Zoology 510, Class Notes for Ridley, Chapter 12
The Units of Selection.

No written assignment, but you should try to answer the Study and Review Questions at the end of the chapter.

Brief Outline

• Overview of this unit (Part 3, Adaptation and Natural Selection, Chapters 11-13).
• Introduction to Chapter 12. The Units of Selection.
• Checklist of important concepts and terms
• Section-by-section comments
  • 12.1. What benefits from adaptation?.
  • 12.2. Benefits occur at various levels.
    • 12.2.1. Gene benefit -- segregation distortion.
    • 12.2.2. Cell benefit.
    • 12.2.3. Organism benefit.
    • 12.2.4. Kin benefit.
    • 12.2.5. Group benefit?
    • 12.2.6. Requirement for heritability.
  • 12.3. Selection acts at various levels.
  • 12.4. Two senses for "unit of selection".

510 index page

Introduction to Chapter 12.

• This chapter introduces a controversial topic in modern evolutionary theory, one rooted historically in a deep misunderstanding that selection always acts "for the good of the species". (see 12.2.5, p. 325).  Before concentrating on details of this chapter, you should once again review the entire unit (Chapters 11, 12, and 13).

• There are basically two questions being asked in this chapter:
  • Who or what benefits from "adaptation"?  Is it species?  Or is it individual organisms?  Or groups of organisms?  Or genes?
  • Who or what does selection act upon?

• Since adaptations are understood to result from selection, there is a presumption that both of these questions must have the same answer.
• But different features of biological organization appear to be "designed" to benefit different levels in the organizational hierarchy.  Some features seem useful only to genes, others only to organisms, others, still others only to species.

• This observation raises more questions.
  • Can all features of biological organization be explained by selection acting at a single level?  If so, how?
  • Or can selection act at several levels?  If so, which ones, and how do the levels interact?

• The resulting problems and controversies are complicated by semantics.  Many of the essential terms (e.g.,adaptation, selection, unit, individual, gene, species) have multiple and sometimes rather fuzzy usages.

• The basic conceptual approach presented in Chapter 11, section 11.2 Methods to study adaptation is quite relevant here.  So also are some of the specific examples, which may be explainable at more than one level of selection.  Also significant are the various definitions and constraints described in Chapter 13 Adaptive Explanation.

CHECK LIST of important TERMS

• Units of selection, or levels of selection
• Biological hierarchy (molecules, cells, organisms, populations, etc.)
• Segregation distortion / meiotic drive
• "Selfish genes"
• Germ cell line vs. somatic cell line (Weismannist life cycle)
• Individual organism
• Altruism
• Kin selection
• Group selection
• Hamilton's rule ( rb > c, or: relatedness x benefit > cost )
• Replicator (gene, broadly defined)
• Interactor (phenotype, broadly defined)

Chapter 12, Section-by-Section Comments

12.1. "For the benefit of which level in the biological hierarchy of levels of organization does natural selection produce adaptations?"

• Do adaptations benefit genes or cells or organisms or groups or species?
• Clearly, many familiar adaptations benefit all levels.
• But does adaptation ever benefit one level at the expense of another level?  If so, when and how?

12.2. "Natural selection has produced adaptations that benefit various levels of organization."

• 12.2.1 "Segregation distortion benefits one gene at the expense of its alleles."
  • Alleles which distort segregation in their own favor are intrinsically more fit to the extent of that distortion.
  • If there is no associated cost, alleles which cause segregation distorter will quickly increase in frequency.
  • But this fitness has no benefit for cell or organism, only for the distorting allele itself.
  • If there is any selective cost to the organism, the outcome of selection will depend on the balance between the direct advantage to the allele based on segregation and indirect cost due to selection against organisms containing distorting alleles.
  • Selection distortion is only apparent when there are competing alleles.  Once either a normal or a distorting allele is fixed, then there is no longer any segregation to distort.
  • Another putative example of gene-level benefit involves transposable elements ("selfish DNA"; see Chapter 10, sections 10.8 and 10.9, pp. 273-276) which replicate themselves within a genome.  For some reason, Ridley avoids discussing selfish DNA in terms of the units of selection.
• 12.2.2 "Selection may sometimes favor some cell lines at the expense of the rest of the body."
  • Cancer demonstrates selection favoring mutant cells at the expense of the organism.
  • The effect is so powerful that selection for organisms has apparently produced mechanisms to suppress such events.
  • Selection among cells can also operate in plants which have multiple growth points,  The favored meristem can overwhelm other branches, but the outcome can be beneficial for the entire plant.

• 12.2.3 "Natural selection has produced many adaptations to benefit organisms."
  • Most adaptations are traditionally understood as features which benefit individual organisms.
  • This view is probably true.  But simply to assert it, as Ridley does ("It can hardly be doubted ... that organismal adaptations exist") begs the scientific method which Ridley espouses in Chapter 11, section 11.2.

• 12.2.4 "Natural selection working on groups of close genetic relatives is called kin selection."
  • Animals sometimes behave in ways which benefit others at cost to themselves.  This phenomenon has been labelled altruistic behavior.  Popular examples include:
    • Helping raise offspring which are not one's own.
    • Giving warning cries which help others to escape but attract a predator's attention to one's self.

  • Much altruistic behavior can be explained by kin selection.
    • Kin selection is a powerful concept for understanding the evolution of behavior in which the actions of one organism benefit other, related organisms.
    • The basic idea of kin selection is that actions which benefit close relatives benefit copies of one's own genes.
    • If the benefit from altruistic behavior for the copies of one's own genes is greater than the cost to one's own genes, selection will favor the altruism.
    • The effect can be quantitifed, for example in Hamilton's rule:  altruism will be favored when rb > c, or: relatedness x benefit is greater than cost.
    • Kin selection can be especially powerful in special cases.
      • In hymenopteran insects (ants, bees, and wasps), the haplodiploid reproductive system causes females to be more closely related to their sisters than they are to their daughters or brothers.  (In ordinary diploid sexual reproduction, the coefficient of relatedness is the same siblings as for parent/offspring.)
      • As a consequence, selection can readily favor traits which increase the production of sisters.
      • This phenomenon is thought to explain the how elaborate social systems have revolved several times among the hymenopterans.
    • Kin selection is explainable at the gene level.  From this perspective, altruistic behavior is not altruistic but selfish.

  • An alternative explanation for some altruistic behavior, not mentioned by Ridley, is that some altruistic behavior may result from genetic or developmental constraints on instincts.  For example, instincts appropriate to caring for one's own offspring may be advantageous enough to be retained even though they might sometimes be extended (at some cost) to offspring of other individuals.

• 12.2.5 "The issue of whether group selection ever produces adaptations for the benefit of groups has been controversial, though most biologists now think it is only a weak force in evolution."
  • Here is an example of a semantic issue (i.e., a controversy based at least in part in how a word is used).:
    • Although kin selection involves groups of related individuals, "kin selection" is NOT usually included as "group selection".
    • The distinction is that kin selection is understood to involve selection which benefits individuals carrying particular genes, not selection for the groups themselves.
    • The fact that colonies of social insects seem to have adaptations "for the good of the group" is thus preferably interpreted as adaptations for the greatest reproductive advantage of the individuals.

  • The classic proposal for "group selection" is Wynne-Edwards example of reproductive restraint.
    • Selection based individual advantage is presumed always to maximize reproductive capacity.
    • Some species do exhibit restrained reproductive capacity.  Why?
    • At least some cases can be explained by individual advantage.
      • To assure resources for reproduction, individuals defend territories.
      • Within each territory, resources are limited.
      • Individuals maximize their own individual reproductive potential by matching reproductive effort to available resources.

  • Ridley explains conditions under which group selection, carefully defined, could occur.
  • These conditions could in principle be met.
  • However, it seems unlikely that the resulting group selection would be strong enough to override individual selection.
  • Not mentioned by Ridley:  Conditions may be more favorable to group selection if groups can recognize and defend against invasion by mutants or outsiders.

• 12.2.6 "The level in the hierarchy of levels of organization that will evolve adaptations is controlled by which level shows heritability."
  • Selection depends on heritability.
  • Genes and individuals do have heritable differences.
  • Ordinary groups, such as flocks of birds, do not often seem to have heritable differences.
  • Ridley uses this concept to argue that adaptations will benefit units which show heritability, and that unit "is usually the organism, with some interesting exceptions."

12.3. "Another sense of 'unit of selection' is the entity whose frequency is adjusted directly by natural selection."

• In this section, Ridley presents a completely different perspective on the idea of "units".
• In section 12.2, "units" were the entities which are benefitted by adaptation.  Here, "units" are entities whose frequencies are adjusted by selection.
• The argument here is largely semantic, over how words should be used.
• One conclusion, favored by many competent biologists, is that with the most careful usage, the only unit of selection is the gene.
• Other competent biologists disagree, vehemently.

12.4. "The two senses of 'unit of selection' are compatible; one specifies the entity that generally shows phenotypic adaptations, the other specifies the entity whose frequency is generally adjusted by natural selection."

• This section does not present fundamental biology, but presents Ridley's own attempt to resolve an apparently fruitless controversy.  Ridley concludes that:
  • "Adaptations can evolve only if they benefit replicators."  ("Replicators" are generally conceived as "genes".)
  • "Adaptations should benefit entities that show heritability--which are usually organisms", and, "those entities will show adaptations that propagate genes efficiently."

510 index page

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