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Natural Selection
Nature of Science
• Science
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method of explaining natural world
assumes universe regularly operates the same
uses logical system to understand regularities in nature
emphasizes logical testing of explanations by gathering
data
– limited to explaining natural world by means of natural
processes
– continually changing as new observations and
discoveries are made
Scientific Theories
• Theories
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Have been tested and retested many times
Supported by lots of data
Have been tested against many other ideas
Can always be tested, modified or discarded
Evolution
• Process of change over a period of time.
• Concept first proposed by other naturalists
(Buffon) before Darwin completed his work
Charles Darwin
• Published On the Origin of
Species by Means of Natural
Selection, or the Preservation
of Favored Races in the
Struggle for Life on November
24, 1859
Charles Darwin
• Presented mechanism by which evolution could occur
natural selection
• Book covered many aspects of biology
• Journey to publication of book was long and influenced
by many people, personal observations and ideas.
Lyle and Hutton
• Geologists
• formulated the ideas that
– the earth was several million years old and that
– the earth had changed during those millions of years
Lamarck
• First to propose a mechanism by which
evolution could occur
• His proposed mechanism is incorrect, but
his experiments paved the way for other
scientists to better understand evolution
• Hypothesis
– By selective use or disuse of organs,
organisms acquired or lost certain traits during
their lifetime. These traits could then be
passed on to their offspring. Over time, this
process led to change in the species.
• Hypothesis included concepts of
– Tendency toward perfection
– Use and disuse
– Inheritance of acquired characteristics
Malthus
• English economist from late 1700’s
• Published a book where he proposed that if the
human population continued to grow unchecked,
sooner or later there would not be enough
resources for everyone
• He also noted that individuals were being born
faster than they were dying
• Darwin read his publication on his voyage and
used this information to clarify his hypothesis of
natural selection
Darwin made two points in his book
• 1. Principle of Common Descent
– Species evolved from ancestral species
– Life is united because all organisms are related through descent from
common ancestor
– Adaptation accumulate as descendants from common ancestor
moved into various habitats over millions of years.
– Descent with modification could account for diversity of life
Darwin made two points in his book
• 2. Natural selection
– Mechanism which could result in evolutionary change
– Process by which individuals with inherited
characteristics well suited to the environment leave
more offspring on average than do other individual
– Idea was evident by observing results of artificial
selection
• Nature provided the variation and humans selected the
variations they found useful and controlled the breeding
Darwin Developed His Theory by
Stating 3 Inferences Based on 5 Facts
• Fact 1: Overproduction.
– All species have such great potential fertility that their
population size would increase exponentially if all individuals
that are born would reproduce successfully
Darwin Developed His Theory by
Stating 3 Inferences Based on 5 Facts
• Fact 2: Stability of
Populations.
– Most populations are
normally stable in size
except for seasonal
fluctuations
Darwin Developed His Theory by
Stating 3 Inferences Based on 5 Facts
Fact 3: Limited Resources.
– Natural resources are limited
Inference 1
• Production of more individuals than the
environment can support leads to a struggle for
existence among individuals of a population, with
only a fraction of offspring surviving each
generation
Fact 4--Variation
• Individuals of a population vary in characteristics
• No two individuals exactly alike
• Ways in which variation is created within a species
1. Mutations.
2. Shuffling of genes during meiosis
3. Sexual Reproduction
Fact 5: Inheritance of Variation
• Much of this variation is heritable
Inference 2
• Survival in the struggle
for existence is not
random, but depends in
part on the inheritable
characteristics of the
surviving individuals.
They have
characteristics that in
some way gives them
an advantage over
others.
Inference 3
• This unequal ability
of individuals to
survive and
reproduce will lead
to a gradual change
in a population, with
favorable
characteristics
accumulating over
the generations
Fitness
• The ability of an organism to survive AND
reproduce is fitness
• Adaptations are inherited characteristics that
increase the fitness of individuals in a population
• Over time, natural selection results in changes in
the inherited characteristics of a population.
These changes increase a species’ fitness in its
environment
Evidence For Evolution
• A. Geographic Distribution or Biogeography
1. Differences and similarities between organisms in
different parts of the world
2. Patterns in geographic distribution of life make sense
in an evolutionary context
3. Example: Marsupials in Australia; Lungfish distribution
Evidence For Evolution
• 1. Fossil Record
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Positions of rock strata reveals relative age
Younger rock layered on top of older ones
Oldest fossil evidence of life--3.8 billion years old
Prokaryotes found in rocks of 3.5 billion years in age
Evidence For Evolution
• 1. Fossil Record
5. Fossil evidence often link past and present life forms
6. Example: Basilosaurus and whales
Evidence For Evolution
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2. Similarities in Structure
a. Homologous structures
--structural similarities in species sharing a common ancestor
b. Example: forelimb structure in vertebrates
Evidence For Evolution
• 2. Similarities in Structure
c. Evolution is a remodeling process
--descent with modification
• especially evident with “less than perfect” structures
• vestigial organs
Evidence For Evolution
• 3. Similarities in
Development
a. Embryos of closely related
organisms often have similar
stages in development
b. Example: Vertebrate
Embryos
Evidence For Evolution
• 3. Similarities in
Development
a. Embryos of closely related
organisms often have similar
stages in development
b. Example: Vertebrate
Embryos
Evidence For Evolution
• 4. Molecular Biology
a. Sequences of bases in DNA are passed from parents
to offspring
b. DNA sequences determine amino acid sequences of
proteins
c. These proteins are the records of an organisms
ancestry
d. The more closely related organisms are, the more
similar their DNA
e. Example: Cytochrome C protein sequence differences