Transcript Natural selection

Chapter 15 Notes, Natural
Selection and Evolution
DARWIN'S VOYAGE
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In 1831, a young amateur naturalist by the name of
Charles Darwin traveled on a ship called the HMS
Beagle.
On his voyage, he collected specimens of rocks,
fossils, animals and plants.
Along the voyage, Darwin stopped at an island
archipelago called the Galapagos Islands.
On the Galapagos Islands, Darwin observed strange
creatures like giant tortoises, marine iguanas, and
flightless cormorants.
GALAPAGOS ISLANDS
DARWIN'S DISCOVERY
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As fascinating as these creatures were, it was not
these creatures that made Darwin famous.
It was actually the discovery of some finches.
Initially when Darwin discovered the finches, he
thought that each bird was a different kind of bird
that was non-related to the other birds.
After returning to England, Darwin showed his
specimens to an Ornithologist who confirmed that
the birds were not different kinds of birds, but
different varieties of finches.
ARTIFICIAL SELECTION
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At the time of this discovery, selective breeding was
common with domestic breeds of animals.
Darwin noticed similarities between the selective
breeding of domestic plants and animals and the
different varieties of finches that he found.
Darwin called the selective breeding of plants and
animals artificial selection.
Darwin made a hypothesis that something was
happening in nature similar to artificial selection but
it was occurring naturally rather than by selective
breeding man. He called this natural selection.
ARTIFICIAL VERSUS NATURAL SELECTION
NATURAL SELECTION
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The only thing Darwin needed was a mechanism to
explain what caused nature to select for traits
without man directing it.
Then Darwin noticed that each finch had a unique
beak size and shape that assisted them in eating a
particular type of food that was most abundant on
each particular island.
Then he developed his hypothesis to explain the
mechanism for natural selection.
MECHANISM FOR NATURAL
SELECTION
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These are the four basic principles Darwin used to
explain what causes natural selection to occur.
1) All living things have variety within species.
2) Traits are inherited from parents to offspring.
3) Species compete with one another for limited
resources (food, shelter, water, nutrients etc.).
4) Those individuals that inherit an advantageous
trait from their parents will be more fit to survive
and therefore reproduce more successfully than
the individuals without the trait.
NATURAL SELECTION AND
EVOLUTION
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Darwin published his book On the Origin of
Species by Means of Natural Selection in 1859.
Darwin also introduced another idea in his
book he called the theory of evolution.
Darwin proposed that if given enough time,
small changes in species could produce much
larger changes.
In other words, he theorized that natural
selection could cause evolution.
NATURAL SELECTION AND EVOLUTION
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Natural selection is not the same thing as evolution.
Natural selection produces small changes in
populations of organisms to produce new species
or varieties of the same species. Natural selection is
observable, repeatable, and testable.
Evolution is a theory, that attempts to offer an
explanation for the origin of all living things. It also
suggests that all living organisms share a common
ancestor in the past. Because it is something that is
believed by some to have happened in the past
we can not directly observe it, repeat it, or test it.
EXAMPLE OF NATURAL SELECTION
Snowshoe
Hare
Desert
Jackrabbit
Cottontail Rabbit
EXAMPLE OF EVOLUTION
Black-Capped Chickadee
Tyrannosaurus Rex
HOW CAN NEW SPECIES FORM?
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First of all we have to define what a species is.
This is more complex than you may realize.
There is the Typological species concept or
morphological species concept, which is based on
similarities and differences of the physical
characteristics of organisms.
There is also the Biological species concept which is
based on whether or not organisms can interbreed
and produce viable and fertile offspring.
WHY USE THE BIOLOGICAL SPECIES
CONCEPT?
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The western meadowlark and eastern meadowlark may look alike
and live in overlapping regions, but they have different songs and
do not breed with one another. Are they the same species? Most
biologists say no. Why? Because they do not breed.
BIOLOGICAL SPECIES CONCEPT
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Some species can interbreed and have offspring,
but their offspring are usually inviable or sterile.
Examples of these hybrids between species: mule,
zonkey, liger, pizzly or grolar bear, beefalo.
SPECIATION
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Natural selection can cause speciation.
Speciation is the separation of one species into
two or more species.
Speciation usually begins with a genetic drift.
When the allele frequencies in a population
change or shift it is called a genetic drift.
A form of genetic drift that can occur when a
small population branches off from a large
population is called the founder effect.
THE FOUNDER EFFECT
SPECIATION
• There are two common ways that speciation often
occurs; allopatric and sympatric speciation.
• Allopatric speciation occurs when a population of
organisms becomes separated and isolated by a
geographical barrier.
• A geographical barrier could be a mountain range,
a large river, a canyon, a desert etc.
ALLOPATRIC SPECIATION
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The Abert's squirrel (found on the south rim)
and the Kaibab squirrel (found on the north
rim) are separated by the Grand Canyon.
SYMPATRIC SPECIATION
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Sympatric speciation can occur when
populations become isolated by ecological
factors, instead of geographic barriers.
For example different habitats may be
occupied, a different niche may be established,
or a different resource may be used.
Any of these factors could cause genetic drift to
occur within a population of organisms living in
the same area.
Cichlid fish species in Africa's Lake Victoria is a
good example of sympatric speciation.
SYMPATRIC SPECIATION
REPRODUCTIVE ISOLATION
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After speciation occurs, reproductive isolation
must occur for a new population of organisms to
become a new species.
In other words, successful reproduction between
two different populations must be prevented.
Prezygotic isolation is when allele frequencies
change dramatically enough that reproduction
between two species becomes unlikely due to
reproductive timing, behavior etc.
A good example of this is with Pacific Salmon.
REPRODUCTIVE ISOLATION
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Another way that populations can become
reproductively isolated is by postzygotic isolation.
Postzygotic isolation is when two organisms can
reproduce, but the offspring are sterile or inviable.
An example of this is a mule. A donkey's diploid
number is 62. A horse's diploid number is 64.
A hybrid mule has a diploid number 63, which
makes the mule sterile or unable to reproduce.
Offspring can also become inviable, or unable to
survive or develop normally to reproduce.
THE HARDY WEINBERG PRINCIPLE
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In 1908, English mathematician Godfrey Hardy
and German physician Wilhelm Weinberg
developed an equation that explains how
allele frequencies remain stable in a population
unless acted upon by certain circumstances.
Basically, in layman's terms they mathematically
demonstrated how natural selection works.
The Hardy-Weinberg principle can be written
algebraically by the following equation
p2 + 2pq + q2 = 1
Or p + q = 1
WHAT ARE THE VARIABLES IN THE
HARDY WEINBERG EQUATION?
SO WHAT DOES THE HARDYWEINBERG PRINCIPLE MEAN?
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Basically, it says that natural selection will not
occur unless the following events cause a
genetic drift in the allele frequencies.
A large population becomes small
Immigration or emigration occurs
Mating becomes nonrandom
A specific trait is selected
A mutation occurs
HARDY WEINBERG EQUATION
WHAT ARE THE DIFFERENT TYPES OF
NATURAL SELECTION?
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natural selection will not occur unless
certain forces cause selection to occur.
Some of these forces can be stabilizing,
directional, disruptive, or sexual.
Stabilizing selection occurs when the
intermediate or average form of a trait is
selected. Stabilizing selection operates to
eliminate extreme expressions of a trait.
STABILIZING SELECTION
FORCES OF SELECTION
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Directional selection is when one extreme form
of a trait is favored and selected.
FORCES OF SELECTION
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Disruptive Selection
is a type of selection
process that selects
for two different
extremes of a trait
and selects against
the average trait.
In this case, both
extremes use
mimicry as a
camouflage
strategy
FORCES OF SELECTION
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Some forces of
selection do not
enhance an
organism's
fitness or ability
to survive, but
rather advertise
an organism's
health or
dominance.
This type of
selection is
called sexual
selection.