This was originally an umbrella term expressing the overall ability of an individual to produce (fertile) descendants in comparison with the other members of the population. Neodarwinists assign biological fitness to individual alleles. In this conception, this is a number that expresses how many fewer descendants on average are produced during its lifetime by the carrier of a particular allele compared to the carriers of the most successful alleles in the particular population. The biological fitness (w) can be used to calculate a selection coefficient (s) as w = 1 – s. The selection coefficient is thus the obverse of biological fitness as it expresses the degree to which the bearers of certain alleles are affected by natural selection. If the carriers of allele A on an average leave the greatest number of descendants, i.e. 10, while the carriers of allele B leave an average of 8 descendants, then the biological fitness of carriers of allele B is 0.8 and the corresponding selection coefficient is 0.2.