Here is something from a few months ago on adaptation:
In the ocean near Antarctica lives a family of fish (within the Notothenioidae) called the icefish which unlike any other fish (or vertebrate) on the planet has no red blood cells in its veins. In fact, even its white blood cell count is only 1%; in other words, their blood is literally ice water. For any other animal, this would mean instant death. But the icefish gets along just dandy because the near-freezing water of the Antarctic is more highly oxygenated than water at warmer temperatures. With so much oxygen in the water, the icefish does not need hemoglobin to circulate oxygen in its system. Moreover, not only is hemoglobin unnecessary, it is also a deteriment at freezing temperatures. Its viscosity increases the colder it gets and increased viscosity naturally interferes with circulation. Thus even red-blooded Antarctic fish have a dramatically lower percentage of hemoglobin in their blood.
With no hemoglobin in their blood, a creationist might reckon that -- of all the fish in the world -- these fish were simply uniquely created that way. But when you look at their DNA, the two hemoglobin genes are indeed present in the code, exactly where they would be found in red-blooded fish. The two genes however are corrupted; the first gene is missing pieces of code whereas the second is almost entirely eroded away. These are mutations, plain and simple, and they explain why the fish do not have hemoglobin: the corrupted DNA cannot produce hemoglobin proteins.
Now, these mutations (which represent not a single mutation event but a series of deformations or copying errors to the DNA) would have been absolutely fatal to a fish -- or person -- who lived in an environment that required the use of hemoglobin. If it was something that happened to a human egg, we would have a great example of how mutations can cause birth defects and death. But this is not what happened with the icefish, for they are all alive and well today. These mutations thus did not have a detrimental effect and indeed were beneficial. Antarctic fish that received mutations preventing the production of red blood cells would lack the viscocity posed by hemoglobin and thus have more efficient circulation systems that would allow a greater input of oxygen from the highly oxygenated ice water. This allowed mutated fish to live in colder waters than red-blooded fish could have lived and also prevented them from living in warmer waters where fish without the mutations could easily live. Rather than causing death, the corrupted genes were passed on to successive generations because they faciliated better adaptation to cold water.
That adaptation was involved is apparent from the history of Antarctica. Prior to 65 million years ago it had a warm climate; dinosaurs lived there and ocean temperatures were warm as well. But the climate began to change in the Cenozoic Era and continental drift brought Antarctica down to the southern pole where it became surrounded all around by ocean. The Antarctic Circumpolar Current that circles this ocean effectively isolates Antarctic fish from those to the north, preventing them from migrating to warmer waters. Characteristics that helped fish to live in an increasingly colder environment, like the hemoglobin mutations but also larger gills (which admit a greater amount of water), would logically became more widespread in the population because such fish were more likely to spawn than less adapted fish. The mutations hindering the production of hemoglobin would have then become more and more common in the breeding population (isolated from their kin in warmer waters) over successive generations, with functioning genes becoming more and more scarce (e.g. selective breeding), until we have the breeding population we see today, with all known icefish in their respective species lacking any hemoglobin in their circulatory systems.
