Ecology and Evolution: Biology 301 Fall 2002 -- Wethey and Woodin
Department of Biological Sciences
University of South Carolina

Natural Selection

• Natural Selection - the preservation of favorable variations and the rejection of injurious variations.

• Direct analogy to Artificial Selection
• Individuals having any advantage, however slight, over others, would have the best chance of procreating their kind

(Modern equivalent view: selection acts at the individual level, not the level of the population or the species).

• Variations neither useful nor injurious would not be affected by natural selection. (Modern term: Neutral Alleles)
• On islands, modifications which favored the individuals of any of the species would tend to be preserved (Modern equivalent: gene flow tends to slow down evolutionary divergence)
• Natural selection should be more powerful than artificial selection because incomparably longer time has been available.
• Nature selects only for the good of the individual, and acts on …the whole machinery of life (Modern view: selection on the individual phenotype)
• Natural selection accumulates changes slowly over whole geologic periods (Modern idea: Gradualism, contrasted with an alternative modern idea of Punctuated Equilibria, in which revolutionary changes occur rapidly, interspersed with periods of little change)
• Examples
• Protective coloration in insects, and birds
• Hairy fruit suffer less damage from beetles than smooth fruit
• Purple plums suffer less from a certain disease than yellow plums
• In nature, such differences would settle which variety would succeed.
• Natural selection cannot … modify the structure of one species, without giving it an advantage, for the good of another species. (Modern view: selection operates at the level of the individual, and does not operate at higher levels of organization – Group Selection is very unlikely)

• Examples of Natural Selection
• Predator – wolf subjected to a reduction in slow prey. Fastest individuals should have greater chance of survival.
• In Catskills:
• light greyhound-like wolves prey upon deer
• stocky wolves prey upon sheep
• Plant-Pollinator interactions
• Flowers visited by nectar feeders – those with more nectar attract more pollinators and produce more seeds
• Flowers visited by pollen feeders – selection should favor greater pollen production
• Coevolution between plants and pollinators – bees and clover
• Intercrossing of Individuals
• Varieties within a species are interfertile – experiments with cabbages 78 out of 233 crosses were “true to their kind”
• Species are not capable of interbreeding (Modern definition of the Biological Species Concept).
• Circumstances Favorable for Selection
• Large amount of inheritable and diversified variability is favorable (Modern view: heritability is important)
• Intercrossing can hardly be counterbalanced by natural selection (Modern equivalent: balance between selection and migration)
• Isolation is an important element in the process of natural selection (Modern equivalent: Allopatric speciation more likely than sympatric)
• Large continental areas favorable because they can be converted to numerous islands by sea level change
• Divergence of character
• Diversification by natural selection yields groups of organisms related by common descent.
• Our system of classification of species by genus, family, and class, is derived from different degrees of ancestry in common among groups.
• Intermediate forms will tend to become extinct, (so we expect the fossil record to be incomplete).