Darwins Finches and Natural Selection

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Darwins Finches and Natural Selection

• by
• Cheryl Heinz, Dept. of Biological Sciences,
  Benedictine University, and Eric Ribbens, Dept.
  of Biological Sciences, Western Illinois
  University

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The Galapagos Islands

• Located approximately 1000km from the coast of
  Ecuador, South America.
• This is just a little closer than the distance
  between Chicago and Philadelphia.
• Mostly ground between the two U.S. cities.
• Mostly deep water between the Galapagos Islands
  and the coast of South America.

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Implications

• Terrestrial species on these islands wont have
  many relatives nearby.
• Neighboring islands will have close relatives
• but new terrestrial species wont arrive on these
  islands from the South American mainland very
  often.
• most of the island species have had plenty of
  time to differentiate from their nearest living
  relatives.
• Another way of saying this is that there is very
  little gene flow between the islands and the
  mainland.

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CQ1 Limited gene flow means

• A Birds rarely move between the mainland and the
  islands.
• B Birds on the island have the same genes as
  birds on the mainland.
• C Birds on the mainland dont like birds from
  the islands.
• D Birds on the mainland wont evolve, but birds
  on the islands might.

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Charles Darwin

• Darwin explored these islands from April through
  October 1835.
• Entire voyage of The Beagle Dec 1831 - Oct 1836
• When and where he started thinking about what was
  to become his theory of evolution by natural
  selection.
• He did not publish his thoughts until the
  publication of The Origin of Species in 1859.

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Charles Darwins Ideas

• Biological evolution is change in species over
  time.
• This was not a new idea at the time
• But there were no good mechanisms to explain how
  these changes occurred
• Natural selection is just such a mechanism, and
  this is what Darwin contributed.

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Galapagos Endemics

• The Galapagos today is an amazing place.
• Animals live there that are found nowhere else on
  earth.
• This makes them endemic
• Perhaps the most famous of the endemic birds are
  the finches, of which there are 13 different
  species
• The islands are a natural laboratory, and one in
  which evolution can be observed.

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Endemics

• Among the kinds of animals found here and
  nowhere else
• 1 penguin species
• 1 giant tortoise species
• 1 marine iguana species
• 7 species of lava lizard
• 14 species of sea cucumber
• 1 species of sea lion
• 1 species of hawk
• several species each of mockingbirds, doves,
  owls, flycatchers, and yellow
  warblers

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CQ2 Endemic means

• A The end is imminent.
• B The species isnt found anywhere else.
• C The species has very specific habitat
  requirements.
• D The species needs to be protected.
• E The species is extinct.

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The Finches

• The 13 finch species include
• 6 species of ground finches
• 3 species of tree finches
• 1 woodpecker finch
• 1 vegetarian finch
• 1 mangrove finch
• 1 Coco Island finch
• A warbler finch that looks more like a warbler
  than a finch (one of the tree finches).
• The woodpecker finch actually uses cactus spines
  to dig grubs out of branches!

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Peter and Rosemary Grant

• Scientists Peter and Rosemary Grant have studied
  many of these species for the past thirty years.
• Spend months at a time on the islands
• Often know every finch on an island
• Lets look at some of their data.

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Graph showing the distribution of beak depths for
medium ground finches in Year 1
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CQ3 What is the average depth of the finches
beaks in Year 1?

• A about 7mm
• B about 8mm
• C about 9.5mm
• D about 10mm
• E about 11mm

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CQ4 How much was the biggest difference in beak
depth?

• A 2 mm
• B 4 mm
• C 6 mm
• D 8 mm
• E 10 mm

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A Change in the Weather

• Year 2
• Like most years, some rain fell the first week of
  January.
• The rest of January, there was one small shower.
• The total rainfall for the entire year 24mm.
• In a normal year, 130mm of rain would fall.
• In Year 1, 137mm of rain fell.

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A Change in the Weather

• The ground finches feed on seeds
• Year 1 June 1m2 of lava on the island
  has over 10 grams of seeds.
• Year 2 June 6 grams of seeds per m2.
• Year 2 December 3 grams of seeds per m2.
• In the drought, the plants conserved their
  resources and did not produce new seeds.
• Similarly, the finches did not mate and did not
  produce eggs in Year 2

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Seeds

• A variety of seeds are produced on the island.
• Finches prefer the softest seeds, which are the
  easiest to open.
• The seeds above are seeds of a plant called
  Caltrop, in the genus Tribulus.
• These are among the hardest to eat.
• It takes a medium ground finch with a beak at
  least 11mm long to open one.
• Ground finches with beaks that are 10.5mm long or
  less havent even been seen trying to eat them.

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Prediction?

• What do you think will happen to the size of the
  finch population between Years 1 and 3?
  (Remember, Year 2 is a drought year.)
• Sketch a rough graph of your prediction

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CQ5 What do you think a graph of population size
would look like for Year 1 to Year 3?
B
A
Time
Time
C
D
Time
Time
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Another Year of Change

• On one day in January of Year 3, more than 50mm
  of rain fell on the island .
• The plants finally flowered and produced new
  seeds.
• The Grants and their colleagues returned to the
  Galapagos.
• They found the finch population had been
  decimated.
• No new finches hatched in Year 2.
• Only one finch born in Year 1 survived to Year 3.

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Year 3 Data

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CQ6 What was the average beak depth in 1978?
(Remember that the average beak depth in 1976 was
9.5 mm.)

• A Just under 7mm
• B About 8mm
• C About 9mm
• D Just under 10mm
• E Just under 11mm

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Evolution is

• A change in the frequency of an allele, such as
  an allele for beak depth, is the basic definition
  of evolution.

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CQ7 Did the finch population evolve from 1976 to
1978?

• A Yes
• B No
• C Maybe
• D I dont know

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Evolution by natural selection

• The Grants first went to the Galapagos to take a
  quick snapshot of finch diversity.
• Within only a few years, they saw natural
  selection.
• In the course of one season, the beaks got 0.54mm
  deeper and 0.39mm longer.
• The sex ratio changed, too.

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Evolution by natural selection

• The beak size and shape was changing, right
  before the Grants eyes!
• This is definitely evolution as we defined it
  earlier.

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Surprises?

• Two things surprised the Grants
• Evolution could occur quickly enough to observe
  within a few field seasons.
• Darwin believed that we did not have a long
  enough lifespan to observe evolution.
• A single weather event can cause evolution, if
  there are traits that affect survival and if
  there is variation in those traits.

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Surprises?

• 2. Evolution can occur at very small scales.
• The Grants measurements were very careful.
• The birds werent used to humans, and so were
  easy to catch and measure
• They couldnt see a difference in even 1mm
  between two finches, but their measurements could
• And due to those measurements, they could find
  that 0.5mm was enough to make a difference
  between survival and death in a drought year

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CQ8 Which do you find more surprising?

• A Evolution occurs at a rate that we humans can
  observe.
• B As little as half of a millimeter can make the
  difference between life and death.
• C Both surprise me.
• D Neither was particularly surprising.

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Heritability

• Its important to note that beak size and shape
  is heritable in these finches.
• A bird with a large, deep beak will have
  offspring with large and deep beaks.
• Natural selection can occur without heritability,
  but evolution by natural selection cannot!
• (think about that for a minute)

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Evolution by Natural Selection

• Steps
• Individuals vary in some traits.
• 2. Some of the differences in traits are passed
  along to offspring.
• This requires a genetic basis to the trait
• The trait is thus heritable
• 
  (more)

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Evolution by Natural Selection

• 3. Different individuals produce different
  numbers of surviving offspring.
• Produce different numbers, or
• Different numbers survive.
• 4. The particular value of a trait is connected
  to the number of offspring produced.
• Traits that allow for more offspring to be
  produced are said to be naturally selected.

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CQ9 If beak depth increased during the drought,
primarily due to selective mortality, can we
really say that this natural selection was driven
by environment favoring the survival of birds
with deeper beaks?

• A No. Beak depth changed due to birds dying, not
  to birds surviving.
• B Yes. Birds with deeper beaks survived at a
  higher rate than birds with shallower beaks.
• C Im really confused.

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Want to find out more of the story? Check out

• The Beak of the Finch A story of evolution in
  our time, by Jonathan Weiner (New
  York Knopf, 1994).