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Chapter 14
Darwin and Evolution
14.1 Darwin’s Theory of Evolution
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Charles Darwin was 22 in 1831.
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Naturalist aboard the HMS Beagle
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5 year voyage through Southern Hemisphere
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Mission to expand navy’s knowledge of natural resources (water and food) in foreign lands
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Observation led Darwin to conclude that biological evolution occurs
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Contrary to current beliefs of his time
Before Darwin
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People had an entirely different way of viewing the world.
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Earth only a few thousand years old
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Species remained the same since creation.
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Explorers brought back fossils (remains of once-living organisms) found in strata (layers
of rock or sedimentary material).
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Georges Cuvier founded paleontology.
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Study of fossils
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He believed in fixity of species yet strata showed a succession of different forms
over time.
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Catastrophism—explains appearance of new forms as replacing old forms due
to local catastrophe
Jean-Baptiste de Lamarck
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Evolution occurred and that adaptation was the cause of diversity
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Concluded more complex forms descended from less complex forms
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Inheritance of acquired characteristics
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Use and disuse of a structure can bring about inherited change
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Long neck in giraffes developed from stretching to reach food
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Not supported—people who were blinded in an accident would have
blind children
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Darwin’s ideas close to Lamarck BUT
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Species are suited to the environment through no will of their own
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Natural selection is the means for speciation
Darwin’s conclusions based on
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Study of geology, fossils, and biogeography
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Evidence that the Earth was subject to slow but continuous cycles of
erosion and uplift
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Uniformitarianism—these slow changes occurred at a uniform rate
Darwin concluded the Earth must be very old
Study of geology and fossils
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Darwin collected fossils during his voyage that differed from modern forms.
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Once he accepted that the Earth must be very old, he began to think there was
time for descent with modification (evolution) to occur.
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Species are not fixed but change over time.
Darwin’s conclusions based on
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Charles Lyell’s book Principles of Geology
Darwin’s conclusions based on
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Modern genetics—phenotypic changes acquired during the lifetime do
not result in genetic changes that can be passed to offspring
Study of geology and fossils
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Darwin collected fossils during his voyage that differed from modern forms.
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Once he accepted that the Earth must be very old, he began to think there was
time for descent with modification (evolution) to occur.
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Species are not fixed but change over time.
Darwin’s conclusions based on
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Study of biogeography
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Study of the distribution of life-forms on Earth
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Compared animals of South America to England
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Did the Patagonian hare resemble a rabbit because the 2 types of animals were
adapted to the same environment?
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Study of biogeography (cont.)
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Convergent evolution
Galápagos Islands—off the coast of South America
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Too far from mainland for most terrestrial plants and animals to
colonize
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Species there slightly different from mainland forms
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Where did they come from? Why are they different?
Finches
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Some were like mainland finches, others quite different
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Ground-dwelling finch beak size dependent on seed they eat
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Tree-dwelling finch beak size and shape dependent on their insect prey
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Could have descended from a mainland finch
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Speciation—formation of a new species—occurred because the isolated
population evolved independently of the mainland population.
Study of biogeography (cont.)
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Galápagos Islands—off the coast of South America
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Too far from mainland for most terrestrial plants and animals to
colonize
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Species there slightly different from mainland forms
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Where did they come from? Why are they different?
Finches
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Some were like mainland finches, others quite different
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Ground-dwelling finch beak size dependent on seed they eat
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Tree-dwelling finch beak size and shape dependent on their insect prey
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Could have descended from a mainland finch
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Speciation—formation of a new species—occurred because the isolated
population evolved independently of the mainland population.
Galápagos Islands tortoises
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Each island also had its own type of tortoise.
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Long-necked tortoises in dry areas where food scarce
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Short-necked tortoises in moist regions with abundant food
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Adaptation—any characteristic that makes an organism more suited to
its environment
Natural selection and adaptation
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Natural selection
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Mechanism for evolution
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Process that results in the evolution of organisms well adapted to their
environment
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Requires:
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Members of a population to have heritable variation
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More offspring produced than can be supported
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Individuals with favorable traits survive and reproduce more than those
lacking the traits
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Across generations, a larger proportion of the population possesses the
favorable traits and become adapted to the environment
Uses only variations resulting from genetic changes
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No direction or anticipation of future needs
Ongoing process—as environment changes, suitability of adaptations changes
Organisms vary in their traits
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Prior to Darwin, variations were considered imperfections and ignored.
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Variations are essential to Darwin’s natural selection process.
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Genes, together with the environment, determine phenotype.
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Mutations, chromosomal rearrangements, and assortment of chromosomes during
meiosis and fertilization can cause new variations to arise.
Organisms struggle to exist
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Thomas Malthus said death and famine are inevitable because the human population
increases faster than the food supply.
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Darwin applied this to all organisms.
Organisms differ in fitness
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Fitness—reproductive success of an individual relative to other members of the
population
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Most-fit individuals capture the most resources and convert into a larger number of
viable offspring
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What determines fitness varies for different populations.
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Artificial selection—humans choose particular traits
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In nature, interaction with the environment determines fitness.
Organisms become adapted
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An adaptation may take many generations to evolve.
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Manatees, penguins and sea turtles all have flippers to move through the water.
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Determined by comparing the number of surviving fertile offspring produced by
each member of a population
Convergent evolution
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Natural selection causes adaptive traits to be increasingly represented in succeeding
generations.
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Other process for evolution (chapter 15)
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Only natural selection results in adaptation to the environment.
Darwin and Wallace
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After the HMS Beagle retuned to England in 1836, Darwin waited 20 years to publish On
the Origin of Species.
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Gathered evidence to support his hypothesis
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Prompted to publish after reading similar hypothesis by Wallace
Alfred Russel Wallace
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Also an English naturalist and collector at home and abroad
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Conceived idea of “survival of the fittest”
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Sent essay to Darwin for comment
Joint paper presented to Linnean Society
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Darwin presented an abstract of On the Origin of Species
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Wallace presented “On the Tendency of Varieties to Depart Indefinitely from
the Original Type”
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Announced to the world that species evolve through natural selection with
common descent
14.2 Evidence of Evolutionary Change
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Theory of evolution states that all living things have a common ancestor but each is adapted to a
particular way of life
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Hypotheses become a scientific theory only when a variety of evidence from independent
investigators supports the hypothesis.
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Theory of evolution is a unifying theme in biology because it can explain so many different
observations in various fields of biology
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Fossil evidence
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Fossils trapped in rock strata are the fossil record that tell us about the history of life.
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Succession of life-forms from the simple to the more complex
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Find transitional links between groups
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Archaeopteryx fossils intermediate between reptiles and birds
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Evolution of whales
Biogeographical evidence
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Study of distribution of organisms throughout the world
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Distribution consistent with hypothesis that, when forms are related, they evolved in
one locale and then spread to accessible regions
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Darwin noted that South America lacks rabbits even though environment is suitable
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Rabbits evolved elsewhere and had no means of reaching South America.
South America, Antarctica, and Australia connected long ago
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Marsupials and placental mammals arose at that time.
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Marsupials only plentiful in Australia because it drifted away separating them
from competition with placental mammals
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Opossum only marsupial in the Americas
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In some cases marsupial and placental mammals resemble each other.
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Supports hypothesis that evolution is influenced by environment
Anatomical evidence
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Vestigial structures
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Anatomical features that are fully developed in one group of organisms but
reduced and nonfunctional in other similar groups
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Most birds have well-developed wings; ostriches do not
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Whales and snakes have remnants of hip bones and legs.
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Humans have a tailbone.
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Presence explained by common descent hypothesis
Homologous structures
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Anatomically similar structures are explainable by inheritance from a common
ancestor.
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All vertebrate forelimbs contain the same set of bones organized in similar ways
despite dissimilar functions.
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Analogous structures—structures serve the same function but are not
constructed similarly—bird and insect wings
Homology shared by vertebrates extends to their embryological development
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At some time during development, all vertebrates have a postanal tail and paired
pharyngeal pouches.
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In fishes and amphibian larvae, pouches develop into functioning gills.
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In humans, first pair of pouches becomes cavity of middle ear and auditory
tube, second pair becomes tonsils, and third and fourth pair become thymus
and parathyroid glands
New structures originate by modifying preexisting structures of one’s ancestors.
Molecular evidence
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All living organisms use the same biochemical molecules, including DNA, ATP, and many
nearly identical enzymes.
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Many developmental genes (Hox) are shared in animals ranging from worms to humans.
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Life’s vast diversity has come about from only slight differences in the same genes.
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Cytochrome c is a molecule used in the electron transport chain of all organisms.
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Data on the amino acid sequence show that the sequence varies consistent with
other data regarding anatomical similarities, and therefore relatedness.