Chapter 3.  Darwinian Natural Selection

  • Darwin's accomplishment: establish truth of evolution, establish a mechanism for evolution (natural selection).  Natural selection (which results in change in species) is an inevitable outcome of four attributes of nature ("Darwin's postulates", according to the text):
    • Variation among individuals within species
    • Some of the variation is heritable
    • Overproduction of young is the rule
    • Non-random survival and reproduction:  the individuals with the most favorable traits survive and reproduce more than do individuals with less favorable traits.  These traits are passed on the next generation, thus resulting in evolutionary change.
  • Some terms:
    • Fitness = ability of an individual to survive and reproduce in its environment (contrast the everyday usage of this term)
    • Adaptation = characteristic of an organism that increases its fitness related to other members of its population.
  • The evolution of flower color in an experimental snapdragon population (Fig. 3.3)
    • Flower color varies, genetically determined (single locus, two alleles).
    • Non-random reproduction:
      • Male function - white flowers receive more visits from pollinator.
      • Female function - white flowers set more seed.
    • Evolution - composition of population (allele frequencies) changes over time.
  • The evolution of beak shape in Galapagos finches
    • 13 species of the genus Geospiza, all descended from one species which colonized the archipelago from the mainland.  There is considerable specialization among the species with respect to food type, and this is reflected in beak shape and size (Fig. 3.1).
    • Geospiza fortis - a seed-eating species chosen by Peter and Rosemary Grant for intensive study.  Each of Darwin's postulates are well illustrated by this population.
      • The populations are variable (Fig. 3.6).  This variation could have a genetic cause, and environmental cause, or a combination of the two.
      • Some of the variation is heritable (Fig. 3.7).  Heritability is the slope of the regression line when offspring value of a trait is plotted against midparent value of the trait.  Note that this is a special meaning of the term "heritability" and that there are many problems associated with estimating heritability (Box 3.1).  Note that (unlike the snapdragon population example) little is known about explicit genetics of traits being selected.
      • There is overproduction of young: 89% of young die before breeding in normal years; this mortality is increased during drought years when seeds are scarce.
      • Survival and reproduction are non-random.  Birds with large beaks survived drought better than birds with small beaks.   Explanation: during drought years, plant species producing small seeds are less frequent than plant species producing large seeds, and birds with large beaks are better able to exploit large seeds.  See Fig. 3.8.
      • This process resulted in evolutionary change - an increase in mean beak size (Fig. 3.9, 10)
  • Some important things to remember about natural selection
    • "Natural selection" is a metaphor.  There is no conscious agent of selection.  If the four postulates are true, than natural selection will inevitably occur and species will change.
    • Natural selection acts on individuals, but it affects populations over time.
    • The make up of a population today is a consequence of natural selection acting in past generations, so natural selection always lags behind environmental change and cannot anticipate the future.
    • Natural selection creates new things.
      • Range of variation in original population can be transcended (examples: domesticated dogs, oil content in maize seeds [Fig. 3.12]).
      • New things can be created by modifying old things (examples: Panda's "thumb", climbing structures in vines).
    • Natural selection cannot produce perfect organisms - there are always constraints and compromises.
    • Natural selection (and evolution) has no goals.  There may be trends in the evolution of life, but these are discovered ex post facto.  For example, the evolution of our species was not inevitable.
    • Evolution by natural selection may result in increased complexity (e.g. eukaryotes evolving from prokaryotes) or reduced complexity (e.g. parasites evolving from free-living ancestors).
    • Is evolution “random”?  This depends on what you mean by “random.”
    • Natural selection does not work "for the good of the species".  Altruism and cooperation can always be explained as resulting from individual or kin selection.
  • Some early problems with the theory of evolution by natural selection.  Note: these were responsible critiques raised by scientists, not creationists.
    • What is the source of variation?  Why wouldn't natural selection stop when variation is exhausted?
    • How is variation passed from generation to generation?
      • Fleeming Jenkin and blending inheritance: if inheritance is blending, than novel advantageous traits will be swamped and cannot be favored by natural selection.
      • Darwin's solution: combination of inheritance of acquired characteristics and "pan-genesis" (good example of  incorrect, abandoned theories).
    • The earth is too young (15-20 million years according to Kelvin's calculations) for evolution to have created nature as we see it.
  • The Modern Synthesis - integration of genetics and natural selection, ca. 1930-50.  This is the current, standard theory of evolution which underpins and informs all current research in  biology.
    • Microevolution: small, gradual changes in species result from natural selection.
    • Macroevolution = microevolution X time.
    • Darwin's postulates have been recast into modern language (mutations, allele frequency, etc.).
  • What do the Creationists say and why do they say it? There are about a half-dozen standard creationist arguments against evolution:
    • The "argument from design": complexity and perfection in nature implies an intelligent designer.
    • Fitness is a circular concept, therefore natural selection is untreatable and cannot be considered a scientific theory.
    • The earth is young (radiometric dating is flawed).
    • Evolution violates the Second Law of Thermodynamics.
    • You can't see evolution happen, so evolution (like religion) must be based on faith.
    • Note that these objections have been answered many times, but you will see them, endlessly re-born, in creationist literature, e.g. websites.