Genetic Determinism and Heritability

Gene selectionismAll instances of selection are instances of selection acting on genes (rather than individuals or groups)

How can selection ‘act on’ genes?

Genetic Determinism and Heritability

How can selection ‘act on’ genes?

It selects genes via their phenotypic effects

What is the relation between genes and their effects?

Received view:Genes are major causes of development and traits

Many traits• have a ‘genetic basis’• are ‘inherited’• are under ‘genetic control’• are ‘genetically determined’

Genetic determinism

the ultrabithorax mutation

causes flies to have a second pair of wings

Genetic determinism

‘G determines trait X’ – what does this mean?

(1) Having G is the only cause for developing X

Rejected by today’s interactionist consensus: All genes interact with environmental factors to produce any trait

(2) Any organism with G develops X

Counter-example: many organisms have G without developing X – all those G-individuals that die before X is being expressed (e.g. flower petals)

Genetic determinism

‘G determines trait X’ – what does this mean?

(3) G has a flat norm of reaction

Change in environment makes no difference to phenotype

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Developmental Systems Theory

Interactionism is an inadequate thesis about the causal role of genes in development and inheritance: it still privileges genes

Genes are just one of many causal components

DST comes in many versions; many, but not all, defend the following more specific theses

1. Organisms inherit more than genes

2. The notion of ‘gene for X’ is incoherent

3. The parity thesis

2. The notion of ‘gene for X’ is incoherent

‘Gene for X’ requires that the gene’s causal contribution can be isolated from that of the environment

But their contributions are inseparable and incommensurable

Lewontin’s brick and mortar metaphor• One builder lays bricks, the other applies mortar• Which is more important to build a house?

Genes and environment lack a uniform kind of effect by which their contributions could be compared

Contrast: two people pushing a car (both exert a force)

2. The notion of ‘gene for X’ is incoherent

Defending ‘genes for’ (Kitcher)

Allele A is ‘for’ root proliferation iff

AB-trees produce more roots than BB-trees, given a standard environment and keeping other things constant

A makes a phenotypic difference in standard environments

‘Standard environment’

• statistically normal (frequently encountered by species)

• does not reduce mean population fitness

2. The notion of ‘gene for X’ is incoherent

The Acorn objection (Griffiths, Gray)

In standard environments for oak trees most acorns rot

Therefore, an acorn’s having A will not make a phenotypic difference in standard environment:

AB offspring will on average produce as many roots as BB, i.e. none

2. The notion of ‘gene for X’ is incoherent

Refinement (Kitcher)

Allele A is ‘for’ root proliferation iff

AB-trees produce more or equal number of roots than BB-trees, given a standard environment and keeping other things constant

• Prematurely dying ABs produce equal number (0)

• Surviving ABs produce more

Heritability

Heritability analysis is a statistical procedure to measure the causal contribution of genes

1) Heritability as resemblance (non-standard, e.g. Sober)

Degree of parent-offspring similarity; independent of the degree to which genes are responsible for similarity

2) Heritability in the sense of heritability analysis

Degree to which phenotypic variation is due to genetic differences – measured as a fraction of total variance

‘broad’ heritability: H2 = VG / VP

‘narrow’ heritability: h2 = VA / VP

Heritability

Genotypic difference makes 3-unit difference on phenotypeEnvironmental difference also makes 3-unit difference

H2 = 50%

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Heritability

Genotypic difference makes 3-unit difference on phenotypeEnvironmental difference just makes 1-unit difference

H2 >> 50%

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Heritability

An important assumption for heritability analyses:

Additivity = lack of statistical gene-environment interaction

Genotype makes the same difference in all environments, i.e. there is an overall genotypic effect

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Heritability

Statistical gene-environment interaction:

Genotype does not make the same difference in all environments

E1: +3

E2: 0

E3: -3

There is no overall genotypic effect

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Objection to Heritability Studies (Lewontin)

Heritability studies are useless for tracing causal relations

1. Additivity rarely obtains: different populations live in very different environments, leading to different phenotypic effects for any given genotype

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Objection to Heritability Studies (Lewontin)

Heritability studies are useless for tracing causal relations

1. Additivity rarely obtains: different populations live in very different environments, leading to different phenotypic effects for any given genotype

C1. Therefore, heritability estimates are ‘local’: • a given heritability value may be different outside the

narrow range of environments that hold in the study population

• it may not be extrapolated

C2. Therefore, heritability cannot provide evidence of general relation between cause and effect

Response (Sesardic)

Ad 1) Additivity rarely obtains

• Empirical data are few and inconclusive• Most philosophers simply take Lewontin’s word for it

Ad C2) Heritability does not trace causation

• Accepts that we cannot make global claims across all or even a wide range of not investigated environments

• But it does not follow that the local claims are not causal:

Provided additivity holds in a given context, heritability does quantify the causal contribution of genotypes within that context