Transcript APBio Feb7 PopGen

Tuesday, February 7th
Genes Within Populations
slash
The Nature of Evolution
What We’ll Learn Today
• How did things get like they are now?
– Evolutionary Change
– Genetic Variation
• Hardy-Weinberg Principle
– The Five Assumptions
– (And how those 5 assumptions are pretty
hypothetical)
Let’s Get Some Definitions Out of the Way:
• Genetic Variation – differences in alleles
of genes found within individuals of a
population
•Let’s pretend our class is a population.
•There is variations among the alleles in our
genotypes (some differences we can see in our
phenotype)
•Thus, there is genetic variation within our
population
Let’s Get Some Definitions Out of the Way:
• Evolution – refers to how an entity (like a species,
a social system, a gas, a planet, etc) changes
through time
– Darwin actually didn’t use the term “Evolution”
– He used the phrase “descent with modification”
Two Evolutionary Ideas
• Jean-Baptiste Lamarck was a prominent
biologist who believed that evolution
occurred by the inheritance of acquired
characteristics.
-Characteristics or changes an individual
acquired over its lifetime were passed onto
its offspring
The Human Equivalent
• Let’s say you were a wood-worker. You are
married and have one child, who, like you, has
5 fingers on each hand.
• One day, tragically, you have an accident at
your work and two of your fingers on your
right hand are cut off.
• Any child you have after this point will inherit
this change you have recently acquired and
will be born with 8 fingers.
The Second Theory: Darwin’s
• Darwin was boasting another idea:
I say dear chap,
Natural
Selection is the
mechanism of
evolution!
Darwin
• Natural Selection - the differential
reproduction of genotypes caused by factors
in the environment
• Wait… What?
• Let’s break it down…
Natural Selection, the Break Down
• “differential reproduction” = some animals are
better at making offspring than others
• “of genotypes” = those new offspring will
inherit the genes of its parents
• “caused by factors in the environment” = the
differential reproduction often depends on an
organism’s ability to live in its environment
(how good is it at hiding from predators?
Reaching or finding food? Protecting itself/ its
young?)
So… What?
• Some organisms are better suited for their
environment, or sudden changes their
environment recently underwent, and they
are more successful at having offspring
• Ability to reproduce successfully is called an
organism’s fitness
• Anyone heard the song “Fit But You Know It?”
Reference: Urban Dictionary
• Fit is defined as:
• The British version of Hot.
– “Dude that girl is so fit!” “Right?”
• Extremely good looking, synonymous with
“hott” (why two “t’s”? IDK)
– “Whoa, that model is fit!”
You should remember this definition:
• Fitness – genetic contribution of an individual
to succeeding generations
• i.e. Passing down your genes
• i.e. Reproducing
• i.e. Having kids…
• You get the idea.
• So that song is really saying that some girl has
a lot of kids…?
…Yeah, I guess so.
Which Giraffe Is More Fit?
Anyway…
• Darwin purposed that Natural Selection is the
mechanism for Evolution
• Some individuals in a population who are
more suited for the environment are more fit,
passing on their advantageous genes to more
offspring than those less-fit losers.
• As a result, the population gradually comes to
include more and more individuals with the
advantageous characteristics
Hardy-Weinberg
(and Castle)
Equilibrium
They asked “Why doesn’t a population, after a long
time, just consist of individuals with the dominant
phenotype?”
Well, they realized the original
proportions of the genotypes will
remain constant as long as….
Five Ideal (and unrealistic) Assumptions
• 1. No mutation takes place. Ever.
• 2. No genes are transferred to or from other
sources (no immigration or emigration takes
place)
• 3. Random mating is occurring.
• 4. The population size is LARGE
• 5. No selection occurs.
So in ideal conditions, let’s check out
the Hardy-Weinberg Equations
• These equations let us calculate the allele
frequencies of the two alleles in a population
• “p” is a variable used to denote the frequency
of one allele (usually the dominant one)
• “q” is the variable used to denote the frequency
of the other allele (usually the recessive one)
• The first equation : p + q = 1
• So if 75% of alleles are dominant, p = 0.75
• That leaves us with 25% recessive, q = 0.25
• Thus 0.75 + 0.25 = 1
So in ideal conditions, let’s check out
the Hardy-Weinberg Equations
•
•
•
•
•
The Second Equation looks at genotype
p2 represents Homozygous Dominant (AA)
q2 represents Homozygous Recessive (aa)
2pq represents Heterozygotes (Aa)
The equation is:
• p2 + 2pq + q2 = 1
• This is derived from squaring both sides of the
first equation: (p + q )2 = 12
Let’s Try an Example
• BB = a black cat
• Bb = a black cat
• bb = a white cat
84% are black
16% are white
Well bb = all white cats,
which is homozygote
recessive (which = q2)
So q2 = 0.16
So q = 0.4
You Finish It
• With a partner / group:
• Find the frequency of p
• Then find the frequency of:
– Homozygote Dominants (BB or p2)
– Heterozygotes (Bb or 2pq)
– Homozygote Recessives (bb or q2)
Let’s Try One More
• RR = Can Roll
• Rr = Can Roll
• rr = Can NOT Roll
Who CAN Roll their
tongue?
Who CANNOT Roll their
tongue?
Use this information to find the allele
frequencies of R and r in this classroom.