Transcript Understanding Our Environment - McGraw Hill Higher Education

Evolution and Natural Selection
Chapter 11
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Outline
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Gene Variation
Pace of Evolution
 Fossil Record
 Molecular Record
 Anatomical Record
Hardy-Weinberg
Forms of Selection
Industrial Melanism
Species Formation
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Gene Variation
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Macroevolution - Evolutionary change on a
grand scale, encompassing novel designs,
evolutionary trends and episodic mass
extinction.
Microevolution - Differential survival and
reproduction due to natural selection.
Gradually alters population to include more
individuals with advantageous
characteristics.
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Gene Variation
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Darwin’s explanation of evolution:
 Adaptation by natural selection is
responsible for evolutionary changes within
a species(microevolution), and
accumulation of these changes leads to
development of new species
(macroevolution).
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Pace of Evolution
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Punctuated Equilibria
 Proposed by Stephen Jay Gould and Niles
Eldridge.
- Evolution normally proceeds in spurts,
with long periods of little movement in
between.
 Contrasted to the theory of gradual
evolutionary change (gradualism).
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Fossil Record
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Fossils are preserved traces of once-living
organisms created when organisms become
buried in sediment and calcium in hard
surfaces mineralizes.
 Often provide evidence of successive
evolution.
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Molecular Record
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Evolutionary theory allows evolutionary
change involves substitution of new versions
of old genes.
 New alleles arise by mutation and come to
predominate through favorable selection.
Molecular Clocks
 The longer the time since divergence, the
greater the number of differences in
nucleotide sequence of cytochrome C.
- Changes accumulate at constant rate.
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Molecular Clock of Cytochrome C
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Molecular Record
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Phylogenetic Trees
 Evolutionary history of a gene can be
mapped as a phylogenetic tree.
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Anatomical Record
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Homologous Structures - Structure and
function have diverged between body parts of
different animals, but are derived from same
part present in a common ancestor.
 Forelimbs of Vertebrates
Analogous Structures - Features resemble
each other as a result of parallel evolution in
separate lineages.
 Flippers of penguins and dolphins
Vestigal Organs - Organs no longer of use.
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Hardy-Weinberg Rule
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From 1920’s onward, scientists began
formulating theory of how alternative gene
forms (alleles) behave in a population, and
how changes in gene frequencies lead to
evolutionary change.
 1908 Hardy and Weinberg pointed out in
the absence of forces, in a large population
with random mating, allelic frequencies
remain constant.
- Hardy Weinberg equilibrium
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Hardy-Weinberg Equilibrium
(p+q)2 = p2 + 2pq + q2
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p = Individuals homozygous for allele B.
2pq = Individuals heterozygous for B and b.
q = Individuals homozygous for allele b.
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Hardy-Weinberg
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Assumptions:
 Large population size
 Random mating
 No mutation
 No immigration
 Absence of natural selection
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Change in Allelic Frequencies
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Mutation
 Error in replication of a nucleotide
sequence in DNA (Very slow rate).
Migration
 Movement of individuals from one
population to another (Dependent on
strength of selective forces).
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Change in Allelic Frequencies
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Genetic Drift
 Change in allelic frequencies due to
random events.
- Founder Effect - Population started by
few individuals and thus a restricted
gene pool (Rare genes may become
common).
- Bottleneck Effect - Gene pool becomes
very small, usually due to small
population size.
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Change in Allelic Frequencies
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Nonrandom Mating
 Individuals with certain genotypes mate
with more or less commonly than expected
on a random basis.
- Inbreeding - Mating with relatives.
 Increases homozygosity
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Change in Allelic Frequencies
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Selection
 Some individuals leave behind more
progeny than others.
- Artificial - Breeder selects desired
characteristics.
- Natural - Environment determines
adapted characteristics.
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Forms of Selection
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Disruptive
 Selection acts to eliminate intermediate
phenotypes.
Stabilizing
 Selection acts to eliminate both extremes
from an array of phenotypes.
Directional
 Selection acts to eliminate one extreme
from an array of phenotypes.
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Three Forms of Selection
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Sickle-Cell Anemnia
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Hereditary disease affecting hemoglobin
molecules.
 Sickle-Cell homozygosity frequently leads
to a reduced life span.
 Heterozygosity causes enough hemoglobin
to be produced to keep red blood cells
healthy.
Very common in Africa.
 Stabilizing selection as heterozygosity
infers less susceptibility to malaria.
- One of leading causes of death in Africa.
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Stabilizing Selection in Sickle-Cell
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Peppered Moths and Industrial Melanism
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Until the mid nineteenth century, Peppered
Moths ,Biston betularia, had predominately
light-colored wings.
 Subsequently, dark individuals became
predominant.
- Industrial smog helped turn lichens on
tree trunks dark.
- Contrasting colors between trunk color
and moth color led to differential
predation by birds.
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Peppered Moths and Industrial Melanism
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Second half of the twentieth century saw
widespread implementation of pollution
controls, thus trends reversed and light
colored moths again dominated.
But, caution must be taken, as the selective
agent could be some factor other than wing
coloration.
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Industrial Melanism
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Species Concept
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A species is generally defined as a group of
organisms unlike other such groups and
does not integrate extensively with other
groups in nature.
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Species Formation
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Local populations adapt to the specific
circumstances each faces.
When they become different enough, the
populations become ecological races.
Natural selection reinforces differences
through isolating mechanisms.
Two races become incapable of interbreeding
and are considered two separate species.
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Prezygotic Isolating Mechanisms
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Prevent formation of Zygote:
 Geographic Isolation
 Ecological Isolation
 Behavioral Isolation
 Temporal Isolation
 Mechanical Isolation
 Prevention of Gamete Fusion
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Geographical and Ecological Isolation In Oaks
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Postzygotic Isolating Mechanisms
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Prevent zygotes from developing into normal,
functional offspring.
 Improper development
- Reduced fertility or sterility.
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Review
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Gene Variation
Pace of Evolution
 Fossil Record
 Molecular Record
 Anatomical Record
Hardy-Weinberg
Forms of Selection
Industrial Melanism
Species Formation
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies