Evolution

1
Evolution

• Chapters 15 - 17

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Important People in Evolution

• Charles Lyell (1833) - wrote Principles of
  Geology
• James Hutton (1785) Proposed that Earth was
  shaped by geological forces that took place over
  extremely long periods of time. He estimates the
  Earth to be millions of years old.
• Thomas Malthus (1798) Predicted the human
  population would grow faster than the space and
  food supplies needed to sustain them.

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Jean-Baptiste Lamark (1809)

• Proposed that organisms changed over time due to
  use and disuse of certain organs/parts. The
  traits that were used in an organisms lifetime
  could be passed on to their offspring.
• His theory was flawed!

4
All of these ideas influenced Darwins thinking

• Charles Darwin (1831) Set sail on the H.M.S.
  Beagle.
• Voyage provided him with much of the data and
  evidence that lead to his theory of evolution.
• Darwins Theory of Evolution would turn the
  scientific world upside down!!!!

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What did Darwin Observe?

• Extreme diversity existed among plant and animal
  species around the world.
• The organisms found within a certain area were
  well suited to survive in that environment.
• Living organisms he observed greatly resembled
  fossils that he found on his voyage.

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The Galapagos Islands

• This tiny group of islands influenced Darwin the
  most!
• Although the islands were close together, the
  climates and environments differed greatly.
• Similar organism found on different islands had
  varying characteristics.
• Examples
• Giant Galapagos Tortoises with varying shell
  shape.
• Darwins Finches beak shape differed among the
  birds.

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Darwin Presents his Theory of Evolution

• 1859 Darwin published his work On the
  Origin of Species
• What is a species? A group of organisms that
  can interbreed and produce fertile offspring.
• What is natural variation? Differences among
  individuals of a species.
• Humans had been employing artificial selection
  for many years!
• Selection by humans for breeding of useful traits
  from the natural variation among the organisms

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Evolution by Natural Selection

• Struggle for Existence
• Survival of the Fittest Fitness results from
  adaptations that give an organism advantages for
  survival. The most fit organisms will survive
  and reproduce passing along the advantageous
  characteristics to their offspring. These
  changes can only be seen after many generations!
  
• Adaptation a trait that makes living things
  betterable to survive in their surroundings.

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Three Types of Adaptations

• Physiological Adaptations traits that involve
  the internal functions or chemistry of an
  organism. (ex weeds resistant to pesticides,
  bacteria resistant to antibiotics, etc.)

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Structural Adaptations

• Structural Adaptations traits that involve the
  physical structure or anatomy of an organism (ex
  quills of porcupine mimicry, camouflage, etc

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Some other structural adaptations are
subtle..MIMICRY
Mimicry enables one species to resemble another
species.
THE FROGS ON TOP ARE DIFFERENT FROM THE FROGS ON
ON THE BOTTOM!
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Behavioral Adaptations

• Behavioral Adaptations inherited traits that
  help an organism survive and reproduce in a given
  environment a behavior or response (ex living
  in hunting groups)

13
Descent with Modification

• As organisms change over time they become
  different, resulting in many varied species.
  This illustrates common descent. All living
  things have a common ancestor.
• Ex Owl ? White Mouse ? Black mouse!

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So

• All species living and extinct were derived
  from common ancestors.
• This concept is called COMMON DESCENT.
• Descent with modification common descent leads
  to the conclusion that there is a single tree of
  life that links all living things on Earth

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So, What is the proof all this happens?

• Darwin argued that living things have been
  evolving on Earth for millions of years and the
  Evidence Used to Support his Evolution Theory was
  based on
• Fossil Record
• Geographic Distribution of living species
• Homologous Body Structures
• Similarities in early development
• Biochemical Evidence

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• Geographic Distribution of Living Species
  -Species evolve differently based on where they
  live.

• Fossil Record - dead remains of an organism that
  has left an imprint in sedimentary rock and/or
  clay.

17

• Similarities in Early Development
• Vestigial Organs organ orstructure you no
  longer need (or has no known function ? ex
  appendix).

• Homologous Body Structures traits from a common
  embryo (one of the first stages of development).

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Evolution Genetics

• Darwin had a disadvantage when he developed his
  theory of evolutionhe did not understand the
  mechanisms of heredity.
• Today, we understand how genes, heredity, and
  evolution all tie together.
• Single-Gene Traits vs. Polygenic Traits
• Both lead to evolution, but polygenic traits
  (wide variety of phenotypes possible) lead to a
  more complex process of natural selection.

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The Three Types of Natural Selection

• Directional Selection, Stabilizing Selection,
  Disruptive Selection

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Three effects of selection on a characteristic

• 1. Directional Selection
• 2. Stabilizing Selection
• 3. Disruptive Selection

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Figure 23.12 Modes of selection
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Directional Selection

• When individuals at one end of the population
  curve have higher fitness than individuals in the
  middle or at either end of the population curve.
• Causes entire curve to move as character trait
  changes
• Ex ?beak size of Galapagos finches
• ?peppered moths
• ? antibiotic resistance

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Figure 23.13 Directional selection for beak size
in a Galápagos population of the medium ground
finch
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Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
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Stabilizing Selection

• When individuals near the center of the curve
  have higher fitness than individuals at either
  end of the curve
• Intermediate forms of a trait are favored and
  alleles that specify extreme forms are eliminated
  from a pop.
• Counteracts the effects of mutation, gene flow,
  and genetic drift preserves the most common
  phenotypes.
• Ex. Weight of human babies at birth

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Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality, high fitness
Selection against both extremes keep curve narrow
and in same place.
High mortality, low fitness
Percentage of Population
Birth Weight
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Disruptive Selection

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle.
• Forms at both ends of the range of variation are
  favored and intermediate forms are selected
  against selection creates two, distinct
  phenotypes
• Ex. Bird beak size no middle sized seeds,
  only large seeds and small seeds thus, small and
  large beaks are favored

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Figure 23.14 Diversifying selection in a finch
population
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Disruptive Selection
Section 16-2
Disruptive Selection
Largest and smallest seeds become more common.
Key
Population splits into two subgroups specializing
in different seeds.
Low mortality, high fitness
Number of Birdsin Population
Number of Birdsin Population
High mortality, low fitness
Beak Size
Beak Size
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Genes Variation

• While developing his theory of evolution, Darwin
  did not know how heredity worked
• Without understanding heredity, Darwin was unable
  to explain 2 important factors
• The source of variation central to his theory
• How hereditable traits were passed from one
  generation to the next
• Today, genetics, molecular biology, and
  evolutionary theory work together to explain how
  evolution takes place

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Gene Pools

• A gene pool is the combined genetic information
  of all the members of a particular population
• Recall that a population is a collection of
  individuals of the same species in a given area
  which share a common group of genes
• The relative frequency of an allele is the number
  of times that allele occurs in a gene pool
  compared to the number of times other alleles
  occur

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Sources of Genetic Variation

• The two main sources of genetic variation are
  mutations and the genetic shuffling that results
  from sexual reproduction
• A mutation is any change in a sequence of DNA
• Most inheritable differences are the result of
  gene shuffling that occurs during sexual
  reproduction

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Genetic Drift

• In addition to natural selection, genetic drift
  is a way by which allele frequencies can change
• In the real world, population sizes fluctuate
• Because populations fluctuate in size, sometimes
  there can be changes in allele frequencies due to
  random chance
• These changes are called random genetic drift
• In small populations, individuals that carry a
  particular allele may leave more descendants than
  other individuals, just by change
• Over time, a series of chance occurrences of this
  type can cause an allele to become common in a
  population

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The Power of Genetic Drift

• Genetic drift is a powerful force when a
  population size is very small
• Can and does lead to allele fixation
• Depends on starting frequency (which allele
  becomes fixed)

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Consequences of Genetic Drift

• Consequences of genetic drift
• Can and does lead to fixation of alleles
• Effect of chance is different from population to
  population
• Small populations are effected by genetic drift
  more often than larger ones
• Given enough time, even in large populations
  genetic drift can have an effect
• Genetic drift reduces variability in populations
  by reducing heterozygosity

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Real World Examples of Genetic Drift

• The Bottleneck Effect
• Occurs when only a few individuals survive a
  random event, resulting in a shift in allele
  frequencies within the population
• Small population sizes facilitate inbreeding and
  genetic drift, both of which decrease genetic
  variation

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Real World Examples of Genetic Drift

• The Founder Effect
• Occurs when individuals from a source population
  move to a new area and start a new population
• This new population is often started by
  relatively few individuals that do not represent
  the population well in terms of all alleles being
  represented

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So

• What determines which variants survive the event
  or get to the new location?
• Random chance
• Genetic drift has the largest effect on small
  populations (10-100 individuals)

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Speciation

• Isolating Mechanisms Reproductive Isolation
  Formation of a Species
• In other words, Isolating mechanisms are objects
  that separate species and prevent them from
  reproducing.
• By isolating a species ? a NEW species can be
  formed!!!

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3 Major Types of Isolation

• Behavioral Isolation Two separate species are
  capable of interbreeding, but they do not because
  of behavioral differences.
• Example ? Meadowlarks, different songs to
  attract mates courtship

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• Geographic Isolation When one species becomes
  separated by some barrier (river, mountain, road,
  mall, etc.) they may develop into two distinct
  species over time.
• Example ? Squirrel population split by Colorado
  River 10,000 years ago

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Temporal Isolation

• This mechanism occurs when species reproduce at
  different times (different mating times).
• Example ?Orchids in the rainforest, only release
  pollen 1 day!