Zoology 510, Class Notes for Ridley, Part 3
Adaptation and Natural Selection.

HOW TO STUDY PART 3.
Please begin by reading Ridley p. 279.

In the previous unit (Part 2, Evolutionary Genetics), some students had difficulty seeing the forest for the trees.  The advice here may reduce that difficulty in Part 3.
Please keep the following points in mind as you study Chapters 11, 12, and 13.

• These chapters comprise a single unit.  Don't expect to understand this unit if you only look at it piece-by-piece.  Preview (skim) the entire unit before you lose yourself in any one part.  Ridley p. 279 is an overview; return to this page as often as necessary.

• The material in this unit weaves together issues at several levels of abstraction.  It contains examples within examples and explanations within explanations.  (It's like a jigsaw puzzle, in which the overall picture matters as much as the individual pieces, and in which a completed picture becomes a piece in a still larger puzzle.)

• There are several levels to this material.  Each level is important.  You certainly can't comprehend the higher (more abstract) levels without some particular examples, but you are also unlikely to appreciate the lower (more concrete) examples without seeing how they fit into the larger picture.

• The only reliable way to understand this material is to engage with it in several repeated passes.  Do not worry too much about mastering each piece (numbered chapter section) as you read it the first time.  Do expect to read the entire unit at least twice and review all the material three or four more times.
  • On the first pass, try to form a "big picture" of what the overriding issues are through the entire unit.  Read lightly (skim) all three chapters.
  • On the second pass, take each chapter one at a time.  Read the chapter carefully, but do not try (yet) to master all the details.  Try to form an impression of why the chapter takes the shape it does.
  • On the third pass, after reading a chapter carefully (above), skim that chapter again while trying to see how the specific examples (sections) in the chapter contribute to the larger context of the chapter.
  • On the fourth pass, skim the entire unit again, to see how issues in all three separate chapters overall (as well as those previously studied) impact on the specific material in each chapter.  Note cross references to earlier and later material.
  • On the fifth pass, try to understand each specific example well enough to see how the deeper issues apply.  Read carefully when necessary (for detail this time), while keeping the deeper issues in mind.

• Do not memorize anything in isolation, but only in relation to the web of issues and arguments that are developed through the entire unit.

• The three chapters address three different ways of looking at adaptation.
  • Chapter 11.  How should any adaptations be analyzed, in order to understand whether and how selection can mold them?  This chapter presents both general strategies for analyzing adaptation (in section 11.2) and examples of how that analysis proceeds (in sections 11.3, 11.4 and 11.5).
  • Chapter 12.   On what units of biological organization does selection operate?  Who or what benefits from adaptation?  Are there any adaptations "for the good of the species"?
  • Chapter 13.  What difficulties arise in attempting to explain adaptation?
• These three approaches are not alternatives and are not sequential.  They are more like three different perspectives (e.g., front view, side view, top view) of the same problem -- How can adaptation be properly understood?
• The examples in each chapter should be thought about not only from the perspective of the chapter in which they appear, but also in the perspectives of the other chapters.
  • For instance, the problem of sex is introduced as a example illustrating how difficult it can be to determine whether and how a significant biological pattern could be adaptive.  But this problem should also be examined from the "units" of selection perspective (Chapter 12), and one should also remember to clarify the definitions and assumptions one brings to bear on the problem (Chapter 13).   The other examples in Chapter 11 should also be examined from the perspective of Chapters 12 and 13.
  • Similarly, the examples given in Chapter 12 (examples such as meiotic drive and altruism), of selection operating at different levels, should also be examined from the perspective of Chapter 11 (are they really adaptations?) and Chapter 13 (what constraints may be operating?).
  • Again, in Chapter 13 should be examined from the perspective of Chapter 11 (are they really adaptations?) and Chapter 12 (what level of organization does the adaptation benefit?).
• Try to apply these interwoven themes throughout this unit.  They will not always be mentioned explicitly.  Sometimes it is when a problem is NOT mentioned that it may be most important to remember to consider it.

The following summarizes some particular points of significance.

• Analysis of adaptation.
  • A particular trait can ONLY be an adaptation IF all of the following apply:
    • Variant forms occur (or have occurred in the past).
    • The trait operates to provide some specified benefit.
    • Selection actually favors the particular trait over its variants (or has done so in the past).  Benefit must outweight cost, in relation to available alternatives.
  • A hypothesis of adaptation can be tested by:
    • Correspondence between trait parameters and engineering criteria for improved fitness (design test).
    • Demonstration that selection favors the trait (experimental test).
    • Correlation of trait occurrence with circumstances when it would be expected to be advantageous (comparative test).
  • The examples of sexual reproduction, sexual selection, and sex ratio each presents some challenge in terms of these criteria for adaptation.  Many other examples could be given, of difficulty determining how a trait can properly function as an adaptation.   But few are as broadly applicable or as challenging as these.

  • Tests for adaptation all have serious limitations; it is actually quite difficult to provide compelling support for a hypothesis of adaptation in any case.  As a consequence, adaptive explanation remains problematic (Chapter 13).
• Units of selection
  • In many (perhaps most) cases, what is advantageous at one level of biological organization is also good at all other levels.  For example, reasonable accuracy of DNA replication is adaptive for genes, cells, organisms, kin groups, populations, and species.  So is efficient metabolism.
  • It is cases when there is potential conflict between levels that the question of "units of selection" becomes significant.
  • Of great historical significance is the question, Can there be such a thing as adaptations "for the good of the species", which do not benefit individual organisms directly?
  • Most adaptations are understood at the level of "gene" or of "individual organism", and usually these two levels seem to be congruent.
  • Nevertheless, certain well-studied cases do pit genes against organisms.  The textbook example is segregation distortion / meiotic drive.
  • An especially clear case, cancer, pits cells against organisms.  This example is not developed in the text, but shall be discussed in lecture.
  • Most controversy regarding "units of selection" occurs over the best or most proper explanation for altruistic behavior in animals.  In some cases, it has become clear that what looks "altruistic" does in fact provide a benefit to the altruist that is greater than the cost.
  • There are no widely accepted examples of adaptation "for the good of the species", although there remains a strong suspicion that sexual reproduction might be one such trait (because it is so hard to identify a benefit to the gene or the individual which outweighs the clear 50% cost).
• Adaptive explanation.
  • Natural selection (and only natural selection) explains adaptation.  Attempts to explain adaptation without depending on natural selection (e.g., "natural theology", "Lamarckism") are unsatisfactory.
  • Not all biological traits are adaptations, and not all adaptations are current or optimal. So biological traits usually need explanation that extends beyond "just" natural selection.
  • "Adaptationism" is the presumption, without adequate testing, that traits are adaptive (or, equivalently, that natural selection provides an adequate explanation for all traits.)
  • Chapter 13 contains a list of other factors, besides adaptation, that can contribute to the form of a trait and should be considered in any adaptive explanation.
• Many of these points are fairly obvious, at least in many cases.  However, most are also more subtle than they appear, with hidden pitfalls for the unwary.

510 index page

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