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Darwin’s 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 won’t have many

relatives nearby.

•  Neighboring islands will have close relatives –  but new terrestrial species won’t 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 don’t like birds from the islands.

D: Birds on the mainland won’t 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 Darwin’s 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 isn’t 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

•  Let’s 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 haven’t even been seen trying to eat them. 18

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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?

A: ""

""""

Time

Time Time

Time

B:

C: D:

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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 don’t 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:

1.  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 weren’t used to humans, and so were easy to catch and measure

•  They couldn’t 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

•  It’s 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:

1.  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: I’m 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).

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