Download Population Genetics Ch 11

The Evolution of Populations
Chapter 11
Genetic Variation in Populations
Microevolution - the observable change
in the allele frequencies of a population
over time.
– Occurs on a small scale
In populations there are a wide range of
phenotypes
– Ex: short/fat penguins and taller/slim
penguins
Genetic Variation in Populations
In order to have a wide range of
phenotypes, there must be genetic
variation
The greater the variation in phenotypes,
the more likely it is that at least some
individuals can survive in a changing
environment
Genetic Variation in Populations
For example, a short/fat penguin might
be better able to stay warm in an
especially cold winter. A tall/slim
penguin might be able to dive better,
thus catching more fish in a period of
food shortage
Genetic Variation in Populations
Gene pool - All of
the alleles of every
individual in a
population
Genetic Variation in Populations
The allele frequency is a measure of
how common a certain allele was in the
population
Genetic Variation in Populations
To calculate allele
frequency for “b”:
– 7b
– 7B
– Total alleles = 14
• 7b/14 TOTAL = 50%
Genetic Variation - 2 Main Sources
Mutations
– A random change in
DNA
– Can form a new
allele
– Mutations that occur
in sex cells can be
passed on to
offspring
Recombination
– Occurs during
meiosis
– Parents’ alleles can
be arranged in new
ways during crossing
over
How do we analyze genetic
variation in a population?
With graphs!
Natural Selection in Populations
“mean” = average
“distribution” = range of data; a set of
data and their frequency
of occurrence
Natural Selection in Populations
Normal
distribution/bell
shaped curve
– frequency is highest
near the mean and
decreases toward
each extreme end of
the range
Natural Selection in Populations
Phenotypes in the
middle of the range
are most common
– Ex: people of
average height (56ft) would fall in the
shaded region
– Anyone shorter or
taller (the extremes)
would fall outside the
shaded region
Directional Selection
Directional selection Favors a phenotype at
one extreme of a trait’s
range.
– Causes a shift in the
population’s phenotypic
distribution
– Extreme/rare phenotype
becomes more common
Directional Selection
Ex: Peppered moth
example
– After the tree trunks
became dark again, the
trait for white bodies was
selected against because
it was no longer
beneficial. This caused a
shift in the distribution.
The mean range now
included mostly black
moths.
Stabilizing Selection
Stabilizing SelectionThe intermediate
phenotype is favored
and becomes most
common in the
population.
– Distribution becomes
stable in the middle
range/decreases genetic
variation
Stabilizing Selection
Ex: Human birth
weight
– Babies with either a
very low or very high
weight at birth are
more likely to
experience
complications than
babies of average
weight
Disruptive Selection
Disruptive selectionBoth extreme
phenotypes are
favored
– Intermediate is
selected against
Disruptive Selection
Ex: The African
swallowtail butterfly has
traits to be either
orange, black (the
extremes), or tan (the
intermediate)
– Predators don’t eat the
orange or black ones
because orange and
black butterflies of
different species are
poisonous. Thus, the
extreme trait is favored.
11.3 Notes
Gene Flow…
– Organisms move to new population &
reproduce --> its alleles are added to new
population’s gene pool and removed from
original one.
– Increases genetic variation receiving of
population
– Lack of gene flow can lead to speciation
11.3 Notes
Genetic Drift
– Small populations = more affected by
chance
– Elimination/sudden increase of alleles in a
small population = drift
– Drift = loss of genetic diversity
11.3 Notes
Bottleneck Effect
– Drift that occurs after an event reduces
population size
• Reduced population size = reduced genetic
variation
11.3 Notes
The Founder Effect
– Small number of individuals colonize a new
area
• Gene pool of new colony is usually very
different (less diverse) from original population
11.3 Notes
Effects of Drift
– Loss of genetic variation
• Less likely to have a large amount of
individuals that can survive a changing
environment
– Higher chance of lethal genes being
passed on instead of eliminated
11.3 Notes
Sexual Selection
– Males continually produce sperm, small
investment, can have many mates
– Females have limited number of eggs, long
gestational periods, invest more
time/resources
• Makes females choosy about mates
11.3 Notes
Sexual Selection
– Certain traits increase mating success
– Intra- : competition between males
– Inter- : showy males to attract mates
– Some showy traits can put animals in
danger, but suggest good
health/fertility/ability to care/defend to
potential mates. GOOD QUALITY.
1/26/12 Bell Ringer!
What kind of sexual selection is feautured in
this video? What traits might the beetle have
to make him successful?
Sexual Selection of Darwin Beetles
What kind of sexual selection is featured in
this video? Why are females so choosy about
mates?
Sexual Selection Birds of Paradise
Hardy-Weinberg Equilibrium
Describes populations that are not
evolving
“Equilibrium” - Genotype frequencies in
a population stay the SAME over time
as long as 5 conditions are met.
5 Conditions
Very large population - no drift
No emigration/immigration - no gene
flow
No mutations - no new alleles can be
added to gene pool
Random mating - no sexual selection
No natural selection - all traits aid in
survival equally
Hardy-Weinberg Equilibrium
Equilibrium is only met if ALL 5
conditions are met!!!!
– Real populations rarely meet all 5
Why study Hardy-Weinberg?
Biologists can compare real data to data
predicted by the model.
– Can learn more about the ways
populations evolve and about how to test
factors that can lead to evolution.
Review on what leads to
evolution
Genetic drift
Gene flow
Mutation
Sexual selection
Natural selection
Speciation Through Isolation
Speciation - the rise of two or more new
species from a previously existing
species
Isolated populations - when gene flow
stops
Isolated populations adapt to their
environments differently, which can lead
to differences in the gene pools
Speciation Through Isolation
Reproductive isolation - members of
different populations can not mate
successfully
– Reproductive organs not compatible
– Producing barren offspring
– Final step of speciation
What causes isolation?
Behavioral isolation - differences in
courtship or mating behaviors
– Bird dances
– Frog songs
– Firefly flash patterns
What causes isolation?
Geographic isolation - Physical barriers
that can separate populations
– Mountain ranges
– Rivers
– Valleys
What causes isolation?
Geographic
– Isthmus of Panama
separated a large
population of wrasse
fish
– Gene pools changed
over time
– Now 2 separate
species
What causes isolation?
Temporal isolation - mating seasons do
not line up
– High competition for mates at one time
Evolution Through Natural
Selection is Not Random
“Chance” and “random” relate to how
easily an outcome can be predicted
– Genetic drift
– Mutations
Evolution Through Natural
Selection is Not Random
Natural selection is NOT random
Natural selection pushes a population’s
traits in an advantageous direction
The response of species to
environmental challenges is not random
Patterns In Evolution
Convergent Evolution - evolution toward
similar characteristics in unrelated
species
Adaptations to similar environments
Patterns In Evolution
Shark and dolphin
tails
Separated by 300
million years of
evolution
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Patterns In Evolution
Divergent Evolution - Closely related
species evolve in different directions
Different appearances due to adapting
to different environments
Patterns In Evolution
Red fox and Kit fox
Red coat, temperate
region vs. Sandy
coat/big ears, desert
regions
Patterns In Evolution
Species interactions can lead to
connected evolutionary pathways
Coevolution - process in which two or
more species evolve in response to
changes in each other
Patterns In Evolution
Acacia plant and
stinging ant
Ant protects plant
from predators and
plant provides
nectar for the ant
Patterns In Evolution
“Evolutionary Arms Race” - coevolution
in competitive relationships
Patterns In Evolution
Garter snake and rough
skinned newt
Newt produces a
neurotoxin that
concentrates on their
skin
Garter snakes have
evolved a resistance to
the toxin
Driven newts to have
extreme levels of toxins
Patterns In Evolution
Extinction - The elimination of a species
from Earth
Most often occurs when a species
cannot adapt to an environmental
change
Patterns In Evolution
Background Extinction - occur
continuously at a low rate
Usually affect one or a few species in a
small area
Patterns In Evolution
Mass Extinction - Very rare, but very
intense, often on a global level
Can destroy many species
There have been at least 5 mass
extinctions in the last 600 million years
Patterns In Evolution
Punctuated Equilibrium - episodes of
speciation occur suddenly in geologic
time and are followed by long periods of
stability
Patterns In Evolution
Adaptive Radiation - Diversification of
one ancestral species into many
descendant species
– Descendant species usually adapted to a
wide variety of environments