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Mechanisms of Population Change

Populations may change through two
major genetic forces:

Natural Selection (includes artificial seltn)
 Genetic Drift

Two other forces that change a population:
 Mutations
 Gene
flow
 Not
much change, mutation is too rare & gene flow
tends to equalize gene frequencies
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
1
Natural Selection Occurs because:


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Organisms with favorable
traits… chance at survival
The longer an organism
lives, more offspring it
may have
More of the next
generation will have the
favorable trait than the
previous generation
2
Natural Selection


Individuals of one
phenotype are
favored
Peppered moths
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
3
Genetic Drift

Differences in reproduction or survival between
organisms due to random chance
 Environmental
disturbances can cause genetic drift as
long as they act at random:


Hurricanes
Volcanic eruptions (Mt. St. Helens)
 Can
be from random events
 seeds from white flowers are blown
to gravel, some seeds from red
flowers blown to good soil.
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Genetic Drift
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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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


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)
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
6
Adaptations

Adaptation –a characteristic that improves
the survival of a population or individual
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Adaptation



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
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
8
What are Species?

Members of a species
generally look alike
 Bald
eagles in Alaska (top) &
Colorado (bottom)
 But not always

Species –are reproductively
isolated from other species,
meaning that one can only
reproduce within one’s own
species
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
9
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
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
10
Walking Sticks

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The striped form 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.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
11
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.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
12
Walking Sticks

“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.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
13
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.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
14
Walking Sticks

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Population is diverging with
respect to alleles for:
 coloration (once were all green),
 digestion (once were generalists),
 dealing with plant toxins,
 mating behavior.
Natural selection is better
adapting each form to their
preferred plant.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
15
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W/ time, natural 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.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
16
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
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
17
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

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
18
Biological Evolution

Only populations can evolve; individuals
cannot
 (on
molecular level, genes can evolve)
Small scale change = microevolution
 Large scale change = macroevolution

© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
19
Biological Evolution

Macroevolution
 can
lead to speciation,
but it does not have to
 has occurred in dogs
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
20
Review
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Understanding evolution has
impacted our use of Antibiotics
and our understanding the
development of drug resistance

Doonesbury cartoon?
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Understanding evolution has
impacted our understanding of the
world (shown at right)
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Review of agents of change video
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