Natural selection is an abstract, a process by which creatures that are not as well-adapted to their environments tend to die and thus fail to reproduce, while creatures with traits more compatible with their environment tend to reproduce fruitfully. This process is ongoing and leads to better-adapted creatures of all species.
DNA is not sentient. It does not "know to change, or even by [sic] mutated to help the species"
DNA, the "memory chemical" that stores genetic information, comprises the genes of every living organism.
The upshot of the analysis is this, says Goodenough: "The fact that the same kinds of genes are found in all the different kinds of organisms on the planet today, including both bacteria and organisms like ourselves, indicates that these genes developed long ago." To put it slightly differently, the presence of similar or identical genes in two organisms is a genetic fingerprint of a common ancestor at some time in the past.
The detailed science of fingering common ancestors and figuring out how their descendants are related is complicated stuff, way beyond my powers of comprehension. But the essential technique is obvious to anyone who has played telephone. You remember: You sit in a circle and whisper a phrase into your neighbor's ear, who then passes the phrase along. When the phrase returns to its starting point, "A pox on both your houses," may be transmogrified into, say, "A box of genes in both kinds of louses."
Obviously, the further you are from the origin, the more distorted the phrase will be (although if you are clever in sorting out the phonemes and the rhymes, you can deduce how the two sentences are related and how they were changed while being passed around).
While taxonomists traditionally analyzed relationships among organisms in terms of shared features, say wings or scales, genetic analysts measure relationships by calculating time to a common ancestor.
There's another way to show the similarity of genes in various organisms. Remember the lowly yeast, the microbes that raise dough into bread and ferment hops into beer? To demonstrate the similarity between yeast and human genes, experimenters remove genes that make proteins the yeast need to live.
What happens? The yeast croaks, that's what. But when the deleted genes are replaced with similar human genes, the yeast lives. These results, says Kansas biologist Robert Palazzo, "argue that common molecules exist in systems as distant as yeast and humans."
Much of this analysis is done with RNA in the ribosome, a cellular unit that assembles proteins under the direction of the genes. Because the genes for ribosomes change slowly, this analysis shows the evolutionary links between humans, worms, fruit flies and other organisms that, to the naked eye or microscope alike, look rather different.
In short, Organisms that are better suited to the environment are more likely to survive and reproduce, forcing species to change, or evolve.
Although not as old as life itself, the debate over evolution goes back quite a way. Even before Darwin wrote his seminal "The Origin of Species," European explorers were returning facts that demanded explanation. New animals challenged the notion that Noah's ark could possibly have held two representatives of every species. New geologic evidence indicated that the Earth was far more ancient than a literal reading of the Bible would permit.
Another discovery demanding explanation was the new races of people from Africa, Asia and Oceana. The question arose: Were humans all one species? "People saw more differences than similarities in these races," says Blair Nelson, a Ph.D. candidate in the history of science at the University of Wisconsin-Madison, "and it was difficult not to construct a racial hierarchy."
Just as Darwin's work made many people uncomfortable calling a monkey "uncle," many Europeans were uncomfortable with the notion of calling an Australian Aborigine "cousin."
Starting around 1840, a scientific explanation called polygenism arose to explain the appearance of so many races. According to polygenism, God created not one race but several. Despite the fact that scientists had developed polygenism, "It was a creationist theory," says Nelson, "and there were more acts of creation than in Genesis."
While polygenism was promoted by scientists who used God to explain nature, its opponents were theologians who pinned the differences among humans on the effects of environment as people migrated away from Mt. Ararat after Noah beached his ark.
In other words, they held that environment could affect appearance.
Nowadays, it's the religious folks who are arguing that God did the creating, while the scientists claim that environment (the testing force for natural selection) is what affects body type and species formation. "There has been a complete switch between Christians and scientists" since the earlier debate, Nelson says.