Transcript The date today (6/2)

Evolution and Natural Selection
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Pre – Darwin “Origin of species”

Earth relatively young
(thousands of years) ; this hypothesis was being replaced

In the 1700’s and early 1800’s, geological evidence
suggested that the earth was not young, but quite old,
and that it had undergone considerable change over its
history. Massive geologic formations, such as the
Grand Canyon, were seen as the result of slow
geologic processes
Pre – Darwin “Origin of species”

There were a limited number of fossils found
and most did not appear dramatically different
from current species; species were thought
not to change between generations and the
number of species on the earth was constant.

Organisms were thought to be perfectly
adapted to their environment.
Post- Darwin “Origin of species”

Species are related by descent

Adaptation to the environment is the result of the
interplay of random variation and natural
selection

The number of species was not constant and
species changed over time.

Descent with modification
Conditions necessary for decent
with modification

Variation within a population
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The variation is heritable

The reproductive success depends on the
available variation.
Evolution theory
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Variation exists among individuals within a species
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Organisms produce more offspring than the environment
can support

Competition exists among individuals
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The organisms whose variation best fit them to the
environment are the ones who are most likely to survive,
reproduce, and pass those desirable variations to the
next generation
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.
Gene Variation

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).
Evidences for evolution:
fossil record

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.
Take out a blank sheet of paper
On the paper write :
1. your name (first and last)
2. The date today (6/2)
3. The letter of the correct answer to the following question
Which of the following Conditions necessary for decent with
modification do you know are conditions of the ecosystem of the
predator-prey (Daphnia and Hydra) project ?
(i.e. exclude those conditions that would require
experimentation with to know that the condition was
being met)?
A.) Variation in traits exists within a population.
B.) The variation is heritable.
C.) The reproductive success depends on the available
variation.
D.) All of the above
E.) Only condition B is known.
Fossilization process limits
available fossils

Requires burial, sedimentary
rock, deposition of minerals
replacing hard parts of an
organism.

Soft parts preserved in
impressions or casts in soft
sediment
Dating the fossil record – isotopic dating
5.00E+09
4.50E+09
4.5 billion
4.00E+09
3.50E+09
3.00E+09
2.50E+09
2.00E+09
1.50E+09
1.00E+09
Origin of mammals & dinosaurs
End of dinosaurs
Humans, Chimps diverge
Cambrian
Land vertebrates
Oldest animals
Multicellular fossils
Oldest fossil
Origin of earth
Timelines
History of Earth
5.00E+08
500 million
0.00E+00
Molecular Record

Evolutionary theory allows evolutionary change
involves substitution of new versions of old
genes.

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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.
Molecular Record

Phylogenetic Trees

Evolutionary history of a gene can be mapped
as a phylogenetic tree.
Mechanisms of evolution
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Genetic drift

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Mutation
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Random fluctuations in the allele frequencies
within a population
A random change in the coding of an allele or
gene
Natural selection (including sexual selection)

the differential reproduction of genotypes
caused by factors in the environment.
Anatomical Record – products of
evolutionary process

Homologous Structures - Structure and
function have diverged between body parts of
different animals, but are derived from same
part present in a common ancestor.


Analogous Structures - Features resemble
each other as a result of parallel evolution in
separate lineages.


Forelimbs of Vertebrates
Flippers of penguins and dolphins
Vestigal Organs - Organs no longer of use.
Products of evolution

Adaptation – organisms are not perfectly
adapted ( have to make due with your
genetics)

Examples: panda thumb
Hardy-Weinberg Rule

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.
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Hardy Weinberg equilibrium
Hardy-Weinberg individuals do not evolve
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Assumptions:

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Large population size
Random mating
No mutation
No immigration
Absence of natural selection
Change in Allelic Frequencies
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Mutation

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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).
Change in Allelic Frequencies

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.

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
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
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Stabilizing
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Selection acts to eliminate intermediate phenotypes.
Selection acts to eliminate both extremes from an
array of phenotypes.
Directional

Selection acts to eliminate one extreme from an array
of phenotypes.
Three Forms of Selection
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Frequency of A allele
Control: no difference in fitness
0.5
Frequency of A allele
0.5
Frequency of A allele
Balancing selection
0.5
Frequency of A allele
0.5
Sickle-Cell Anemnia

Hereditary disease affecting hemoglobin
molecules.


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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.
Stabilizing Selection in Sickle-Cell
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Peppered Moths and Industrial
Melanism

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.
Peppered Moths and Industrial
Melanism


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.
Industrial Melanism

Example of
directional selection
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Species Concept

A species is generally defined as a group
of organisms unlike other such groups and
does not integrate extensively with other
groups in nature.
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.
Prezygotic Isolating Mechanisms
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Prevent formation of Zygote:
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Geographic Isolation
Ecological Isolation
Behavioral Isolation
Temporal Isolation
Mechanical Isolation
Prevention of Gamete Fusion