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Evolution
Chapter 15
Darwin’s Theory of Evolution
15-1 The Puzzle of Life’s
Diversity
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Biological Diversity- variety of living things.
Evolutionary Theory- a collection of scientific
facts, observations & hypotheses; it can
explain the diversity of life on Earth.
Evolution- change over time; the process by
which modern organisms have descended
from ancient organisms.
Scientific Theory- a well supported testable
explanation of natural events; supported by
evidence & can be tested with new evidence.
Charles Darwin
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Contributed more than any other scientist to our understanding
of evolution.
Voyage of the Beagle- sailed around the
world stopping at the Galapagos Islands.
He made observations & collected evidence.
He noticed many plants & animals were well
suited to their environment.
He collected fossils- preserved remains of
ancient organisms.
He collected many different species he had
never seen before.
Galapagos Islands
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Islands west of South America that are close
together but have very different climates.
Darwin studied shell shape of the many
different tortoises.
He discovered many similar but different birds
that turned out to be finches.
Each species of finch had a different shaped
beak which was a structural adaptation.
Darwin felt the animals on all the different
islands once came from the same species or
common ancestry.
15-2 Ideas that shaped
Darwin’s Thinking
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He felt that many would find his ideas
too shocking to accept.
He began to put all of his work on
paper.
Several scientists influenced Darwin:
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Hutton- earth is shaped by geological forces
Lyell- earth is shaped by geological forces
Lamarck- in inherited acquired traits; organisms
change over time.
Malthus-predicted that human pop. Would
grow faster than space and food would
15-3 Darwin’s Case
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He was reluctant to publish his ideas
because they were so radical.
He was disturbed by his own findings
which challenged fundamental scientific
beliefs.
When he heard a scientist name
Wallace had the same idea as his,
Darwin published his work.
Darwin Published his Book
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On the Origin of Species – 1859.
The book provided evidence that
evolution has occurred.
His theory on how evolution comes
about was based on artificial selection.
He believed that natural selection
occurs in nature.
Artificial & Natural Selection
Artificial Selection
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Animal breeders select for breeding only animals with
the desired traits.
Ex. Crops, livestock & horses.
Natural Selection
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Individuals differ & some of the differences can be
passed on to their offspring.
More offspring are produced than can survive or
reproduce.
Individuals best suited to their environment survive &
reproduce most successfully.
Ex. run fast, outsmart with behavioral tactics.
Survival of the Fittest
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Animals prey on other animals. Some die and
some survive.
Fitness- the ability to survive & reproduce in a
specific environment.
Only the fittest organisms pass on their traits,
thus the species change over time.
Adaptation- an inherited characteristic that
can be physical or behavioral.
Principle of Common Descent
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All species come from common
ancestors.
It links all organisms on Earth into a
single tree of life.
Ex. Tigers-panthers-cheetahs,
felines-horses-dogs-bats, mammalsbirds-alligators-fish.
Types of Evidence to Support
Evolution
1. Fossil record- comparing fossils from older &
younger rock layers documents the fact that
evolution has occurred; it is a detailed
record of evolution. Pg. 382,15-13
2. Geographical distribution of living speciesthe presence of similar environments
suggests natural selection. Ex.-the finches
could have descended from common
ancestors on the mainland. Pg. 383, 15-14
4 Types of Evidence to
Support Evolution
3. Homologous structures of living organismsthey have different mature forms but
develop from the same embryonic tissues.
Bone structure is the same but they look
different. All are adapted to survive in
different environments. Ex. Legs, wings,
fins. Pg 384, 15-15
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Vestigial organs- some homologous structures
no longer serve important functions in
descendants. Their size is reduces.
4 Types of Evidence to
Support Evolution
4. Similarities in early development, or
embryology- early stages, or embryos, of
many animals are very similar. Pg. 385, 15-17
5. DNA evidence
Evolution
Chapter 16
Evolution of Populations
16-1 Genes & Variations
What we already know:
 Traits are controlled by genes.
 Many genes have at least 2 forms or
alleles.
 Individuals of all species are
heterozygous for many genes.
Genetic Variation
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Studied in populations
Population- a group of individuals of the same species that
interbreed; share a common group of genes.
Gene Pool- results from interbreeding in populations; it’s
all the genes that are present in a population; contains 2
or more alleles for each inheritable trait.
Relative Frequency- the # of times the allele occurs in a
gene pool, compared with the # of times other alleles for
the same gene occur; it’s often a %. Pg. 394, 16-2
Ex. An allele makes up ¼ of population’s allele for a given trait. The
relative frequency is 25%.
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In genetic terms, evolution is any change in the
relative frequency of alleles in a population.
2 Main Sources of Genetic
Variation
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Mutations
Gene Shuffling
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Occurs during gamete production; results in
many different combinations.
Single gene trait controlled by 1 gene.
2-3 different genotypes are possible.
Compare populations gene pool to a deck of
cards. Shuffle & you get a different hand each
time but it never changes the # of kings, aces,
etc.
Polygenic Trait
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Can have many possible genotypes,
producing many possible phenotypes.
Controlled by 2 or more genes, each
may have more than one allele. Ex. Height.
Bell Shaped Curve Pg. 396, 16-4
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How phenotypes are expressed.
Most people are in the middle of the curve
16-2 Evolution as Genetic
Change
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Natural Selection acts directly on
phenotype. Ex. Lizard color. Pg. 397, 16-5
Natural selection on Polygenic Traits are
more complicated.
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Directional
Stabilizing
Disruptive
Directional Selection
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When individuals at one end of the bellshaped curve have higher fitness than
individuals near the middle or the other end
of the curve.
The curve shifts toward the higher fitness
end.
Ex.- birds with larger beaks will have higher
fitness if a small-medium seed shortage
occurs. Pg. 398, 16-6
Stabilizing Selection
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When individuals near the middle of the curve
have higher fitness than those on either end
of the curve.
This is where the average form of trait is.
The curve narrows around the middle
Ex.- a babies birth weight. Less wt., lower
survival rate. Higher wt., difficult birth.
Pg. 399, 16-7
Disruptive Selection
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When individuals at the upper and lower ends
of the curve have higher fitness than the
individuals near the middle.
The curve develops a peak at each end and a
low point in the middle.
Ex.- Medium bird seed has a shortage. Small
& large bird seed are abundant. Birds with
small & large beaks will have greater fitness.
Pg. 399, 16-8
Genetic Drift
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Allele frequencies change.
Occurs in small populations.
Individuals with a particular allele leave more
descendants than other individuals.
Alleles become more or less common by
chance.
The only similarity to natural selection is that
they involve a change in a population’s allele
frequencies.
Genetic Equilibrium
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A situation in which allele frequencies of
a population remain constant.
5 conditions required to maintain genetic equilibrium:
Random mating.
 Population must be very large.
 There can be no movement into or out of the population
(migration).
 No mutations.
 No natural selection
When all 5 are met, evolution will not occur.
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16-3 Process of Speciation
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Speciation- formation of new species
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For 1 species to evolve into 2 new species:
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Gene pools of 2 populations must become
separated.
They must be reproductively isolated.
Ex. – finches on the Galapagos islands.
Reproductive Isolation
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When members of 2 populations cannot
interbreed & produce fertile offspring.
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Behavioral- different courtship rituals.
Ex. Songs.
Geographical- populations are separated by
geographic barriers. Ex. Mountains, rivers.
Temporal- populations reproduce at
different times. Ex. Pollination.
Speciation of Darwin’s Finches
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The species evolved on the Islands from a
single species of founding birds.
They traveled from mainland South America to
one island to found a new population.
They survived & reproduced then crossed to a
2nd island.
The became geographically isolated & gene
pool was no longer shared.
Seed size on the island favored large beak
birds.
Large beak birds reproductively isolated &
evolved into a new species.