Question

how does random fertilizationadd to genetic variaton

Answer 1

When an egg is fertilized it gets half a set of genes from each of its parents. Most reproductive cells differ from each other genetically, even within the same organism. When eggs are fertilized randomly, they get one of millions of possible sets of genes, since there is the possibility for great variation from both parent's reproductive cells. So basically, each parent contributes a random reproductive cell, and they come together to create an even more random offspring.