BIO 416Evolution

Lecture #12

Population Genetics III

DestabilizingThe Equilbrium

February 9, 2009

Dr. Karen Schmeichel Oglethorpe University

Hardy

Weinberg

Business:

•Lit Rev #1 Due 2/11•Darwin’s Birthday 2/12 –(10 pts)

•Exam 2/13 – Study Guide on Wednesday

•Exam – Through Ch 7

Objectives:

•Reconsider H-W equilibrium using Card Simulations

•Discuss Factors that destabilize H-W•Cover the ways in which selection can alter changes in allelic and genotypic frequencies

Hardy-Weinberg Equilibrium: a Null Model

1. Allele frequencies in a population will not change, generation after generation

2. If the allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq and q2

5

Hardy-Weinberg-Castle Equilibrium:

Describes how allele and genotype frequencies do not change during the course of many generations, unless

destabilizing conditions exist

p2 +2pq + q2 = 1f(AA) f(Aa) f(aa)

= 0.8 = 0.2

= 0.8

= 0.2

= 0.8 x 0.8 = 0.8 x 0.2

= 0.8 x 0.2 = 0.2 x 0.2

f(AA) = .64 f(Aa) = .32 f (aa) = .04

Another Way to conceptualize HW:

7

In a given population with alleles A and a:

p + q = 1

A Population in H-W Equilibrium will show same allelic frequency generation after generation (RARE, if ever!)

A Population Genetics Simulation

HW Equilibrium

is Predictive

HW Equilibrium: a Null Model

1. Allele frequencies in a population will not change, generation after generation

2. If the allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq and q2

How to “mind your

p’s and q’s”?

14

Alloenzymes &

Orchid Exercise

Heterozygosity as a Measure of

Genetic Diversity:The average individual is

heterozygous at 4-15%

of its genes

Hardy Weinberg Conditions: Allele and genotype frequencies will not change from generation to generation if:

1.No selection2.No mutation3.No migration 4.No genetic drift5.Mating is random

Can Selection Change allele frequencies

from one generation to the next?

(Game Rule: 75% of the heterozygotes survive

and 50% of the homozyg recessives survive)

Violation ofConclusion #1

Violation ofConclusion #2

Papua New Guinea’sFore Tribe and Kuru

23

Dogma Following the Modern Synthesis:

Natural Selection should preserve the allele most conducive to

survival and reproduction and eliminate the rest.

The one best allele was called wild type and any other alleles

were considered mutants (extremely rare).

Natural SelectionIs most potent

When a recessive

Allele is common,

Not rare(think: heterozygote)

FAST! SLOW!

Selection Favoring Heterozygotes: “Overdominance”

(ex, Fig. 6.18 in Fruit Flies)A way of maintaining recessive alleles

Selection Favoring Homozygotes: “Underdominance”(ex, Fig 6.19 in Fruit Flies)

In this case, one allele tends to go to fixation, while the other allele is

lost (may be different in different populations)

For 2/11: •Ch 7 & Prairie Chicken

Paper

Mutation Alone

Causes SlowChange Over

Time – Generally

A weak Mechanism for

Evolution

Mutation + Selection: The 1-2 Punch

Mutation/Selection Balance(or frequency of deleterious allele

at equilibrium) is Described Mathematically:

q = s