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Lecture 6

BIO220H1 Lecture Notes - Lecture 6: Mathematical Model, Exponential Growth, Coefficient Of Determination


Department
Biology
Course Code
BIO220H1
Professor
John Stinchcombe
Lecture
6

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Lecture 6: Ecology and Evolution of Harvested Populations II
Today:
oWhat does harvesting do to evolution of targeted species?
How is harvesting determined?
Often apply size threshold
oEX: Put net in ocean to catch fish
oImagine net has 4-inch diameter holes
All little fish can swim in and out of net no problem, escapes
Any fish that are larger get caught and can’t swim through and eaten
oEX: Trophy hunting:
Size of antler racks/size of horns of bighorn sheep
Incentive to get the individuals with biggest antlers
May even be regulation of hunting that says no shooting bucks with fewer
than this many points on antlers
Example of harvesting being determined by a continuous trait
Continuous Traits vs Discrete Traits
Phenotypes that vary continuously
oQuantitative traits
oEX: Size, mass, length, time to maturity, behaviour, BMI
oDetermined by many genes -most that have small effect individually on
phenotype
oBut they cumulatively produce variation we see b/c there are a lot of them
oEX: Variance in female hand size in EEB department
Discrete/Mendelian traits
oAttached or detached ear lobe
oTongue rolling
oBlood groups
Human height
Examples of Quantitative Traits
Human height photo
oMen are in black, women are in black
oContinuous distribution of height
oSee sexual dimorphism in human height
Men usually taller than women
oHeight influenced by nutritional environment (access to food when you are
young)
oHeight also influenced by genes
oTraits like this that determine harvested organisms that die
EX: Size of fish caught, or size of antlers on hunted deer
Swimming speed of Olympic swimmers
PC general cognitive:
oAbility to do well on standardized test

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oShow continuous variation
Size of fish getting through net
Size of antlers on deer
What is the architecture of quantitative traits?
Quantitative Traits
oLots of genes
oEach gene has small individual effect on phenotype
oInfluenced by environment (nutritional status, body mass)
oSimple rule of thumb:
P = G + E + G X E
Phenotypes are the product of genetics + environment and the interaction
between the two
Issue with Quantitative Traits:
Unlike discrete Mendelian traits:
oCan’t infer genotypes from phenotypes
There are tonnes of genes making phenotype
i.e. Can’t tell you the multi-locus genotype just from seeing someone is
6”7’
But if you have detached ear lobes, or AB blood group, can tell you
genotype from phenotype because those are Mendelian traits
With Mendelian traits can understand genotypes from phenotypes
Doesn’t work with quantitative traits
oCan understand overall distribution of phenotypes but don’t anything about
alleles
oConsequence: Need to work with statistical descriptions
A brief aside: Medical relevance of quantitative traits
Most medical things are genetic traits
oHeart disease risk, cancer risk, BMI
Traits that are classic quantitative traits
oLots of genes, strong environmental influences
Not nature or nurture, it’s nature and nurture
What type of selection is this?
Graph of frequency in population against size
oSome individuals above threshold die
oi.e. get caught in net and harvested out while the ones before if live
Special form of directional selection called truncation selection
oTake distribution of size and truncating it by eliminating all these individuals
oMakes selection favour smaller individuals since all the bigger individuals die
= Statistical distribution
Truncation Selection
Strong form of directional selection
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oIndividuals w/ phenotype above threshold die
oBelow threshold live
How to measure intensity of selection?
oBecause can’t infer genotype from phenotype, need to use a proxy to determine
how strong selection is
oUse selection differential
o= Difference in mean of population after selection, compared to before selection
oS = z* - z
oz* = mean of population after selection
How will traits involved in this threshold evolve?
Slide 9
Frequency verse size
Imagine the mean was 10 before
Then kill off all the big individuals
Mean of population size decreases
oWe are calculating an average of the smaller individuals
If there is any genetic variation for size, then smaller size will evolve
oImposing directional selection strongly for smaller size
Selection for small size
Calculate selection differential
oMean after minus mean before = -2
oNegative sign tells us that selection favours smaller individuals
Clicker Question
A major ice storm leads to mortality in grindylows. Before the ice storm mean body size was
8 g. After ice storm, the mean body size was 10 g. What is the correct pairing of the
selection differential, and the direction of body size evolution we would predict? (small have
died)
a. S = -2, we predict decreased size to evolve
b. S = 2, we predict decreased size to evolve
c. S = 0.8 we predict decreased size to evolve
d. S = 1.25, we predict increased size to evolve
e. S =2 we predict increased size to evolve
ANS: e)
Tracking the evolution of quantitative traits
For really simple case:
oCould just measure average phenotype of population over time and track
evolution and natural selection
oi.e. use equation s = z* - z
But down side: Changes in environment over time
oCauses changes in phenotype
Recall equation: P = G + E + GXE
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