Lab 9: Darwinian Snails

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Lab 9 Darwinian Snails

• An Introduction to Natural Selection

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Heres Chuckie!!!Charles DarwinDead white
dudeCollege problems at EdinburghFamily
pressure to buckle down and get seriousExtended
sea voyage as a paid companion, not naturalist
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Alfred Russell Wallace1823-1913 Co-discoverer
of natural selection simultaneous with Charles
Darwin

• Both men observe island biotas and conclude they
  are descendents of species originating on
  adjacent mainland areas.
• Both men read Thomas Malthus economic essays on
  human population dynamics
• Island species observed are different from
  mainland species. How does such a change happen?
• Neither man knows that heredity is carried by
  genes but they do perceive that heredity is how
  plant and animal traits are transmitted between
  generations.
• If plants and animals vary enough to affect their
  ability to survive and reproduce then it is
  rational for Wallace and Darwin to assume such
  characteristics would increase over time among a
  population of organisms.

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Natural Selection of the 19th century

• Darwin and Wallace both encounter island species
  that appear to be radiations from mainland
  sources
• Evolution itself is not a new idea at this time
  but natural selection is the first rational
  method explaining how it occurs
• Darwins book on the topic discusses the origin
  of species from other species, not an origin of
  life
• Natural selection appears to be even more
  convincing to Darwin after he considers how
  easily artificial selection works in
  domestication of agricultural species

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What is natural selection?

• Recall the recent labs on genetics and meiosis,
  specifically the probability of certain genetic
  combinations of alleles and crossing over.
• Consider the variation between individuals
  present in a human population
• Changes in the allele frequency of populations
  over time are reflections of differential
  survival and reproductionakanatural selection.
• Descent with genetic change from common ancestry
  describes the relationship shared by all life as
  natural selection works over long periods of time.

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Todays lab

• Periwinkles are a class of the phylum Mollusca
  known as Gastropoda, (stomach-foot).
• Crabs are a class of the phylum Arthropoda known
  as Malacostracam (gr. soft-shell)
• Predation of periwinkles by crabs occurs
• Crab predation is a factor in determining which
  periwinkles survive to reproduce

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Carcinus maenas vs Littorina obtusata(aka crab
vs snail)

• C. maenas arrive in North America during the
  1950s, perhaps as larvae in ballast water
  discharged by ships
• L. obtusata is not adapted to living in habitats
  with C. maenas
• Predation of L. obtusata by C. maenas is an
  outcome of introducting C. maenas into N.
  American habitats
• Response of L. obtusata to predation by C. maenas
  conforms to an outcome predictable by a
  hypothesis modeled from natural selection

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Homework assignment

• Page 145 in lab manual
• Part V questions
• Compose lucid and thoughtful responses to the
  questions.
• Using up to one page of text (single spaced 12 pt
  Times New Roman font) type your answers to the
  questions up and turn in as homework.