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Genotype fitness measures the reproductive or viability success assigned to a genotype in a specified selection model. Relative fitnesses weight Hardy-Weinberg proportions after viability selection; continuous-time fitness differences instead act as selection-rate differences. The convention and time units must be specified before comparing selection with migration.

Deviation in inherited allele proportions from the Mendelian expectation. Embryonic viability selection can also distort offspring counts and must be separated from gametic transmission bias.

Viability selection is natural selection through differences in survival to a specified life stage. If two inherited types contribute equal numbers initially but one survives more often to maturity, their frequencies among survivors change even before mating or differences in offspring number are measured.

For initial type frequency $p$ and survival probabilities $s_A,s_B$, the surviving frequency is $p{s}_A/(p{s}_A+(1-p)s_B)$ whenever the denominator is positive. The denominator is the overall survival probability. This is the same normalization principle used in the haploid selection recurrence, applied specifically to survival rather than complete lifetime genotype fitness.