Genetic Variation & Natural Selection Within Populations11.1 & 11.2

Variation & Natural SelectionWhat have we learned?At your table, take 2 minutes to

discuss what the take away message from our last chapter was. Determine your tables response & be prepared to share. 2:00

1:301:00

:30TIME!

11.1 Genetic VariationScenario:

◦The zombie apocalypse has begun.◦The entire US is in horror and trying

to find ways to survive.◦What type of traits make you more

likely to be caught and changed into a zombie?

◦Does everyone you know have these traits?

◦What traits will make you less likely to be zombie food?

Genetic VariationGenetic Variation

increases chance that some individuals will survive.◦Natural Selection acts on

phenotype in a population

◦Must have genetic variation to have different phenotypes

◦The greater the variation in phenotypes the greater the chance of survival.

Socially Awkward Penguin

Gene PoolDefined:

◦The combined alleles of all the individuals in a population.

Why is it important:◦Where genetic variation is

demonstratedReview:

◦What is an allele?How do we determine the

frequency of an allele in a gene pool?

WITH MATH!!!!

YAY!

Allele FrequencyA class of 20 students all have either

brown (Bb or BB) or blue eyes (bb).◦8 students have blue eyes (bb = 8)◦8 students have a parent with blue eyes

(Bb = 8)◦4 students have no parent with blue eyes

(BB = 4) Assume these parents don’t carry recessive allele

◦What is the frequency of the blue eye allele in this class?

◦Hint: each individual allele counts as one in your total. (a blue eyed individual has 2 alleles for blue eyes)

Allele FrequencyHow many blue eyed students?

◦8 students x 2 b alleles = 16 b allelesHow many brown eyed students with

recessive allele?◦8 students x 1 b allele = 8 b alleles◦8 students x 1 B allele = 8 B alleles

How many students (BB)◦4 students x 2 B alleles = 8

Total b alleles = 24Total B alleles = 16Total alleles in pop = 40

Allele Frequency(B) allele freq is 16 / 40 = 40%(b) allele freq is 24 / 40 = 60%What would happen to the

frequency of B alleles if suddenly having blue eyes became an advantageous trait to humans?

Sources of Genetic VariationMutation:

◦DNA change can lead to change in allele.

Recombination:◦Rearrangement of parent’s alleles

during gamete production

Brain Break – The Music VersionParents hate your music….you hate

their music…why?As you age, your brain becomes

unable to handle as much dopamine.◦Dopamine: plays a major role in reward

driven learning. Gives you the “chills” when a new song you like comes on.

Because you fail to get that same sensation as you age, your music taste will become stagnate.

11.2 Nat. Selection on Populations

Distribution of TraitsNormal Distribution:

◦Frequency is highest near the mean and decreases towards the extreme ends.

Range of Variables

Frequency

But Mr. Wilson, how does this Apply to Natural

Selection?Thanks for Asking!

Actually, for some traits, all phenotypes provide an equal chance of survival.

These phenotypes generally show a normal

distribution

Those that are the most common appear in the middle of the range.

Those that are less common are at the

extremes.

But…If a trait in this distribution

becomes most favorable in nature…

The favorable phenotype will increase in frequency.◦This is natural selection

Can allele frequencies be observed over time? You betcha!

MicroevolutionDefined:

◦The observable change in allele frequencies of a population over time.

Natural Selections Role:◦Directional◦Stabilizing◦Disruptive Selection

Directional SelectionDefined:

◦Selection that favors one extreme of a trait’s range.

Example:◦Methicillin Resistant Staphylococcus

Aureus (MRSA)

Lets Use Our Example

Stabilizing SelectionDefined:

◦The intermediate phenotype is favored.

Example:◦Size of a newborn baby. ◦Too small = increased risks◦Too big = increased risks

The Likely Trend

Disruptive Selection Defined:

◦ When both extreme phenotypes are favored.◦ Intermediate phenotypes are selected against in

nature. Example:

◦ London’s Peppered Moth

Rural Industrial

At this point:

End of Notes!!!