Schizophrenia as one extreme of a sexually selected fitness indicatorShaner, 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 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 PowerGeneral 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 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 effortSchizophrenia 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???
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)
ConclusionMany key features of schizophrenia explained:Age of onset, disparity between sexesReduced reproductive rateSubstantial heritabilityPolygenic basisFrequent developmental abnormalitiesIncreased reproductive success of unaffected relativesTreatment 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?