chapter 16 and chapter 17. how does evolution work? individual organisms cannot evolve. populations...
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Chapter 16 and Chapter 17
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How Does Evolution Work?Individual organisms cannot
evolve. Populations of a particular species evolve.
Natural selection acts on the range of phenotypes in a population.
Microevolution occurs as the frequency of alleles in a population changes.
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Evolution-What Happens?Macroevolution or Evolution occurs when there is a change in allele frequency which
produces a new species.
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DefinitionsGene pool: All alleles of the
population’s genes.Allelic frequency: % of a specific
allele in the gene pool. Example: Approximately 75% have
dominant allele for tongue rolling. 25% non-rolling
Genetic Equilibrium: This exists when the frequency of alleles remains the same over generations. The population is not evolving.
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When Does Evolution Occurs?Evolution results when there are Forces
that change allelic frequencies.
Forces that cause Evolution:1. Gene flow: Transport of genes by
migrating individuals.2. Nonrandom Mating: Mating based
on preferencesExample: a female may choose a mate
based on male size, color, or ability to gather food.
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Forces of Evolution Continued3. Mutation: Change in DNA4. Genetic Drift: chance
event changes allelic frequencies – Greatly affect small populations such as the animals of the Galapagos Islands or Amish.
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Causes of Genetic Drift1. Mating over a long time
period in a small population.
2. Little movement of males or females into or out of the population.
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3 Types of Natural SelectionStabilizing selection – favors average individuals
Directional selection – favors one of the extreme variations of a trait
Disruptive selection – favors individuals with both extremes of a trait (eliminates intermediate phenotypes)
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What is a Species?A population or
group of populations whose members have the ability to breed with one another and produce fertile offspring
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Evolution of Species (Speciation)Significant changes in the gene pool can lead to evolution of a new species over time.
Speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.
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Artificial Speciation
Diane Dodd’s fruit fly lab, 1989
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Why Don’t the Populations Interbreed?
1. Geographic isolation – physical barrier divides a population.
2. Reproductive isolation – formerly interbreeding organisms can no longer mate to produce offspring..
3. Change in niche -- Change in food source. Example Finches
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1. Geographic Isolation
A physical barrier that separates a population into groups.
Can be1. Mountains or Rivers2. Islands with water in between
Darwin’s 13 finches on Galapagos3. Valleys caused by lava flow4. Roads/Highways
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1. Geographical Isolation
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2. Reproductive IsolationPrevents closely related species from interbreedingTimingBehaviorHabitat
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TimingSimilar species have different breeding seasons
Eastern Spotted Western SpottedSkunk Skunk
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Behavior
Similar species may have different courtship or mating behaviors.
Ex: Eastern & Western meadowlarks almost identical in color shape and habitat, but difference in courtship rituals differ different species
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HabitatSpecies remain reproductively isolated because they are adapted to different habitats.
Ex: Stickleback fish one is a bottom feeder, one spends time in the top open layers of lakes in British Columbia, Canada
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Patterns of Evolution
1. Divergent Evolution – evolution where species diverge or become less and less alike as they adapt to different environments.
2. Adaptive Radiation-ancestral species evolves into an array of species to fit diverse habitats. This is a type of divergent evolution
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Both the wooly mammoth, which occupied parts of North America, and the elephant, still found in Asia and Africa are presumed to have evolved from a common ancestor.
Their geographical isolation and environmental selection pressures caused further evolution of the species.
Each, in its own location, occupies(d) a similar niche.
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Patterns of EvolutionContinued2. Convergent Evolution – Unrelated species occupy similar environments in different parts of the world.
Similar pressures of natural selection lead to similar adaptations.
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A Hummingbird Moth
A Humming Bird
Example of Convergent Evolution
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Rhea
Emu
Ostrich
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Speciation can occur quickly or slowlyGradualism – idea that species originate through a gradual accumulation of adaptations.
Punctuated equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.
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GradualismGradual changes in species over time
Evidence of many intermediate forms in fossil records
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Punctuated EquilibriumScientists
found remains of intermediate forms
Also saw that populations remained the same over large periods of time then suddenly changed
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The End