Download Mechanisms for Evolution

Mechanisms for Evolution
Populations and Evolution
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Population – a group of organisms that interbreed
Each population shares a gene pool
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The gene pool contains the different alleles present in the
population
Populations and Evolution
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The allele frequency is
the number of times each
allele occurs in the gene
pool.
The frequency of alleles
in a population tends not
to change unless there is
an outside force causing
it
Allele frequencies change because
of Mutations
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Mutations can affect allele
frequency in a population
by
1. Adding new alleles
for a trait
2. Changing the
amount of each allele
present
It can take a long time to
eliminate a mutation and
a long time for a new
mutation to become
prevalent
Evolution occurs because of changes
to the equilibrium
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1.
2.
3.
4.
There are 4 mechanisms that can change the
allele frequencies in a population
Natural Selection
Migration
Genetic Drift
Non-random Mating
1. Natural Selection
1. Natural Selection
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Natural selection: the
process by which
individuals who are more
fit for the environment
survive and reproduce
There are 4 different
types of natural
selection:
a. Stabilizing Selection
b. Directional Selection
c. Disruptive Selection
d. Sexual Selection
1a. Stabilizing Selection
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Stabilizing selection occurs when
individuals with the average form of the
trait are most fit for the environment and
extreme traits are eliminated
This is the most common form of selection
and works in all populations at all times
Example of stabilizing selection
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Lizard body size:
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Large lizards are easily seen by predators, but smaller
lizards cannot run as fast to escape the predators
Mid sized lizards are most fit in the environment, so
they survive and reproduce more often, changing the
allele frequencies in the population
1b. Directional Selection
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Directional selection occurs when
individuals with one extreme of variations
are the most fit in the environment.
This causes a gradual shift in allele
frequency to that extreme.
Example of Directional Selection

Anteater tongue length:
 Anteaters with long tongues are most fit
because of the depth of the nests of the
termites they eat.
1c. Disruptive Selection
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Disruptive selection occurs when both
extremes of variations are the most fit
Therefore, there is selection against the
middle variations (in other words, those
with neither extreme tend to die out)
1c. Disruptive Selection

Ex: Limpet shells
 Dark limpets blend with bare rocks
 Light limpets blend with barnacle covered
rocks
 Tan limpets are visible in both situations
and get preyed upon by birds
1d. Sexual Selection
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Sexual selection is natural selection arising
through preference by one sex for certain
characteristics in individuals of the other sex.
Mates tend to be chosen for their phenotypes
and females tend to choose the males
1d. Sexual Selection
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Sexual selection not only includes mate choice
but also encompasses male-male competition
and female-female competition.
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Ex: larger females out-compete smaller females
Sexual Selection cont’d
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Ex: Peacocks
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Male peacocks have large tail
feathers that make it difficult to
fly and escape from predators
Female peacocks choose males
based on their tail feather
length and fullness
Over time males with larger tail
feathers reproduce more
causing large tails to be
selected for
Sexual Selection in Peacocks (3
min.)
2. Migration

Movement into and out of a population can
change the allele frequency in a population’s
gene pool
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Immigration can ADD individuals with variations to
the population
Emigration can REMOVE individuals with variations
from a population
2. Migration
3. Genetic Drift
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Genetic drift is the change in the gene pool
of a small population that takes place
strictly by chance.
In smaller populations the allele frequency
can change more rapidly than in larger
populations
3. Genetic Drift cont’d
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The founder effect is a special case of genetic drift
The founder effect is the loss of genetic variation
that occurs when a new population is established by
a very small number of individuals from a larger
population
3. Genetic Drift cont’d: Founder Effect
4. Non-Random Mating
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Non random mating:
any mating system
in which males are
not randomly
assigned to females
This occurs in
populations in which
individuals select
mates based on
observable traits
4. Non-Random Mating
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The result of non random mating is that
some individuals have more opportunity to
mate than others
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Thus, those deemed “more fit” produce more
offspring, and their traits are more likely to be
passed on
Genetic Drift Simulation
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This simulation, like evolution itself, is
based on random events.
You will roll a die to choose a random
number
To complete the manual version of the
Genetic Drift Simulation you need the
Worm Worksheet and colored pencils in 5
different colors
Genetic Drift Simulation
Genetic Drift Simulation
Genetic Drift Simulation
Genetic Drift Simulation
Keep rolling the die until you have 5 dots
Genetic Drift Simulation
Genetic Drift Simulation
Genetic Drift Simulation
Genetic Drift Simulation
Genetic Drift Simulation
1. What is genetic drift?
2. Today you looked at 5 populations made of 5 individuals each.
If your populations were made of 500 individuals instead of 5,
would the allele frequencies have changed more or less rapidly
than what you saw in your simulation? Why?
3. Describe the founder effect.
4. How many generations did it take each of your populations to
become homogenous? (homogenous = all the same color)
5. Based on what you did in the simulation, describe the role
randomness plays in genetic drift.