modern two major groups of whales, toothed and baleen (the figure summarizes the historical relationships among these major groups of animals).
It is important to understand that the fossil forms we know of are not necessarily the direct descendants of each other, since evolution proceeds more like a bush with many branches than as a linear orderly sequence. This bush-like pattern is exactly what one would expect from a natural process that doesn’t have an internal direction (as opposed, say, to a divinely inspired plan directed toward a predetermined end). However, for evolutionary theory to be confirmed all we have to see is that some of the intermediate forms are found when and where they are expected. Badly out of sequence fossils are one way to disprove evolution. Unfortunately for creationists nobody has yet found a human fossil together with a dinosaur, or a dinosaur in pre-Cambrian times.
Not only fossil record, but also modern molecular biology illuminates the history of whales, telling us essentially the same story from a completely different viewpoint (the convergence of different types of evidence is the hallmark of a correct scientific conclusion). It turns out from DNA evidence that among the close (though not quite the closest) living relatives to whales are hippos, which share the last common ancestor a bit earlier then the time of the split between Cetaceans and Artiodactyls, 55 million years ago.
Interestingly, hippos are also right now on their way to becoming fully aquatic animals, and show many similarities to whales: their babies swim before they walk, the mother nurses the young underwater, their testicles remain inside their body, they are hairless, and they do not sweat. These are all adaptations to a prevalently aquatic lifestyle, and show us some of the intermediate stages that whales’ ancestors probably went through (it is hard to say for sure because behaviors and soft body parts such as testicles do not fossilize). Next time somebody ridicules the idea of gradual evolution of marine from terrestrial forms, just go to the zoo and point out sea lions and hippos.