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Biology 1201A Lecture Notes - Genetic Drift, Allele Frequency, Macroevolution

Course Code
BIOL 1201A
Jennifer Waugh

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An untested idea or opinion; speculation
- Explanation of a set of natural phenomena based on proven/ testable hypothesis/
observations (restricted correct scientific definition)
Early Evolutionary Thought
Classification + Characterizing
Aristotle Scala Natura
- Hierarchy of things compared to the Gods (ladder).
Carl Von Linne Linnean System
- Wanted to name everything; Linnean nomenclature
Le Comte de Buffon Vestigal Traits
- Discovered some organisms had vestigial traits
- Maybe useful to ancestors but no longer relevant
Jean Baptiste Lamarck Evolved species to best-fit environment.
- No longer accepted theory -> tested but proven wrong
Georges Cuvier “Catastrophism”/ fixity of species
- Catastrophism: massive events changing everything but other than that everything
remains constant
Charles Lyell “Uniformitarianism” – naturally changing
- Earth has been changing gradually over time due to natural agents
Darwinian Evolution
Charles Darwin Evolution by natural selection
- Species change gradually b/c of interactions between individuals’ traits and their
Natural Selection
Differential survival and reproduction of individuals in a population due to current
environmental influences:
Evolution by natural selection is observable:
Antibiotic resistance in bacteria
Pesticide resistance in insects
Heavy metal tolerance in plants
Beak size in Darwin’s finches
The degree to which an individual contributes offspring (genes) to future generations
- In this definition, genes means alleles

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- Parent A has a W of 2, parent B has a W of 4
- Relative fitness (w) is the fitness of the individual relative to others in the (highest
fitness of population gets a w of 1)
- Then all other parents get rankings compared to that highest w of 1
- In the example parent B has a w of 1, and parent A has half the fitness of that (0.5)
Another example: Squirrels: (BB) = 4, (Bb) = 6, (bb) = 2
- (bb) have a relative fitness of w=0.33
- 2 is one third of 6 (the max absolute fitness)
Traits that increase the probability that an individual with that trait will survive or
reproduce in a particular environment
- An adaptation is a trait that is associated with fitness
- They are things that help and make individuals reproduce and survive in their
environment such as thorns on a rose
- Could also be behavioral traits, such as schools of fish (they have a lower chance of
being eaten when there is so many of them)
- Selection pressures are things that influence fitness (crabs were a selection
pressure on the shell thickness)
Constraints on adaptation
Available variation for selection to act upon
Changing environments over time
Conflict between selection pressures (trade-offs in fitness)
Natural Selection link to genes
‘Beach mice’ are light colored; ‘mainland mice’ are darker
Blending with native soil color reduces predation
Light color a result of a single amino acid change in Mc1r
Frequency of ‘light’ allele correlates with degree of lightness in populations
Small-scale changes in genetic make-up of a population
- This is the definition of evolution (micro)
- Population level process that causes changes in mean (most frequent) phenotype
- In the example, over time if the predators eat all the blue and green flowers, or
some other factor occurs that leaves mostly yellow flowers, our end population will
look much different than that of our beginning one
Hardy-Weinberg Principle
Background on pig population & color inheritance:
- In the Mendelian pigs experiment we tried to get pure bred pigs (pure bred brown,
and pure bred black)

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- Artificial selection = selective breeding of animals or plants to ensure that
desirable traits appear in successive generations
- From the experiment we found out that black (B) is a dominant trait, and brown
(W) is recessive
- We also found out that phenotype ratios of offspring for a particular cross are
predictable (with clean ratios 1:0, 1:1, 3:1)
Calculating proportions (frequencies)
Proportion = # of items of interest
# of items in total
A large population exists in which homozygotes and heterozygotes have the same
fitness: allele frequencies are: f(B) = 0.3, f(W) = 0.7. What happens to allele frequencies
- The frequencies hovered around what they were to begin with; there weren’t any
big changes
- Nothing is changing because none of the pigs are experiencing evolution; since the
fitness is the same they all have the same rate of reproduction
- The allele component of the next generation is the same as the preceding one
because alleles are just passed down to offspring
- Knowing whether an allele is dominant or recessive does not tell you if it is going
to be common or rare (and the same vice versa)
- Phenotype ratios for populations aren't "nice" (clean ratios)
- Just because an allele becomes common in a population, it doesnt mean it
becomes dominant
Punnett Squares
Allele frequencies give probabilities of gamete ‘genotypes’ and expected offspring
genotype frequencies based on probability
- p2 = probability of getting an A1 egg and an A1 sperm
- 2(p x q) = getting heterozygotes
- q2 = probability of getting a homozygous recessive
Genotype frequencies of offspring:
P2=p2 A1A1
2(p x q) = 2pq A1A2
q2=q2 A2A2
Investigating the HW Principle
Are genotype frequencies predictable?
- Initial allele frequencies:
- f(B) = 0.4, let p = f(B), f(W) = 0.6, let q = f(W)
- Predicted genotype frequencies:
- f(BB) = p2 = 0.42 = 0.16
- f(BW) = 2pq = 2(0.4 x 0.6) = 0.48
- f(WW) = q2 = 0.62 = 0.36
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