Warm-Up2/23/07

• What are the 2 sources of Genetic Variation that we talked about?– Mutation– Gene Shuffling (Sexual Reproduction)

• How many phenotypes are usually produced by a single-gene trait?– 2, a single gene trait generally has 2 alleles, like the

widow’s peak hairline in humans.

• How many phenotypes are usually produced by a polygenic trait?– More than two - usually many variations like human height.

Warm-Up2-26-07

In a population of mice, fur color is a single-gene trait controlled by two alleles: B (black) and b (brown). B is dominant over b. There are 8 homozygous black (BB) mice, 24 heterozygous black (Bb) mice, and 18 homozygous recessive brown (bb) mice.

1. How many total alleles are there in this population?• 100

2. How many B alleles are there? How about b alleles?• B = 40, b = 60

3. What is the Relative Frequency of B in this population? How about the Relative Frequency of b?

• B = 40%, b = 60%

In a population of mice, fur color is a single-gene trait controlled by two alleles: B (black) and b (brown). B is dominant over b. There are 8 homozygous black (BB) mice, 24 heterozygous black (Bb) mice, and 18 homozygous recessive brown (bb) mice.

Warm-Up, continued2-26-07

1. How many black mice are there in this population? And how many brown mice are there?

• Black = 32, Brown = 18

2. How is it possible that there are more black mice than brown mice when the b allele for brown has a higher Relative Frequency than that B allele for black?

• The heterozygous individuals have black fur, but carry one b allele.

Challenge Question

• Lets say that being homozygous recessive (bb) was lethal - all the baby mice born with a bb genotype die before they can reproduce.

• What do you think would happen to the b allele in that population?

Warm-Up (Write the question in your own words)

The plasma membrane of a cell consists of • A protein molecules arranged in two layers

with polar areas forming the outside of the membrane.

• B two layers of lipids organized with the nonpolar tails forming the interior of the membrane.

• C lipid molecules positioned between two carbohydrate layers.

• D protein

Brag Sheet• Name

• GPA

• Class Rank

• Expected Graduation Date

• Honors Classes

• Extra curricular Activities– Leadership

• Community Service

Evolution as Genetic Change

16.2

Natural Selection and Genetics

• _________ Selection leads to E_________• Natural Selection acts on the _____otype

– Why?

• But we know that an organism’s genetics determine their phenotype

• In genetic terms, evolution is any change in the __________ frequency of _______ in a population’s gene pool.

relative

volution

Phen

Natural

alleles

Normal Distribution of Phenotypes

Fre

qu

enc

y o

f P

hen

oty

pe

Phenotype (height)

Page 396, remember human height?

One of Darwin’s many evolution ideas was:

A. Descent with modification (Current species descended from previous generation, and each generation exhibited major changes. Over time, large changes scale down to smaller ones.)

B. Descent with modification (Current species descended from previous generation, and each generation exhibited no changes. Over time, small changes don't add up to large ones.)

C. Descent with modification (Current species descended from previous generation, and each generation exhibited minor changes. Over time, small changes add up to large ones.)

• The combined genetic information of all members of a particular population is the population’s

• phenotype.

• relative frequency

• genotype.

• gene pool.

Types of Selection• Natural Selection can affect the

distribution of phenotypes in any of three ways:

• Directional Selection

• Stabilizing Selection

• Disruptive Selection

Directional Selection• When Natural Selection favors

phenotypes at one end of the curve.

The whole curve shifts in the direction of selection

Stabilizing Selection• When Natural Selection favors

phenotypes near the center of the curve

KeyP

erce

nta

ge

of

Po

pu

lati

on

Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Disruptive Selection• When Natural Selection favors

phenotypes at both ends of the curve.

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Results in two peaks with a dip in the middle.

Think

• There are three populations of mice in the forest; white, grey, black.

• A hawk spots the white and black only.

• What happens to each population of mice?

• What type of selection is working here?

• Graph the results

• Species of birds have short, medium, and long beaks.

• The food source is insects in a tree.

• What beak will survive?

• What type of selection is this?

• Graph the results

Define

• Homologous structures

• Adaptation

Genetic Drift

• Genetic Drift is random changes in allele frequencies.

• It relies on pure chance.

• Generally occurs in small populations

• Decreases genetic diversity in small populations

• Lets look at an example

A small bag of marbles

Imagine that a small population is represented by a bag of 20 marbles, 1/2 brown and 1/2 green. You only get to select 4 individuals from the popluation to reproduce to form the next generation.

A small bag of marbles

We see in this example that, simply by chance, the brown allele disappeared from the population completely!

Lets try it ourselves!

The effects of Genetic Drifton Diversity

• Generally has a big effect on small populations

• Greatly reduces genetic diversity in small populations

• What happens when a population loses genetic diversity?

Warm-up

• Three people are walking through a forest. One person is Andre the Giant, another is Ms. Luna, and the third is the little guy from Jacka**. The forest is full of bushes and small trees. There is a predator above the trees.

• Who will be killed? Why?

• What type of selection is this?