Classical and Balanced Schools

In population genetics, the classical and balanced schools were two schools of thought with differing views on the frequency of genetic polymorphisms in a population, and on how natural selection acts on those polymorphisms. Because both schools considered natural selection to be the most powerful driving force of evolution, both of these models can be considered selectionist models.

The classical school believed that the incidence of genetic polymorphisms in a population is rare. They believed that individuals in a population are homozygous at most loci, with only the occasional deleterious allele that is removed by natural selection. The classical school thus believed in natural selection as a mostly 'purifying' force, removing deleterious alleles in order to maintain homozygosity at most loci. Although some new alleles may come into existence, 'classicists' would consider them transient in nature, existing for a few generations as a consequence of genetic drift , and then either being lost or very rarely drifting to fixation. (Fixation meaning a new allele permanently replaces the original allele in the population).

The balanced school, on the other hand, believed that genetic polymorphisms are common. They believed that polymorphisms are maintained by individuals being heterozygous at most loci. Thus the balanced school viewed natural selection as a 'balancing' force, seeking to maintain heterozygosity at most loci, while occasionally acting as a purifying force that removed the low frequency of harmful alleles.

In the 1960s, electrophoresis of proteins showed us a large amount of variation in protein structure. This suggested that genetic polymorphisms are common, apparently validating the approach of the balanced school. However, many geneticists believed that the balanced school's idea of how genetic polymorphisms are maintained - i.e. by organisms being predominantly heterozygous - was flawed.

If heterozygosity is to be considered the fittest genotype, then when two heterozygotes reproduce, according to Mendelian segregation principles, on average 50% of the offspring would be homozygous and thus less fit. This is an example of genetic load (also segregational load), whereby harmful alleles that are maintained in one generation can be detrimental to the next. This could have a cumulative effect, in theory leading to to the extinction of a population, as 'less fit' homozygotes accumulate in number from successive heterozygous crosses.

A way around this problem was to consider that genetic polymorphisms are neutral in their effect on the organism. This means that polymorphism could be maintained in heterozygotes and homozygotes alike, with no advantage or detriment to either. This lead to the neutral model of molecular evolution.