Assignment tests

The fourth method for inferring sex-biased dispersal is based on the assignment tests that were introduced in Chapter 4. These tests can be used to compare the number of males and females that are assigned to a population other than the one

Figure 6.10 A marine iguana (Amblyrhynchus ciistatus) on Santa Cruz Island in the Galápagos archipelago. A comparison of FST values that were calculated separately for each sex provided evidence of male-biased dispersal in this species (Rassmann et al., 1997). Author's photograph

from which they were sampled, the rationale here being that members of the more dispersive sex will be misassigned most often. A slightly modified assignment test was used first by Favre et al. (1997) to infer sex-biased dispersal in the greater white-toothed shrew (Crocidura russula). For each individual they calculated an assignment index, which reflects the probability that a particular individual's genotype originated in the population from which it was sampled. Differences in population genetic diversity were then corrected for by subtracting assignment population means from the log-transformed assignment indexes, which provided corrected assignment indexes (Alc). These corrected indexes reflect the expected frequency of each individual genotype in the population from which it was sampled. Individuals with negative Alc values have rare genotypes (i.e. with a low expected frequency) and therefore are likely to be recent immigrants. A preponderance of negative Alc values in the female greater white-toothed shrew led Favre et al. (1997) to conclude that dispersal in this species is female-biased, an

Females 1=1 Males

Females 1=1 Males o

Figure 6.11 Corrected assignment indexes (Ac) in the white-toothed shrew. A higher proportion of females compared with males have negative AJc values, which means that females are more likely to disperse. Redrawn from Favre et al. (1997)

unusual pattern in mammals that may be explained by the fact that, unlike most mammals, this species is socially monogamous and therefore may be expected to show dispersal patterns that are more typically found in birds (Figure 6.11).

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