chapter 16 evolution of populations. two main sources of genetic variation 1.mutations—change in...
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Chapter 16Evolution of Populations
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Two main sources of genetic variation
1. Mutations—change in genes (DNA sequence) or chromosomes
2. Gene recombination—mixing of genes that result from meiosis
and sexual reproduction
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Gene Pool—the combined genetic information of all the
members of a particular populaiton
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Speciation—formation of a new species through reproductive
isolation
Example: Galapagos Island finches
Ground Squirrels
**Quick Speciation Activity**
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Fig. 24-6
A. harrisi A. leucurus
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Types of reproductive isolation1. Behavioral isolation (sympatric)
2. Geographical isolation (allopatric)
Which type did we demonstrate in our activity?
If one of the Earth’s plates moves 1.9 cm a yr., in 1 million years it would
move 12 miles
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Geographic Isolation
Some birds from species A cross to a second island.
The two populations no longer share a gene pool.
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Seed sizes on the second island favor birds with large beaks.
The population on the second island evolves into population B, with larger beaks.
Changes in the Gene Pool
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Fig. 22-6
(a) Cactus-eater (c) Seed-eater
(b) Insect-eater
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Species—a group of similar organisms that can breed and
produce fertile offspring
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Fig. 24-2a
(a) Similarity between different species
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Exit Slip
• List the conditions required for a population to become a species.
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Types of selection
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1. Disruptive Selection • Selection that splits a population into 2
groups.
• Removes individuals with average traits, but keeps those with more extreme traits.
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2. Stabilizing Selection• Eliminates extreme expressions of a trait
when the average expression leads to higher fitness.
• Most common form of natural selection.
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3. Directional Selection
• An extreme version of a trait makes an organism more fit.
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Height Selection Activity:
•Only extremely tall and extremely short •Only medium•Only extremely tall
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4. Sexual Selection
• Operates in populations where males and females look very differently.
• Typically, males will be larger and more colorful.
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Types of Evolution
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Adaptive radiation (divergent evolution)—
a single species evolves into several new species that live
in different ways
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Convergent evolution—
unrelated species independently
evolve similarities when adapting to
similar environments
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Coevolution— two species evolve in response to changes
in each other over time
Example: mutualism
Moth pollinates the comet orchid
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Rate of Evolution
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Catastrophism
• Evolution occurs after a catastrophy
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Gradualism
• Evolution proceeds in small, gradual steps
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Punctuated Equilibrium
• Rapid spurts of genetic change that cause species to diverge quickly.
• These periods disrupt much longer periods when the species exhibit little change.
• Instances of abrupt transitions.
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Chapter 17The History of Life
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If one of the Earth’s plates moves 1.9 cm a yr., in 1 million years it
would move 12 miles
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Fossil record—information about past life that has been obtained
from fossils-it is incomplete
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1. Most organisms are now extinct
2. fossils occur in a particular order
3. groups of organisms have changed over time
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Extinct—a species that has died out
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Where do most fossils form?
Most fossils form in sedimentary rock as weight compresses layers
of sediment in bodies of water
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Index Fossil—an easily recognized species used to compare the relative ages of
fossils
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Sedimentary rocks form in horizontal layers.
When part of Earth’s crust is compressed, a bend in a rock forms, tilting the rock layers.
As the surface erodes due to water, wind, waves, or glaciers, the older rock surface is exposed.
New sediment is then deposited above the exposed older rock surface.
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Water carries small rock particles to lakes and seas.
Dead organisms are buried by layers of sediment, which forms new rock.
The preserved remains may later be discovered and studied.
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Relative dating—the age of a fossil is determined by comparing its placement with fossils in other
layers
The oldest layers are on the bottom
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Half-life—the length of time required for half of the
radioactive atoms in a sample to decay
Carbon-14 5770Uranium-235 713 million yrsPotassium-40 1.3 billion yrsUranium-238 4.5 billion yrs
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Radioactive dating—scientists use half-life to calculate the age
of fossils based on the amount of remaining radioactive isotopes
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Microfossils—microscopic fossil
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Mass Extinction—many types of living things become extinct
in a short period of time
Example: Dinosaurs
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