Transcript Evolution of Populations

Evolution of Populations
Chapter 11
Warm Up 1/21 & 1/22
1. Compare Larmark and Darwin in their
theories of evolution.
2. List the 5 evidences of evolution.
3. Define the following: Vestigal, fitness,
mutation and adaptation
KEY CONCEPT
A population shares a
common gene pool
Genes and Variation
Gene pool - all the genes that
exist within a population
Genetic variation in a population increases
the chance that some individuals will
survive
Genetic variation leads to phenotypic variation.
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
 made up of all alleles in a population
 allele combinations form when organisms have
offspring
Allele frequencies measure genetic
variation.
measures how common allele is in
population
can be calculated for each allele in
gene pool
Genes and Variation
Relative (allelic) frequency - the percentage of
a particular allele in a gene pool.
KEY CONCEPT
Populations, not individuals, evolve.
Natural Selection in Populations
Natural Selection- In nature, unequal
ability to survive and reproduce... Survival
of the fittest.
Natural Selection ACTS ON
PHENOTYPE but influences
genotype (thus, allelic frequency)
Artificial Selection- Mankind selects for
desired traits. Also known as “selective
breeding”
Over time, the zebra herd becomes faster as
the slower zebra (and their genes) are
removed from the population… survival of
the fittest
Darwin’s Theory = Evolution by means of
natural selection
ADAPT OR DIE!
Camouflage- organisms blend-in
with surrounding environment
Mimicry- species copy another to
insure their own survival
NatGeo
Natural Selection effects Genetic
Change in Populations
Natural Selection has three
affects on phenotype
distribution
1.Directional Selection
2.Stabilizing Selection
3.Disruptive Selection
Artificial Selection
Normal Distribution of traits
A normal distribution graphs as a bellshaped curve.
Phenotypes near the
middle range tend to
be most common.
Examples- height and
weight
Directional Selection
This type of selection favors phenotypes at
one extreme of a trait’s range.
An extreme phenotype that was once rare
becomes more common.
Ex. Drug resistant bacteria
Directional Selection
1. Directional SelectionIndividuals on one end
of a curve are “better
fitted” than the middle
or other end
Peccaries naturally choose to consume those
cactus plants with the fewest spines As a result,
at flowering time there are more cacti with
higher spine numbers; thus, there are more of
their alleles going into pollen, eggs, and seeds
for the next generation.
Stabilizing Selection
Stabilizing selection favors the intermediate
phenotype.
Selection against both extremes
decreases the genetic diversity of a given
population.
Stabilizing Selection
2. Stabilizing SelectionIndividuals near center
of a curve are “better
fitted” (have highest
fitness) than both ends
Peccaries are consuming the low-spine
number plants, and the insects are killing
the high-spine-number plants. As these
gene combinations are removed from the
cactus gene pool, there is less and less
variety possible in subsequent
generations.
Disruptive Selection
This type of selection occurs when both
extreme phenotypes are observed.
Individuals with the intermediate type are
selected against.
By favoring both extreme phenotypes,
disruptive selection can lead to the
formation of a new species.
Disruptive Selection
3. Disruptive SelectionIndividuals at upper and
lower ends are “better
fitted” than the ones in the
middle
Years of collecting have left their toll on
the roadside cacti. In this environment, it is
maladaptive to be good looking and have
a reasonable number of spines. Low
spine-number plants are not picked
because they don't "look right", and high
spine-number varieties are left alone
because they are too hard to pick.
Gradually, the gene pool changes in favor
of the two extreme spine number types.
Other mechanisms of Evolution
Natural selection is not the only
mechanism through which populations
evolve
3 other mechanisms of Evolution:
Gene Flow
Genetic Drift
Sexual Selection
Gene flow – movement of alleles into
or out of a population
Immigration – new alleles move IN
Emigration – alleles move OUT
Genetic drift - change
in allelic frequencies
by chance
Ex: sudden extinction of
a dominant species;
small populations
most affected
Genetic drift is a change in allele frequencies due to
.
chance
Genetic drift causes a loss of genetic
diversity It is most common in small
populations.
A population bottleneck can lead to
genetic drift.
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It occurs when an event
drastically reduces
population size.
The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
The founding of a small population can
lead to genetic drift.
–
–
It occurs when a few individuals start a new
population.
The founder effect is genetic drift that occurs
after start of new population
Sexual selection occurs when certain traits
increase mating success.
Sexual selection occurs due to higher cost of
reproduction for females.
 males produce many sperm continuously
 females are more limited in potential offspring each
cycle
Genetic equilibrium - when alleles stay the same
from generation to generation
1.
2.
3.
4.
5.
The Hardy Weinberg Principle: Allele
frequencies will remain constant under five
conditions
Random Mating
Large Population
No movement (immigration or emigration)
No Mutations
No Natural Selection: equal change of
survival
5 Factors that can lead to
evolution
Genetic Drift
Gene Flow
Mutation
Sexual Selection
Natural Selection
Genetic drift changes allele frequencies
due to chance alone
Gene flow moves alleles from
one population to another
Mutations produce the genetic variation
needed for evolution.
Sexual selection selects for traits that
improve mating success.
Natural selection selects for traits
advantageous for survival
KEY CONCEPT
New species can arise when populations
are isolated
The isolation of populations can lead to speciation
speciation - evolution of a new species
.
Reproductive isolation can occur between isolated
populations
Populations can become isolated in several ways:
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1. Behavioral
2. Geographic
3. Temporal
1. Behavioral Isolation: Two populations
capable of breeding but cannot because of
courtship rituals
2. Geographic Isolation: Two populations are
separated by geographic barriers
Ex: Rivers, Oceans, Mountains
3. Temporal Isolation: Two or more
populations reproduce at different times
Patterns of Evolution
1.
2.
3.
4.
Extinction
Divergent Evolution (adaptive radiation)
Convergent Evolution
Coevolution
1.Extinction
Why do species go extinct?
Extinction
Natural selection, climate
changes, and
catastrophic events have
caused 99 percent of all
species that have ever
lived to become extinct.
Mass extinctions –
caused by continents
moving, sea level
changing, volcano
eruptions, large meteors
Predict what each ecosystem will look like after the event.
Catastrophic
Event
Catastrophic
Event
Question
When a mass extinction happens, what do
you think will happen next?
Divergent Evolution
(adaptive radiation)
Divergent evolution – natural selection
causes 1 species to evolve into many
species with many different adaptations
(homologous structures)
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After mass extinctions, many environments
will be open for inhabitation
Species will migrate to that area and new
environmental pressures will cause the
population to change over time
This is also known as Adaptive Radiation
Adaptive Radiation in
honeycreepers
Evidence of Evolution
Homologous structures - similar
structures found in related organisms that
are adapted for different purposes.
Ex: human arm and bat wing or whale
flipper
---DIVERGENT EVOLUTION--the process of two or more related species
becoming more and more dissimilar.
Homologous structures 
Divergent evolution
Convergent Evolution
Convergent Evolution – when unrelated
organisms come to resemble one another
(analagous structures)
Analogous structures - structures found
in unrelated organisms that have a similar
function but may be structurally different
Ex: bird wing and insect wing
---CONVERGENT EVOLUTION--independent evolution of similar
features in species of different
lineages
Analogous structures 
Convergent evolution
Coevolution
When 2 species
evolve in response to
one another
Coevolution can occur in competitive
relationships, sometimes called
evolutionary.
Speciation occurs in patterns
Punctuated equilibrium: species show little
evolutionary change for millions of years,
followed by periods of rapid speciation
Gradualism- Species evolve slowly, over
long periods of time.