Human Adaptation - ad aptos Definitions and perspectives ......6 1 Novel environments 2 ‘Novel’...
Transcript of Human Adaptation - ad aptos Definitions and perspectives ......6 1 Novel environments 2 ‘Novel’...
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1 Novel environments 2 ‘Novel’ genes, genotypes (via mutation, drift, inbreeding, gene flow,selection)3 Tradeoffs between opposing selective pressures (eg autoimmune vs infectious diseases)4 Extremes of adaptations (eg overgrowth, extreme male brain)5 Conflicts within and between species6 Constraints on optimization (evolutionary legacies)7 Trait involves benefits to own reproduction, or to kin, that offset costs to phenotype (genes that increase reproduction spread even if they decrease health, happiness or longevity)8 Trait is not a disease but a beneficial protective response (eg cough,fever,pain,nausea,vomiting,anxiety,fatigue)
Human Adaptation - ad aptos: ‘good fit’ between trait and environment
-produced by natural selection-better than alternatives in immediate circumstances
How are health and disease related to human adaptations?
The BIG 8 can give rise to apparent maladaptations
Definitions and perspectives on adaptation (from Crespi 2000)
(1) Teleonomic - focus on functional design - how the trait or formof trait has been ‘designed’ by selection for function in some context
(2) Phylogenetic - infer origin of trait using phylogeny,infer ‘selective regime’ under which trait arose, testperformance of trait in ancestral and current selectiveregimes. If trait arose under current selectiveregime, & exhibits higher performance thanantecedent, then is called an adaptation; mostgenerally, this approach is the comparative method
(3) Population and quantitative genetic -relate allelic and genotypic variation tophenotypic variation and fitness variation.‘Purifying’ selection and stabilizingselection are evidence of adaptation,positive and directional selection areevidence for adapting. Adaptive peakviewpoint is conceptually useful
Trait value
Fitness
Observe a Trait, Trait Variation (cognitive, emotional,behavioral,morphological, physiological,molecular)
What is its adaptive significance? -> does presence or form of trait or phenotype increase survival and reproduction of its bearers,and if so, how? -> does trait involve interactions between individuals, within or between species? Cooperation or conflict?
Information on adaptive significance is crucial to understanding both proximate and ultimate causesof the forms of traitsNOTE: adaptive significance is often conditionalon environmental variation
ALTERNATIVE HYPOTHESES
*(1) Adaptive (function optimal)Very close fit between trait andenvironment of trait
*(2) Adaptive compromiseTradeoff between opposing selectivepressures - what are they? costs ANDbenefits present
*(3) Non-adaptive (neutral) byproduct ofanother trait, or other cause
*(4) Mal-adaptive
HOW DO WE TELL THE DIFFERENCE?
APPROACHES (1) Functional design
what does trait do? with what aspect of environment does it interact?
(2) Analysis of selection
measure genotype or phenotype, and fitness, andanalyze their relationship
(3) Phylogenetic and comparative analysis
analyze relationship between traits, or traits and environments, over evolutionary time, across species or across populations, in a phylogenetic context
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METHODS OBSERVATION
What are processes and patterns in nature?Correlation vs causation: x,y, and zgiven x <-> y, causal relation is:x-> y or y-> x or z -> x and y
EXPERIMENTSPerturb the system, predict the outcome based on hypothesis under test, differentiate between correlation and causation - KO
MODELLINGMake assumptions, explore mathematical consequences
Components: (1) strategy set(2) optimization criterion (eg fitness)(3) maths for determining which ‘strategy’ is optimal under what circumstances
Combining the approaches and methods
(1) Functional design Observation, experiments, modelling: what aspect of environment does trait interact with, and how?
(2) Measuring selection and response to selectionObservation: how does trait covary with components of fitness?What is the form of selection? Is the trait evolving now?What is its genetic basis?
(3) Comparative, phylogenetic methodObservation: How have trait and environment changed over evolutionary time? Has the trait evolved in parallel with the environmental factor? Have two traits evolved in association with one another? Can do between species or between populations.
• EXAMPLE: more sperm competition should select for largertestes (and more sperm)
- Experimental evolution: often not practical
- Interspecific comparison: test whether traits are correlatedacross species
• Problem: related species may share the same traits due toshared ancestry = phylogenetic non-independence
• Result is that species cannot be taken as independentdata points
Phylogenetic, comparative tests of evolutionary theories
Example
Degree of sperm competition
Test
es s
ize
ABC
DE F
Plain correlation doesn’t mean much – if species D, E and F are closely related they could have evolved larger testes sizes only once
Independent contrasts
Trait 1 Contrast
Trai
t 2 C
ontra
st
51
62
62
95
Trait 1: (6-5=1)Trait 2: (2-1=1)contrast: (1,1)
Felsenstein 1985
Felsenstein 1985, 1988
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Trait 1 Contrast
Trai
t 2 C
ontra
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62
62
95
Trait 1: (9-6=3)Trait 2: (5-2=3)contrast: (3,3)
Independent contrasts
Trait 1 Contrast
Trai
t 2 C
ontra
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62
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5.51.5
7.53.5
Averageof
descendents
Trait 1: 7.5-5.5=2Trait 2: 3.5-1.5=2contrast: (2,2)
Independent contrasts
SOME CAUSES OF LACK OF ADAPTATION
(1) Evolutionarily-novel environments (mismatches!)e. g., moths at lights, humans with novel technology, candy bars
(2) Time lagse. g., fruit dispersal by extinct megafauna, rapid change in human environments, long time needed to fix advantageous mutations; LCT
(3) GeneticsLack of sufficient genetic variationHeterozygote advantage Linkage, pleiotropy
(4) Local, not global, optimization
Case study: laryngeal nerveLaryngeal nerveanatomy
1. down the neck
2. behind the aorta3. up neck
4. to larynx
Is it adaptive?
For giraffes?
EXAMPLES OF ANALYSES OF ADAPTATION
Darwin’s finches Human brain size
Recognizing and demonstrating maladaption is challenging and it is best conducted by combining analyses of proximate mechanisms AND ultimate causes
Darwin’s finches
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Beaks as functionally-designed ‘tools’ for food acquisition Measuring selection on beak size
Phylogenetics of finches and their beaks
No comparative analyses done yet!
EXAMPLES
Darwin’s finches
Human brain size (and shape)
FUNCTIONAL DESIGNMEASUREMENT OF SELECTIONCOMPARATIVE ANALYSIS
Facialperception
Affectrecognition
Socialjudgement
Mentalizing
Gazedetection
FUNCTIONAL DESIGN OF THE BRAIN:
‘Social Brain’: Distributed, Integrated Neural System ‘for’Acquiring and Processing Social Information
Funct IMAGINGLESIONSTMSNatural variation
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SELECTION AND HERITABILITY:Human brain size increase:
Tripled in about 4 million yearsMainly in the neocortex
Selection on genes ‘for’ human brain sizeMicrocephaly genesASPM and MCPH1
Heritability of brain size is high
BENEFITS: BRAIN SIZE IS POSITIVELY CORRELATED WITH MEASURES OF ‘INTELLIGENCE’, AMONG PRIMATES and WITHIN HUMANS
Brain, 2006
Narr et al.CerebralCortex, 2007
p<0.01
Brain Behavior Evolution, 2007
p<0.01
EARLY LIFE-Malnutrition-Pathogens-Infectious diseases
Austria,Upper Paleolithic
Now
When and how has most natural selectiontaken place in humans?
AGE
%SV
Sandiford et al. 1997
Cvorovis et al. 2008
Wachs et al. 2006
Martin & Kubzansky 2005
Webb, Sellen et al. 2009
In humans, maternal ‘intelligence’ is positivelycorrelated with measures of child survivorship and health
Implications for public health?
COSTS
Brain is VERY LARGE and energeticallyCOSTLY to grow and maintain:
Is 3X larger than expected forprimate body size
Has 22X the metabolic rate of skeletal muscle
PHYLOGENETIC, COMPARATIVE ANALYSIS OF BRAIN FUNCTION
1996 2007
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1 Novel environments 2 ‘Novel’ genes, genotypes (via mutation, drift, inbreeding, gene flow,selection)3 Tradeoffs between opposing selective pressures 4 Extremes of adaptations (eg overgrowth, extreme male brain)5 Conflicts within and between species6 Constraints on optimization (evolutionary legacies)7 Trait involves benefits to own reproduction, or to kin, that offset costs to phenotype (genes that increase reproduction spread even if they decrease health, happiness or longevity)8 Trait is not a disease but a beneficial protective response (eg cough,fever,pain,nausea,vomiting,anxiety,fatigue)
Human Adaptation - ad aptos: ‘good fit’ between trait and environment
-produced by natural selection-better than alternatives in immediate circumstances
How are health and disease related to human adaptations?
The BIG 8 can give rise to apparent maladaptations
TP53
Codon 72: Pro/Pro compared to Pro/Arg, Arg/Arg
Lower apoptotic potential, leading to Higher implantation failure (& lower reproduction)Higher cancer riskHigher overall survival (longevity)
Other polymorphic genes in TP53 pathway mediate same tradeoffs
Kang et al. 2009 PNAS; Corbo et al. 2012 PLoS ONE
Genes ultimately mediate medically-important tradeoffs I
Functional design? knockouts, molecular mechanismsSelection - at molecular levelComparative - within/among species variation
p53 is a nuclear phospho-protein which, in response to DNA damage,slows progression through the cell cycle and initiates apoptosis if damage issevere. Tumour-specific point mutations occur in many forms of human cancerwith as many as 50% of cancers containing a p53 mutation. 20% of mutationsare concentrated at 5 'hot-spot' codons.
APOE
E4 alleles, compared to E3 and E2
Better verbal, memory skills (especially when young)
Increased risk of schizophrenia and Alzheimer’s
Alexander et al 2007 Biol Psy; Chang et al. 2011 NeuromageJochemsen et al. 2012 Neurob. Aging
Genes ultimately mediate medically-important tradeoffs II
COMT
VAL158MET polymorphism
Mediates prefrontal, striatal dopamine levels
VAL - better cognitive flexibility, updating;worse cognitive stability
MET - worse cognitive flexibility, updating; better cognitive stability
Fallon et al. 2012 Cerebral Cortex
Genes ultimately mediate medically-important tradeoffs III
Combining the approaches and methods to recognize adaptation (e.g., health) and maladaptation (e. g. disease and its risk)
(1) Functional design Observation, experiments, modelling: what aspect of environment does trait interact with, and how?
(2) Measuring selection and response to selectionObservation: how does trait covary with components of fitness?What is the form of selection? Is the trait evolving now?What is its genetic basis?
(3) Comparative, phylogenetic methodObservation: how have trait and environment changed over evolutionary time? Has the trait evolved in parallel with the environmental factor? Have two traits evolved in association with one another? Can do between species or between populations.
Causes, effects and evolution of allelic variation: how to THINK about genes and genic variation, and their relation to disease
Inclusive fitness: how to THINK about situations whererelated individuals interact
Adaptive significance: how to THINK about variation in phenotypes - benefits and costs, tradeoffs, evolutionary history, relation to maladaptation/disease
NOW
How to THINK about patterns of evolutionary change across generations in genes and phenotypes -> phylogenetics, thehistory of biological entities