Tuesday September 11 – lecture one & Thursday September 13 – lecture two
Chapter 3 (pg. 73-96 and 97-105)
Chapter three – Darwinian Natural Selection
3.1 Artificial Selection: Domestic Animals and Plants
Mechanisms of evolution under domestication helped Darwin understand
mechanism of evolution in nature
Darwin studied the mechanisms that plant and animal breeders used to select for
Darwin himself was a pigeon breeder
Domestic tomato: Solanum lycopersicum
Can be found around the world (cultivation & weedy escapee)
Domesticated tomatoes can still interbreed with some species of wild tomatoes
It was first cultivated by Native Americans and the early explores introduced the
tomatoes to Europe (where it began to spread globally)
The wild tomato is small (less than a cm in length & weighs only a few grams)
The domesticated tomato can now grow large (15cm in length and 1 kg in weight)
Molecular Explanation: on chromosome 2 tomatoes carry a gene known as fw2.2,
which encodes for a protein, which represses cell division. The size of the fruit
depends on the amount of proteins being made. Increased number in protein causes
smaller size. Cultivated fruits have fewer proteins, which results in a larger size.
Anne Frary & Steven Tanksley demonstrated this phenomenon through
experimentation (30% more growth)
By selecting for only the large plants (cultivate) they were eliminating the small-
Other examples of domesticated plants include: broccoli, Brussels sprouts,
cauliflower, kale, & kohlrabi they were derived from Brassica oleracea
3.2 Evolution by Natural Selection
Similar processes to artificial selection occurs within nature
Darwin wrote his book – On the Origin of Species by Means of Natural Selection
Theory of Evolution by natural selection holds that descent with modification is the
logical outcome of four postulates
1. Individuals within populations are variable
2. The variations among individuals are, at least in part, passed from parents to
3. In every generation, some individuals are more successful at surviving and
reproducing than others
4. 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
1 Darwinian Evolution: Gradual change in populations over time
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 reproducing, then some traits will be passed on more frequently than others.
This results in the characteristics of the population to change with each succeeding
Darwinian fitness – the ability of an individual to survive and reproduce in its
Fitness refers to how well an individual survives and how many offspring it
produces compared to other individuals of its species.
Adaptation – refers to a trait or characteristic of an organism, like a modified form of
reverse transcriptase, that increases its fitness relative to individuals
Both Darwin and Alfred Wallace discovered the mechanism for evolution
independently around the same time. Darwin is given more credit because he
thought of it first and had massive documentation in his book. Darwin and Wallace’s
four postulates are testable!
3.3 The evolution of Flower Color in an Experimental Snapdragon Population
Experimenters: Kristina Niovi Jones & Jennifer Reithel
Question: whether natural selection by bumblebees could influence the evolution of
a floral trait controlled by alleles of a single gene
Experiment: population of 48 snapdragons (postulates one and two were true) then
they watched to see if postulates three and four would come true.
Postulate one: there is variation among individuals
¾ - pure white with 2 spots of yellow on the lower lip
¼ - yellow all over
Varied in colour
Postulate two: some of the variation is heritable
The gene has 2 alleles (S and s). Genotype SS or Ss will have white flowers while ss
will have yellow flowers. Out of the 48 snapdragons 12 were SS, 24 were Ss, and 12
were ss. The colour traits are heritable.
Testing Postulate 3: Do individuals Vary in their Success at Surviving or
Experiment occurred in a meadow in Colorado, Snapdragons were kept in pots to
prevent dying. The experimenters did not intervene for the reproductive aspects.
The free-living bees pollinated the plants. The experimenters tracked the number of
times the bees visited the plants. To measure the plants’ success at reproducing they
counted the number of seeds. Plants showed variation in reproductive success, both
as pollen donors and seed mothers.
2 Testing Postulate 4: is reproduction nonrandom?
White flowers attracted twice as many bees
The white plants produced slightly more seeds per fruit
Therefore reproductive success was not random. White plants had a higher
reproductive success than yellow plants.
Testing Darwin’s Prediction: did the population Evolve?
The bees selected particular individuals and granted them high reproductive
success. Since the white flowers were favoured, the next generation of snapdragon
plants is more likely to be white. And they were more likely by 2% for each
3.4 The Evolution of Beak Shape in Galapagos Finches
Observers: Peter Grant & Rosemary Grant
What: studies finches in the Galapagos Archipelago
Darwin’s finches derived from a small flock of
grassquits from Central and South America
(2.3 million years ago)
Fourteen different species
All of them are closely related to one another
All of the finches range from about 4 – 6 inches.
They are either black or brown
The variation exists in beak size and shape
The beaks are important for feeding
The different beak sizes and shapes allows them to eat different foods
Warbler – insects, spiders, and nectar
Woodpecker & mangrove – twigs or cactus spine are used to pry insect larvae or
termites from dead wood
Ground finches – ticks, iguanas, & tortoises
Vegetarian finches – leaves & fruits
The data was gathered on medium ground finch on Isla Daphne Major
The climate is seasonal
Vegetation consists of dry forest and scrubs as well as cactus
The ground finch is ideal to study because their population is small and lived up to
about sixteen years. The generation time is 4.5 years
They are seed eating birds
Testing Postulate 1: Is the Finch Population Variable?
Researchers captured and marked the finches (allowed identification)
The scientists weighed, and measured the tail length, beak width, beak depth, and
beak length for the captured birds. The variables varied.
3 Testing Postulate 2: Is some of the Variation among individuals Heritable?
Beak depth varied due to the difference in environments they experienced or if their
genotype differed. Variations in food fed to the birds at infancy can alter or effect
beak depth (environmental). Injuries can also affect beak depth (environmental)
Peter Boag estimated the heritability of beak depth – strong correspondence
Large proportion of the observed variation in beak depth is genetically based and
can be passed on to offspring. Other factors such as maternal effects, environment,
conspecific nest parasitism, and misidentified paternity can underplay the