 However, the observed levels of genetic polymorphism presented an insurmountable problem for the mathematical theory of evolutionary genetics. The amount of genetic variation was much too large to be explained by the type of balancing selection observed by Ford and his colleagues for conspicuous phenotypic polymorphisms in natural populations. The basic problem was that the numbers of selective deaths necessary to account for the observed levels of allozyme polymorphism exceeded the reproductive excess of almost all species. This where the focus on genetics must be coupled with the ecology of reproduction and viability. Haldane (1957) called this the “cost of natural selection” and it is also referred to as the substitutional load. Differently put, the mortality of homozygous genotypes, if independently selected, (also known as the “segregation load”) would exceed the total numbers of offspring produced by a population. For this reason, Kimura (1983) proposed his neutral theory of molecular evolution, founded on the theoretical observation that the probability of fixation of a novel mutant allele with selective coefficient, \(s \gt 0\), was approximately \(2s\) (Wright 1931: 133). Since \(s\) was generally believed to be in the range of 0.001 to 0.0001, the likelihood of loss of a new favorable mutation by chance is quite high, only slightly smaller than the probability of loss by chance for a truly neutral allele.