Schizophrenia as one extreme of a sexually selected fitness indicator Shaner, Miller, & Mintz 2004 Katelyn & Hana

Intro: Background  Shizophrenia 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 abnormality

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 fitness  brain 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  Prediction: Across different human groups schizophrenia’s average age of onset will be correlated with courtship’s average age of onse t  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 abnormalities are increased  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 causes complicates Schizophrenia because the unattractive extreme of an SSRI 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 will be evolutionarily transient, lineage-specific mutations that reduce general 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 SSRIs  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?