Evolution is one of the most important ideas ever thought of by mankind. It explains virtually everything in biology, and has countless uses, from computer science to philosophy. It is a crucial tool for understanding the world around us. Yet, despite evolution's importance, public opinion (though not the opinions of scientists) is divided on whether it's true. People have a surprisingly poor understanding of evolution, and hold a number of misconceptions about the idea. This is not their fault. Despite evolution's importance, it's hard to find information about it that's easy for non-scientists to understand. That's what this text is designed to help with. This text is aimed at anyone, no matter their age or education, who wants to learn more about the idea of evolution. This text will not treat you like you're dumb (because odds are really good you're not), nor does it require any scientific knowledge.  In short, this is an introduction to evolution, and will explain what evolution is, how it works, the evidence for it, and whether the arguments against it hold up. That said, let's see what evolution is all about.
   
Five Observations

    Evolution is based on five observations. First, no two organisms, or living things, are identical. Look at your dog, your best friend's dog, and your neighbor's dog. Even if they're all the same gender and breed, they are all noticeably different.

    Second, these differences can be passed down from parent to child. Look at horses. Individual horses tend to be very different from one another, but the children, or offspring, tend to be quite similar to their parents.

    Third, a population produces more children, more offspring, than will survive to reproduce. Look at oak trees. They make countless acorns each year, but only a handful of those acorns will ever grow into large enough trees to make their own acorns.

    Fourth, some individuals have more offspring, more children, that survive to reproduce than others. Some individuals are barren, and others die young. The survivors have different numbers of offspring. Even those with identical numbers of offspring will have different numbers that survive long enough to reproduce on their own. One butterfly could have no offspring that survive to maturity. Others may have dozens.

    The fifth and most important observation is that some individuals have more offspring that survive to reproduce because of their inherited differences. A dog that is resistant to disease will probably pass that on to its pups, and so have more pups that survive to reproduce than another dog. A wild horse that can run faster will probably have more foals than one that runs slowly. The slow one may have a foal or two, and then be eaten by wolves. The fast one will probably survive to have many foals, most of which will be able to outrun the wolves long enough to have offspring of their own. An oak tree that makes acorns that are more resistant to rot will probably have more offspring than another oak. When animals like squirrels and birds bury acorns to save to eat during the winter, they always forget to dig many of them up. When spring comes, the oak with the rot-resistant acorns will have lost fewer to decay, and so will have more offspring. A butterfly that doesn't need to eat as much nectar will be less likely to starve before it can reproduce, and be able to spend more time looking for mates. It will probably have more offspring, many of which will probably have to eat less than normal.

    It doesn't take much probing to that realize these observations are true, and scientists draw an interesting conclusion from them: some members of a species will outcompete others. There are never enough resources to go around, whether the resource is food, shelter, or safety. Some members of a species will be better equipped to get those resources, or to make better use of them. Because of this, they will have more offspring than the others. The better-equipped living things, or organisms, pass their abilities on to their offspring. Over time, the better-equipped organisms outcompete the other ones.

    This will be easier to understand with an example. You have a population of lobsters. Some have stronger claws than others. There is only so much food, and only so many places to hide. The strong-clawed lobsters are better able to hunt, and better able to fight the weak-clawed lobsters for hiding places. After one generation, many weak-clawed lobsters starved because they couldn't get as much food. Other weak-clawed lobsters were eaten, because they couldn't fight off the strong ones for their hiding places. Most of the strong-clawed lobsters, though, were able to find enough food and hide, and so they survived. Because, at the end of the generation, you have many more strong lobsters than weak ones, when the lobsters reproduce, they make a lot more strong-clawed young than weak-clawed ones. After a few generations of this, the weak-clawed lobsters are mostly wiped out, while the strong-clawed lobsters are doing just fine. They outcompeted the weak-clawed ones, and lobsters are now different things from what they were before. The nature of lobsters was changed over time.

Genes and Selection Pressures

    An organism's traits are controlled by its genes, which are coded instructions found in its cells. An organism passes on many of its genes to its offspring. If it reproduces sexually, like humans, it passes on half its genes. If it reproduces by splitting itself in half, it passes on all its genes. Because every individual is different, a population has a lot of different genes in it. If you count every gene in a population, and add up every gene that appears more than once (genes for brown hair, for example), you get something called a gene pool. Gene pools are controlled by things called selection pressures.

    A selection pressure is anything that affects which individuals have more offspring that survive to reproduce. One example of a selection pressure is the food supply. Individuals with genes that help them get more food are probably going to survive longer, and so have more offspring. Another example is antibiotics. If a colony of bacteria is exposed to the drug penicillin, those resistant to being killed by the drug will survive to have offspring that reproduce. There is a selection pressure towards traits that help you get more food, and towards traits that help bacteria resist penicillin.

    Sometimes, an individual is born that has a mutation, or change in its genes, that makes it different from the rest of the population. Sometimes the change is bad, and selection pressures stop it from spreading. Other times, the change is good, and selection pressures usually spread it around the population. For example, Lance Armstrong, the famous bicyclist, has a mutation from a few generations ago that gives him an oversized heart. Mutations and selection pressures are constantly changing the gene pool.

    Selection pressures select towards a gene that helps an organism have more offspring that survive to breed, and more and more of the population gets it over time. Eventually, better genes outcompete genes that didn't do the job as well, and the entire population is changed over several generations. This can be a bit of a tricky concept, so let me give a few examples.

    Antelope live on the plains of Africa, and their only protection from being eaten is running away. One antelope is born with a mutation that lets it contract its muscles more quickly. This lets it run faster than the other antelopes. We'll call this antelope "the mutant antelope," because it has a mutation. Because it is faster, it is able to outrun the other antelopes in its herd, so that the predators eat the slower antelopes, and not the mutant one. Because of this, the mutant antelope lives a long life, and so is able to have many offspring. It passes on its mutation to some of its offspring, who also survive to have mutant offspring. We see how the selection pressure of not being eaten is spreading the mutation through the population. The predators are still eating antelope, so the size of the herd isn't getting any bigger overall, but the number of mutant antelope in the herd is increasing. Eventually, all the non-mutant antelopes are eaten, and all the members of the herd have the mutation. After a few mutations, the herd is so different from all the other herds that it can no longer breed with members of other herds. It's now a totally different species of antelope.

    Let's look at a fungus as another example. This fungus breaks down rotting wood in a forest to get nutrients. When a fungus reproduces with another fungus, it shoots out fungus seeds, or spores. One fungus has a mutation that lets it tolerate acid better than all the others. This mutation neither helps nor hurts the fungus. There is no selection pressure for or against it, similar to eye color in humans.  The mutation survives in the population, but it doesn't really spread, because there is no selection pressure. A few thousand years after this, an area of the forest gets wet and turns into a swamp. Most of the forest remains unchanged, but that new swamp area has a totally different set of selection pressures. Swamp water is a little bit acidic, and most of the fungus population can't handle this acid. A few, however, have that acid-tolerating gene, and spread into the swamp. There are no other funguses in the swamp, so there's no competition for food. The acid-resistant fungus quickly occupies the entire swamp. There are now almost as many acid-resistant funguses in the swamp as there are normal ones in the forest. Because the swamp and the forest have completely different sorts of selection pressures, different mutations will spread through the swamp and forest fungus populations. Over time, and after a few more mutations spread, the forest fungus and swamp fungus will become completely different species.

    Here are some other, briefer, examples of evolution. For example, a new disease forces organisms to evolve a better immune system. Or, a species arrives on an island, which doesn't have many resources, and so evolves to be smaller so as to use fewer resources. Or, new organisms arrive in an area, and compete with pre-existing organisms, and force both to evolve. Eventually, either one is wiped out or one evolves to get its food from a different source. All of these are quite common examples of evolution.

    All populations have diversity in their gene pool, because genes mutate naturally when errors are made copying the genes to pass them on to offspring. These mutations cause the gene pools to change over time. Selection pressures change over time, because nature changes over time. A combination of changing selection pressures and diverse, changing gene pools creates new species over time.

    You have to remember, though, that an organism does not control its evolution. Evolution is simply a process, like water flowing across a tabletop. Selection pressures force the gene pool in different directions; the gene pool has no control over its future. Evolution is not planned, guided, or marching towards a predetermined goal. Evolution is simply what happens when the environment places selection pressures on the gene pool. Again, think of water flowing across a tabletop.

    The way all of evolution works is the phrase "mutations at random cause nonrandom reproduction."
   
Descent with Modification
   
    Because evolution is organisms changing over time, and eventually creating new species, the process has been called "descent with modification." One species may give rise to a half-dozen new ones, while others may die out as a dead end. A species does not have to go extinct to give rise to another. Remember that evolution acts at the population level, so one population in a species can evolve, while the others remain unchanged, or evolve in different directions. If you were to map the lines of descent, and show what species evolved into what other species, you wind up with a sort of bushy tree. You start with some species at the bottom of the tree. Some die out without evolving into new species. Others evolve into several. Thus, some branches stop, others keep going, and others split into several branches.

    Continuing with the idea of "descent with modification," any change in a population must be advantageous in order to spread quickly. Otherwise, selection pressures could not speed it along, and it often disappears. If it's harmful, it simply will not spread. Even if the harmful mutation, combined with another mutation, would be very helpful, it still won't spread. That's why animals don't have springs for legbones. That would require at least two mutations: one to make their bones springy, and another to warp their legbones into spring shapes. But having non-springy spiral bones would prevent you from moving well, and having non-spiral springy bones would just make you flop around like a fish out of water. New features have to evolve by steps, and one step along the way is hugely disadvantageous, so spring-legs cannot and do not evolve.

    However, new body parts regularly form from old parts that served a different function. For example, feathers first evolved in dinosaurs as a sort of modified scale that kept the animal warmer. Some of the smaller feathered dinosaurs moved into the trees. There, those with longer feathers used them as parachutes. Over time, mutant feathers developed that could be used to glide. Those with the mutant gliding feathers were successful and spread the gene very quickly. A series of more mutations created the modern bird's flight feather. The mutations that created the flight feather were coincidences, but the fact that they spread throughout the population was not. Since the dinosaurs with better feathers survived longer to produce more offspring, the dinosaurs with mutant feather genes outcompeted the others. Again, mutations at random cause nonrandom reproduction.