(cycle 6) gsa : chapter 18. 3-18. 4 evolutionary agents and. If a lethal allele is dominant, than both the homozygous and heterozygous carriers will die from its effects. It therefore causes allele and genotype frequencies to differ from those predicted by the. Change in allele frequencies as individuals join a population and reproduce. Random changes in allele frequencies caused by chance events. Differential survivorship or reproduction of individuals with different phenotypes. Introduces new genetic variation into population; does not change allele frequencies quickly. > it introduces genetic variability and therefore, increases it. > if two gene pools are quite different, then a little gene flow causes. > but if populations are genetically similar, then a lot of gene flow will have no effect. Reduces genetic variation, especially in small populations; can eliminate rare alleles. > greatly reduces ge(cid:374)eti(cid:272) (cid:448)ariatio(cid:374) (cid:894)i(cid:374)(cid:272)l. (cid:862)bottle(cid:374)e(cid:272)k(cid:863) & (cid:862)fou(cid:374)der. One allele replacing another or allelic variation being preserved.