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Chapter 3

BIO342H5 Chapter Notes - Chapter 3: Macroevolution, Microevolution, Modern Synthesis (20Th Century)

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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).
oIf 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.
oRead 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.
oIf these four postulates are true then composition of the population
changes from one generation to the next (see Figure 3.4 on page 76).
oIf 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.
oDarwin 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
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
oThe 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
oThe 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?
oFree living bumblebees pollinate the plants and tracked the number of
times bees visited each flower.
oThe researchers counted the seeds produced from each fruit.
oThe plants showed considerable variation in reproductive success, both
as pollen donors and as seed mothers.
Postulate 4: Is Reproduction Nonrandom?
oThe scientists expected that one colour would attract more bees than
the other
oThe yellow spots on otherwise white snapdragons are thought to serve
as nectar guides, helping bees find the reward the flowers offers
oAll-yellow flowers lack these guides and may be less attractive to the
oIt 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
oOne 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
oWhen 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 way.
oThey simply lived through the selection event while others died or
reproduce more than the competing virions or birds
oWhat changed after the selection process was the characteristics of the
populations of virions and finches, not the individuals themselves.
oA higher frequency of HIV virions in the population were able to
replicate in the presence of AZT and a higher proportion of finches had
deep beaks.
Natural Selection Acts on Phenotypes but Evolution Consists of Changes in
Allele Frequencies
oSelection would have altered the frequencies of the phenotypes in the
population, but in the next generation the phenotype distribution might
have gone back to what it was before selection occurred.
oOnly when the survivors of selection pass their successful phenotypes to
their offspring, via genotypes that help determine phenotypes, does
natural selection cause populations to change from one generation to
the next.
Natural Selection Is Not Forward Looking
oNatural selection adapts populations to conditions that prevailed in the
past, not conditions that might occur in the future
oAdapting for the future is impossible and is often a misconception
New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits
oThe evolution of new traits is possible for two reasons:
During reproduction in sexual species, meiosis and fertilization
recombine existing alleles into new genotypes.
Mutation and recombination yield new suites of traits for selection
to sort
Read example on page 91.
oPersistent natural selections can lead to the evolution of entirely new
functions for existing behaviour (e.g. the giant panda’s thumb; read on
page 92).
oA trait that is used in a novel way and is eventually elaborated by
selection into a completely new structure is known as a preadaption.
Preadaption improves an individual’s fitness fortuitously –not
because natural selection is conscious or forward looking.
Natural Selection Does Not Lead to Perfection
oEvolution does not result in organisms that are perfect
oEach population evolves a phenotypes that strikes a compromise
between opposing agents of selection
oNatural selection cannot simultaneously optimize; it leads to adaptation,
not perfection
Natural Selection is Nonrandom, but It Is Not Progressive