Say you have a population of 5,000 people. That makes 10,000 copies of each gene. Imagine a gene where 3,000 of those copies are of one particular allele or type. In the next generation, there won't necessarily be exactly 3,000 copies again. There may be 3,050 or 2,960 copies instead. Some gametes get randomly picked out of all the possible gametes that could have been used. This is a bit like tossing a coin, and 100 coin tosses rarely yield exactly 50 heads. Natural selection happens when individuals developed from certain gametes are more likely to survive and reproduce. Genetic drift, together with mutation and recombination, randomly produces the gametes that selection can act on. Or, if there is no selection, allele frequencies can change by mutation and genetic drift alone.
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