processes evolution
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8/4/2019 Processes Evolution
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Processes of EvolutionHow gene pools change across generations
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The wisdom tooth question
• Remember this question?
• Some evidence exists that over several centuries, the number of people born with small wisdom teeth or no wisdom teeth has increased. Using your best
understanding of Natural Selection,explain how selection could cause this.
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How selection works:
• Variation exists
• Traits are inheritable
• Some traits have more survival valuethan others (differential survival).
• Some individuals have a better chance
of reproduction than others and a betterchance of passing on their traits(differential reproduction).
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Is this selection?
• “Humans in the past needed wisdomteeth, but since our way of eating isdifferent, we no longer need them.Because we no longer need them,
they’re getting smaller.”
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Is this selection?
• “Our parents may have had genes for large wisdom teeth. Because they’re too
big for the child, the child may get amutation that causes him to have smaller
wisdom teeth.”
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Is this selection?
• “Way in the past two people who hadsmall wisdom teeth probably mated andsince then people have had smallerwisdom teeth.”
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Is this selection?
• “We inherited wisdom teeth from our ancestors even though they have nopurpose. Because they have no purpose,the genes are recessing from our
population.”
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Is this selection?
• “People could be born with smaller or no
wisdom teeth because of naturalvariation that exists. Sometimes themouth is too small to contain the wisdomteeth without proper dentistry, so people
with large wisdom teeth may die fromimpactions and infections.”
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Natural Selection
• There must be variation in the original population:
• Variation in size of wisdom teeth
• A few people are born without wisdom teeth (random
mutation)
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Natural Selection
• There must bedifferential survival(selection) due to the
variation:
• People with largewisdom teeth are
more likely to getimpacted teeth, whichcan get infected.
Infections can be fatal.
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Natural Selection
• There must be differential reproduction:
• Young people who have impactedwisdom teeth may die of an infectionbefore they reproduce. Their genes
that produced large wisdom teeth arenot passed on.
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Thinking Question:
• People in Western culture often have access
to good dental care, and can have wisdom
teeth removed before they cause problems.
• How does this affect the process of natural
selection?
• Will wisdom teeth continue to get smaller inWestern nations? (The words “need” and
“purpose” should not be used in your
answer!)
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Antibiotic Resistance
• We will watch a short video on the rise ofantibiotic resistance.
• At the end, you should be able to explainhow natural selection works has causedthe rise in antibiotic-resistant bacteria.(Note it is bacteria that become resistant,
not people . Bacteria do not become“immune” — they do not have immunesystems.)
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Thinking Question:
• Use the principles of natural selection toexplain antibiotic resistance. Be sure toinclude these in your answers:
• Variation in the original population.
• Differential survival.
• Differential reproduction
• “Need,” “purpose,” and “immune” should
not be in your answer!
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Genes and Evolution
• Genes are the units of heredity.
• Genes code for proteins, which result inour set of traits.
• Genes are passed from parent tooffspring through the sex cells.
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Genes in Individuals
• Different “versions” of
genes are alleles.
• Dominant alleles areexpressed in thephenotype (expressed
trait) even if only onecopy is inherited.
• Recessive alleles areexpressed only if two
copies are inherited.
“Genotype” is a description of the
alleles for a given trait in an
individual: BB, Bb, or bb
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The Gene Pool Concept
• The “gene pool” of a population is the
entire collection of alleles for a given traitthroughout a given population.
• The word for all genes for all traits in an
individual or population is genome .
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• Allele ratios in a gene pool will not change
from generation to generation (that is, noevolution) only if all these things are true:
• No mutation
• Large population
• No migration
• No selection
• Random mating
Hardy-Weinberg Equilibrium
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Mutations
• Small mutations appear randomly inpopulations.
• The appearance of mutations changesallele ratios by
• “breaking” functional alleles (as in
genetic disorders)
• adding new alleles
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Population size
• In large populations, random eventshave a very small effect.
• In small populations, because fewerindividuals have any given trait, randomevents can have a larger effect. Changes
in gene ratios caused by random eventsare called “genetic drift.”
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A population bottleneck is genetic drift
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A population bottleneck is genetic drift.
F d ff i i d if
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Founder effect is genetic drift.
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• Migration into and out of a populationcan change gene ratios.
• Immigrants can bring in new mutations,or a different ratio of alleles.
• Emigrants may take away a high
proportion of a certain allele.
• Small population are more affected thanlarge populations.
Migration
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Selection
• Selection may increase or decrease thefrequency of certain alleles:
• Directional selection: favors one end ofa range over another.
• Disruptive selection: disfavors the
midrange.• Stabilizing selection: favors the mid-
range.
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• Mate choice among most organisms isselective, not random.
• Sexual selection may favor traits that arein conflict with natural selection. Forexample, bright-colored male guppies
attract more females, but are also morevisible to predators.
Mating Behavior
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• Because perfect Hardy-Weinbergequilibrium is never met with in nature,all populations experience small shifts in
gene ratios with each generation.
• Gene ratio shifts may fluctuate withcyclical changes in climate. Long-term
changes in habitat (such as globalclimate change) can shift the gene ratiosfar enough to bring about speciation.
Evolution Happens
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