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Mechanisms of Population Change
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Populations may change through two major genetic forces:
Natural Selection (includes artificial seltn)
Genetic Drift
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Two other forces that change a population:
 Mutations
 Gene flow
 Not
much change, mutation is too rare & gene flow tends to equalize gene
frequencies
Natural Selection Occurs because:
Organisms with favorable traits have a better chance at survival
 The longer an organism lives, the more offspring it may produce
 More of the next generation will have the favorable trait than the
previous generation
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Natural Selection
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Individuals of one phenotype are favored
Peppered moths
Genetic Drift
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Differences in reproduction or survival between organisms due to random
chance
Environmental disturbances can cause genetic drift:
Hurricanes or Volcanic eruptions
Clearing land for development or Over-hunting
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Can be from random events
 seeds
from white flowers are blown to gravel, some seeds from red flowers blown to
good soil.
Genetic Drift
How Populations Change
Mutation creates genetic variation (genes)
 Gene flow, genetic drift and natural selection act on this genetic
variation to change allele frequencies in populations
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Gene flow tends to equalize allele frequencies between 2 pops.
Genetic drift and natural selection tend to diverge allele frequencies
between 2 pops. (populations tend to diverge)
Changing Populations
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Adaptation of populations; better adapted to their environments
Diversity of life
Shared inherited characteristics between organisms
Adaptations
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Adaptation –a characteristic that improves the survival of a
population or individual
Adaptation
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An improvement in a population over time, changing a population to better
fit its environment
Only the result of natural selection acting on genetic variation, no other
force adapts a population
Environments change and new genetic variation arises, so adaptation never
reaches perfection
Adaptation:
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Matches organisms to environment
Can be complex
Accomplishes specific functions
Adaptations Are Not Perfect
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Natural selection has limitations:
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Genetic constraints
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Multiple effects of
genes in development
Ecological trade-offs
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No variation, no adapting
What is good for one thing may hurt in another
What are Species?

Members of a species generally look alike
 Bald eagles in Alaska
 But not always

(top) & Colorado (bottom)
Species –are reproductively isolated from other species, meaning that one
can only reproduce within one’s own species
Walking Sticks
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Adapting to match their environment (two mating pairs)
Striped form is well-hidden among the needle-like leaves w/ stripes, solid
form among solid leaves
Walking Sticks
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The striped form of the species prefers needle-like leaves of the chemise
(sheh-mēz) bush.
The solid-green form prefers the solid green leaves of the wild lilac.

The two forms are more likely to survive on their preferred plant.
 Predators
(birds) more likely to catch them if they are not on their preferred plant.
Walking Sticks
Is a complex adaptation, more than coloration.
 Lilac and chemise bush leaves have different pulp, fiber, chemical contents,
and plant toxins.
 The solid-green walking sticks have enzymes to better digest the leaves of
the wild lilac.
 The striped form has enzymes adapted to better digest the leaves of the
leaves of the chemise bush.
Walking Sticks
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“Hybrid offspring” of a cross between solid and striped parents
are less fit than offspring of a single form mating.
 Do
not blend as well on either Lilac or chemise bush leaves; predators more
likely to eat them.
 Do not digest either plant as well as single forms.
Walking Sticks
Because “Hybrid offspring” (a solid & striped cross) are less fit than “single
form” offspring, the population is adapting in one other way:
 Individuals prefer to mate with others of the same color pattern.
Walking Sticks
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Population is diverging with respect to alleles for:
 coloration,
 digestion,
 dealing with plant toxins,
 mating behavior.
Natural selection is better adapting each form to their preferred plant.
W/ time, disruptive selection will further diverge the two forms & further
decrease their interbreeding.
When the two forms loose the ability to interbreed, they will become two
different species.
Speciation
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When one species splits to form two species
Results from reproductive isolation
Often, it is a secondary consequence of changing populations
Occasionally, is a direct consequence of a diverging population
 The
hybrid is less fit, walking sticks
Rates of Speciation differ in different organisms
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Walking sticks, have a reason to evolve reproductive isolation quickly,
hybrids are less fit
speciation may occur in a couple thousand years.
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European and N. American sycamore trees do not have a reason to evolve
reproductive isolation
they were separated geographically 20 mya & do not normally interbreed
speciation has not yet occurred
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Speciation
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The splitting of one species into two has never been seen in
animals
 check
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 It
back in a couple thousand years
Walking sticks
Apple maggots (worms)
has been seen in plants (2 species split into 3).
Biological Evolution
Is a change in the genetic characteristics of a population over time
 If any allele frequency in a population changes w/ time, the
population is evolving
 Most people think of evolution as Darwin’s speciation, but this is
not correct
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Biological Evolution
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Only populations can evolve; individuals cannot
 (on
molecular level, genes can evolve)
Small scale change = microevolution
 Large scale change = macroevolution
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Biological Evolution
Impact of Evolutionary Thought
Understanding
evolution has impacted technology:
 Development and use of pesticides
 Antibiotics and the development of drug

resistance
Doonesbury cartoon?
I have assigned the readings on Evolution, but I have only lightly covered this
topic