Transcript Evolution

Evolution
A Scientific Explanation for
Similarities and Differences
Between Species
Evolution
• Evolution = progressive change in
characteristics of organisms as a
result of changes in genetic
composition
• Two important aspects
– Descent from a common ancestor
– Adaptation to the environment
• Adaptation = characteristic that makes it
more likely that an organism will survive
and reproduce in its environment
A Flowchart of Evolutionary Reasoning
Potential for
rapid reproduction
Competition for
survival and
reproduction (1)
Struggle for
existence
Formation of new
genotypes leads to
phenotypic variation
Relatively constant
resources and
population over time
Variability in
structures and
behaviors
NATURAL SELECTION
On average, the fittest
organisms leave
the most offspring (2)
Some variability is
inherited; adaptations
increase in future
generations
(observations)
(conclusions)
Survival of
the fittest
EVOLUTION:
The genetic makeup of the
population changes over time,
driven by natural selection (3)
Adaptation
An increase in
frequency of
genotypes that
confer a favorable
advantage in a
given environment.
Natural Selection As the
Mechanism for Evolution
Applying Your Knowledge
1. Adaptation
2. Evolution
3. Natural selection
A. The mechanism for evolution is
B. A progressive change in the
characteristics of organisms is
C. A trait that makes a species
survival more likely is called a(n)
Evidence for Common Descent From the Fossil Record
Progressive changes from simpler to more complex
organisms can be seen in the fossil record.
Biogeographical
Evidence for
Common Descent
Different island species resemble each other.
Biogeographical Evidence for
Common Descent
Island Populations resemble
those on nearby land.
The Galapagos finches resembled the
grassquit found on the coast of Ecuador.
Anatomical Evidence for Common
Descent: Homologous Structures
Flying
Swimming Running Grasping
Anatomical Evidence:
Vestigial Structures
Remnants of
hindlimb seen in
boa and whale
Functional hindlimb
in salamander
Evidence for Common Descent
from Biochemistry
Evidence for Evolution from Biochemistry
Similarities in sequence measured by
ease of separating DNA strands by heat
Evidence for Evolution: Genetics
• Mutation generates diversity
• Meiosis and Fertilization generate new
combinations due to
– Crossing Over
– Alternate patterns of chromosome
segregation
– Unique genotype of fertilizing sperm
combined with unique genotype of egg
Two Types of Evolution
Microevolution
Macroevolution
Change within a
population or species
Change to a new
species
Microevolution led to
an increase in darkwinged Pepper Moths
in industrial regions of
Britain.
Evolution As a Change in
Genotypes
Individuals carrying the S’ allele were
more likely to survive when malaria
is the selecting agent.
Genotype
SS
SS’
S’S’
Phenotype
No disease
Susceptible to Malaria
Sickle Cell Trait (mild
symptoms)
Resistant to Malaria
Sickle Cell Anemia
Die from anemia
Malaria as an Agent
of Natural Selection
SS
X
S’S’
X
SS’
Malaria
Eliminates SS
Remaining
Genotypes
SS’
SS
X
S’S’
X
SS’
SS
X
SS’
S’S’
X
Anemia
Eliminates S’S’
SS’
SS’
Hardy-Weinberg Equilibrium
• A condition where allele frequencies and
genotypic frequencies remain constant
from generation to generation
• Changes from equilibrium values are
used to determine if natural selection is
occurring
Hardy-Weinberg Equilibrium
Allelic
Frequencies
p+q=1
p = frequency of dominant allele (eg. A)
q = frequency of recessive allele (eg. a)
Genotypic
Frequencies
p2 + 2pq + q2 = 1
p2= freq. of homozygous dominants (AA)
q2= freq. of homozygous recessives (aa)
2pq = frequency of heterozygotes (Aa)
Large population size
Random mating
No migration
No mutation
No selection
Conditions
Example Using
Hardy-Weinberg Equilibrium
• If the frequency of albinos in a
population is 9%, what is the frequency
of AA and Aa genotypes?
• Let A = allele for normal skin
pigmentation
• Let a = allele for albinism
Frequency of Albinos  q2  0.09
q  q2  0.3
p  1  q  0.7
.7A
.3a
.7A
.49AA
.21Aa
.3a
.21Aa
.09aa
Frequency of AA  p 2  0.49
Frequency of Aa  2pq  0.42
Frequency of aa  q2  0.09
Applying Hardy-Weinberg Equilibrium
Values to RFLP Analysis
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
Large
Population
Size
Genetic Drift: Changes in allele
frequency due to small population sizes
1. Founder effect
2. Population Bottleneck
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
Random
Mating
Non-random mating: Alters genotypic
but not allelic frequencies
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Migration
Migration: Can add new alleles, remove
alleles or change allele frequency
Leads to Gene Flow between populations
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Mutation
Mutation: Alters allele frequency,
causes formation of new genotypes
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Selection
Natural Selection: Increases frequency
of genotypes with higher fitness
Molecular Evolution
Two Hypotheses for the Origin of Modern Humans
DNA Analyses
Related to Human Origins
Visit http://www.dnalc.org/ and choose
Genetic Origins
Mitochondrial Control Region
Media and Animations
Solving the Mystery of the
Neanderthals
Other Applications of DNA Analysis
can be found at http://dnai.org
Choose Applications
Types of Selection
• Stabilizing: eliminates extremes
Types of Selection
• Disruptive: increases both extremes
Types of Selection
• Directional: increases one extreme
Applying Your Knowledge
1. Stabilizing Selection
2. Disruptive Selection
3. Directional Selection
Which type of selection has occurred if
• The background is sandy with dark rocks
and snails are found with either dark or
light shell colors?
• After spraying with malathion, more fruit
flies are found to be resistant to this
insecticide?
Species Formation
• Species = Group of actually or potentially
interbreeding natural populations which
are reproductively isolated from other
such groups
• Speciation depends on
– isolation (lack of gene flow)
– genetic divergence
Mechanisms for Speciation
• Allopatric Speciation
– Occurs as a result of geographical isolation
– Most common mechanism
• Sympatric Speciation
– Occurs in the same location
– Can be due to ecological isolation
– Can be due to Polyploidy
• Occurs for plants that have a sudden change
in numbers of chromosome sets
Allopatric Speciation
Single species
(white mice);
homogeneous habitat
(a)
Geographical barrier
(impassable river);
isolated populations
(b)
(c)
(d)
Genetic drift;
genetic divergence;
tan vs. white mice
Barrier removed
(river dries up);
Mice mix but don’t interbreed.
Summary of Allopatric Speciation
• One group separates from the population.
• Separate evolutionary pressures cause
different genetic changes in both groups.
(Is this (1) microevolution or (2) macroevolution?)
• Sufficient genetic changes accumulate so
that interbreeding cannot occur if groups
are rejoined.
(Is this (1) microevolution or (2) macroevolution?)
Sympatric Speciation
Single species
(white mice);
homogeneous habitat
(a)
Climate change;
two habitats;
isolated because don’t mix
(b)
(c)
(d)
Environmental pressure to adapt;
genetic divergence;
tan vs. white mice
Sufficient divergence;
now different species
Speciation by Polyploidy
Diploid with
chromosome set A and
chromosome set B.
Chromosomes duplicate
but do not separate
Tetraploid with two sets of
A and B.
Cross between diploid and
tetraploid species 
Triploid with one each of
chromosome sets A, B and D.
Modern Wheat
Chromosomes duplicate
but do not separate 
Hexaploid with three sets
of A, B and D.
Applying Your Knowledge
1. Sympatric Speciation
2. Speciation by Polyploidy
3. Allopatric Speciation
A. Which process involves a sudden, large
change in chromosome number?
B. Which process requires geographical
separation?
C. Which process can occur as a result of small
differences within the same local environment?
Patterns of Evolution
• Divergent
– different phenotypes arise as related
species encounter environmental
differences
Patterns of Evolution
• Convergent
– similar phenotypes arise in unrelated
species as a result of environmental
similarities
North American
Desert Plants
Cactus
African
Desert Plants
Euphorbs
Patterns of Evolution
• Coevolution: species adjust together to
maintain relationship
Predators
and their Prey
Flowering plants
and their Pollinators