past. After calibrating the clock the next step is to calculate divergence time by dividing the estimated time since the sequences diverged by the amount of sequence divergence. The resulting number is the estimated rate at which molecular evolution is occurring. The most widely cited molecular clock is a ‘universal’ mtDNA clock of approximately two percent sequence divergence every million years. Although referred to as a universal clock, this idea of the "universal" clock is not possible considering rates of evolution differ within DNA regions. Another drawback to using molecular clocks is that they ideally need to be calibrated from an independent source of data other than the molecular data. This poses a problem for taxa that don't fossilize/preserve easily, making it almost impossible to calibrate their molecular clock. Despite these inconveniences, the molecular clock hypothesis is still used today. The molecular clock has been successful in dating events happening up to 65 million years ago. Mate choice hypotheses The concept of mate choice explains how organisms select their mates based on two main methods; The Good Genes Hypothesis and Genetic Compatibility. The Good Genes Hypothesis, also referred to as the sexy son hypothesis, suggests that the females will choose a male that produce an offspring that will have increased fitness advantages and genetic viability. Therefore, the mates that are more 'attractive" are more likely to be chosen for mating and pass on their genes to the next generation. In species which exhibit polyandry the females will search out for the most suitable males and re-mate until they have found the best sperm to fertilize their eggs. Genetic compatibility is where mates are choosing their partner based on the compatibility of their genotypes. The mate which is doing the selecting must know their own genotype as well as the genotypes of potential mates in order to select the appropriate partner. Genetic compatibility in most instances is limited to specific traits, such as the major histocompatibility complex in mammals, because of complex genetic interactions. This behavior is potentially seen in humans. A study looking at women's choice in men based on body odors concluded that the scent of the odors were influenced by the MHC and that they influence mate choice in human populations. Sex-biased dispersal Sex-biased dispersal, or the tendency of one sex to disperse between populations more frequently than the other, is a common behavior studied by researchers. Three