Observations:

1) There is individual variation among the members of a species.

2) At least some of this variation is heritable (passed from parents to offspring).

3) Because of these inherited advantages and disadvantages, some individuals are better suited for survival than others.

4) Because of the scarcity of resources, there is a struggle for existence. Those individuals with advantageous traits are more likely to survive and reproduce.

5) The survivors pass down these advantageous traits to their offspring.

Conclusion: The makeup of a species changes over time as undesirable traits are eliminated and desirable traits become more common. Therefore the species adapts. Sometimes referred to as "micro-evolution", it can happen over a relatively short period of time and involves relatively small, incremental changes. Microevolution is frequently observed in natural populations as the environment changes, and in lab experiments with short lived organisms like fruit flies and bacteria.

Comments: Individuals do not evolve. Species do. An individual is born with a set of inherited characteristics that make it either well suited or poorly suited for a particular environment. If it was born with the right traits, it is more likely to live and pass them down. If it was born with the wrong traits, it is more likely to die before passing them down. Note that while a characteristic (a thick coat of fur, for example) might convey an advantage in one environment, it might actually be a disadvantage in a different environment. Adaptations can only be discussed in the context of a particular set of environmental conditions. An adaptation is defined as an inherited characteristic that makes an organism better suited for its environment than the competition. Evolution is defined as a change in a species over time, and natural selection is one mechanism that can cause evolution to occur.

It was once thought that there were an unchanging number of species on the earth; it was believed that species neither appeared nor disappeared over time. The study of fossils has shown that some species which are alive today cannot be found beyond a certain point in the past. In other cases, the fossils look somewhat different or dramatically different from the most similar things alive today. Finally, there is evidence that some species that lived in the past, such as the dinosaurs, have gone extinct. This raised the question: "How do new species appear?"

Definitions: A species is defined as a group of organisms that are theoretically (if they had the opportunity) capable of mating and producing viable offspring. Populations are subsets of a species that, because of proximity, have individuals which are realistically capable (because they have the opportunity) of reproducing and producing viable offspring.

What separates two closely related species? Closely related species are prevented from mating by physical or behavioral differences that make them incompatible. These are called Reproductive Isolating Mechanisms (RIMs). Attempted matings will result in failure.

How does speciation occur? An entire species can gradually be transformed into a distinctly different species (as compared to their ancestors) over time as small changes eventually accumulate in response to natural selection. Alternatively, one species can transform into two or more sub-species by the following process, called allopatric speciation. Members of the same species disperse into different environments. Since the local environments differ (think about desert and mountain, or lake and river, or grassland and forest for example), each of the populations is selected in different evolutionary directions. If the environmental differences are extreme enough, or if enough time passes, the two reproductively isolated populations will accumulate enough genetic differences that it would be disadvantageous or behaviorally or physically impossible for them to hybridize and produce viable offspring.

It's not just that humans are related to apes, but that all living things are related to each other. All living things on earth appear to be descended from common primitive ancestors. How do we know? Our belief comes from the fact that all life on Earth has certain things in common, so we assume that it came from the same source. The best evidence comes from examination of our DNA or genetic blueprint. All living things transmit their genetic code using the same molecule called DNA, and all living things share some gene sequences. Over the 3.5 to 4 billion years of which there has been life on Earth, these common ancestors diverged into a wide variety of species and gradually, some of those species became more complex. The best way to think about macro-evolution is that life is like a branching bush. Life got started on Earth a long time ago, and branched in many different directions. Only the tips of the branches represent species that are alive today. Humans are not direct descendants of chimps or gorillas or any other species alive today, but humans and the other primates are on the same branch of the tree. We are more closely related to chimps and gorillas (about 99% of our DNA is identical!) than anything else alive today, but they are not our ancestors; better to think of them as "distant cousins". The non-primate mammals, such as cats, dogs, rats, etc. are more distant relatives, but the evidence of our relatedness shows in our common features: we all have hair and we feed our babies with milk. No other group of living things has those characteristics. There are more genetic differences between us and them than there are between us and the great apes.

Although evolution is still controversial in some segments of society, it is an accepted fact among biologists that nothing else explains life so elegantly as the idea that we have all evolved from common ancestors. It is the foundation on which the science is based. Modern medicine depends upon the fact that when we learn something about an animal, we can generally apply that knowledge to humans (and vice versa), and that the more closely related the animal is to us, the more likely that things will work the same way in us. In the treatment of disease, we watch as strains of bacteria evolve resistance to our antibiotics, and as the common cold evolves to avoid destruction by our anti-bodies. We have discovered genes that perform the same vital functions in organisms as different as humans, yeast, and giant trees. These gene sequences are lengthy, complex, and convoluted and yet they are virtually the same. On certain branches of the tree, they even contain the same unique changes. The only explanation that makes sense is that at some point in history, our ancestor developed a solution which has been conserved and passed down along all the various branches of the evolutionary tree because it works!

Are we still evolving? Slowly, and not in the ways you might think, but yes. Humans are a special case because we use technology to try to escape natural selection. Many of us have genetic characteristics that would have reduced our chances of survival if not for the intervention of modern medicine and the fact that humans tend to take care of each other. Selection against bad vision was probably more intense when humans were hunter-gatherers than it is today, for example. The superficial differences between human races are due to the fact that until recently, geographic isolation prevented people from regularly interbreeding with those in other parts of the world since our original dispersal from the cradle of Africa. Even in the early 1900s, the fastest sustainable speed of travel was only about 5 miles per hour, and most people married someone who lived less than 10 miles away. All of that has changed, but that's only the beginning. Technology continues to progress. A map of the entire human genome is now virtually complete. It won't be too long before we will be able to repair defective genes in human beings and finally take control of our own biological destiny. In the very near future, our technology will transform us far more rapidly and perhaps even more dramatically than nature ever did.

Visit the Talk.Origins website for lots of information on "the creation/evolution controversy."

Scientific American's "15 Answers to Creationist Nonsense" gives a good synopsis of the current scientific understanding of evolution.

An excellent hypertext glossary of evolutionary terms.