Evolution of schizophrenia

The evolution of schizophrenia refers to the theory of natural selection working in favor of selecting traits that are characteristic of the disorder. Positive symptoms are features that are not present in healthy individuals but appear as a result of the disease process. These include visual and/or auditory hallucinations, delusions, paranoia, and major thought disorders. Negative symptoms refer to features that are normally present but are reduced or absent as a result of the disease process, including social withdrawal, apathy, anhedonia, alogia, and behavioral perseveration. Cognitive symptoms of schizophrenia involve disturbances in executive functions, working memory impairment, and inability to sustain attention.

Given the high numbers of individuals diagnosed with schizophrenia (nearly 1% of modern-day populations), it is unlikely that the disorder has arisen solely from random mutations. Instead it is believed that, despite its maladaptive nature, schizophrenia has been either selected for throughout the years or exists as a selective by-product.

Hypotheses

Balancing Selection and Positive Selection Hypothesis

The balancing selection hypothesis suggests that balancing selection, an evolutionary mechanism, has allowed for the persistence of certain schizophrenia genes. This mechanism is defined as maintaining multiple alleles of a gene in the gene pool of a population despite having selective pressures. Heterozygote advantage, a mechanism of balancing selection, is when the presence of both the dominant and recessive allele for a particular gene allows for greater fitness in an individual as compared to if the individual only expressed one type of allele.