Lecture 2 evolutionary fact and theory
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Transcript of Lecture 2 evolutionary fact and theory
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Anthropology 101: Human Biological Evolution
Lecture 2: Evolutionary Fact and Theory
Dr. Leanna [email protected]
Office AHSDrop In Hours:5:00-6:30 PM AHS 303
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Darwin and the history of evolutionary thinking
• Historical Context
• Darwin’s theory of natural selection
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Organisms are well-suited to their environment
• Viceroy butterfly
• Monarch butterfly
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Complex adaptations require special explanation
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The Scientific Revolution in Europe
• Global exploration
• Round Earth - 1492 etc.
• Biological Diversity
• Earth orbits the sun - 1541 Copernicus
• Solar system in motion - 1600’s Galeleio
• physics, medicine, chemistry advanced
• Scientific methods
• Measuring instruments
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A problem emerges from the growing body of knowledge
• Early Geology = Study of stratigraphy• Stratum (strata) = Layers of earth
Within these layers
find fossils of non-
existing animals
And many resemble
living ones
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A solution is proposed: James Hutton (1794)
• Strata = gradual process erosion accumulation
• Proposed “Deep Time” Earth profoundly old
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The solution is resisted: Georges Cuvier (1796)
• Proposed “Catastrophism”
Strata formed by series of divine catastrophes
Species go extinct
Multiple creation events
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The solution is refined: Lyell (1830)
• Proposes “Uniformitarianism”
• Natural processes same in past
and present
• Slow accumulation of changes
Requires deep time
• Darwin’s friend & mentor
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So if the earth isn’t young…maybe life isn’t either
• Jean-Baptiste Lamarck
• First to propose an evolutionary mechanism for species change
• He was wrong in the details, but important nonetheless
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Prior to Lamarck: Fixity of Species
• Species: A group of organisms that can interbreed to produce viable off-spring
Biblical understanding:• Every species was a unique
act of creation• Once created, never changes
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…so perhaps species aren’t fixed
Lamarckian Evolution – Inheritance of Acquired Characteristics• Traits determined by their usefulness in an environment.• If environment changes, organisms acquire new traits• pass new traits on to their offspring
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Meanwhile… • Thomas Malthus (1798)• An Essay on the Principle
of Population• Population growth outstrips
food supply• Population limited by
limited resources• Writing about human
population
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Charles Darwin influenced by these ideas
• Born to rich family in 1809• Grandfather Erasmus proposed
early idea of species evolution
• Dropped out of med school• Studied religion• 1832 sailed on HMS Beagle• 5 year voyage to map coast of
South America
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The Voyage of the Beagle(1831-1836)
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The HMS Beagle visited the Galapagos Island• Animals unique to each
island
• Similar to mainland species
• Islands are volcanic, recent formations
• Appearance of NEW FORMS
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Darwin collected many birds on the Galapagos • A variety of finch species
• Similarity in beak structure, body shape, and coloring
• Perfect gradation between species
• One species modified for different purposes
Adaptive Radiation
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Darwin returned to England in 1836
• Sort collections • Talked to scientific
colleagues • Started a series of notebooks• Talked to plant and animal
breeders• Growing evidence of
gradation of forms in the fossil record
• Read Thomas Malthus
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Enter Wallace• Alfred Russel Wallace
• Working-class family, b.1823
• little formal education
• Expedition to Amazon in 1848
• Developed similar ideas in
response to variety of natural
phenomena
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Publication
• 1858: Enter Wallace… Yawn…
• 1859: Darwin published On the Origin of Species.
People noticed
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The Theory of Evolution by Natural Selection
• Evolution: Change over time Can apply to many things besides biological organisms
• Natural Selection: Process by which species evolve Analogous with artificial selection
• Darwin interested in change over time of biological forms via the process of natural selection
• What is the process? How do biological forms change?
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Darwin’s Theory of Natural Selection
1. The ability of a population to expand is infinite, but the ability of the environment to support populations is always finite.
2. Organisms within populations vary, and this variation affects the ability of individuals to survive and reproduce.
3. Variations are transmitted from parents to offspring.
Darwin called this process natural selection
Of course, long before you mature, most of you will be eaten
“Bummer of a birthmark, Hal.”
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The Theory of Evolution by Natural Selection
1. Competition:
Malthus: Capacity of populations to increase is unlimited. Resources are not
2. Variation:
In all species, individuals vary in some of their physical or behavioral characteristics
3. Fitness:
Some members of a population have a variant that grants an advantage over others
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The Theory of Evolution by Natural Selection
4. Heritability: Some portion of this trait variation is inherited5. Differential Reproductive Success:Individuals with favorable traits produce more offspring thanthose who don’t, leading to more members of the population whoinherited the trait6. Adaptation:Highly favorable traits will eventually become widespread in thepopulation, building new species-typical features7. Speciation:Over time, as new variants accumulate and old ones disappear, new species develop
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Time
x
NO
YES
Evolution changes the characteristics of a population, NOT an individual
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The Vocabulary of Evolution by Natural Selection
• Evolution: Change over time
• Natural Selection: Process by which species evolve
• Reproductive Success: The number of offspring an individual produces and rears to reproductive age
• Fitness: A measure of relative reproductive success• Adaptation: An anatomical, physiological, or behavioral trait
which improves an organism’s fitness in a given environment• Selective Pressures: Forces in the environment that influence
reproductive success in individuals
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Birds of the Galapagos show us how natural selection works
Daphne Major
Medium Ground Finch
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Postulate 1: Environment constrains population growth• Severe drought
occurred 1976-78
• Drought affected seed availability and quality
• Many birds died of starvation
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Postulate 2: Individuals vary in ability to survive and reproduce
• Beak size varies• Small beaked birds have trouble with large seeds• During drought, larger beaked birds were at advantage
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Postulate 3: Variation is transmitted from parents to offspring• Beak size is inherited
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Characteristics of population changed over time
• Large beaked adults survived better• Large beaked birds had large beaked offspring• Mean beak size increased in population
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But, natural selection can: produce change (Directional Selection) or maintain status quo (Stabilizing Selection)
• Small beaked birds can’t
find enough food
• Large beaked birds have
higher juvenile mortality
• Selection favors
intermediate beak size
• At equilibrium, selection
will maintain stasis (no
change)
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Adaptations evolve in many small steps each favored by natural selection
• The key to natural selection is variation + selective retention
• Small changes occur
• Selection retains beneficial changes
• Complexity emerges
• Each step must be favored by natural selection
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We can see these steps in the fossil record and among living creatures
Abalone:
Crude image
Spiny Murex:
Better image
Limpet: Directional information
Beyrich Split Shell:
Better directional information
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Evolution can produce rapid change
• Examples in nature Radiation of Darwin’s
finches ( 500,000 yrs) Radiation of African
cichlids ( 12,000 yrs)
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Loss and Imperfection
• Natural Selection can remove complexity
Cave fish lose eyesight Penguins have small
wings
• Imperfect features Human fondness for
sugar, fat, salt Peacock’s tail Human appendix
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MYTH: ‘Survival of the fittest’ justifies everyone for themselves
• Survival of the fittest is NOT natural selection• Fittest does NOT always mean most aggressive, strongest,
most selfish
• Don’t use the NATURALIST FALLACY “because its natural its the right way to behave, or its
moral or good”
• Natural selection describes what happens in the world, NOT how we should chose to live our lives
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MYTH: Evolution cannot be disproved
What would disprove evolution?
• Major jumps or different ordering in the fossil record
• Novel combinations that counter the predicted trajectory of
evolutionary history
• Animals & plants that don’t change
• Young earth = not enough time for all that we see to have
evolved
• Variety in the building blocks used by life and perfect forms
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Key points
• Without variation, there can be no evolution
• Characteristics acquired during life are not heritable
• Whether a trait is favorable or not is determined by the
environment
• Natural selection acts on individuals
Even at a cost to the population or species
No “good of the species”
• Populations evolve
• Small steps add up to more complex traits