A third way to compare the dispersal patterns of males and females is to calculate genetic differentiation (typically an FST value) separately for each sex based on data from the same biparentally inherited loci. The sex that disperses more should show lower levels of among-population differentiation than the philopatric sex. One drawback to this technique is that, because only biparentally inherited markers can be used, all of the relevant loci will be passed down to both male and female offspring once the dispersed adult reproduces. This means that, unless data are collected from dispersed individuals before they have reproduced, any sex-specific signals will be weakened. Discordant FST values in males and females of the marine iguana Amblyrhynchus cristatus (Figure 6.10) on the Galapagos archipelago provided evidence of male-biased dispersal because the same nuclear loci, when analysed separately for each sex, revealed average FST values of 0.14 in females and 0.04 in males. These data were supported by field observations of males actively dispersing between islands. Males also spent more time than females swimming off-shore to forage, a behaviour that could increase their likelihood of being passively dispersed by ocean currents (Rassmann et al., 1997).
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