Download chapter 15 POPULATIONS

Chapter 15
POPULATIONS
POPULATIONS
 Population

one species in one place at one time.
Units of evolution – how scientists study
and predict evolution.
 Demography

statistical study of populations.
Used to predict how the size of a population
will change.
What Drives Evolution:
There are 5 Forces of Change
Only natural selection
makes a population
better adapted (more
fit) to its environment.
Mutations Provide Raw Material For Evolution
mutation at the gene level
mutation at the chromosome
level
Mutations are usually neutral or harmful in their effects; only rarely are they beneficial.
Mutations “Just Happen”
Mutations occur at random without regard to whether they have a beneficial,
neutral or harmful
Mutations
are effect.
a randomly acting evolutionary forces.
Gene Flow or Migration
Gene flow – movement of genes from one
area to another.
Immigration & Emigration are key
factors in the human gene flow.
Gene flow in plants –
wind-dispersed
pollen moving
between Monterey
pines.
Gene Flow
Immigration: moving INto an area
• My mother immigrated to America when
she was 18 years old.
Emigration : EXITing an area.
• She emigrated from England after WWII.
Gene Flow or Migration
Genetic Drift
Genetic Drift - random fluctuation in allele
frequency between generations.
The effects of genetic drift are pronounced in small populations.
FYI
A Genetic Bottleneck
is a Form of Genetic
Drift
In a genetic bottleneck,
allele frequency is altered
due to a population crash.
FYI Genetic Bottleneck – A Historical Case
Note: A genetic bottleneck creates random
genetic changes without regard to adaptation.
A severe genetic bottleneck occurred in northern elephant seals.
Other animals known to be affected by genetic bottlenecks include the
cheetah and both ancient and modern human populations.
Endangered Species Are in the Narrow Portion of a Genetic
Bottleneck and Have Reduced Genetic Variation
The Effect of Genetic Drift is
Inversely Related to Population Size
Large populations = small effects.
Small populations = large effects.
FYI
Founder Effect
A founder effect occurs when a small number of individuals from one population
Another
Variation
of Geneticisolated
Driftfrom the original one.
found
a new population
that is reproductively
FYI
Founder Effect - Another Variation of Genetic Drift
The South Atlantic island of Tristan da Cunha was colonized by 15 Britons in 1814,
one of them carrying an allele for retinitis pigmentosum. Among their 240
descendents living on the island today, 4 are blind by the disease and 9 others are
carriers.
FYI - The Founder Effect
Old Order Amish populations are derived from a few dozen colonists who
escaped religious persecution in Germany in 1719 to settle in Pennsylvania.
The community
closed. frequencies in Amish are significantly different
Allele
and geneticisdisease
from the German ancestral and the surrounding local populations.
FYI - The Founder Effect
Non-Random Mating
Non-random mating;
Mates are chosen – as in maintaining a dog pedigree
Inbreeding is mating with close relatives.
• increases the frequency of recessive genetic disorders.
Natural Selection
An adaptation (favorable trait) allows nature
to choose who will have a better chance of
surviving and reproducing.
Natural
Selection
works on the
phenotype not
the genotype.
It’s not natural – but this is one
outcome of strong selection.
Copy this Graphic Organizer
Populations
Can evolve through
Gene
Mutation
Gene
Flow
Non-random
Mating
Gentic
Drift
Natural
Selection
End of Day One
KEY FEATURES OF
POPULATIONS
1. Size - how many individuals there are

–effects the ability of the population to survive.
A small population is more likely to become extinct:

Why? Drastically affected by:
 -random events or natural disaster
 -inbreeding. Recessive traits are more likely to appear.
 reduced variability it’s harder to be adapted to changes.
KEY FEATURES OF
POPULATIONS
2. Density – how close together in a given area.
 –too far apart: may only rarely encounter
one another resulting in little reproduction.
 too close together: resources may be scarce.
KEY FEATURES OF POPULATIONS
Population size is limited by:
density-dependent factors






density-independent factors
Disease
Competition
Predators
Parasites
Food
Crowding





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

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The greater the population,
the greater effect these
factors have.
Ex. Black plague in the
Middle Ages – more deaths
in cities

Volcanic eruptions
Temperature
Storms
Floods
Drought
Chemical pesticides
Major habitat disruption (as
in the New Orleans
flooding)
Most are abiotic factors
KEY FEATURES OF
POPULATIONS

3. Dispersion

–how are individuals arranged.
Most common
PREDICTING POPULATION
GROWTH
 Nearly all populations will tend to grow
exponentially as long as there are resources
available.
 4 basic factors affecting population growth:

birth rate & Immigration (growth)


(into)
death rate & Emigration (decline)

(exiting)
 r(rate of growth)=birth rate – death rate
FYI Exponential Growth - Problem
 Suppose you work for the month of July
 (31 days)
 If you earned a penny on July 1stand your pay
doubled each day; what would be your final
paycheck on July 31st?
 Write down an estimate in your notes.
Exponential Growth




Day 1 - 1¢
Day 2 - 2¢
Day 3 - 4¢
Day 4 - 8¢
 Day 10 - $5.12
 Day 20 - $5,242.88
 Day 31 - $10, 737,418.24
PREDICTING POPULATION
GROWTH – The “J” Curve
 Exponential growth curve: As
a population gets larger, it also
grows at a faster rate.
Maximum population
growth under ideal
circumstances.
FACT: No population exhibits this type of growth for long.
“S” Curve – Logistic Growth
Population numbers stay around the
carrying capacity.
 Carrying capacity (K): number of organisms that
an environment can support.
PREDICTING POPULATION GROWTH
Two modes of population growth.
The Exponential curve (J-curve) occurs The Logistic curve (S-curve) shows the effect
when there is no limit to population
size.
of a limiting factor (in this case the carrying
capacity of the environment).
POPULATION GROWTH
STRATEGIES
2 ways (Strategies) a population can grow:
r strategists: “J” curve.
insects, bacteria
Exponential growth; then crash
K strategists: “S” curve.
mammals, fish, birds
Live within the carrying capacity
Who are “r” and “K” strategists?
 r strategists (J curve)
 Small organisms/some plants
 Rapid maturations
 Quick to reach reproductive age
 Require no parent nurturing
 Have periods of time where eggs/endospores can
remain dormant
 K strategists (S curve)
 Larger mammals, fish, birds
 Require parenting
 Slower to grow & mature
 Few offspring
Natural Selection Shapes Populations
Natural selection acts on phenotype, NOT
genotype.
THEREFORE: selection against unfavorable
recessive alleles is SLOW.
Hardy-Weinberg Principle
 In 1908 G.H. Hardy and Wilhelm
Weinberg showed that dominant alleles
do not replace recessive ones.
 The Hardy-Weinberg Principle states:
populations do not change unless
evolutionary forces act upon them.
Hardy-Weinberg Principle, con’t
• An important way of discovering why
real populations change with time is to
construct a model of a population that
does not change.
• This is just what Hardy and Weinberg
did.
• Their principle describes a hypothetical
situation in which there is no change in
the gene pool hence no evolution.
Hardy-Weinberg Principle
• Hardy-Weinberg Principle:
– Predicts allele frequencies in a
population
– True only for a stable population
– Not applicable in “real” life. Some force
of change is always at work.
Hardy-Weinberg Principle
• p² + 2pq + q² = 1
• Frequencies of individuals that are:
• p² = homozygous dominant allele
• 2pq = heterozygous
• q² = homozygous recessive allele
Stabilizing Selection
Favors the Average
Directional Selection
Favors one extreme
Disruptive Selection
Favors both Extremes
Natural Selection Shapes Populations
NATURAL SELECTION
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
Favors one extreme
phenotype
Favors the average
phenotype
Favors both extreme
phenotypes
Possible reason:
Predators can identify
easier and eat the
average type organism
Crash Course populations
 http://www.youtube.com/watch?v=RBOsqmB
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9w4WxbX&index=2&edufilter=uD4dsTAhNKX
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