Coffin wanted to know if a virus could affect an animal's evolution through genetic recombination--a process whereby chromosomes in sperm and egg cells exchange genes. Genetic recombination is the reason why you look different from your siblings (and, for that matter, from everyone else). Inside the cells that make up sperm and eggs, chromosomes from each of your parents pair up. While they are pressed together, the chromosomes often break, and each chromosome swaps a portion of its genetic material with the other. Then the chromosomes glue themselves back together and separate, as they go on to become sperm and eggs. Since each has picked up new genetic material, the constellation of physical characteristics they carry has been substantially altered.