How does evolution change a population?
As you may know, the species is the one affected by evolution—not the population.
First, when an organism experiences a mutation during development, it changes the whole organism. Let's say there are ten red lizards and ten green lizards. If they are in a very grassy area, the green lizards would be less likely to die because they camouflage better. The red ones would more likely die because they are more noticeable. So, a better mutation becomes more common in a population.
Next, sexual reproduction involves itself into evolution. When organisms undergo meiosis and have crossing-over, their DNA changes and is more diversified. When organisms have crossing-over, they evolve. Here's a picture that simply explains it how it works:
These are some ways that organisms can evolve, but the population itself does not evolve. The allele frequency — the rate of alleles in a given population — of population changes as more organisms change the gene pool — the different types of genes in a population.