Ridley, Chapter 4
Natural Selection and Variation

510 index page

Brief Outline

• Introductory Note.

  Section-by-section comments on Ridley, Chapter 4.

• 4.1  Struggle for existence.
• 4.2  Necessary conditions for selection.
  "Survival of the fittest."
• 4.3  Natural selection as explanation
  Other mechanisms.
• 4.4  Types of natural selection
• 4.5  Natural variation.
• 4.6  Variation in fitness.
• 4.7  Mutation and Recombination.
• 4.8  Randomness of variation.
  Mutations as "errors".

INTRODUCTORY NOTE:  Be prepared to come back to this chapter again and again, to review the essence of selection.  Why so much emphasis on enumerated lists of conditions necessary for selection?  

• First, in order to create predictive models about how evolution works, each and every one of these factors must be included in the model.  That's what the next chapter will be about.
• Second, in order to determine whether natural selection has shaped any particular trait, it must be demonstrated that all of the necessary conditions do indeed apply.  One such example (e.g., peppered moth) will be considered in the next chapter.  Many more will be considered later.
• The success of any evolutionary explanation depends on how well the explanation can document in detail how each element in the process contributes to the outcome.

SECTION-BY-SECTION COMMENTS ON RIDLEY, Chapter 4.

4.1  "In nature, there is a struggle for existence."
4.2  "Natural selection operates if some conditions are met."

Together these two statements constitute one of the most fundamental principles in biology.

The first of these two statements is sometimes called the Malthusian principle, since it was Thomas Malthus whose "Essay on the Principles of Population as it Affects the Future Improvement of Society" stimulated Darwin's thoughts toward natural selection.

Ridley lists four necessary conditions for natural selection to operate:

1. Reproduction,
2. Heredity,
3. Variation in Characters, and
4. Variation in Fitness according to character state.
   

1. Excess reproduction (i.e., an intrinsic reproductive rate greater than one offspring per parent, and
2. Finite resources (or, equivalently, limited carrying capacity of the environment).

There is no single best way to state the necessary conditions.

Compare Ridley's list with Darwin's summary in the concluding paragraph of the Origin  (the complete text of this chapter is included on the CD that accompanies Ridley's textbook):

"These laws, taken in the largest sense, being

1. Growth with Reproduction;
2. Inheritance, which is almost implied by reproduction;
3. Variability from the indirect and direct action of the external conditions of life, and from use and disuse;
4. a Ratio of Increase so high as to lead to a Struggle for Life,

Note that Darwin also lists four conditions, as does Ridley, but Darwin's list looks rather different.  Although the two lists are not packaged quite the same way, their essential content is identical.  Darwin assumes as obvious the dependence of fitness on character state, and so includes it as an unspoken correlate of variability.  And he leaves finite resources as an unspoken but self-evident fact of life in the real world.

But any way it's worded, Natural selection is what happens whenever these basic conditions obtain.  Note that "natural selection" is not a "force".  It is a label given to the necessary outcome of life under these conditions.

A simpler version of these lists of conditions is the declaration that:

• Natural selection happens whenever there is heritable variation in traits that affect fitness.

Here is my own list of "essential conditions for natural selection":

1. A process of reproduction.
2. Potential for population growth.  (A rate of reproduction which, if unchecked, would yield indefinite population growth.)
3. Finite resources which constrain indefinite population growth.
4. Heredity.
5. Mutation.  A source for variation in heritable traits.
6. A causal relation between heritable trait variation and realized reproductive potential.)

The first three of these yield a struggle for existence in which some reproductive potential is not achieved.  The second three establish that this struggle will affect fitness, which is a quantitiative measure of realized reproductive potential.

"Survival of the fittest."
A NOTE OF CAUTION may be appropriate here.  Natural selection is sometimes given a caricature definition as survival of the fittest, with "the fittest" in  turn being defined as simply those which survive.  The result is rather obviously circular reasoning.  If natural selection is defined too simply in terms of fitness, and if fitness is inappropriately defined as the product of natural selection, then this circularity is hard to avoid.

It is thus important to remember that fitness can better be defined as good engineering design -- traits which confer efficiency at surviving, making a living, and reproducing -- without any reference to natural selection as a set of causal processes.  Fitness can thus be related to the original non-evolutionary meaning of "adaptation", whose root is "apt", or fit.  That well-established, meaning, predating Darwin, refers only to immediate contribution to ecological success, with no necessary implication of any evolutionary explanation.

4.3  "Natural selection explains both evolution and adaptation."

This is the great success of the Darwinian explanation.  It explains BOTH descent with modification AND adaptation.

Note that none of above lists made any mention of adaptation.  Like natural selection, adaptation is a label for a complex phenomenon.  Indeed, the term "adaptation" is sometimes used AS IF it were a SYNONYM for "natural selection", referring to the evolutionary process by which fitness increases.  And when "adaptation" refers to a particular structure or behavior or overall life strategy, it is generally presumed that these traits have resulted from natural selection.  In either case, the meaning of "adaptation" is very close to the meaning of "natural selection".  But both of those usages take for granted the workings of Darwinian evolution.  The actual definition of adaptation -- fitness for a way of life - does NOT depend on natural selection.  Rather, the phenomenon of adaptation is explained by natural selection.

Another NOTE OF CAUTION:  There are additional processes at work in evolution besides natural selection, most notably mutation and the random, statistical process of genetic drift.  Mutation (or at least variability) is listed as one of the necessary conditions for natural selection, because without mutation, variability would eventually be exhausted and then natural selection must cease.

Mutation and genetic drift can yield evolutionary change independently of and parallel to selection, either by maintaining deleterious variants in spite of selection, or by producing variation that is invisible to selection.  Together, these two processes can explain some aspects of descent with modification, arguably just as well as natural selection, including a lot of modification that matters to biologists.  But mutation and drift cannot explain adaptation without the additional agency of natural selection.

4.4  "Natural selection can be directional, stabilizing, or disruptive."

This is a very basic concept for later understanding.  Make sure you understand the distinction among these three, and can interpret the graphical presentations (Ridley, Figures 4.2, 4.3, 4.4).

• Stabilizing selection:  Maintains the value of a character state in a population, by favoring the current value over variation which would increase or decrease that value.  
• Directional selection:  Shifts the predominant character state in a population, by favoring variation in one direction while acting against variation in the opposite direction.  
• Disruptive selection:  Acts against a central trait value by favoring variation that increases or decreases that value.  
• Absence of selection:  No selective pressure on trait value.  

Note that these are all easily represented graphically and are most clearly applicable to microevolution of quantitative traits.

Although a core presumption of evolutionary theory is that macroevolution occurs by the same mechanisms as microevolution (i.e., uniformitarianism), extrapolating from directional, stabilizing, or disruptive selection to the creation of evolutionary novelty is NOT a trivial problem.  One might therefore consider yet another form of selection:

• Indirect selection.  Selection upon genotypic characteristics which do not affect the phenotype but which do affect the quantity or quality of mutations which appear in future generations.

4.5  "Variation in natural populations is widespread."

Remember, in Ridley's list of necessary conditions, natural selection depends on reproduction, heredity, character variation, and fitness variation.  We looked at reproduction and heredity in Chapter 2.  Here we take note of the extent of variation at many levels of biological organization.

Note that measures of variation are relatively easy compared to demonstrations that the variation is hereditary or that the variation affects reproductive success.

4.6  "Organisms in a population vary in reproductive success."

"The struggle for existence and phenotypic variation are both universal conditions in nature."  But, as Ridley points out, it is logically possible for variation to exist but not affect reproduction, or for variation in reproductive success to exist but not be hereditary.  Thus natural selection is plausible, but not inevitable, unless the additional conditions of heredity and affect on fitness are also met.

Again, note that measuring variation in reproductive success is relatively easy compared to uncovering what trait variation is responsible for that differential success or to determining whether the variation is indeed hereditary.  In other words, demostrating the presence of variation in reproductive success is necessary but not sufficient for proving that natural selection may be operating; demonstrating that natural selection is, in fact, operating can be quite onerous.

4.7  "New variation is generated by mutation and recombination."
4.8  "The variation created by recombination and mutation is random with respect to the direction of adaptation."

Randomness is an important concept, but the word lacks a clear definition.  The many possible types of mutation are certainly NOT uniformly distributed, neither in rate nor in genomic location nor in impact on fitness.  Certain types of mutation are much more frequent than others, certain loci are much more prone to mutation than others, and certain phenotypic traits more likely to be affected than others.  So mutation is not random in any ordinary mathematical sense.

For this reason, many researchers prefer the term undirected to describe this property of mutation.  That is, particular mutations are NOT directed toward particular adaptive goals.  This is a most important concept for our modern understanding of evolution.  Whenever evolution follows a particular adaptive direction, that direction comes from selection alone.  It is NOT determined by the process of mutation.

This concept distinguishes modern evolutionary theory from certain alternatives, such as Lamarck's (and Darwin's) ideas of inheritance of acquired characters, with variation itself being directed by the circumstances of each individual life, or orthogenesis, with some unspecified guiding force determining the long-term trends of evolution (such as increasing size or complexity).  However, no evidence has been found to suggest that any such alternative could occur.

Randomness of variation is NOT a NECESSARY condition for natural selection.
NEVERTHELESS...

• Darwinism does require that adaptation be directed by selection, not by variation.
• IF variation is limited to certain dimensions, then selection in other dimensions will not be possible.
• One of the outstanding questions regarding evolution concerns the extent to which the directions that evolution has taken have been constrained by the availability of variation.
• The impressive success of life in adapting to a multitude of niches in a wide range of habitats suggests that any such constraint must still permit many degrees of freedom for adaptation.  

Mutations as "accidents" or "errors".
Mutations are often characterized as "mistakes", "errors", "accidents", or "failures" of accurate DNA replication.  While that is almost certainly the case for certain types of mutation, notably those caused by external agents such as x-rays and mutagenic chemicals, it is just as certainly NOT the case for other important classes of mutation which are caused and regulated by internal enzyme systems.  There is growing evidence, particularly in bacteria, for processes variously called "genetic intelligence", "natural genetic engineering", and "adaptive mutation" which facilitate adaptive evolutionary change and thereby achieve a selective advantage termed "evolvability" or "evolutionary amenability".

This topic, the evolution of evolvability through regulated mutational mechanisms, has not yet reached mainstream evolutionary biology.  For further information, see mutational landscape or recent publications by Prof. King.

Comments and questions:   dgking@siu.edu
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