How do organs evolve?

Answer 1

Evolution is something that happens to an entire species, however originates from a select few of the specie.

What happens is a mutation occurs in one of them, this mutation may or may not be beneficial to it. If it helps it according Darwin's term 'survival of the fittest' it is most likely to reach adulthood and reproduce which then gives its offspring the same trait. These then have an advantage over the rest of the specie.

Therefore overtime (multiple generations) this phenotype becomes more common in the specie until it out 'performs' and outlives the rest in the specie without the trait leaving it with a new characteristic.