Schizophrenia is not
something new to humanity, but rather a mental illness that has affected
individuals throughout history. It is characterized by a loss of reality
through severely distorted beliefs, perceptions, and thought processes that
typically develop in early adulthood. Schizophrenia
is found in all cultures across the world and occurs in about 1% of the
population, suggesting its heritability, as its occurrence is much greater than
mutation can explain. The disorder undoubtedly causes great disadvantage to
individuals who exhibit its symptoms, from reduced social aptitude to
anxiety-driven behavior. Not only does it affect behavior, but also
reproductive ability, which is reduced to 50% in male Schizophrenic individuals.
The puzzling question about schizophrenia is then: how has it persisted
throughout time if it has had such disadvantageous effects on those affected?
Evolutionary theories have
proposed several potent explanations of how genes associated with schizophrenic
individuals have remained in the human gene pool. Deleterious alleles have been
shown to survive as a result of heterozygote advantage, genetic polymorphism,
and pleitropy (when deleterious genes are associated with other advantageous
genes). Studies have shown that these three evolutionary concepts do indeed
pose strong explanations for the persistence of schizophrenia.
As in the case of
Sickle-Cell Anemia, where heterozygous genotyped individuals experienced
increased resistance to malaria causing their reproductive advantage, schizophrenia
could exhibit the same effect. Studies have revealed that schizophrenic females
experience increased fecundity. Their male offspring had a higher rate of
survival in their first year of life compared to male offspring of non-schizophrenic
females. Further, female offspring of schizophrenic mothers had a higher rate
of survival for the first fifteen
years of life, suggesting that heterozygosity in offspring causes an advantage
over homozygosity in offspring. It has also been discovered that immediate
relatives of schizophrenic individuals have lower rates of viral infections,
lower rates of accidents, and increased fertility (Brune 2003). This again
suggests that genes associated with schizophrenia cause a greater survival and
reproductive advantage, directly tying in with the evolutionary theory of
natural selection.
Apart from heterozygote
advantage, schizophrenia may also be a result of the evolution of sociability. Although
schizophrenic behavior is not typically seen as particularly pro-social or
progressive, it could have been beneficial to our early ancestors’ development.
For example, the over-protective nature of schizophrenic mothers would have
certainly caused a greater likelihood of survival of her offspring.
Additionally, in evolutionary scenarios where establishing territoriality would
be more important than pro-social behavior, individuals with Schizophrenic
alleles would have an advantage over more socially adapt individuals (Brune
2003). More recently, studies have shown that relatives of schizophrenic
individuals exhibit higher levels of creativity, a characteristic key to
survival and progression. This would certainly cause a greater chance that
schizophrenic alleles would be passed on.
The fact that schizophrenia
is not particularly beneficial to individuals yet it has persisted through time
suggests that evolutionary principles could very well be at work. Heterozygote
advantage has been found in several other cases where deleterious alleles
remain in a gene pool and evidence strongly suggests that this is the case for
schizophrenia-coding genes. Furthermore, behavior associated with schizophrenia
seems to have been beneficial in certain situations where pro-social behavior
was not favorable. Lastly, genes associated with schizophrenia have been linked
to survival and reproductive advantage time and again, supporting gene
polymorphism and pleitropy. Evidently, evolutionary principles provide a convincing
explanation for the presence of schizophrenia in populations throughout todays
world.
Sarah Bakhiet
Word Count: 535
Work Cited
Brune, Martin. "Schizophrenia—an Evolutionary Enigma?" Neuroscience and Biobehavioral Reviews 28 (2004) 41–53. Sunburst.usd.edu. Web. 23 Oct. 2003.
Greta post! I wondered why females with schizophrenia have a much higher fecundity rate than males. Is it a strictly biological trait, or is the societal acceptance of schizophrenic women greater than that of men?
ReplyDeleteGreat question! I think societal acceptance of schizophrenic women could definitely play into their greater fecundity. I'd like to see more research on that!
DeleteI wonder if other mental illnesses have a heterozygote advantage? Also, do you know if it is a mutation that leads to the onset of the illness, or is something else that is heritable, like methylation?
ReplyDeleteMost researchers agree that it is heritable, rather than a mutation, since its occurance is so high in populations.
DeleteI find it especially interesting that schizophrenia impairs the reproductive ability of males but enhances it in females. This would seem to indicate that the female contribution to reproduction is more important and thus more strongly acted upon by selection forces.
ReplyDeleteI suppose this means that we might never be able to completely "cure" mental illnesses?
Very interesting. I always thought schizophrenia was always deleterious to an individual. I wonder the same thing as the first commenter: why do the females have much higher fecundity?
ReplyDeleteDo you know how they measured the offspring's increased rate of survival? There are so many other environmental, social, and biological factors that come into play when it comes to an offspring's survival. How do they know that the benefit is from having a schizophrenic mother? Was there a control group to compare to? If so, were they part of the same community? Same age of the parents? Same relative fitness of the parents, schizophrenia-withstanding? etc.
ReplyDeletePerhaps this example will be used in future evolution classes when talking about the heterozygote advantage instead of the proverbial Sickle-cell anemia case.
ReplyDeleteI had no idea that this was associated with schizophrenia! I think it's an important idea to consider when addressing mental illness as a health issue because there are always many factors involved in a disease's development and how we manage it in society. I wonder if this phenomenon will decrease, however, in today's world because of the negative stigmas associated with mental health.
ReplyDeleteIs the presence of schizophrenia dependent on the alleles of a single locus or is it determined by multiple genes? Does schizophrenia manifest in degrees over a spectrum, or is it more of a yes-or-no question as to whether it is present or not, like sickle cell? The fact that relatives of schizophrenic individuals have advantageous traits without showing schizophrenic behavior suggests that the interactions between multiple genes determines whether one has schizophrenia, and to what degree.
ReplyDeleteInteresting stuff but how exactly do relatives of schizophrenic individuals exhibit higher levels of creativity?
ReplyDeleteI loved this post I thought it was so interesting! I actually had no idea that schizophrenia was inheritable, I thought it was disease of the mind that happened over time! However the fact that it gives the heterozygous advantage of female fecundity does explain why natural selection has not made this gene extinct. However I agree with Martin. How can you really attribute being highly creative with schizophrenia? Are there tests that show a correlation? Even so, with creativity being successful would these tests be valid?
DeleteThanks for the question Martin and Nkemji! Researchers have done correlation studies on families and observed this trend; however, as you mentioned the validity of such studies are something to be considered.
DeleteAre the factors resulting in "lower rates of viral infections, lower rates of accidents" for immediate relatives of schizophrenic individuals related to schizophrenia via pleitropy? "Lower rates of accidents" seems really arbitrary and like something that is heavily influenced by the environment rather than genetics. How was "lower rates of accidents" quantified experimentally?
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ReplyDeleteNow when I think about psychological disorders, they no longer seem to be solely "diseases", but more like evolutionary adaptations. It does make sense that most traits have positive and negative consequences. I have yet to hear of a "perfect" adaptation!
ReplyDeleteI have read that marijuana use can increase your likelihood of becoming schizophrenic if you are genetically predisposed. I wonder if any research has been done into how this may play into the evolution of this disease. If, as this article suggests, it confers a fitness advantage to be schizophrenic, perhaps we may be selecting for a population predisposed to smoke pot! Conversely, if it turns out that now the social stigma associated with the disease has made it a fitness disadvantage to suffer from it in modern times, maybe the opposite it true.
ReplyDeleteGreat point Sarah! I'd love to see how the correlation between marijuana use and schizophrenia could influence selection for drug-users!
DeleteThis article was very interesting! I thought it was very interesting that schizophrenia could have been an advantageous trait in that it increased the likelihood of offspring survival and helped organism to establish territories.
ReplyDeleteHi Sarah, thanks for the article and great insight you provided. So many people, including myself, have not had positive experiences with Schizophrenic individuals and media, of course, does not help that situation. People never think to ask why this disease persists in the population, but from an evolutionary standpoint things really can, and do, make a lot more sense. Have their been any growth that you can find in popular, mainstream literature that would spread these findings out for more people to consider? Do you think doing that would make a difference in the way people understand and perceive schizophrenia in our society today?
ReplyDeleteHi Rebecca, thanks for your insight! I definitely agree that spreading such findings could help reduce the stigma associated schizophrenia. I haven't heard of any popular, mainstream literature of the sort, but I think its certainly important for researchers to present data in a way accessible to the general population.
DeleteIt's very intriguing to think that schizophrenic females have increased fecundity, with the first male offspring having a high fitness. I have never thought about schizophrenia before, but maybe there is, indeed, a reason for everything. It's taking apart a concept that is widely accepted and viewed as deleterious to the human society, but the article that you provided just reminded me see things from an objective point of view.
ReplyDeleteWow, it's very interesting to compare Schizophrenia to Sickle-Cell Anemia. I've always wondered how Sickle-Cell Anemia persists in the population when its intended evolutionary function is outdated. Based on this article, it seems that Schizophrenia plays a bigger role in survival than Sickle-Cell Anemia. Does this mean that Schizophrenia will take longer to weed out from the human genome? This article presents many interesting questions.
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