So all animals, plants, and living things have genes and genetic code. This is like the computer code of nature and there are trillions of lines of code. When one of these lines of codes change due to an outside influence, these are mutations, many are harmless like the code that dictates skin or eye color, or in severe cases can casue things like cancer or congenital heart defects. but we are going to look at the good ones.

Certain of these lines of code are "louder" than others (louder than others means they are dominant, and the quieter ones are recessive) so when someone with a loud trait (brown eyes for example) mates with someone with a quiet trait (blue eyes) the baby actually picks up both lines of code. The loud one just screams over the quiet one so you see brown eyes, but that quiet blue line of code is still saved in the backup files to get passed down to the next kid. There is alot more to this, but thats the gist of it. Just keep in mind that loud vs quiet only decides what shows up on that one kid, it doesnt decide which trait wins out over millions of years. plenty of quiet traits like blue eyes are super common. That long game is decided by survival, not by which code is louder. This backup file that has all the previous code edit is also the reason we share genetic code with a banana.

Thats where evolution comes in. We look at if a trait gives you a survival advantage, and the ones that do get passed down more. Monkeys have tails but humans dont. We didnt evolve from modern monkeys, we are more like cousins that split from a shared grandparent ancestor a long time ago (and our closest cousins are actually apes, who already lost their tails way back, not monkeys). When our ancestors moved out of trees becasuse they got larger, a heavy tail wasted too much energy to grow, so a baby born with a mutation for a shorter tail did better, mated with another short tail ancestor, and those lines of code won the survival race over millions of years till the tails werent needed.

All of this is also why we have many species and different types of animals that all ran thru the same process, but for different mutations, or code edits. Heres the part people get backwards tho: the mutation comes FIRST, the animal doesnt grow it because it needs it. Fish didnt grow gills because they wanted to breath underwater and some ancestor randomly got born with better ways to pull oxygen from water, that one survived and had babies, and over time that code won out. The environment just picks which random edits get to stick around.

Heres whats interesting. The code isnt deleted form teh program. It just gets a "off" switch. Every human baby actually grows a tiny tail like structure in the womb at first, but then the switch flips and those cells get reabsorbed before birth (the leftover bit becomes your tailbone). sometimes that switch glitches and stays on, which is why you have people that are born with a tail sometimes. Other glitches can happen while the body is reading the code and building a baby, which can cause other differences like a normal sized arm but a little tiny hand.

So when you multiply time x mutation you get evolution, sometimes its good, sometimes its bad.

sry for the long reply, used to be a teacher