First of all, let's get on the same page here.
Charles Darwin was a naturalist who spent several years in the Galapagos Islands in the 1830s, where he observed lots of variation among the flora and fauna. This got him thinking and theorizing and observing even more, and in 1859, he published "On the Origin of Species" and basically turned the world upside down. Before this, just about everyone (scientists included) tended to believe that all creatures put on earth were formed perfectly and immutably. Fossils were assumed to be from creatures that still existed somewhere, not creatures that had gone extinct.
Darwin offered a different explanation: creatures evolved through natural selection. Here's how natural selection works: Let's say, millions of years ago, creatures that we'd now call giraffes were wandering around Africa, but they've got short necks. One day, a giraffe is born with a genetic mutation that makes his neck just a little bit longer than the other giraffes'. Because of his long neck, he can reach leaves higher up in the trees. He's better nourished and that makes him a little bit stronger and healthier than the others. And in the animal kingdom, that means more procreation! So he finds a lady giraffe and they have a giraffe baby. And that giraffe baby inherits the longer neck from her father. So she gets to procreate, and she passes the longer neck on to her offspring. With each generation, the longer the neck. And the longer the neck, the better the chances of survival, since long-necked giraffes can reach leaves that are higher, see predators approaching from far away, etc. The giraffes with shorter necks don't procreate as often, simply because they don't survive. And after several thousand/million years, you've ONLY got long-necked giraffes. Bam. New species.
Generally speaking, a new species is official when it can no longer reproduce with the old species. Because there's usually more than one mutation going on...giraffes aren't only getting longer necks with every generation, they might be getting taller legs, or different coat patterns, etc. Eventually, long-necked, spotted giraffes' sperm and eggs are no longer compatible.
One of the biggest proofs of evolution is the fossil record. We can see how species changed, adapted, and went extinct because their remains were left behind, and we can date them based on how deep in the earth they are and carbon dating. There are millions of species that no longer exist. We can also see natural selection at work today, not only in the Galapagos Islands, but everywhere. Check out this famous moth example.
Okay, now let's do some de-bunking. Here are some common arguments against evolution, and my response to them.
"Evolution is just a theory."
Here's a delightfully sassy quote by modern biologist Richard Dawkins to address this point: "So is gravity, but I don't see you jumping out of buildings."
In science, the word "theory" has a slightly different meaning than in everyday life. In everyday life, a theory is an idea or a guess...something that hasn't been tested yet. In science, a theory is the end result of lots of observation and experimentation. A theory both explains past observations and makes predictions future events and patterns. Scientists don't throw the term around lightly.
So when scientists say something is a scientific THEORY, they mean that lots of testing and observation has been done to prove it.
"I can't imagine something as complex as the human eye just EVOLVING."
This is an argument that I've heard a lot, and I admit that there is a poetry to it. A lot of it seems to stem from this quote from Darwin himself: “To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I confess, absurd in the highest degree."
BUT, it seems that people often end the quote there. In reality, the very next sentence in this Darwin quote is, "The difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered subversive of the theory.”
And the reality is that although true evolution takes many many many years, this kind of complexity has been observed. Scientists have proposed how light-sensitive patches of skin eventually became the complex eye. Check out a summary of the idea here.
"Wouldn't we see species changing before our eyes?"
Well, that's not actually how evolution works. Evolution doesn't say that individual creatures will transform. It says that over time, generations of a species change. (See explanation above.)
"But Darwin didn't even believe his own theory."
I'm not sure where this one comes from. I don't know a ton about Darwin's personal history...I do know he struggled with aspects of his theory, partly because of his religious uncertainty and partly because it was a CRAZY BRAND NEW THEORY that was not widely accepted at the time.
And regardless of whether or not he personally believed it, it has been proven by countless other scientists.
"Fossils are just placed on earth by God to test our faith."
Here's why this doesn't jive with me, personally. Because God doesn't seem to work like that...I just mean that there doesn't seem to be precedent. Faith is hope for things not seen...not the denial of things seen. I can't think of any other examples in scripture where God showed us something with physical evidence and asked us to deny it. So it doesn't make sense that He would do that with fossils.
"If humans descended from monkeys, why are there still monkeys?"
Darwin never actually claimed that human are descended from modern monkeys. He claimed that humans and modern monkeys share a common ancestor.
Most of us are familiar with the famous "March of Progress" illustration of evolution:
But it's actually a little bit misleading...it seems to imply that a monkey just grew up into a man. A better illustration is this:
A monkey swinging through the trees didn't suddenly lose its hair and start walking upright. Somewhere around 12-16 MILLION years ago, some ancient primates had babies who could walk upright more easily, and they survived to have babies who did the same thing, and eventually, those upright-walking primates could no longer reproduce with the other primates and they were officially a new species.
"Evolution can't explain how life FIRST began on earth."
It doesn't claim to. Life spontaneously beginning on earth is one of the great mysteries of science, and there are several hypotheses out there. There are also several amazing experiments that are getting closer and closer to showing that life can be created from non-organic ("non-alive") compounds.
(AND WHY IT MATTERS)
Creation of Man
One of the doctrines of the restored Gospel is that we are spirit children of Heavenly Father. Our spirits existed before we came to earth, and we inhabit our bodies like a hand inhabits a glove. So while there could have been generations of humanoid beings on earth, Adam was the first being who was a spirit son of God. Here's an illustration of what I mean. (And I know I just said that this illustration isn't a very accurate one, but it is a good one for illustrating this point). Like this:
The Bible says that "man was created in God's image" and "from the dust of the earth." But these are vague and poetic terms. It's totally possible that God just sort of snapped His fingers and swirled together a human being from dirt. But the fossil record says differently. The Bible doesn't give us the details of how man was created, only why. I don't think that scripture is the only source of truth...that it's the comprehensive guide to everything in the universe. The purpose of scripture is to bring us closer to God and help us know how to live good lives. I think that God wants us to study and find truth in all places, including the world of science.
Idea of Eternal Progression
When it comes down to it, the entire theory of evolution is all about how species change to meet adversity. And the entire plan of salvation is about how individuals change to meet adversity. To me, evolution is the physical and scientific parallel to the idea of spiritual growth. It's just that spiritual growth happens on an individual level and physical growth happens through generations. It's not a perfectly clean parallel. But it's one that resonates with me.
My Place In the Universe
This is the "so what?" part of things. There are a handful of people out there who would ask "If you're just trying to be a good Christian, why do you need to know where human beings came from? Why do you need to understand any of this? Just be kind and move on."
And here's my reply.
While that may be enough for some people, it isn't for me. I think it's important to understand how the world around us works, because then we can better understand our place within it. And understanding our place within the world helps us know how to treat God's creations--how our actions affect the ecology around us. Understanding the scientific world reminds me that we are subject to the laws and patterns of the natural world. (And one of those things is evolution.)
There's also the awesome part of Mormon doctrine that says that I have the potential to be a God myself someday--to create worlds without end. But how am I to do that if I don't study how my own God did it? I realize that there will probably be thousands of years worth of curriculum on "planet creation" in the next life before I'll be ready to give it a go. And there are plenty of people who have lived on earth who didn't get a chance to study science and evolution in this life. But I do have the chance. And it strengthens my testimony to learn of these amazing things now.
Because here's the most amazing thing about the theory of evolution to me:
It reveals a God of immense complexity.
From all that I've studied and felt and learned, I've come to see God as the Greatest Scientist. The God who loves me and knows me as an individual is also the master of the nuclear fusion that creates starlight. He is more than just an invisible man in the sky, a mere human being with magical creation powers. He understands the diverse and constantly shifting universe. That's incredible.
A God who can snap His fingers and create a living being is one worthy of worship, but one who can orchestrate a complicated machine of a universe is one worthy of emulation.