Why are government health officials in the process of frantically putting together a national response plan to a possible, even though unlikely, pandemic of the bird flu? In a word, the answer is evolution.
What Darwin's theory addresses is not the origin of life, but rather describes the process of how life forms can change and even develop into new species, over time intervals that cover numerous generations of the organism in question. Seeing evolution in action is routine when it comes to single-celled organisms, including bacteria and viruses. The reason for this is that certain tpes of bacteria, for example, reproduce at incredible rates, with new generations forming perhaps every hour. In relatively short periods of time one can see the characteristics of dozens of generations. In addition, the way new types/species of bacteria or organism arise is through random mutations of the DNA and/or RNA insie the cell. Mutations can arise via random mistakes made during the transcription of DNA molecules, or it may occur from bombardment of the DNA with different types of chemicals or radiation. Whatever the mechanism, a genetic mutation, even to single genes, can in fact lead to a different type of offspring when compared to the parent. This is commonplace with single-celled critters. How often have you heard people saying that the use of antibacterial soaps has had an unintended consequence of helping lead to a new stran of the original baceria that is resistant to the soap? Or to medications? It happens all the time, to the point where each year the mix of flu vaccines we can get in the U.S. vary since the flu strains each year are a bit different. Evolution is absolutely a fact because we see it all the time in these systems. New offspring that have a genetic makeup that provides characteristics that are more conducive to survival in a particular environment allow that organism to in fact survive, while organisms with the original genetic akeup may in fact die off. Nature selects the better-suited organisms for survival over poorly-suited strains.
This is precisely the fear with the Asian bird flu. As the present strains reproduce and mutate, it is possible that a new offspring develops that can be transmitted from human to human, rather than the more likely (at least for now) bird to human through close contact or consumption. The main processes that are included in evolution, which include speciation and natural selection, take time (over many generations), which is why we won't likely see the process for multicelled organisms in the lab. This process has, however, mountains of hard evidence in the fossil record, comparative anatomy of all sorts of animals and plants, computer simulations, and by lookng at polymorphisms in the genetic code (is it simply chance that humans and chimps are something like 99% the same genetically? Probably not...). The processes involved in evolution are observed everyday in various situations, which is why scientists have almost universally agreed that the theory of evolution is actually fact (even Pope John Paul agreed that the physical evidence supports evolution as the correct model for understanding the development and variety of life), and it is evolution that has once again made the possibility of a pandemic real.