Short Answer 
Gene flow introduces new genetic material between populations, while natural selection favors traits that enhance survival and reproduction. Additionally, genetic mutations create new traits, and genetic drift results in random changes in the gene pool, impacting a population’s overall genetic diversity.
Step 1: Understanding Gene Flow
Gene flow occurs when members of one population migrate to another location and interbreed with a different population of the same species. This movement and interbreeding result in a new combination of genes in the gene pool. An example is when a zebra joins another herd, introducing new genetic material into the group.
Step 2: Natural Selection and Its Effects
Natural selection, as described by Charles Darwin, is the process where organisms with favorable traits have a better chance of surviving and reproducing. This leads to a gradual change in traits within a population over time. An illustration of this process is when competition for sunlight encourages trees to grow taller, as those that can reach more light survive and reproduce.
Step 3: Genetic Changes from Mutation and Drift
Genetic mutation refers to alterations in the DNA, which can create new traits, such as a snake developing stronger venom through these changes. Additionally, genetic drift occurs due to random events, like a grassfire that decimates a population, leading to the loss of specific genes as individuals die. This randomness can significantly influence the overall traits present in a population.