hardy-weinberg equilibrium measuring evolution at the population level

HARDY-WEIN

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EQUILIBRIU

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QUIZ:

In the H-W equation: what are p2 , 2pq, and q2? p & q?

Genotypic/allele frequencies remain constant when a population (is/is not) evolving.

In order for a population to be in equilibrium, what must be true?

1)

2)

3)

4)

5)

P2 + 2PQ + Q2 = 1 & P + Q = 1

P2 ; Homozygous dominant

2pq = heterozygous

Q2 = homozygous recessive

P= dominant allele

Q= recessive allele

ALLELE FREQUENCIES CONSTANT=POPULATION IS NOT EVOLVING!

Frequencies of genotypes can change but alleles cannot : WHY?

IN ORDER TO BE IN EQUILIBRIUM THERE MUST BE:NO natural selection

NO sexual selection (nonrandom mating)

NO net mutations

LARGE POPULATION

NO gene flow

WHAT ARE EXAMPLES OF NATURAL SELECTION & SEXUAL SELECTION THAT WE’VE SEEN?

NET MUTATIONS

A mutation that affects a particular allele frequency.

Ex. Albinism in a population may affect whether or not a certain allele (ex. Red hair) appears.

LARGE POPULATIONS

Small populations are at risk for:

Genetic Drift

Founder effect

Small population inhabits an island with relative small gene pool.

A certain trait could be selected for by chance.

Bottleneck effect

A small population is subject to a natural disaster

A certain trait (whether or not more successful) gets selected for by accident.

GENE FLOW

Immigration: Individuals, along with their gene pool enter a population. Eg. Italian genes in NYC in the 1920s.

Emigration: Individuals, along with their gene pool leave a population. Eg. Native Americans forced out of Georgia to Oklahoma in the trail of tears in the 1800s.

Migration: seasonal movement. Since the population usually reproduces in the same spot, this will not affect the gene pool.

EQUILIBRIUM PROBLEMS

In a population of Rainforest frogs, 88% are non-albino and it has been determined that albinism is a recessive trait.

What are the frequencies of phenotypes?

What are the frequencies of genotypes?

What are the frequencies of alleles?

PHENOTYPE FREQUENCY:

Non-albino: .88

Albino: .12

GENOTYPE FREQUENCY:

Determine frequency of homozygous recessive first:

frequency of recessive genotype = q2

Then you determine the frequency of the q allele: square root of p2

Then you determine the frequency of the p allele: p-q = 1

Finally you can determine:

heterozygous: 2pq

homozygous dominant: p2

IN A SUBSEQUENT GENERATION, THE PROPORTION OF ALBINO WAS REDUCED TO .10

What are the frequencies of each genotype if the population remains in equilibrium?

THE POPULATION OF FROGS WAS REDUCED BECAUSE THEY HAVE BEEN HUNTED AND SOLD ON THE FROG MARKET!

An incredible flood occurs in this rainforest. The frequency of albino frogs is now .40 What are the new values for p & q?

How could such an event have happened?

MODEL

Natural Selection on “Goldfish”

Let’s set up a model for natural selection on goldfish.

The population of goldfish has approximately 50% regular and 50% pretzel.

Module A: You are sharks and show no preference for goldfish.

Module B: You are sharks and show preference for pretzel goldfish.

Make a hypothesis to determine what will happen to the allele frequencies in each module!