Download How do we know the population is evolving?

Introduction to HardyWeinberg Principle
How
do we know the population is
evolving?
We can tell if the population is evolving if
we measure genetic change using HardyWeinberg equilibrium equation.
Think back to
yesterday’s bird beak
lab. What beak
survived better than
other beaks while
foraging for cat food?
Did the bird population
change over time?
Now, in real populations in nature, it’s not that
easy to observe change.
 If we were to go into Sauerman Woods, and
attempt to capture all the rabbits, it’s not
possible. We need to capture as many rabbits
as possible because we need a large sample.
However, capture is difficult, so we use a
Population Sampling.
 Population Sampling is a technique that uses
part of a population to represent a whole
population.

 Once
we have a population sample, we
can evaluate whether the population is in
fact, evolving.
 Scientists attempt to measure genetic
change using the Hardy-Weinberg
principle. “Under certain conditions,
frequency of the dominant and recessive
alleles will remain the same generation
after generation.”
Common Misconceptions



The dominant allele of a
trait will always have the
highest frequency in a
population.
The recessive allele of a
trait will always have the
lowest frequency in a
population.
Polydactyl is a condition
of having more than 5
digits—inherited as an
autosomal dominant
phenotype.

Once we have the
population sample to
actually measure HardyWeinberg principle, we
can use a gene pool (all
of the genes of every
individual in a population
sample) to study the
frequency with which
certain alleles change
over time.
Hardy-Weinberg Equation

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Allele frequency (gene frequency) = for example, A:80% a:20%
Genotype frequency = AA Aa aa
(the number of specific types of alleles / the total number of alleles in
the gene pool)
 P = frequency of the dominant allele
 Q = frequency of the recessive allele
 P+Q=1
 P2 + 2pq + q2 = 1
 P2 = homozygous dominant individuals
(percentage of homozygous dominant individuals)
 2pq = heterozygous individuals
(percentage of heterozygous individuals)
 Q2 = homozygous recessive individuals
(percentage of homozygous recessive individuals)
Does Hardy-Weinberg Exist in
Reality?

What does the Hardy-Weinberg principle say?
 Hardy-Weinberg says that genes and gene
frequency remain stable and at equilibrium.
 Equilibrium means that the genotypes or
phenotypes in a population remain constant over
time.
 If this principle is in fact, true, NO EVOLUTION
is occurring in the population!
Does Hardy-Weinberg Exist in
Reality?
In order for Hardy-Weinberg principle to be
actually occurring, the following conditions
must be present in order for Hardy-Weinberg
equilibrium to be actually occurring:
 The population is large (no emmigration or
immigration)
 Mates are chosen at random
 Mutation is not present
 Natural Selection forces are absent (no
“Survival of the Fittest”

Hardy-Weinberg is a Hypothetical
Situation!
This is a hypothetical or imaginary situation because…..



Mates are really not chosen at random.
Mutations are often present in a population.
Natural Selection forces are often present in a population.
Are the 4 conditions of Hardy-Weinberg
principle ever met in nature?

Very rarely are these
conditions met in
nature—so evolution
is usually occurring.

However, sharks and
turtles come very
close to meeting the 4
requirements.
Would you expect sharks and turtles to exhibit relatively
constant allele frequencies? Yes! Researchers have
determined that genetic information in mammals changes
about 3% every 1 million years, while genetic information of
sharks changes at a rate 7 to 8 times slower than other
mammals.
 Do
any populations ever fulfill all of the
conditions perfectly? No. The HardyWeinberg principle describes an imaginary
situation in which no selection forces are
acting on organisms.
 Each
population has a life-span of 5
million years—when genes change too
much, the population may die out.
 Scientists often attempt to prove
something is not true—in order to prove
something else is true! This is why HardyWeinberg principle is so convenient.