I think evolution is true. But many of its fans don't understand it very well (and some are mean to people with doubts). So I'm going to help explain it, from a philosopher's perspective.
Evolution is a general purpose idea about how knowledge can be created. Think of knowledge as useful, good information.
When something appears to be designed for a purpose, there is knowledge there. For example, hawks' eyes can see far – they seem designed for the purpose of long range vision; there is knowledge in a hawk's eye. And a wrist watch keeps time; it seems designed for the purpose of keeping accurate time; there's knowledge there.
There used to be a great mystery about how knowledge could be created. Some people thought the answer must be a designer. It's easy to understand how knowledge can be created by a designer because he already has knowledge and uses it for his creation. A watch does have a human designer, and that isn't mysterious. So people thought hawks were designed by God.
But where does a designer's knowledge come from? Even if you say human designers were designed by God, then where did God's knowledge come from? If designers are the only source of knowledge, God must have had a designer, and God's designer must have had a designer, and so on. Which doesn't work.
Evolution solves this mystery of where knowledge can come from without a designer. It's the only idea which has ever solved this mystery.
A big idea of evolution is replicators. Some things make copies of themselves. It's in the context of this repeated copying that evolution happens.
Replicators aren't simple, but they aren't a huge mystery like knowledge creation. It's not that hard to imagine building a robot which is programmed to construct more robots using the same design as itself (assume that people come by periodically and give it raw materials).
Some types of crystals, placed in the right circumstances, create more crystals of the same type. If there was no crystal there, none would be created. But if the wind blows a crystal to a place with the right circumstances, or a human places one there, then it creates more crystals. (If you're curious about this, look up the idea of a "seed crystal" to get started.)
Variation and Selection
Replicators aren't enough for evolution. Variation and selection are also needed.
Variation is easy. Replicators aren't perfect. Errors happen. Some copies are a little bit different. This can be random or accidental. The result is some amount of change, some new things are created.
Selection is more interesting than variation. The general principle is that replicators which are better at replicating end up existing in larger numbers. Replicators which are inferior end up existing in smaller numbers, even zero.
The better something is at making copies of itself, the more copies of it will exist in the future. (On average. It could get unlucky.)
When you put variation and selection together, evolution happens. Lots of different replicators get made (due to copying errors). Because they are made randomly, not designed, most of them are inferior replicators. They make fewer copies of themselves. But a few variations happen to be improvements, and make more copies.
Inferior at what? An improvement by what standard? The standard of making more copies.
And not just immediate copies, but also copies of copies. And copies of copies of copies. In other words, great grandchildren count. In fact, it's better to look at great grandchildren than regular children. That's a good rule of thumb: the more great grandchildren a replicator creates, the better a replicator it is.
What if we aren't dealing with people or animals? Then instead of "children" think "copies". And instead of "great grandchildren", think "copies of copies of copies". I'll speak of great grandchildren for convenience, but the more general concept is copies of copies.
With genes and animals, the replicators die off regularly, and decompose, and the resources (like the atoms they are made out of) get reused. So a replicator which doesn't do very well ends up at zero copies, and better replicators use its resources. But this is just one possible scenario.
We could also imagine replicators which aren't living creatures, which don't die, and which don't have the ability to take resources (like building material) from other replicators. Then the inferior replicators wouldn't die off, there'd just be fewer of them compared to superior replicators. Over time, better replicators will create way way more copies.
Suppose a replicator is able to create 10 copies per year (they all take the full year to be created). But another replicator can do 20. After 20 years with no replication errors, do you think the better replicator will have twice as many total copies?
It's actually far more. The better replicator will have 413,554
times as many copies after 20 years. Over time, better replicators dominate, even if they're only a little better and nothing ever dies or runs out of resources to keep making copies. Direct competition between replicators is not required.
So, replicators have copying errors and then over time there are more replicators that are better at replicating, and fewer that are inferior at replicating. Where's the knowledge? Where's the useful information, the appearance of design?
Well, over time these replicators get good at creating lots of great grandchildren. So, they appear designed for (approximately) the purpose of creating lots of great grandchildren. So there is knowledge there. There is useful information that's able to achieve a specific purpose.
It's not just any purpose. It's not knowledge about anything. It's knowledge specifically about (roughly) replicating great grandchildren. But that is knowledge.
So how are other types of knowledge created? Like a hawk's eyesight.
Knowledge About Other Topics
So where does knowledge of a hawk's long distance eyesight come from? Or a tiger's sharp claws, an ant's scent trails, a cow's ability to create milk, a fly's ability to land softly.
The general principle is that creating knowledge about one topic often creates knowledge about other topics too. If I want to be a good physicist, I'll have to learn some math too. If I want to be a good doctor, I should know some chemistry. If I want to be a good lawyer, I should learn how to read. Pursuing one topic leads to knowledge about many topics.
Getting great grandchildren is a complicated problem. Let's consider animals. They don't just have to have babies. They also have to get food, and not become food. Animals do not have knowledge about just anything. But they do have knowledge about many things relevant to having great grandchildren besides fertility.
Hawk eyes and tiger claws are relevant to their survival, and survival is relevant to having children. That's why evolution was able to create these eyes, claws, and so on.
Some critics portray evolution as randomness, and question the ability of randomness to create knowledge. And some of their opponent's laugh in their faces and call them ignorant. But, actually, there's an interesting issue here. It's a good topic to bring up.
The variation part of evolution is random. (Actually not exactly, but that's complicated, I'm not going to get into it.) And randomness doesn't create knowledge. Randomness doesn't design things (like eyes) for purposes (like seeing).
Although part of evolution is random, part isn't. The selection part of evolution isn't random. It designs replicators (like genes or ideas) for a purpose. What purpose? Roughly, to have a maximum number of great grandchildren.
How can something designed for one purpose (great grandchildren) be good at other purposes? This is an important question which some intolerant anti-religious evolution-proponents do not understand, let alone have an answer to. I discussed it above.
Animals are not replicators. And an animal's offspring are not copies of the parent animal.
What's actually copied are genes. Genes are tiny little sequences of information (made out of DNA). What they mainly do is something like control proteins to build baby animals. Genes are copied to child animals pretty much perfectly (except for infrequent tiny errors).
If you want to know more about genes, a good place to start is by reading The Selfish Gene
A "meme" is a word that means an idea which is a replicator. It's used for talking about the evolution of ideas.
If you want to know more about memes, you can look at my archives
, or ask at the Fallible Ideas discussion group