Schizophrenia as one extreme of a sexually selected fitness indicator
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Schizophrenia as one extreme of a sexually selected fitness indicator Shaner, Miller & Mintz 2004 Katelyn & Hana Intro: Background Schizophrenia is an evolutionary puzzle—many evolutionary theories have attempted to explain why it persists at a global rate of 1% Too persistent to be caused by a single mutation Intro: Possible Explanations Huxley (1964): schizophrenia is one manifestation of a pleiotropic gene; the same gene may confer advantages in relatives: Resistance to infection (Erlenmeyer-Kimling, 1968) Healthy suspiciousness (Jarvik & Deckard, 1977) Increased fertility (Huxley 1964) Superior Language Skill (Crow 1995, 2000) Other abilities that foster group splitting & migration (Stevens & Price, 2000) Intro: Contradicting Evidence to Previous Studies Only 2 studies have reported possible survival advantages in relatives (Carter & Watts 1971; Lichtermann et al., 2001) Results on reproductive advantage have been unreliable Statistical modeling in twin & family studies has shown that schizophrenia is unlikely to be due to a single a gene or a small collection of single-gene disorders If it is polygenic, this could explain its persistence despite reproductive disadvantages Intro: Why would so many genes dispose to Schizophrenia? Brain systems that fail in schizophrenia are usually vulnerable to “developmental instability” (DI) When manifest in brain development, DI can result in psychopathology Still does not explain how humans evolve this vulnerability Intro: Sexual Selection Sexual Selection Concerns relative reproductive success rather than survival success Most informative fitness indicators show the highest variability Human indicators have evolved Intro: Sexual Selection Crow (1993, 1998): single gene model Randal (1998): females perpetuate susceptibility alleles by reproducing before illness onset Shaner’s study is the first to use fitness indicator theory to explain the evolutionary origin, genetic basis, & characteristic symptoms of schizophrenia Hypothesis Schizophrenia is the unattractive & dysfunctional extreme of a highly variable trait that evolved as a fitness indicator Processes of neural development that go awry should show high sensitivity to fitness & condition Individual with poor fitnessbrain prone to unsuccessful courtship behavior that repels mates Hypothesis: Mechanism of Sexual Selection Symptoms are maladaptive versions of uniquely human verbal courtship behaviors Mate choice mechanism of sexual selection that drove evolution of human language as a fitness indicator Explanatory and Predictive Power General claim: schizophrenia is the unattractive extreme of a sexually selected fitness indicator (SSFI) Courtship & Mating SSFIs are displayed during courtship Across different human groups schizophrenia’s average age of onset will be correlated with courtship’s average age of onset Explains typical age of onset (post-puberty) Anything that stimulates courtship and sexual competition will worsen schizophrenia symptoms Dopamine agonists/antagonists SSFIs affect the probability of mating Explains reduced rate of marriage and reproduction Sex Differences SSFIs show predictable sex differences Males invest more time energy & risk in mating effort Schizophrenia imposes more frequent and severe symptoms on males than on females Females generally prefer older males and males generally prefer younger females Schizophrenia has earlier age of onset in males despite earlier puberty in females Fitness & Condition Sensitivity Neurodevelopmental increased abnormalities are Disordered development is one mechanism by which SSFIs indicate poor fitness & condition Polygenic inheritance underlies schizophrenia SSFIs typically show high additive genetic variance to serve as “good genes” fitness indicators Fitness & Condition Sensitivity Environmental hazards increase risk for Schizophrenia Sensitivity to environmental hazards is one mechanism by which SSFIs indicate genetic quality Increased mortality from a range of natural somatic because the unattractive extreme of an SSFI indicates poor fitness & condition Mutations Most heritable variation in general fitness comes from individual differences in # of “evolutionarily transient, lineage-specific” mutations “Mutation Load” Suggest that most susceptibility alleles for Schizophrenia will be “evolutionarily transient, lineage-specific” mutations that reduce fitness Mutations Why schizophrenia shows substantial heritability in twin & adoption studies but geneticists have failed to find any susceptibility alleles that replicate across human populations The evolutionary half-life of these mutations may only be a few hundred generations, so they will not spread across populations Why does Schizophrenia run in families??? Genetic differences b/t lineages may affect fitness-sensitivity of SSFIs Some families may have higher than average fitness-sensitivity in the SSFIs that go wrong in schizophrenia An allele might increase the verbal courtship of high fitness family members but increase the susceptibility of low fitness members to Schizophrenia If the benefits in high fitness individuals balance the costs in low fitness individuals the allele for higher fitness sensitivity persists Co-Evolution of Mate Preferences Mate Preferences for SSFI co-evolve with the SSFI As a trait evolves greater fitness-sensitivity is should become a more informative fitness indicator Preferences should evolve to pay more attention to the high quality versions Low quality versions will then be viewed as sexually repulsive Co-Evolution of Mate Preferences Explains why there is so much stigmatization surrounding schizophrenia across cultures Predicts the bias to increase after puberty (when mate choice systems mature) and to me more severe in females (choosier sex) Conclusion Many key features of schizophrenia explained: Age of onset, disparity between sexes Reduced reproductive rate Substantial heritability Polygenic basis Frequent developmental abnormalities Increased reproductive success of unaffected relatives Treatment efficacy of dopamine antagonists Future Studies Clarify the nature of the SSFI that goes awry in schizophrenia What type of empirical study could be done?
