Sexual selection is a selection on genetic traits, determined by the preference of an organism that is given the opportunity for mate selection. Some have maintained that the sexually selected characteristics in an organism will be secondary representations of their fitness, such as elongated tail feathers in a bird, or a longer, deeper call in a frog. However, it is generally noted in these examples that the feathers, or the call, are selective disadvantages, and that as a result the organisms that perform the mate selection are conferring disadvantages upon their mates by preference. Consider, for instance, a human population. The female side of the population has (for the purposes of simplicity) a gene called BB, which gives them a big butt. Having a big butt has adaptive significance, for we can say that it makes childbearing easier. Some members of the population have another form bb, which gives them a small butt. Now, let us say that on the male side of the population, there is a gene called the MXLT gene, which causes them to like big butts. Those with the MXLT allele like big butts, and those with the mxlt allele (also known as the Sandler mutation) like small butts. Now, since childbearing is easier on BB females, we would expect that they, and the MXLT males - who breed preferentially with them - would have more children. As a result, the more successful BB and MXLT alleles would grow and eventually become fixed in the population, whereas the deleterious bb and mxlt alleles would dwindle and disappear. However, we are told that this is not the case. Many animals have obvious sexually selected characteristics, and these characteristics are demonstrably disadvantageous in a number of examples. So, how is it that these characteristics became fixed in these populations of organisms?