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Natural Selection
Professor Andrea Garrison
Biology 11
Illustrations ©2010 Pearson Education,
Inc. , unless otherwise noted
Natural Selection
• Mechanism for change in species over time
• Proposed by Charles Darwin in 1859
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Historical Perspective
• Concept of change in plant and animal species
not new with Charles Darwin
• Presence of fossils recognized as evidence of
former living organisms by ancient Greeks
• 1600s – 1800s views of church predominant
– Archbishop Ussher: earth ~6,000 years old
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Historical Perspective
• Theory of Uniformitarianism (Hutton, late
1700s)
– Same geological processes mold the earth today
as in the past, at same rate
– Gradual change is the norm
– Implies very old age for earth
• Charles Lyell (1830) -- Principles of Geology
• Earth millions of years old
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Historical Perspective
• William Smith (1769-1839)
– Surveyor during Industrial Revolution
– Went into mine shafts and surveyed
for canals to move coal
– Older rock strata covered by younger
rock strata
– Fossils can be used to identify same
rock layers in different parts of
England
• Presence of different fossil species in
different rocks strata evidence of change
in species over time
Natural Selection; picture from
http://en.wikipedia.org/wiki/William_Smith_(geologist)
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Historical Perspective
• Geologists established a view of a very old
earth built layer by layer and molded by slow
geological processes
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Historical Perspective
• Geologists set stage for
biologists
• Buffon (1707-1788)
– Divine creation for many
species
– “Lesser families” developed
over time by degenerative
processes
Natural Selection; http://en.wikipedia.org/wiki/Georges-Louis_Leclerc,_Comte_de_Buffon
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Historical Perspective
• Erasmus Darwin (mid- to late1700s)
– Species historically related
– Species changed with
environment
– Offspring could inherit those
changes
– No model to explain how this
happened
Natural Selection; picture Erasmus Darwin by Joseph Wright of Derby (1792),
http://en.wikipedia.org/wiki/Erasmus_Darwin
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Historical Perspective
• Inheritance of Acquired
Characteristics (Jean Baptiste
Lamarck, 1801)
– Changes in species induced by
environment
• Environmental changes cause changes in
organisms’ needs
– Demands on certain body parts → better
developed parts
– Body parts no longer needed →
degenerate from lack of use
– Example: giraffe stretches neck to get at
leaves in tree tops
» Offspring inherit the longer neck
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Historical Perspective
• Inheritance of Acquired Characteristics (Jean
Baptiste Lamarck,1801)
What’s wrong with this concept?
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Historical Perspective
• Inheritance of Acquired Characteristics (Jean
Baptiste Lamarck,1801)
What’s wrong with this concept?
If I work out every day, my baby will have strong
muscles (really ???)
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Historical Perspective
• Inheritance of Acquired Characteristics (Jean
Baptiste Lamarck,1801)
What’s wrong with this concept?
If I work out every day, my baby will have strong
muscles (really ???)
TRAITS MUST BE GENETIC TO BE PASSED ON TO
OFFSPRING!
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Historical Perspective
• Cuvier 1769-1832
– Foremost expert in fossils
– Established extinction as a fact
• Many fossils have no living
members of the species
• Many scientists had believed that
fossils were remnants of species
which still existed somewhere
– Strong opponent of change in
species
Natural Selection; picture from
http://en.wikipedia.org/wiki/Georges_Cuvier
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Historical Perspective
• Theory of Catastrophism (Cuvier, 1796)
– Predominate view of early 1800s
– Extinctions caused by series of natural disasters
– After each extinction, new species used resources
and filled ecological vacancies
• Never explained where the new species came from
• They had already been created, but fossils hadn’t yet
been found in other strata, so appeared new
• Cuvier’s followers taught the a series of divine creations
replaced species lost with each catastrophe
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Natural Selection
• Charles Darwin (1809-1882)
– Studied for medicine and
clergy
– Enjoyed observing and
collecting wildlife
– Age 22 became naturalist for
HMS Beagle (1831-1836)
– Was given copy of Lyell’s book
Principles of Geology before
he left
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• Darwin impressed by
– sheer number of different species
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Natural Selection
• Darwin impressed by
– sheer number of different species
– striking adaptations for organisms’ lifestyles
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Natural Selection
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Natural Selection
• Darwin impressed by
– sheer number of different species
– striking adaptations for organisms’ lifestyles
– Fossils in S. America more closely related to local
living species than fossils on other continents
• Seemed to show relationship over time
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Natural Selection
• Darwin impressed by
– sheer number of different species
– striking adaptations for organisms’ lifestyles
– Fossils in S. America more closely related to local
living species than fossils on other continents
• Seemed to show relationship over time
– Animals in different climates of So. America
seemed more similar to each other than to
animals of similar climates on other continents
• Seemed to show relationship based on location
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• Galapagos islands were especially interesting
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Natural Selection
• Galapagos Islands were
similar in climate and
topography to the Cape
Verde islands off No.
Africa
– Plants and animals very
different from those of
Cape Verde Islands
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Natural Selection
• Species on Galapagos
Is. similar to those on
mainland
– But there were
differences
• Species similar on
different islands, but
with their own
differences
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Natural Selection
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• Darwin noticed that plants and animals
appeared to have come to the Galapagos
Islands from the mainland, then gone through
a series of slightly different changes on each
island
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Natural Selection
• Darwin read Lyell’s book on the journey
– Earth much older than 6,000 years
– Gradually changing physical environment
• Back in England, Darwin looked at his collections
and notes
– Began to see a pattern that could be explained by
species changing over time as their environment
changed
– Could not see a mechanism for such a change in
species
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Natural Selection
• 1838—Darwin read Thomas Malthus’ Essay on
the Principle of Population (1798)
– Malthus wrote that plants and animals
reproduced more offspring that could survive, and
humankind would do the same if left unchecked
– Darwin recognized the forces he’s seen at work in
nature
• Realized this was the basis of the mechanism to explain
changes in species
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Theory of Natural Selection
•
Members of a population vary in structure or
behavior; traits hereditary (differed from Lamarck in
recognizing this)
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Theory of Natural Selection
•
Members of a population vary in structure or
behavior; traits hereditary (differed from Lamarck in
recognizing this)
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Theory of Natural Selection
• Members of a population vary in structure or
behavior; traits hereditary (differed from
Lamarck in recognizing this)
• Organisms have high reproductive capability
– Produce more offspring than environment can
support
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Theory of Natural Selection
• Members of a population vary in structure or
behavior; traits hereditary (differed from
Lamarck in recognizing this)
• Organisms have high reproductive capability
– Produce more offspring than environment can
support
• Resources are limited in environment
– Therefore competition exists between species
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Theory of Natural Selection
•
•
Members of a population vary in structure or
behavior; traits hereditary (differed from
Lamarck in recognizing this)
Organisms have high reproductive capability
– Produce more offspring than environment can
support
•
Resources are limited in environment
– Therefore competition exists between species
•
Individuals whose traits best adapted for
environment survive, others die
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Theory of Natural Selection
• Those “best-fit” for environment more likely
to survive and reproduce to pass on inherited
traits which make them so well suited for
that particular environment
– Unsuitable traits not passed on
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Theory of Natural Selection
• Those “best-fit” for environment more likely
to survive and reproduce to pass on inherited
traits which make them so well suited for
that particular environment
– Unsuitable traits not passed on
• THUS, ENVIRONMENT SELECTS FOR THOSE
ORGANISMS WHICH ARE BEST SUITED TO IT
(NATURAL SELECTION)
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Theory of Natural Selection
• BUT . . .
• Organism only best fit for the environment
that selected for it
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Theory of Natural Selection
•
•
Organism only best fit for the environment that
selected for it
If environment changes, so will traits necessary
for survival
– Thus, species will slowly change as environment
selects for those members of the population which
are best suited to the environmental changes
-OR– Species will die out
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Theory of Natural Selection
•
•
Organism only best fit for the environment that
selected for it
If environment changes, so will traits necessary
for survival
– Thus, species will slowly change as environment
selects for those members of the population which
are best suited to the environmental changes
-OR– Species will die out
•
Eventually the new species may be very different
from the original species ( = evolution)
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Theory of Natural Selection
• Natural selection supported by strong
evidence
• Natural selection = microevolution
• Most biologists today agree that natural
selection is the mechanism for major changes
over time (= macroevolution or evolution)
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Theory of Natural Selection
• Important to note:
– Natural selection does not make a change in an
individual
•
Entire species changes over time – takes many
generations
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Theory of Natural Selection
• Important to note:
– Natural selection does not make a change in an
individual
•
Entire species changes over time – takes many
generations
– Natural selection does not create new traits
•
New traits originate by chance and environment
selects for or against them
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Theory of Natural Selection
•
Important to note:
– Natural selection does not make a change in an
individual
•
Entire species changes over time – takes many generations
– Natural selection does not create new traits
•
New traits originate by chance and environment selects for
or against them
– Natural selection does not result in new, intricate
organs all at once
•
Each step is a very small change
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Theory of Natural Selection
– EXAMPLE: vertebrate eye
•
•
Probably originated as patch of light sensitive cells
Gradually accumulated modifications that increase
survival value
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Theory of Natural Selection
– EXAMPLE: vertebrate eye
•
•
Probably originated as patch of light sensitive cells
Gradually accumulated modifications that increase
survival value
– Idea of irreducible complexity
•
•
Gradual changes would not be selected for if they
didn’t provide new value
Organs so complex as eye require many different fully
formed parts all functioning at once
–
Any one of them does nothing on its own to be selected for
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Theory of Natural Selection
• Darwin’s theory of natural selection worked
out in 1838
• Worked on it for next 20 years for perfection
• 1858 Alfred Russell Wallace sent Darwin a
manuscript for comment
– He had developed same theory as Darwin
– Two men presented in same issued of Journal of
Linnaean Society in 1858
• Darwin published Origin of Species in 1859
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Theory of Natural Selection
• 1865 Mendel published his research on
genetics
• Today we know about DNA, chromosome and
genes
– Recognize that changes in outward traits due to
chance changes in genetic material (mutations)
and also new combinations of traits due to sexual
reproduction
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Theory of Natural Selection
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Theory of Natural Selection
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Theory of Natural Selection
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Adaptive Radiation
• Major pattern of change in species seen by
Darwin in Galapagos finches
– Sudden (in geological time) emergence of several
species from a common ancestor
– Caused by sudden availability of new or unused
resources
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Adaptive Radiation
• 13 different species of finches in Galapagos
– Now believed to have arisen from single pair or
pregnant female from So. American mainland
• Rafted to islands by storm
• Population size increased and resources decreased
• Individuals capable of using other resources did so to
survive
• Natural selection gradually selected for those groups
that could best use the several available resources
• Result: several different species
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Adaptive Radiation
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Figure 22.6
(b) Insect-eater
(a) Cactus-eater
(c) Seed-eater
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