Download Natural Selection - Liberty Union High School District

Slides 1-13
Evidence for Evolution
1. Fossils show change over time
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scientists can date fossils & use them to
support the theory of evolution
common ancestors reveal whether
species are related
– Anatomy of living species also
shows relatedness
How Anatomy supports Evolution
2. Homologous Structures
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Traits similar in different species
because they share a common ancestor
Ex: human arm, dog front limb, horse
leg, whale fin
“ Look the Same”
How Anatomy supports Evolution
3. Analogous structures
• Distantly related species have structures
that have the same function but are
different in structure
• Ex: wing of butterfly & bird
“ Work the Same”
4. Vestigial structures
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Structures reduced in size & often
unused
Remains of functional structures
inherited from an ancestor
Ex: leg & hip bones in pythons &
whales
How do new species form?
1. Geographic Isolation
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When members of a population are
separated
Ex: polar, grizzly, & black bears
2. Reproductive Isolation
• When members of a population can’t
breed even though they live nearby
• Ex: different mating seasons or different
mating calls
Different Types of Evolution
1.
2.
3.
4.
Divergent evolution
Convergent evolution
Coevolution
Adaptive radiation
1. Divergent Evolution
Isolated populations evolve independently
Ex: polar & grizzly bears changed
independently due to different habitats
2. Convergent Evolution
Unrelated species become more alike
because they live in similar environments
Ex: shark & dolphin
3.Coevolution
Species that interact closely adapt to one
another
Ex: flowers & hummingbirds
4. Adaptive Radiation
Evolution of many diverse species from one
common ancestor
Ex: Galapagos finches discovered by Darwin
Population Genetics:
Evolution at the Gene
Level
Population all of the individuals of a
species that live together in one
place at one time.
Natural Selection: populations
changing in response to their
environment as individuals better
adapted to the environment leave
more offspring
4 Steps of Natural Selection:
1.
2.
3.
4.
Overproduction
Genetic Variation
Struggle to survive (limited resources)
Successful reproduction
Allele Frequency
• The relative amount of an allele of a
gene within a population (%)
• Example
• If 100 individuals make up a population
and 50 are homozygous dominant, 25 are
heterozygous, and 25 are recessive, what
is the allele frequency of the dominant
allele?
Allele Frequency
• You Try!
• If 300 individuals make up a population,
150 are homozygous dominant, 100 are
heterozygous, and the rest are
recessive, what is the allele frequency
of the recessive trait?
How does Natural Selection Affect
Allele Frequency
• Mutations provide the raw material
on which natural selection acts
• Evolution depends on variations
because it is the only way that
differences among organisms are
created
• Acts on Populations not individuals
by changing the % of alleles in the
population
Survival of the Fittest
If individuals having certain genes are
better able to produce mature offspring
than those without them, the frequency
(#) of those genes will increase
Survival:
Fitness:
Making it to reproductive age
Producing enough offspring
Different Lines of Evidence
•Wolf Lineage: all dogs are descendent
from the Grey wolf
•Darwin’s Finches: all 13 species are
descendent of a South American
species
•Whale DNA
Genetic Drift
Mutations that take place because of random
fluctuations of gene frequencies due to
small population size
• Example: Ellis-van Creveld syndrome
• A rare form of dwarfism that includes extra
fingers and toes–
• Amish population of eastern Pennsylvania,
which has intermarried over many
generations.
Gene Flow
Changes in the gene pool of a population
because of the introduction of genes
from another population by migration.
• Example: the gene pool in southeast Asia
was changed when U.S. Soldiers had
children with Vietnamese women during the
Vietnam war.
Founder Effect
Changes in a population when a small
population moves to a new location
bringing only a small fraction of the
genes and variation of the parent
population.
• Example: Galapagos Finches
Bottleneck Effect
Over-hunting, predation or natural disaster
leads to a population that no longer
represents the original.
• Organisms are recessive at almost all alleles
• Mutations: deformed Claws, low sperm count,
coat variation
• Example: Cheetahs (2 bottleneck events)
1st :10,000 years ago
2nd: 1980’s
Types of Natural Selection
• Stabilizing
• Directional
• Diversifying
Stabilizing Selection
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Extremes from both ends of the
frequency distribution are eliminated.
The most common form of natural
selection.
A real-life example is that of birth
weight of human babies.
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Too small or too big
you die in childbirth
Directional Selection
• Individuals at either end of the
distribution are favored. The
distribution shifts towards the left or
right, depending on which one is
favored.
• What we usually think of as natural
selection
Diversifying (Disruptive)
Selection
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Both extremes are favored at the
expense of middle phenotype
A mechanism for species formation
without geographic isolation
CLADISTICS
•A Cladogram is a branching diagram that
depicts species divergence from common
ancestors.
•They show the distribution and origins of
shared characteristics.
•Cladograms are testable hypotheses of
phylogenetic relationships.
Questions?