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Population
Genetics
and
Speciation
MISCONCEPTION
ALERT
• Individuals DO NOT evolve!
• Individuals do not change to match changes
in the environment
• POPULATIONS are acted upon by natural
selection, where less fit individuals are less
likely to pass on their genes
• This causes a change in the genetic makeup of the population as a whole
Genetic Variation within Populations
• A healthy population is one with a wide range of
genetic diversity, without which evolution
CANNOT occur
• A wide range of genetic diversity causes there to
be many different phenotypes
Genetic Variation within Populations
• The greater the variation in phenotypes, the more
likely it is that some individual can survive in a
changing environment
• Genetic variation is stored
in a population’s
gene pool – the
combined alleles
of all the individuals
in a population
• An allele frequency is a
measure of how COMMON
a certain allele is in a population
Genetic Change within Populations
• Genetic change can occur because of the
variations within populations
• Genetic change causes a shift in the
allele frequencies of certain traits
• Genetic change within populations over
time can occur in several ways…
Mechanisms of Genetic Change
Over Time (Evolution)
1. Natural Selection
• Occurs when nature
selects for or
against certain traits
Mechanisms of Genetic Change
Over Time (Evolution)
2. Gene Flow
• When genes are added or removed from a
population
• Caused by migration (emigration or
immigration)
Mechanisms of Genetic Change
Over Time (Evolution)
3. Genetic Drift
• A CHANCE event that can cause the allele
frequencies in a population (especially a
small population) to change drastically
• Can be caused by natural disasters such as
fires, volcanoes, earthquakes and floods
• Why?
Mechanisms of Genetic Change
Over Time (Evolution)
4. Non-Random Mating
• The cost of reproduction between males and
females is different
• Males produce many sperm continuously,
while females are born with the number of
eggs that they have
• This allows females to be more choosy when
it comes to selecting a mate
• Non-Random mating occurs because certain
traits in males increase mating success
Mechanisms of Genetic Change
Over Time (Evolution)
5. Mutations
• A random mutation can add a new allele to a
population
Patterns of Natural Selection
• When natural selection is NOT occurring, most
populations have a normal distribution
• If you were to collect one shoe from each student
in your class and ordered and grouped them by
size, you would probably form a hill-shape curve
shown below, also known as normal distribution
Patterns of Natural Selection
• When natural selection DOES occur, populations
can respond in three major ways…
Patterns of Natural Selection
• In directional selection, the “peak” of a normal
distribution moves in one direction along its range
• In this case, selection acts to eliminate one
extreme from a range of phenotypes
Patterns of Natural Selection
• In stabilizing selection, the bell-curve shape
becomes narrower
• In this case, selection eliminates individuals that
have alleles for any extreme type
Patterns of Natural Selection
• In disruptive selection, the bell-curve is pushed
apart in two peaks.
• In this case, selection acts to eliminate individuals
with average phenotype values
Speciation Through Isolation
• Do you remember what defines a species?
• A species is defined as a group of
organisms that can successfully interbreed
• It is important to remember that species
are not permanent, stable things…
Speciation Through Isolation
• If gene flow between two populations
stops for some reason, the populations are
said to be isolated
• As these populations adapt to their
different environments, their gene pools
may change
• All of these changes add up over
generations
Speciation Through Isolation
• With time, the isolated populations
become more and more genetically
different from each other and may no
longer be able to mate with each other
(they become reproductively isolated)
• Reproductive isolation between
populations is the final step of becoming
separate species
Speciation Through Isolation
•
•
The rise of two or more species from one
existing species is called speciation
There are three ways in which population
may become isolated
1. Behavioral Isolation: Caused by a
difference in mating behaviors
2. Geographic Isolation: Involves physical
barriers that divide a population into 2 or
more groups
3. Temporal Isolation: Exists when timing
prevents reproduction between populations
Which type of isolation is this?
Convergent vs. Divergent
Evolution
• Convergent Evolution
– Occurs when different species evolve
in similar environments
– When this occurs, the different species
often evolve to have similar traits
– These traits are said to be
ANALOGOUS
Examples of Convergent Evolution
Bird
Insect
ANALOGOUS STRUCTURES!
(circle them in your notes!)
Dolphin
Shark
Convergent vs. Divergent
Evolution
• Divergent Evolution
– Occurs when closely related species
evolve in different environments
– When this occurs, the similar species
evolve to have different traits
– These traits are said to be
HOMOLOGOUS
Examples of Divergent Evolution
HOMOLOGOUS STRUCTURES!
Coevolution
– The process in which two or more
species evolve in response to changes
in each other
– Species that involve predator-prey or
parasite-host relationships often develop
adaptations in response to each other