Transcript Hardy-Weinberg Genetic Equilibrium

Hardy-Weinberg Genetic Equilibrium
Flamingo population
Remember: to maintain equilibrium
1.
2.
3.
4.
5.
No change due to
mutations.
Individuals do not
move in or out of
the population.
The population is
and remains large.
Random mating.
No selection
Tule Elk population at Pt Reyes
Disruption of Genetic Equilibrium
•
If any of the 5
factors occur,
they can cause the
gene frequency to
change in a
population.
•
If the gene
frequencies
change evolution
has occurred.
Tule Elk population at Pt. Reyes
Mutations
•
•
Mutations occur normally but at extremely low rates.
Mutations do not cause gene frequencies to change.
Migration
•
•
Immigration (movement in) or emigration
(movement out) can effect gene frequencies.
Movement of individuals from one population
to another results in gene flow.
Genetic Drift
• Small populations
are subject to
genetic drift.
• Random events lead
to changes in gene
frequencies.
Each line represents a different population. Each starts with the same number of allele
A. over time the number of individuals with that allele changes. If a population is small
enough over several generation the allele will either be 0 or 100% present.
Nonrandom Mating
•
Random mating
throughout a population
does not occur.
– Geographical regions.
– Sexual selection
• Ex: birds of
paradise
– Assortive mating
• Ex: Fruit flies.
Directional Selection
• Wild pigs impose
selection upon the
cactus population.
– Cactus that have
fewer needles do
not survive to
reproduce.
– Cactus with more
needles survive to
reproduce.
Cactus and wild pigs
Directional Selection results
•
Result of directional
selection is a shift in
the population
towards more spines.
•
One extreme is
selected for and one
against.
Ex: cliff swallows
after a cold snap.
•
I’ve posted a short article on
how cliff swallow wing length
has shorted due to new
selective pressures. Read it
and refer to it in your summary.
Stabilizing Selection
• Both extremes are
selected against.
• The average is
selected for.
• Population becomes
more similar due to
average being
selected for.
• Ex: human birth
weight
Disruptive Selection
•
•
•
•
•
Extremes are
selected for.
Results in two
distinctly different
populations.
Disruptive selection
can lead to two
species.
Average is selected
against.
Ex: finches in west
Africa, eat different
sized seeds.
Calculating allele frequencies
• Use the Hardy-Weinberg equation:
– p² + 2pq + q² = 1
– “q” is the recessive trait and q² is homozygous
recessive
– “p” is the dominate trait and p² is homozygous
dominant.
– pq is heterozygous
– added together you have 100% of the
individuals in a population
Disruption of Genetic Equilibrium Summary of Selection
Summary Review
1. Describe the 3 types of selection.
2. What is genetic drift, what causes it, an what
results from it?
3. What can cause new genes to enter a population?
4. What do these two pictures illustrate?