Zoology 510, Class Notes for Ridley, Chapter 13
Adaptive Explanation.

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 13. Adaptive Explanation.
• Checklist of important concepts and terms
• Section-by-section comments
  • 13.1. Adaptation can only be explained by natural selection.
  • 13.2. Other evolutionary processes do not explain adaptation.
  • 13.3. Natural selection should explain all adaptations.
  • 13.4. Different definitions for adaptation.
  • 13.5. "Function" distinguished from "effect".
  • 13.6. Imperfect adaptation, because of:.
    • 13.7. Time lags.
    • 13.8. Genetic constraints.
    • 13.9. Developmental constraints.
    • 13.10. Historical constraints.
    • 13.11. Trade-offs.
    • 13.12. Concluding comment on constraint.
  • 13.13. Recognizing adaptations.

510 index page

Introduction to Chapter 13.

• This chapter has two distinct purposes.
  • On the one hand, it explains how well adaptive explanation can work.
  • On the other hand, it points out limitations to adaptive explanation.
• This chapter is about the singular nature of natural selection as an explanatory principle.
  • However, although Ridley's introductory manifesto about natural selection as the only explanation for adaptation makes several important points, it is also philosphically flawed (see notes for 13.1).
• This chapter is also about clear thinking, particularly the need for intentional consideration of several factors which affect adaptive explanation.  It explains how several classic puzzles can be resolved.
  • Some of the issues addressed in this chapter are emphasized in creationist attacks on evolutionary theory.  These include:
    • The problem of organs of extreme perfection (13.3).
    • The problem of incipient stages for complex adaptations (13.3)
  • Other issues are of greater concern for contemporary biologists.  These include:
    • Problems of "preadaptation" (13.4).
    • Problems of imperfect adaptation (13.6).
• One of the most important conceptual tools for addressing these issues is a clear distinction between the historical process which created a trait and the current utility for the trait (13.5, 13.12, 13.13).

CHECK LIST of important TERMS

• Adaptation
  • Definition based on history
  • Definition based on current use
• Proposed explanations for adaptation
  • "Supernatural" explanation
  • "Lamarckian" theory
  • Directed mutation / mutation by design
  • Orthogenesis
  • Natural selection
• Pluralism in evolutionary process
  • Genetic drift
  • Mutation
  • Natural selection
  • Exaggerated effectiveness of adaptation ("adaptationism")
• "Gradualist" (step-by-step) requirement
• Coadaptation ("organs of extreme perfection")
• Disadvantageous rudimentary stages
• "Preadaption"
• "Exaptation"
• "Function" vs. "effect"
• Evolutionary constraints
  • Temporal constraint (time lags)
    • Anachronism
  • Genetic constraint
  • Developmental constraint
  • Historical constraint
  • Engineering constraint (trade-offs)

Chapter 13, Section-by-Section Comments

13.1. "Natural selection is the only known explanation for adaptation."

• In this section, Ridley tries to celebrate the effectiveness of evolutionary explanation while dismissing opposing explanations.
• There are several important points in this section.
  • Natural selection can explain adaptation, given well-known facts or widely accepted assumptions about biology.
  • Alternative explanations are famously less successful.
    • Natural theology begs the question (but see below).  That is, it is not so much an explanation as an assertion that further scientific explanation is unnecessary.
    • Contemporary creationism not only begs the question but frequently ignores or misrepresents evidence for evolution.
    • Larmarckianism (the inheritance of acquired characters) also begs the question.
      • Lamarckianism presumes that useful characters will be acquired through experience.
      • Certainly, some useful traits do develop in response to practice, exercise, wear, etc.
      • But Lamarckianism leaves unexplained how useful characters should come to be acquired in the first place.
    • Orthogenesis (consistent variation/mutation in one preferred direction) also begs the question of how characters come to be useful.

• But Ridley's presentation also includes also several examples of of sloppy philosophy.
  • The assertion heading this section, that only natural selection can explain adaptation, risks being either false or circular.
    • If an adaptation is defined simply as a trait which is useful (i.e., which contributes to fitness), then there are certainly traits which are useful but which were not shaped by natural selection for that value.  Ridley provides examples in later sections.
    • If an adaptation is defined as a trait shaped by natural selection, then the assertion is obviously circular.
    • One avoids this trap by recognizing that most useful traits can be explained by natural selection, and that most exceptions also have explanations that fit comfortably into evolutionary theory.
  • Ridley's rejection of natural theology as "viciously circular and unscientific" is inappropriate.
    • It is important to distinguish between theories which are "unscientific" in principle vs. those which are "unscientific" because they ignore current knowledge.
      • Natural theology could, in principle, be true.  Prior to many discoveries in biology, it was at least conceivable that genetics and development and even metabolism might require explanatory principles outside ordinary chemistry and physics.  Indeed, many reasonable people still adhere to that opinion regarding "mind" and "consciousness".
      • Natural theology was once a proper scientific paradigm.  It was largely abandoned in the nineteenth century because it was not consistent with new evidence, not because it was "unscientific".  (Of course, it then becomes unscientific to adhere to a paradigm that has been tested and rejected.)
      • Modern "creationism" and "intelligent design theory" are unscientific, because they ignore or deny well-established evidence and argument.
    • In other words, one should distinguish between explanations which cannot work and those which happen not to be true.
  • Lamarckianism and orthogenesis do seem unsatisfactory in principle, at least to the modern eye.  But either one might, conceivably, be true.  They have been rejected because they are inconsistent with evidence.
  • There is nothing fundamentally unscientific about hypotheses of directed mutation and intelligent design.  Both are demonstrably possible.
    • There is nothing in evolutionary theory to say that the Darwinian process cannot create mechanisms which are no longer Darwinian.
    • Indeed, human intelligence seems to be just such a product, and we can now produce useful traits in a variety or organisms, by intelligent design and/or directed mutation.
    • A small minority of biologists believe that capacity for directed mutation might have evolved in other organisms, particularly prokaryotes.
  • Thus Ridley repeatedly conflates an abstract, philosophical argument (about what is and is not scientific) with a more concrete, practical argument (about what explanations are or are not consistent with past experience and evidence).

13.2. "Pluralism is appropriate in the study of evolution, not of adaptation."

• In other words, evolutionary processes other than natural selection, such as mutation and neutral drift, do not produce adaptation.

13.3. "Natural selection can, in principle, explain all known adaptations."

• This section is intended to counter a popular creationist argument that certain adaptations are fundamentally impossible to explain by natural selection.
  • This argument is exemplified in a recent book, Darwin's Black Box, by Michael Behe.
  • Once a trait has been declared impossible to explain by natural selection, "intelligent design" is usually asserted as the necessary alternative.
• This argument typically takes one of two forms, both of which are fundamentally related.
  • An organ is too perfect to have evolved by small steps.  The vertebrate eye is a favorite example.
  • The incipient stages of a complex organ would be disadvantageous.  This is often represented as "What is the use of half a wing?"
• Behe's book introduces many molecular examples.
• Any particular example of this argument can be countered simply by imagining a plausible scenario.  As an example, Ridley offer a study showing how the optical properties of an eye could arise by small steps.
• Many of Behe's examples have not yet been explained.  However, the basic argument has been refuted so often, the argument itself is often ridiculed.  Richard Dawkins nicely characterizes it as "argument from personal incredulity".
• Ridley concludes with the assertion that "there are no known adaptations that definitely could not have evolved by natural selection."
  • While that is true, the word "definitely" renders it a weak statement.
  • There are still some deeply puzzling cases, such as sexual reproduction.

13.4. "Adaptation can be defined either historically or by current function."

• Many biological traits are puzzling, if they are assumed to have originated to serve the role they play in currently living organisms.
• A famous example involved Alfred Russell Wallace's concern that many human intellectual abilities (music, mathematics, etc.) would have no utility under the conditions under which humans are thought to have evolved.
• Many traits, it must be realized, originated under very different circumstances from those in which they are currently expressed.
• Traits which originated to serve one function and which were then co-opted for other uses are sometimes called preadaptations.  
  • Because the word "preadaptation" has implications of prior design, Gould and Vrba introduced the word exaptation as a label for traits which happened to have some utility distinct from that which led to their origin.
  • Preadaptation/exaptation often provides a basis for solving problems such as "What use is half a wing?"
  • Darwin understood that the intellectual abilities of modern civilized humans could have arisen as an incidental byproduct (i.e., as an exaptation) of selection for basic intelligence that is used in hunting, social interaction, etc.
• The problem posed by "preadaptations" has led to awareness that "adaptation" can be ambiguous, with two distinct meanings.
  • One meaning refers simply to utility.  A trait is an adaptation if it contributes usefully to fitness.
  • A second, more restricted, meaning refers the action of natural selection in shaping a trait.  A trait is an adaptation if it was shaped by natural selection.
  • Ridley suggests an even further restriction, to the continuance of original function.  A trait is an adaptation if it continues to serve the function for which it originally evolved.  I have never encountered this usage elsewhere; it seems to represent a misunderstanding of Gould and Vrba's recommended terminology.

13.5. "The function of an organ should be distinguished from the effects it may have"

• This section seems to represent Ridley's sense of what the word "adaptation" should mean.  "A character is an adaptation in so far as natural selection is maintaining its form in modern populations and ... originally brought it into existence.  The word "function" in the section heading refers to adaptation in this sense.  "Beneficial consequences that are independent of natural selection are not adaptations."  The word "effect" refers to such incidental consequences.

13.6. "Adaptations in nature are not perfect."

• Adaptation is necessarily limited by the variants which actually occur and by the population-genetic processes by which gene frequencies change.
• This is a very important point, often forgotten by biologists (including Ridley) in other discussions of adaptation.
• Adaptation is often discussed as if any useful trait will be found by natural selection.
  • This error typically takes the form, "Trait X evolved because it would be useful."
  • In fact, adaptations evolve because variation appears which happens to be favored by selection.
• Natural selection is not some irresistable force.  Natural selection is simply the consequence of heritable variation in traits that affect fitness.
• Factors which limit or prevent perfect adaptation are often referred as constraints.
• Each of the following sections 13.7 - 13.11 describes one category of constraint, one basic reason why adaptation may not be perfect.

• 13.7. "Adaptations may be imperfect because of time lags."
  • The fact that natural selection operates slowly might be called "temporal constraint".
  • Time lags create "evolutionary anachronisms", in which conditions of selection have changed suddenly, leaving obsolete adaptations until natural selection can catch up.

• 13.8. "Genetic constraint may cause imperfect adaptation."
  • As effective as it clearly is, natural selection is not impotent.  Selection can work only with available variation.
  • If appropriate mutations do not arise, selection cannot operate.
  • In Ridley's words, "Natural populations can be imperfectly adapted because a superior mutation has not arisen."
  • As used here, genetic constraint refers to the failure of a possible mutation to occur.

• 13.9. "Developmental constraints may cause adaptive imperfection."
  • The idea here is not much different in principle from that of "genetic constraint".  Natural selection can work only with available material.
  • Because of the nature of a complex developmental program, some forms of variation may not be possible.
  • [Since genomic reorganization may alter developmental possibilities (i.e., since genetics and development are so closely related), genetic and developmental constraints are often treated together as "ontogenetic constraints".]

• 13.10. "Historical constraints may cause adaptive imperfection."
  • Regardless of what is genetically and developmentally possible, gradual, step-by-step adaptation can proceed only along pathways where each step is individually advantageous.
  • So possibilities for future adaptation are always limited by structures put in place by prior history of adaptation.

• 13.11. "An organism's design may be a trade-off between different adaptive needs."
  • As in any engineering project, the adaptive design of an organism necessarily involves compromises among conflicting needs.
  • Examples are abundantly easy to imagine.  For example, both large size and small size have intrinsic advantages.  No organism can maximize both large size and small size; actual size will necessarily be a compromise.

13.12. "Conclusion: Constraints on adaptation."

• In this section, Ridley surveys the consequences of recognizing evolutionary constraints.  He concludes:
  • First, differences in adaptive function must be distinguished from differences in adaptive history and from adaptively insignificant differences.
  • Consequently, second, the methods for studying adaptation must be informed by recognition of constraints.
• Biologists differ in the relative importance they ascribe to adaptation vs. its constraints.  Those who (over)emphasize adaptation while minimizing constraint are called (often pejoratively) adaptationists.

13.13. "How can we recognize adaptation?"

• History suggests that this is a difficult problem.  Ridley offers several standard answers, but also acknowledges pitfalls.
  • "Adaptations can be recognized as characters that appear too well fitted to their environments for the fit to have arisen by chance."  This criterion is not failsafe, as witnessed by the dispute between Wallace and Darwin over human intellect.  Exaptation can introduce confusion, and "chance" is poorly understood.
  • "If a character is an adaptation, then natural selection will work against mutant alternatives."  This should be true, but does not usually lend itself to practical test.  Not does it help us understand how a character works as an adaptation.  The puzzle of the function of sex is illustrative.
• Ridley's concluding sentence captures the difficulty, even if it is not especially helpful: "We have a clear theoretical concept of what an adaptation is; but that concept implies that adaptation cannot have a universal, foolproof, practical definition." If a definition for adaptation is universal, it will not be not practical; if it is practical, it will not be universal; and nothing is foolproof, not even when the fools are trained as scientists and philosophers.

510 index page

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