Selfish genetic elements are genes or groups of genes that appear to promote themselves in an evolutionary sense irrespective of the organism they are a part of. The idea being that evolution is a competition for survival of the fittest gene and that it is the most successful genes that will gradually become more numerous and be transmitted to later generations.
Sexual reproduction is the means by which the genes of two organisms that have successfully survived to the point of being able to reproduce, are combined in a somewhat haphazard way to produce new organisms that will go on to face the trials of evolution. The fittest of these new organisms going on to recombine their genes in the next generation and so on.
As genes are the architects of the phenotypes that face the trials of evolution it is necessary that the most evolutionary successful phenotypes will have been the result of having certain genes and this will be reflected in the frequencies of the occurrence of a population's genes.
However, although it may appear that evolution is acting at a genetic level, evolution can only