 We also presume that these genetic loci are not affected by natural selection and that the major process responsible for changes in base pairs has been mutation (which can be calculated).The smaller effective population size of the NRY and mtDNA enhances the consequences of drift and founder effect, relative to the autosomes, making NRY and mtDNA variation a potentially sensitive index of population composition.These biologically plausible assumptions are not concrete; Rosser suggests that climatic conditions may affect the fertility of certain lineages.