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.
– Gene flow = animals of the same species but a different
gene pool are introduced into a population
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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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The Finches
• The 13 finch species include:
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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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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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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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Types of evolution
• Microevolution – changes in the gene
(allele) frequencies in a population over
a short period of time
• Macroevolution – changes in the gene
(allele) frequencies in a population over
a large amount of time that essentially
differentiates a population from the rest
of the species.
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Evolution
• Changes in the population are due to 4
main mechanisms:
– Mutation
– Natural selection
– Gene flow
– Genetic drift
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