 The one-locus model outlined above is unlikely to apply to many real-life populations, due to the simplifying assumptions it makes. In reality, selection is rarely the only evolutionary force in operation, genotypic fitnesses are unlikely to be constant across generations, Mendelian segregation does not always hold exactly, and not all evolving populations are large. Much effort in population genetics has been put into making more realistic models which relax these assumptions and are thus more complicated. But the one-locus model illustrates the essence of the population-genetic analysis of evolutionary change.