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Mechanisms for Evolution
Chapter 15
Populations and Evolution
• Population – a group of organisms that interbreed
• Each population shares a gene pool (the different alleles present
in the population)
• Each population has a relative frequency of each allele, or the
number of times the allele occurs in the gene pool.
• The frequency of alleles in a population tends not to change
unless there is an outside force causing the change
Evolution occurs because of changes to the
equilibrium
•
There are 5 mechanisms that can change the
allele frequencies in a population
1.
2.
3.
4.
5.
Natural Selection
Genetic Drift
Mutation
Migration
Non-random Mating
1. Natural Selection
•
Natural selection is a process by which individuals
who are more fit for their environment survive and
reproduce more often
•
Allele frequencies change because of interactions
between the population and the environment
•
There are 4 different types of natural selection:
a. Stabilizing Selection
b. Directional Selection
c. Disruptive Selection
d. Sexual Selection
1a. Stabilizing Selection
• Stabilizing selection occurs when individuals
with the average form of a trait are most fit
for the environment and extreme traits are
eliminated
• This is the most common form of natural
selection
Example of stabilizing selection
• Lizard body size:
– 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
• Directional selection occurs when individuals
with one extreme variation of a trait are the
most fit in the environment.
• This causes a gradual shift in allele frequency
to that extreme (shift in one “direction”).
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
• Disruptive selection occurs when both
extreme variations of a trait are the most
fit.
• There is selection against the middle
variations.
• This type of selection often results in
new species being created.
Example of 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 more often by birds
1d. Sexual Selection
• Sexual selection is the competition for mates
within a population causing differences to occur
in the allele frequencies of the two genders
• Often results in extreme differences in the
physical appearance of males and females.
• Mates tend to be chosen for their phenotypes
and females tend to choose the males.
1d. Sexual Selection cont’d
• Ex: Peacocks
– 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 more common.
– Sexual Selection in Peacocks
2. Genetic Drift
• Genetic drift describes changes in allele
frequency due to population size.
• Allele frequencies can change more rapidly in
smaller populations
Genetic Drift Demo
• 1. What do the marbles in the bottle represent?
• 2. Describe the allele frequencies of the original
“population” in the bottle.
• 3. How are the allele frequencies in the final
population different from the original population?
• 4. What sorts of events might cause genetic drift to
occur?
• The founder effect
occurs when a few
individuals from a larger
population colonize a
new area.
– Ex. Amish community
• The allele frequency
of this population may
differ from the larger
population because of the
limited number of individuals
2. Genetic Drift cont’d
3. Mutations
• Mutations are inheritable changes to the
genotype of an organism
• Mutations occur randomly and spontaneously within a
population
• Many mutations are harmful, but some are useful
• 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
4. Migration
• Movement into and out of a population can change
the allele frequency in a gene pool
– Immigration can ADD individuals with variations to the
population
– Emigration can REMOVE individuals with variations from a
population
• Gene flow which is the process of transferring genes
among different populations
5. Non-Random Mating
• Having a limited number of individuals can
also impact mating.
• Non-random mating can influence allele
frequencies because:
– Mates can be limited by geography
– Mates can be chosen for their traits
– Mates can be more closely related to one another