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 offspring
• 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 naturally selected.
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 fit:
• 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 its species.
• 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 flowers.
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 setting.
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
selected individuals changed in any
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