Transcript File

Chapter 15: Evolution
15.1 Darwin’s Theory of Evolution by Natural Selection
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Charles Darwin
• Charles Darwin was a naturalist, and explored
the Galapagos islands, collecting rocks, fossils,
plants,and animals
– Entire voyage of The HMS 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
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publication of The Origin of Species in 1859.
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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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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Q1: 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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Peter and Rosemary Grant
• Scientists Peter and Rosemary Grant have
studied many of the various Finch 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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Q2: 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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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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Weather effects on Finch
population
• The ground finches feed on seeds.
• 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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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.)
• Talk with your group and sketch a graph
on your whiteboard of what the finch
population will look like
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Q3: What do you think a graph of population
size would look like for Year 1 to Year 3?
B:
A:
Time
C:
Time
D:
Time
Time
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Year 3 Data
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Q4: 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 Defined
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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Q5: 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.
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Any Surprises?
Evolution can occur at very small scales.
The Grants’ measurements were very
careful.
• The birds weren’t used to humans, and so they
were unusually 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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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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Q6: 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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Evolution by Natural Selection
(4 Principles)
1. Variation Individuals in a population differ from
one another. I.e some sunflowers are taller than
others Individuals vary in some traits.
2. Heritability Some of the differences in traits are
passed along to offspring.This requires a genetic
basis to the trait .The trait is thus heritable. i.e Tall
sunflowers produce tall sunflowers, and short
sunflowers produce short sunflowers.
(more…)
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Evolution by Natural Selection
• 3. Overproduction-Populations produce more
offspring than can survive. i.e. each sunflower has
hundreds of seeds, most of which will not germinate.
• 4. Reproductive Advantage- Some
variations allow the organism that possess them to have
more offspring then the organism that does not possess
them. i.e. In different habitats shorter sunflowers
reporduce more successfully.
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Artificial Selection
• Artificial Selection-the process of
directed breeding to produce offspring
with desired traits. (also proposed by
Darwin.)
• This can be implemented by humans i.e
new dog breeds or new strains of crop
plants, and also naturally by nature
(natural selection.)
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Artificial Selection
• in the beef industry- beef producers
“select genes” and phenotypic traits that
produce them winning results and a
better beef product.
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