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Chapter 15 Section 3
SHAPING EVOLUTIONARY THEORY
SLIDE 1
slide 2
What is an allele
again?
It’s an alternative form that a single gene may
have for a particular trait.
Pictured are two homologous chromosomes.
Each chromosome has the same type of gene at
the same spot. Different versions of this gene are
called alleles. These two genes make up this
organisms genotype for the trait of flower color.
Slide 3 What does it look like when
there’s no evolution?
Population of Owls
Generation
#1
Generation
#2
ff
ff
Ff
FF
Ff
FF
Number of each allele in
Generation #1
F: 3
f: 3
Ratio F:f 3:3 or 1:1
Number of each allele
in Generation #2
F: 6
f: 6
Ratio F:f 6:6 or 1:1
Slide 4What would NO evolution look
like?

When there’s no evolution, allelic frequencies remain constant.


This is called being in genetic equilibrium.
This is the basis for the Hardy-Weinberg principle which describes what conditions
need to be met in order for NO evolution to occur and a population to remain in
genetic equilibrium.
Ex: although the number of owls doubled, the ratio of
gray to red owls is the same and the allelic
frequencies are the same.
Slide 5

To be in equilibrium, the population must meet 5 conditions
(no mechanisms of evolutionary change):
 1.
no genetic drift
 2.
no gene flow
 3.
no mutation
 4.
mating must be random
 5.
no natural selection
 *populations in nature might meat some of these requirements, but
hardly any population meets all 5 conditions for long periods of time.
Slide 6: Mechanisms of Evolution: genetic
drift
Genetic Drift: Any change in the allelic
frequencies (# of each kind of allele in
the population) that results from
chance.
 These effects are more pronounced
in smaller populations.
Slide 8: Mechanisms Video: 5 Fingers of
Evolution

http://ed.ted.com/lessons/five-fingers-of-evolution#watch
Pinky- Shrinking Population (aka genetic drift- which is more pronounced in a smaller populationfounder effect & bottleneck
Ring- Nonrandom Mating
Middle- Mutation
Pointer- Gene Flow
Thumb (up/down)- natural selection
Slide 7: Mechanisms of Evolution: genetic drift
continued

Founder Effect- small sample of a population settles
in a location separated from the rest of the
population.



Alleles that were uncommon in the original population
might become common in the new population
Bottleneck- population declines to a very low #
then rebounds.
https://www.youtube.com/watch?v=Q6JEA2olNts
Slide 9
Mechanisms of
Evolution:
Gene Flow

Mechanism of evolution

Can occur during migration of individuals from one population to another

When the migrating individuals breed with the new population, they
contribute their genes to the gene pool of the local population

Makes gene pools of the same species more similar to one another

Ex: wind carrying seeds from parent population to another population,

Ex: animals driven off of herd join a new population
Slide 10: Mechanisms of Evolution:
Nonrandom Mating

Mechanism of Evolution

What would completely random mating look like?


Individuals regardless of environment, heredity, or social interaction. Potential mates
have an equal chance of being selected. As long as mating was random and no
other mechanisms of evolution were happening, no evolution would occur in this
population.
The result of nonrandom mating is that some individuals have more opportunity
to mate than others and thus produce more offspring (and more copies of their
genes) than others. It is simply easier to mate with a nearby individual, as
opposed to one that is farther away. Also, especially in animals, individuals
compete for mates and active selection of mating partners occurs. This goes
directly against the concept of randomness.
Slide 11: Mechanisms of Evolution:
Mutation

Mechanism of evolution

Mutation, a driving force of evolution, is a random change in an
organism’s genetic makeup, which influences the population’s gene pool.

Mutations give rise to new alleles; therefore, they are a source of genetic
variation in a population.

Mutations may be harmful or benign, but they may also be beneficial.
Slide 12: Mechanisms of Evolution:
Natural Selection

Mechanism of evolution

Individuals in a population are not equally adapted to the environment

Best traits survive

Types of natural selection

Directional

Stabilizing

Disruptive

Sexual
Slide 13 Graphs of Natural Selection