Evolution by Natural Selection

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Chapter 23

• Evolution by Natural Selection

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Outline

• Theories of lifes origins
• Evidence for evolution
• change over time
• relatedness of species
• Theories of evolution
• Darwin
• How natural selection works
• Case studies

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Theories of Species Creation Prior to Evolution

• Theory of Special Creation
• species are immutable/unchanged through time and
  independent of one another
• all species were created independently by God, as
  recently as 6000 years ago
• Theory of Spontaneous Generation
• new living organisms appear suddenly when
  suitable conditions
• flies appear on rotting meat, microbes in old
  milk/puddles
• some life-forms arise spontaneously
• not all life arises directly from living organisms

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Theory of Evolution by Natural Selection

• Theory of Evolution by Natural Selection
• key theory in biology
• jointly proposed by Darwin and Wallace (1858)
• Darwin described evolution as descent with
  modification
• ancestral sp. changed over time to become modern
  sp.
• evolution change in genetic characteristics
  (allele frequency) in population over time

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Evidence for Change Over Time

• Fossils traces of organisms that lived in past
• fossil record fossils that have been found and
  described in the scientific
  literature
• most fossils found in sedimentary rocks
• rocks that form in layers from sand/mud deposited
  at locations such
  as beaches or river mouths
• layers associated w/ different intervals in
  geologic time
  scale
• radioactive isotopes used to assign ages to
  geologic time scale
• Earth 4.6 BYA, first signs of life 3.85 BYA

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Evidence for Change Over Time

• Extinction of Species
• fossil record provides evidence for sp. unlike
  any living sp.
• Darwin - evidence that species are dynamic and
  change
• Law of Succession
• particular geographic area
  frequently
  contains fossils w/
  striking similarity to
  living sp.
  in same area
• Darwin - evidence that extinct
  sp.
  are ancestors of modern sp.
  and sp. change over
  time

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Evidence for Change Over Time

• Transitional forms
• many fossils discovered w/ traits intermediate
  btwn older and younger sp.
• sp. change over time
• Environmental change
• Earth's topography and climate changed
  drastically over time
• planet and species are dynamic

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Evidence for Change Over Time

• Vestigial Traits
• rudimentary structures that have no function but
  are similar to functioning structures in similar
  sp.
• human coccyx/primate tailbone, human
  appendix/vertebrate caecum
• trait structure and function change over time

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Evidence That Species Are Related

• Geographic proximity of similar but distinct
  species
• similar but distinct sp. often found living in
  close geographic area
• Darwin collected mockingbirds from Galápagos
  islands
• superficially similar but different islands had
  distinct sp.
• Darwin proposed that different island sp. were
  similar b/c they had descended w/ modification
  from common ancestor
• pattern of similar yet distinct sp. common among
  islands
• related species share phylogeny (family tree)
• phylogenetic tree graphical depiction of
  relationships

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Evidence That Species Are Related

• Homologies similar traits in differing
  organisms
• structural homology similar morphological
  traits in different sp.
• bone structures in human arm, horse leg and bat
  wing
• Darwin interpreted as product of descent w/
  modification
• developmental homology similar embryo
  morphology and/or pattern of tissue
  differentiation
• all vertebrates have gill pouches and tails early
  in development
• genetic homology similar DNA sequences of genes
  of different sp.
• evidence suggests all sp. descended from single
  common ancestor
• almost all organisms use same 64 mRNA codons to
  code for AA

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Evidence That Species Are Related

• Distinguishing homology from analogy
• homologous structures appear similar and are
  similar in structure/organization/function
• due to similar descent
• analogous structures appear similar but are
  different in structure/organization/function
• due to convergent evolution
• occurs when natural selection leads to similar
  solns to challenges posed by environment

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Theories of Evolution

• Idea of evolution predates Darwin
• Inheritance of Acquired Characteristics (Lamarck)
• prior Darwin
• individuals change in response to environment and
  changes are passed to next generation
• giraffes neck

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Theories of Evolution

• Theory of Evolution by Natural Selection (Darwin
  and Wallace, 1858)
• sp. are related to one another and change over
  time
• thus sp. existing today descended from other
  preexisting sp. (descent with modification")
• natural selection acts on individuals and
  individuals w/ certain favorable characteristics
  will survive and reproduce more
• if the advantageous characteristics are
  heritable, traits may be passed to offspring thus
  leading to change in population over time

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Darwin and Wallace

• Darwin
• travels on HMS Beagle
• writes paper on evolution in 1842
• does not publish for 17 years
• Wallace
• studies natural history specimens in Malaysia
• writes brief paper outlining evolution by NS
• sends it to Darwin for review in 1858
• Pressure from Wallace spurs Darwin to publish
• Both papers read simultaneously at
  Linnean Society of
  London in 1858

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How Natural Selection Works

• Populations undergo natural selection when 4
  conditions occur
• individuals in population vary in traits (i.e.,
  size, shape, color, etc.)
• variations are heritable (can be passed on to
  offspring)
• some individuals survive and reproduce better
  than others
• differential survival and reproduction (Darwinian
  fitness)
• individuals w/ traits that confer advantage more
  likely to survive and reproduce
• if favorable traits are heritable then evolution
  of population will result (nonrandom selection)
• traits that provide reproductive advantage will ?
  in frequency
• Evolution genetically based change in
  population over time
• change in allele frequency over time

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How Natural Selection Works

• Fitness ability of individual to survive and
  reproduce
• fitness estimated by measuring of offspring
  produced by one individual vs. another
• Adaptation heritable trait that increases an
  individual's fitness in a particular environment
  relative to individuals lacking that trait

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Case StudyAntibiotic Resistance in
Mycobacterium tuberculosis

• Historical background of TB
• prior to antibiotics TB caused 25 of all
  deaths in NYC and 33 of all deaths in Paris in
  800s
• 1950 to 1990 sanitation, nutrition, and
  antibiotics greatly reduced deaths due to TB in
  developed nations
• late 1980s resurgence in TB in developed
  countries due to strains carrying antibiotic
  resistance
• Patient admitted to hospital in Baltimore with
  active TB
• patient given rifampin for 40 wks, released when
  lung cultures show no bacteria
• patient readmitted w/ TB symptoms 2 months later,
  lung cultures show TB bacteria
• patient given further antibiotic treatment but
  dies in 10 days

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Case StudyAntibiotic Resistance in
Mycobacterium tuberculosis

• Evidence that evolution of resistance in TB
  bacteria occurred
• TB bacteria from lung cultures at wk 1 sensitive
  to rifampin
• TB bacteria cultured at wk 48 resistant
• DNA of TB bacteria from wk 1 and 48 sequenced and
  compared
• differ by 1 nucleotide (point mutation) in gene
  for RNA polymerase
• wk 1 rifampin binds to RNA polymerase of TB
  bacteria, preventing growth of bacteria
• wk 48 rifampin does not bind to mutant RNA
  polymerase in bacteria, thus functional enzyme
  and bacterial growth

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Case StudyAntibiotic Resistance in
Mycobacterium tuberculosis

• development of antibiotic resistance in TB
  patient exhibits 4 conditions of natural
  selection
• variability in their characteristics (1
  variation)
• variation is heritable (coded in rpoB gene) (2
  heritable)
• nonrandom selection occurred
• only bacteria w/ resistance survive to reproduce
  (3 survival)
• pass resistance to offspring, thereby altering
  allele frequency (4 pass traits to offspring and
  population evolves)

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Case StudyBeak Size and Shape and Body Size of
Galápagos Finches

• Long-term research on changes in body size and
  beak size and shape in medium ground finches show
  that drought ? in average beak depth
• Another change occurred btwn 1977-8 after seven
  mo. rain

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Case StudyPollinator Effects on Variation in
Alpine Skypilot Plants

• Research on alpine skypilot plants (Polemonium
  viscosum) illustrate pollinator preferences led
  to habitat-specific natural selection favoring
  individuals w/ traits preferred by pollinator
• variability (1)
• heritable traits (2)
• quantitative traits (i.e., size,
  
  height, color) usually involve
  1
  gene and environmental effects
• small-flowered plants produce
  
  small-flowered offspring, etc.
• strong genetic effect

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Tundra above timberline - pollinated by bumblebees
28 24 20 16 12 8 4 0
individuals
10 12 14 16 18 20 22
Big, long stalks, sweet-smelling
Flower size (mm), Ave 16 mm
Forests below timberline - pollinated by flies
10
8
6
individuals
4
2
0
10 12 14 16 18 20 22
Small, short stalks, skunky
Flower size (mm), Ave 14 mm
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Case StudyPollinator Effects on Variation in
Alpine Skypilot Plants

• Hypothesis NS produced differences in plants
  due to bees preference for large, sweet-smelling
  flowers
• prediction 1 individuals with large flowers
  attract more bees
• prediction 2 individuals that attract more bees
  have ? fitness
• Experiment Do bees have preference for larger
  flowers?
• MM - randomly picked, sweet-smelling flowers
  transferred to enclosure, pollinated by bees,
  seeds produced counted
• results - large flowers w/ tall stalks ? bee
  visits ? seeds
• conclusions - evidence supports NS postulates 3
  (survival) and 4 (more large-flowered offspring)

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Case StudyPollinator Effects on Variation in
Alpine Skypilot Plants

• Can natural selection of alpine skypilots be
  observed in field?
• MM allow bees to pollinate plants in field and
  hand pollinate different set of plants, collect
  seeds, plant them at random locations in field
• results bee-pollinated plants offspring w/
  large flowers, hand-pollinated plants offspring
  w/ small flowers
• conclusion bees exert NS pressure
• offspring of bee-pollinated plants had
  significantly larger flowers than offspring of
  hand-pollinated plants

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Clarifications Regarding Evolution

• Selection acts on individuals but evolution
  occurs in populations
• individuals dont change during NS
• some simply produce more viable offspring causing
  change in allele frequency in population
• acclimation occurs when individual changes in
  response to changes in environment
• adaptation occurs only when population changes in
  response to NS

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Clarifications Regarding Evolution

• Evolution is not progressive
• no such thing as "higher" or "lower" organisms
• instead more ancient and less ancient w/
  different
  adaptations that allow the groups
  to thrive in
  different environments

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Clarifications Regarding Evolution

• Not all traits are adaptive
• adaptation is not perfect process
• Some structures may be subject to genetic or
  historical constraints
• genetic constraints
• when selection on alleles for one trait causes
  correlated but suboptimal change in another trait
  (genetic correlation)
• lack of genetic variation can constrain evolution
• historical constraints
• b/c all traits evolve from previously existing
  traits, adaptations constrained by previous
  history