They've been under the purview of balancing selection. This type of selection occurs when two alleles – one being the harmful variant and the other being the beneficial variant – both confer some sort of benefit in the presence of each other. This can work in two ways. Sometimes the balance of deadly and beneficial alleles can create a balance in the population, allowing for genes to persist over time. In other cases, the lethal genes can be beneficial if there is an environmental change and the deadly gene can offer protection from the new environment. Both of these scenarios create a balance of selection, allowing for the persistence of the gene over time. In some cases, the lethal genes are essential for the maintenance of certain gene complexes and, if lost, could result in decreased fitness. This type of selection is known as evolutionary pressure and it can also contribute to the longevity of certain genes. A third possibility is that some of the genes simply survive due to random genetic drift.