Darwinian Natural Selection (Pages 74 – 97)
3.1. Artificial Selection: Domestic Animals and Plants
To increase the frequency of desirable traits in their stocks, plant and animal breeders employ
artificial selection (Example of Darwin and his pigeon’s on Page 74).
o If the desirable traits are passed from parents to offspring, then the next generation,
consisting of the progeny of only the selected mates, will show the desirable traits in a
higher proportion than existed in the previous generation.
o Read example of Tomato on page 74 and 75.
3.2. Evolution by Natural Selection
Darwin realized that a process like artificial selection occurs in nature
His Theory of Evolution by Natural Selection suggests that descent with modification is the
logical outcome of four postulates:
Individuals within populations are variable
The variations among individuals are, at least in part, passed from parents to
In every generation, some individuals are most successful at surviving and
reproducing than others
The survival and reproduction of individuals are not random; instead they are
tied to the variation among individuals. The individuals with the most
favourable variations, those who are better at surviving and reproducing, are
o If these four postulates are true then composition of the population changes from one
generation to the next (see Figure 3.4 on page 76).
o If there are differences among the individuals in a population that can be passed on to
offspring, and if there is differential success among those individuals in surviving and/or
reproducing, then some traits will be passed on more frequently than other.
The characteristics of a population will change slight with each
succeeding generation Darwinian evolution.
o Darwin referred to the individuals who are better at surviving and reproducing, and
whose offspring make up a great percentage of the population in the next generation as
The ability of an individual to survive and reproduce in its environment
An important aspect is its relative nature. Fitness refers to how well an
individual survives and how many offspring it produces compared to others of
Adaptation refers to a trait or characteristic of an organism that increases its
fitness relative to individuals without the trait.
Read on Alfred Russell Wallace (Page 77)
The interesting fact is that the Darwin-Wallace theory’s four postulates and their logical
consequence can be verified independently (testable).
3.3 The Evolution of Flower Color in an Experimental Snapdragon Population
K.N Jones and J. Reithel (2001) did an experiment using 48 snapdragons to see if Darwin’s four
postulates are true. Postulate 1: There is Variation among Individuals
o The snapdragons varied in flower color. ¾ of the plants had flowers that were almost
pure white; the rest had flowers that were yellow all over.
Postulate 2: Some of the Variation is Heritable
o The variation in color among the plants was due to differences in the plant’s genotypes
for a single gene.
The gene has two alleles, S and s. Individuals with either genotype SS or Ss have
white flowers with just two spots of yellow. Individuals with genotype ss are
yellow all over.
Among the 48 plants in the experimental population, 12 were SS, 24 were Ss,
and 12 were ss.
See Figure 3.5a on page 79.
Postulate 3: Do Individuals Vary in Their Success at Surviving or Reproducing?
o Free living bumblebees pollinate the plants and tracked the number of times bees
visited each flower.
o The researchers counted the seeds produced from each fruit.
o The plants showed considerable variation in reproductive success, both as pollen donors
and as seed mothers.
Postulate 4: Is Reproduction Nonrandom?
o The scientists expected that one colour would attract more bees than the other
o The yellow spots on otherwise white snapdragons are thought to serve as nectar guides,
helping bees find the reward the flowers offers
o All-yellow flowers lack these guides and may be less attractive to the bees.
o It was discovered that the white flowers attracted twice as many bees visits as yellow
Testing Darwin’s Prediction: Did the Population Evolve?
Since white plants had higher reproductive success than yellow and since flower color is
determined by genes, the next generation of snapdragons should have had a higher proportion
of white flowers.
The starting population had 75% had white flowers and their offspring 77% had white flowers.
Darwin’s prediction that the population would evolve as a result
3.4 The Evolution of Beak Shape in Galapagos Finches
Read Pages 80 to 89. It contains the same ideas as the previous section (3.3), however, it is with
Darwin’s finches and in a natural setting. The previous experiment was in a control, experimental
3.5 The Nature of Natural Selection
Although the Theory of Evolution by Natural Selection can be stated concisely, tested rigorously in
natural populations, and validated, it can be difficult to understand thoroughly.
o One reason: Darwin’s theory descent with modification is essentially a statistical
process: a change in the trait distributions of populations.
Natural Selection Acts on Individual, but Its Consequences Occur in Populations
o When HIV strains were selected by exposure to AZT, or finch populations were selected
by changes in seed availability; none of the