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?