Levels of selection Flashcards
Levels of Selection
Background
‘New’ group selection
Empirical studies
Conclusion
‘Old’ group selection:
‘Animal Dispersion in Relation to Social Behaviour’ (1962)
‘Evolution through group selection’ (1986)
Reproductive self-restraint to avoid over-exploitation of resources
`VC Wynne-Edwards
Demolished by:
‘Adaptation and natural selection: a critique of some current evolutionary thought’ (1966) George Williams
‘The Selfish Gene’ (1976)
Richard Dawkins
The conditions for old group
selection are
unlikely to be met in nature
In theory, group selection can work if some groups die out faster than others. But, individuals usually die at a faster rate than groups, so individual selection will be more powerful.
Plus, for group selection to work, groups must be isolated so that selfish individuals can’t migrate into cooperative groups.
‘New’ group selectionists
Wilson, ,Nowak, David Wilson
Cooperative traits spread when between group selection is stronger than within group selection
Wilson’s concept of multi-level selection
Genes, Cells, Organism, Group
Populations are divided into ‘trait groups’ or ‘demes’
Selection acts for or against cooperative traits within groups
Groups mix together before splitting into new groups and the next round of selection…
‘New’ group selection
Cooperation is favoured by:
- increasing group benefits - reducing individual cost - reducing within-group genetic variance, relative to between-group genetic variance
Cooperation is favoured by:
- increasing group benefits (B) - reducing individual cost (C) - reducing within-group genetic variance, relative to between-group genetic variance (r)
New group selection models are mathematically equivalent to Hamilton’s rule for the evolution of altruism: rB > C
If new group selection and inclusive fitness models are mathematically equivalent, are they also equally useful?
NO!
Inclusive fitness theory tells us that quantity which is maximised by natural selection under all conditions (i.e. inclusive fitness) – this idea underpins ALL that we have talked about in this module.
New group selection models tell us when cooperation might evolve only under certain very narrow conditions – the theory has low generality.
The debate is particularly heated among theoreticians…
A. Gardner vs. Goodnight
Pruitt & Goodnight (2014): social spider Anelosimus studiosus
Site-specific selection at the level of groups
Colonies adjusted their composition to match mixture seen at native site
Suggests group trait is locally adapted
BUT….
Individual-level selection not investigated
Successful groups = groups of successful individuals
Genetic basis of group trait unknown
Colony extinction frequent
Limited dispersal
Colonies have a mix of docile and aggressive phenotypes which are site-specific and affect reproductive success
Experiment
Artificial colonies of varied mixes of docile/aggressive phenotypes introduced to different sites
Some introduced to their ‘native’ site, others to ‘foreign’ site
Monitored after two generations
MUCH scepticism
Grinsted et al. (2015) Nature 524: E1-E3
Gardner (2015) Nature 524: E3-E4
Smallegange & Egas (2015) Trends in Ecology & Evolution 30: 379-381
Advocates of group selection cite other studies:
e.g. Gordon et al. (2013) The reward of restraint in the collective regulation of foraging by harvester ant colonies. Nature 498: 91-93
colonies show restraint in foraging that does not affect colony survival
a colony’s foraging behaviour may be inherited by daughter colonies
But, results are entirely consistent with individual-level selection
Conclusions
Conditions for group selection (old or new) to operate are extremely limited in nature
Group selection models that work and inclusive fitness models are mathematically equivalent
Inclusive fitness theory is far more useful because it gives us the quantity that natural selection maximises
Advocates of group selection often appear to have an agenda of denial:
e.g. Nowak et al. (2010): “empirical evidence [for IFT] is meagre”
….so while their evidence should be evaluated fairly, advocates of group selection should do likewise.
