Levels of Selection
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© 1997
David H.A. Fitch
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Lecture notes

Adaptation:  Levels of selection and "altruistic" features

I.  Strength of selection is predicted to decrease with increasing taxonomic hierarchy

A.  Selection requires (results from) heredity, variation, and multiplication (i.e., selection works with entities that demonstrate genealogical relationships and have heritable variations affecting that entity's ability to survive to reproduce)

B.  With increasing levels of groupings (e.g., from species to genera to families...), there are fewer units (and thus less variation among units) and slower rates of multiplication, thus reducing the strength of selection (i.e., the response to selection)

C.  Thus, attempts to explain features of organisms that appear to benefit a group rather than an individual (i.e., "altruistic" traits) should first consider whether or not selection mechanisms at lower hierarchical levels (e.g., at the level of individuals) can provide an explanation.

D.  That is, the decreasing strength of selection with higher taxonomic level is the main argument against "group selection" mechanisms:
1.  Under group selection, a trait might be expected to increase in the species as a whole if the rate of extinction of a group is lowered by a high frequency of the trait.  (An example of such a trait might be recombination and mutation.)
2.  Alternatively, under group selection, the frequency might increase of a trait that somehow enhances the proliferation of new populations carrying the trait.  (An example of such a trait might be cooperative behavior.)

II.  Explanations for evolution of "altruistic" features that provide an alternative to "group selection"

A.  "Altruism" may be an illusion based on our anthropocentric viewpoint

B.  Some features purportedly advantageous to a population may actually be favorable to individuals

C.  Kin selection can provide an explanation for some features that increase the fitness of an individual's relatives, even if the trait is disadvantageous to the individual
1.  According to Hamilton's "rule", the frequency of the trait depends on both the direct influence of the trait on an individual and on its indirect effect, and can increase as long as it confers sufficient advantage on the bearer's relatives:

where wi is the fitness value that the trait bestows on individual i, ai is the direct effect (which could be negative to what point?) of the trait on i, bij is the beneficial effect of i's trait on a related individual j, and rij is the coefficient of relationship between individuals i and j.

2.  A testable prediction of this model is that inbreeding should favor the evolution of such traits

III.  Group selection models

A.  In the "trait group" model, small demes (subpopulations) segregate into small patches after random mating (e.g., the pattern of patching may even be related to the trait itself)

B.  In this model, Random sampling stochastically results in some demes with higher frequency of an altruistic trait (or allele specifying such a trait)

C.  Because "trait groups" with this trait (or allele) have a higher chance of non-extinction (due to the "altruistic" nature of the trait), they contribute relatively more to the next generation than would groups that have a greater chance of extinction (because of a lower frequency of the trait)

D.  The difference between this model of group selection and kin selection is that members of "trait groups" are not related

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Exercises

  1. In many eusocial insect species, a "worker caste" (or castes) comprise(s) individuals that dedicate their entire lives to the care of another individual's (the "queen's") offspring or to the care of the colony itself.  This seems like an extremely altruistic behavior.  How might you (or can you) explain this behavior as an adaptation that has arisen by individual selection?  by kin selection?  by trait group selection?  Which explanation do you think is most valid?  Why?
     
  2. One "explanation" for the existence of sex is that it provides a "long-term advantage" to a population by providing genetic variation (through recombination); the population is thus less subject to future extinction from any one particular environmental factor.  What are some problems with this "explanation"?  How would you invoke selection at different levels (individual, kin, group) to provide a better explanation for this widespread and clearly important adaptation?
     
  3. In a year in which food is scarce and the population is dense, a bird avoids mating.  This might appear to be an altruistic behavior, since this individual is giving up an opportunity to reproduce.  How might such a trait be maintained in a population through individual selection?
     
  4. Many disease agents that infect new hosts and display a high virulence eventually display reduced virulence.  While one obvious explanation is that the host develops resistance, another possibility is that the disease agent becomes more "altruistic" (e.g., allowing the host to live).  How might group selection be legitimately used to explain the evolution of lowered virulence in this case?

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 Examples of
Adaptations  Defining
Adaptation  Levels of
Selection  Optimal
Models  Tradeoffs  Sexual
Selection
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[Examples of Adaptations] [Defining Adaptation] [Levels of Selection] [Optimal Models] [Tradeoffs] [Sexual Selection]

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