Download Natural selection

Chapter 15 Notes, Natural
Selection and the Theory of
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.
At first, when Darwin discovered the finches, he thought
that each bird was an completely different nonrelated kind
of bird.
But, 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 the same kind of bird.
Artificial and Natural Selection
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At the time of his discovery, selective breeding was
common with domesticated breeds of animals.
Darwin noticed similarities between the selective breeding
of domestic plants and animals and the different varieties
of finches that he found.
The selective breeding of domesticated plants and animals
that Darwin observed is often referred to in science as
artificial selection.
Darwin made a hypothesis that something was happening
in nature that was similar to artificial selection (selective
breeding), but it was occurring naturally rather than by the
direction of man. He called this phenomenon natural
selection.
Artificial versus Natural Selection
Natural Selection
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The only thing Darwin needed was a mechanism to explain
what causes natural selection to occur.
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 discovered the mechanism he needed to explain
natural selection.
Mechanism for Natural Selection
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These are the four principles Darwin used to explain
what causes natural selection.
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 more successful at reproducing and passing
on their genes to the next generation.
Natural Selection and Evolution
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Darwin published his book On the Origin of Species by Means
of Natural Selection in 1859.
In his book Darwin introduced a controversial idea called
the theory of evolution.
Darwin’s theory proposed that if natural selection produces
small changes, then perhaps those small changes could
gradually produce much larger changes in a very large
amount of time.
In other words, he theorized that natural selection might
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 to produce new species or new
varieties of the same species. Natural selection is
generally accepted as a fact and can be directly
observed, repeated, and tested.
Natural Selection and Evolution
• Evolution is a theory, that offers an explanation
for the origin of all living things. It suggests
that all living organisms share a common
ancestor in the past.
• Because this theory is something that is believed
by some to have happened in the past, we can
not directly observe it, repeat it, or test it.
Examples of Natural Selection
(Bergman’s and Allen’s Rule)
Snowshoe
Hare
Desert
Jackrabbit
Cottontail Rabbit
Examples of Natural Selection
Examples of Evolution
Black-Capped Chickadee
Tyrannosaurus Rex
Archaeopteryx
Similarities and Differences
Between Birds and Reptiles
Reptiles
Birds
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Cold-blooded
Skin with scales
Bones similar to mammals
Leathery Amniotic Eggs
Respiratory System with fully
functioning lungs
• 3-Chambered Heart
• Mostly Quadruped
• Teeth
Warm-blooded
Feathers (scales on feet)
Hollow bones
Brittle Amniotic Eggs
Air-Sac Respiratory System (No lungs)
4-Chambered Heart
Bipedal with Wings
Muscular gizzard
The Fossil Record
• During a period of time that is often referred to as
the “Cambrian Explosion” almost all representatives
of every animal phylum alive today suddenly appear.
What is a Species?
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The definition of a species has historically been a topic of
mixed opinions and varying interpretations.
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 typically unviable 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 genetic drift.
Genetic drift occurs when the frequencies of alleles in a
population change.
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
separate 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
populations 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 unviable.
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 unviable, 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 will most
often remain stable unless acted upon by certain
circumstances.
In other words, they mathematically showed how and
when natural selection takes place.
The Hardy-Weinberg principle can be written
algebraically by the following equation
p2 + 2pq + q2 = 1
Or p + q = 1
Hardy Weinberg Equation
So what does the Hardy-Weinberg
Principle Mean?
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Natural selection will not occur unless the following
situations occur.
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A large population becomes small.
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Immigration or emigration occurs.
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Mating becomes nonrandom.
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A specific trait is selected.
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A mutation occurs.
Hardy Weinberg Equation
What are the four different forces of
Natural Selection?
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The four forces of natural selection can push or drive
natural selection in a specific direction are referred to
as 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 traits.
Stabilizing Selection
Forces of Selection
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Directional selection is when one extreme form of a trait
is favored or selected.
Forces of Selection
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Disruptive Selection
is a form of selection
that selects for the two
different extreme forms
of a trait and selects
against the average
form of a trait.
In this example, 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.