Answer:
Natural selection is when the higher, more sophisticated animals of a species survive and the less intelligent, less physically adapt, and ones with other disadvantages die off. Thinking about this logically, if natural selection is slowly making the animals that previously were disadvantaged die off, the gene pool will be filled with more advanced and "better" genes. As these survival genes becomes more and more popular, the animals that have them will be the ones that are mated with, making the new generations also have these needed genes to survive. As this process continues to repeat itself throughout time, evolution will occur and the species will become more advanced. Hope this helps and let me know if you have any questions! :)