Transcript How Populations Evolve20102011

Change overtime:
how populations evolve
Isn’t Evolution Just A Theory?
• How does the
scientific meaning of
a term like theory
differ from the way it
is used in everyday
life?
• Can the “facts” of
science change over
time? If so, how?
Evolution
• Scientific theories
– Concepts based on
the conclusions of
observations and
experiments.
– Reserved for a
conceptual scheme
supported by a large
number of
observations and not
yet found lacking.
Scientific Theories
Cell
All organisms are composed of cells.
Biogenesis
Life comes only from life.
Cells of a
plant leaf
Animal Liver cells
bacteria
protists
Scientific Theories
Gene
Organisms contain coded
information that dictates their
form, function, and behavior.
Scientific Theories
Evolution
All living things have a common ancestor.
Genetic changes in a population or species
over generations have led to the diversity of life
on Earth.
How Do We Know Evolution Happens
Fossils from
50 mya
Change over many generations
• How do fossils give us a
picture of change over time?
• What distinguishing feature of
the fossil Pakicetus skull
identified it as related to a
whale? Why was this
surprising?
• Why do scientists seek fossils
that are intermediate in form
and time between modern
forms and their probable
earliest ancestors?
Modern
Baleen whale
Fossil Called Missing Link from Sea to Land Animals
• What evidence from the
fossil fish skeleton
suggests it is an
intermediate form
between fish and land
animals?
Molecular clocks
• Comparing differences in DNA and/or protein
sequences reveal past mutations
How Does Evolution Really Work?
• What are the 4 components of
natural selection?
• What determines an individual
hummingbird’s beak length?
• What factors in the environment
might select for beak length and
shape within the hummingbird
population?
• How can hummingbird DNA help
Dr. Schindler determine the
evolutionary history of
hummingbirds?
Natural Selection
Natural selection
• The process by which
organisms with favorable
variations survive and
reproduce at a higher
rate.
• The process that results
in evolution.
Natural Selection
Natural selection is based on variation:
• Individuals in a population differ from one
another
• Many of these differences are genetic
– Genetic variation occurs because of
mutations and genetic recombination during
meiosis
Natural Selection
• Some variations may
– help individuals in a population to survive
– Increase the amount of offspring the
individuals have
• These traits if inherited would be passed
on to the offspring
• Over time, most individuals in the
population will have these traits.
Natural Selection
• Examples:
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–
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Peppered moths of England
Snails in England
Flat-tailed horned lizard
Pesticide/antibiotic resistance
Sickle cell disease
• Example of heterozygous advantage
– individuals who are heterozygous
for sickle cell disease are resistant to
malaria. This allows for these
individuals to survive better.
2 key points about
natural selection
1. Natural selection is
more of an editing
process than a creative
mechanism.
–
A pesticide does not
create resistant
individuals, but selects
for resistant insects that
were already present in
the population
2 key points about natural selection
2. Natural selection depends
on time and place
–
–
–
It favors those
characteristics in a varying
population that fit the
current, local environment.
Environmental factors vary
from place to place and
time to time
An adaptation in one
situation may be useless or
even harmful in different
situation
Microevolution
Gene pool
• The total collection of
genes in a population
at any one time.
• Consists of all the
alleles in all the
individuals found in a
population.
Microevolution
• A change in the gene pool.
• Occurs when the frequency of the alleles in a population
change over a number of generations.
Predict what it will the gene frequency be at Generation 40
Microevolution
How do you determine if microevolution is occurring?
•
The population is changing or in “disequilibrium”
Hardy-Weinberg Equilibrium
•
States what conditions must exist in a population so
that it doesn’t change (no evolution)
1. The population must be very large
2. Individuals do not migrate into or out of the population.
3. Mutations do not alter the gene pool
4. Mating is random
5. Natural selection does not occur
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf
How does the following Deviate from
Hardy-Weinberg equilibrium
•
1.
2.
3.
4.
5.
States what conditions
must exist in a population
so that it doesn’t change
The population must be
very large
Individuals do not migrate
into or out of the
population.
Mutations do not alter the
gene pool
Mating is random
Natural Selection does not
occur
• A windstorm blows
in hundreds of seeds
from a nearby
meadow, where
nearly all the
flowers are yellow.
What is wrong?
How does the following Deviate from
Hardy-Weinberg equilibrium
•
1.
2.
3.
4.
5.
States what conditions
must exist in a population
so that it doesn’t change
The population must be
very large
Individuals do not migrate
into or out of the
population.
Mutations do not alter the
gene pool
Mating is random
Natural Selection does not
occur
• A cosmic ray hits one
of the red flowers just
as a developing egg cell
is replicating its DNA.
By chance, a red allele
is transformed into a
yellow allele. What is
wrong?
How does the following Deviate from
Hardy-Weinberg equilibrium
•
1.
2.
3.
4.
5.
States what conditions
must exist in a population
so that it doesn’t change
The population must be
very large
Individuals do not migrate
into or out of the
population.
Mutations do not alter the
gene pool
Mating is random
Natural Selection does not
occur
• The flowers tend to
grow in red or
yellow patches. A
landslide buries and
kills a huge patch of
red flowers. What is
wrong?
How does the following Deviate from
Hardy-Weinberg equilibrium
•
1.
2.
3.
4.
5.
States what conditions
must exist in a population
so that it doesn’t change
The population must be
very large
Individuals do not migrate
into or out of the
population.
Mutations do not alter the
gene pool
Mating is random
Natural Selection does not
occur
• The red pigment in the
petals of red flowers is
poisonous and tends to
protect them from
beetles that eat the
developing seeds. The
yellow flowers are not
protected in this way.
What is wrong?
How does the following Deviate from
Hardy-Weinberg equilibrium
•
1.
2.
3.
4.
5.
States what conditions
must exist in a population
so that it doesn’t change
The population must be
very large
Individuals do not migrate
into or out of the
population.
Mutations do not alter the
gene pool
Mating is random
Natural Selection does not
occur
• The bees that pollinate the
flowers tend to develop a
“search image.” Once they
start visiting flowers of a
certain color, they stick to
that color.
• So pollen from red flowers is
more likely to be delivered to
other red flowers, and pollen
from yellow flowers is more
likely to fertilize other
yellow flowers. What is
wrong?
Microevolution
One of the possible causes of microevolution is
Genetic drift
•
change in the gene pool of a small population due to
chance
Microevolution
•
Bottleneck effect – an event
drastically reduces the
number of individuals in the
population. The surviving
individuals do not have the
same genetic makeup of the
original population
LE 13-9a
Microevolution
• Founder Effect – a small group of
individuals from the original population
colonize a new location.
13.16 Natural selection can alter variation in
a population in three ways
– Stabilizing selection: maintains variation for a
particular trait within a narrow range; it selects
against extremes in either direction -- ex:
human birth size between 6.5-9lbs
– Directional selection: acts against individuals
at one of the phenotypic extremes ex:
pesticide resistance
– Disruptive selection: favors individuals at both
extremes of the phenotypic range ex: light
striped snails v. dark snails
Speciation
Species
A population or group of populations whose
members can breed and produce fertile
offspring.
Speciation
The evolution of a new species
Speciation
Causes of speciation
include
1. Reproductive isolation
•
Prezygotic barriers
prevent mating
between populations.
•
Postzygotic barriers
prevent the
development of
offspring that can
survive and reproduce
Temporal Isolation
Seasonal isolation
Western Skunk
Fall breeder
Eastern Skunk
Winter breeder
East meets West
Territory overlap
• Courtship ritual in blue-footed boobies is an
example of one kind of prezygotic barrier,
behavioral isolation
• Many plant species
have flower structures
that are adapted to
specific pollinators
– This is an example of
mechanical isolation,
another prezygotic
barrier
Figure 14.2A, B
• Postzygotic barriers
prevent the development
of offspring that can
survive and reproduce
Speciation
2. Geographical Isolation
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•
•
Allopatric speciation
A population is separated from other
populations of the same species.
Gene flow is blocked.
Physical separation – mountain range, river.
LE 14-4
A. harrisi
A. leucurus
• Example: White-tailed antelope squirrel and
Harris’s antelope squirrel separated by the
Grand Canyon
14.7 Reproductive barriers may
evolve as populations diverge
Maltose medium
Results of
mating experiments
Female
populations
Same
Different
9
8
20
Mating frequencies
in experimental group
Different
22
Same
Female
Starch
Maltose
Male
populations
– laboratory
studies
(fruit flies)
Initial sample
of fruit flies
Starch medium
Male
Maltose Starch
• This has
been
documented
by
18
15
12
15
Mating frequencies
in control group
Figure 14.7A
LE 14-10b
Time
New lineage
Punctuated Equilibrium
Speciation
• The fossil record shows
how speciation has
occurred over geological
time.
• There are four eras
(Precambrian, Paleozic,
Mesozoic, and Cenozoic)
in which mass extinctions
were followed by rapid
speciation.
Speciation
• This evidence
demonstrates punctuated
equilibrium – the concept
that speciation occurs in
spurts followed by long
periods of little change.
LE 14-10a
Time
Gradualism
Speciation in the Fossil Record
The gradualist model of
species formation
• Populations evolve
differences gradually
as they become
adapted to their local
environments
• Speciation occurs by
the steady
accumulation of many
small changes