What was Darwin trying to explain?

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What was Darwin trying to explain?

• Observations
• Hypotheses

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Charles Darwin

• 1809-1882
• Englishman bound for medicineI mean the
  clergybut sidetracked by biology
• 1831-1836 Beagle
• 1839 Journals
• 1859 The Origin of Species!
• Outlines the process of natural selection

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Observations

• Fossil Record
• Extinctions
• Transitional forms
• Environmental/Geological change
• Apparent relatedness of species

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Fossil record

• Layers of preserved material (bones, dung,
  tracks) suggested age differences between them
• Darwin had huge collections to examine, even in
  early 1800s

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Extinctions

• Some unearthed fossils clearly belonged to
  creatures no longer walking the Earth
• Irish elk
• T-rex

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Transitional Forms

• Darwins predecessors encountered fossils buried
  beneath similar looking contemporaries
• Law of Succession

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Transitional Forms Environmental Change

• Darwins predecessors catalogued fossil
  sandwiches, with middle fossils intermediate in
  form between fossils above and below.
• documents a change in traits through time
• Cetaceans loss of legs pelvic girdle.

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Vestigial traits

• Tiny, useless, remnant traits on contemporary
  species

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Legless lizards skinks
Jon Sullivan wildherps.com
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Apparent Relatedness

• Darwins contribution came after and almost
  certainly as a result of his voyage through the
  Galapagos island archipelago
• 1831 - Charles Darwin (22) joins crew of HMS
  Beagle as gentlemans companion (ships
  naturalist) collects catalogues everything.
  (1831-36)
• Plants, insects, fossils Galapagos mockingbirds
• After his return he and his naturalist friends
  noted some things about his collections

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Different islands have distinct (but very
similar) species
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Weight of evidence lead to his heretical proposal

• Species on neighboring islands look similar
  because they are all descended from a common
  ancestor The small differences between them due
  to changes over time.
• A bold assertion dangerous for the time.
  Species are not static!!
• So, he waited about 20 years after developing the
  idea to actually publish

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On the Origin of Species by Means of Natural
Selection, or the Preservation of Favoured Races
in the Struggle for Life-Charles Darwin, 1859

• Darwins blackbirds, gross-beaks, wrens and
  finches

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Origin Figure 1

• What does this figure explain?
• Diversity
• Unity

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We recognize 3 types of similarities (homologies)

• Structural
• Outward physical appearance
• Developmental
• Sequence timing of developmental events or
  shared germinal tissues
• Genetic
• DNA sequence
• Darwin had these first two at his disposal.

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Structural

• Why would a wing, a grasping hand, a paddle, a
  rudder a hinged pendulum be built from the same
  bones, in the same orientation relative
  positions, unless theyre built from a common
  model?

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Developmental

• Why should human embryos ever have gills and a
  tail why should chick embryos ever have gills
  and why should they appear at similar
  developmental stages and in same relative
  positions as those of fish?
• Fish human jaws look very different, but
  develop from same population of embryonic cells.
  Why, unless?

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Genetic

• Why should the same 64, 3-molecule codes specify
  the same building blocks in ALL organisms?
• Why should these strings of code, or genes, be
  the same across dissimilar organisms?

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What was Darwins contribution?

• Others had proposed evolution (of some variety)
  as a pattern that required explanation.
• Darwin provided the process, natural selection,
  that explained the pattern of descent with
  modification.

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Darwins postulates (hypotheses)

• Traits in a population vary among individuals
• Some traits are passed on to offspring (i.e.
  traits have a genetic basis and are heritable)
• Some individuals produce more offspring than
  others (perhaps due to beneficial traits)
• The subset of all offspring that survive are
  those that possess certain traits these traits
  are naturally selected

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Recall the mockingbirds

• Imagine ancestral mockingbird population
  colonizes 4 Galapagos islands
• If all the postulates are satisfied, then Darwin
  predicted that mockingbird populations on each
  island would become different from their
  ancestral population, increasingly well adapted
  to their respective environments, as better
  adapted individuals reproduced at
  disproportionately high rates.

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Darwin introduced fitness to explain natural
selection

• Fitness the ability of an individual to produce
  fertile offspring, relative to that ability in
  other individuals.
• This is a measurable quantity, and allows us to
  define adaptation
• Adaptation a heritable trait that increases the
  fitness of an individual relative to individuals
  lacking that trait.
• These are time and location (environmentally)
  dependent

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Are Darwins hypotheses testable?

• Pollination of alpine skypilots (flower)
• Above treeline Bees are main pollinator of
  skypilots flowers are large
• Below treeline Flies are main pollinator of
  skypilots flowers are small
• H1 Pollinator preferences for different sized
  flowers (selection) lead to observed differences
  in flower size

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Experiment

• Prediction If bees prefer large flowers, then
  the offspring of bee-pollinated flowers will be
  larger than offspring of randomly pollinated
  flowers

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Results

• Have we proven our hypothesis?
• Can we accept it for now?