Evolution Chapter 22. In part, this has been paraphrased from our textbook Bio.
Most mutations are disadvantageous, but some are advantageous.
If all the individuals of a population that were born survived populations would grow exponentially. Most individuals that are born do not reach adulthood and as a result population size is generally stable. Survival to reproductive maturity is not random. Whether or not an individual survives to reproductive maturity depends, in large part, on the genetic makeup of the individual. The individuals that are best at leaving descendants are, on average, those whose inherited characteristics fit them best to their environment.