The Diversity of Life
There are an uncountable number of species living on Earth today. Some studies estimate there could be about 8.7 million total, with only a small fraction of those being catalogued so far. One may wonder where they all came from, and how they all seem to be so well adapted to their environment. However, it's easy to see that species are actually not that different from each other, and if one were to categorize species into groups with similar traits, a graph in the shape of a tree can be formed. A family tree.
All living things reproduce. Offspring are never an exact copy of their parents. They are a combination of their parents' traits, but also subtle, random variations. If these random variations didn't occur, then all siblings would be clones! (they'd have identical DNA)
These variations will obviously have an effect on the individual's life. For example, an animal with longer legs would be able to run away from predators faster. An animal with speckled fur may camoflauge themselves, making them less noticable to predators. An animal with a brain containing more neurons may be smarter, and able to predict and avoid danger. These changes do not have to be dramatic, and can be caused by just a random little change in DNA, as always happens when a new organism is being formed. A change is not always benefical either. After all, it is random. For example, an animal born blind will not be able to avoid capture. An animal with a mental disability may not learn to find food. The members of a species who succeed in their environment are the ones who survive, and in turn, reproduce. This next generation of offspring will be similar, and keep the beneficial traits, but they may also be different in some ways, producing variety, and another opportunity for improvement to the species.
This depressing concept of weak organisms being less likely to reproduce is referred to as "natural selection". It reminds us that nature is a merciless game with species forming competitive or also co-operative relationships. This is also why a species may seem to be made specially for its habitat. Many small random changes occur over long periods of time, but only the animals with modifications that are useful in that environment will survive to reproduce, creating more creatures like them.
It is important to note that a single organism's DNA does not change during its life; its physical form is relatively constant. Only during reproduction does the DNA have an opportunity to change.
Some people do not see how a process so automatic, yet natural could yeild beings as complex as humans. Yet, here we are. We may not remember our family history all the way back to the first bacteria, but fossils, DNA, and our similarities to other animals give us a pretty clear picture. It's not like we just dropped into existance one day!