evolution evolution of populations. three cases of variation what specifically causes the change in...
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Evolution
EVOLUTION OF POPULATIONS
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Three Cases of Variation
• What specifically causes the change in each of these populations of organisms:– Samurai crabs– Galapagos finches– Peppered moth
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Example
Change Cause of change
crabs
finches
moth
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The Peppered Moth• The Peppered Moth
from England exists in two forms – black and mottled gray.
• Before the industrial revolution (in 1850) most trees were light colored because of a lichen that grew on their bark.
• White moths were common and black moths were rare.
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The Peppered Moth…• During the industrial revolution, as
soot and pollution increased, light colored lichens on the bark darkened or died off.
• It was observed that the dark moths increased until they became the more common form.
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In order for changes such as these to occur in a
population, what two things need to be present in the
population?
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• Variation (between individuals)
• Selection (on those individuals)
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Variation of Traits in a Population
• Within a population, individuals vary in observable traits.
• Often these traits are cover a range of appearances (ex. height) that can be represented by a bell curve.
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Sources of Variation
• Variations arise in two main ways:– Mutation – the only way to get a new
trait– Genetic Recombination
• Why no two siblings (except twins) are identical
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Selection • There are two main
types of selection:• Artificial selection
is used by humans for breeding useful traits into a population.– Example – seedless
fruit; turkeys with more white meat
Breeds of dogs
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Natural Selection• Natural selection is the process by
which individuals that are better suited to their environment survive and reproduce most successfully.– Acts only on populations not individuals– Acts only on appearances (phenotype)
not genes (directly)
Examples: pesticide resistance; antibiotic resistance in bacteria
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Natural Selection• There are three types of selection on
multiple gene traits:– Directional selection - the range of
shifts toward one extreme. • examples include the peppered moth;
Galapagos finch, antibiotic/pesticide resistance
– Typical of a changing environment
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Natural Selection–Stabilizing
selection - the range of phenotypes narrows; extremes are eliminated.• Examples include
birth weight in humans, clutch size in birds, hatching weight in birds
–Typical in a stable environment
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Natural Selection– Disruptive selection - the range
of phenotypes “splits” and moves toward extremes; intermediate form is eliminated.• examples include shell color in
limpets, Galapagos finches with large or small beaks (none with medium size!)
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Other Causes of Change• Summary: any type of selection will
cause population to change over time.• In addition to selection, there are other
factors that will cause a population to change – Genetic drift – change due to chance in small
populations• Bottleneck effect• Founder effect
– Gene flow = immigration and emigration– Nonrandom mating– Mutation
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Bottleneck Effect• Disasters such as fires
may drastically reduce the size of a population reducing the size of the gene pool.– By chance, certain alleles
may be more or less prevalent in the survivors.
– Some alleles may be eliminated altogether.
• This decreases genetic variation in a population.
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Founder Effect• Founder effect –This occurs when a
small subunit of a population begins a fresh population (due to migration, or a large die-off). The “founders” don’t have the same allele frequency as the original population.
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Immigration & Emigration• Migration - movement of
individuals (and their genes) from one population to another = gene flow
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Nonrandom Mating– Nonrandom mating -
any type of mate selection
– Example: sexual selection• Most species have distinctly
male and female phenotypes• Differences include fur,
plumage, antlers• Organisms select mates that
provide resources or “good genes” for the chooser
• It is usually the female that is the “agent of selection” causing males to develop elaborate decorations
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Population Stability
• To review: the conditions that WILL cause change are:– natural selection– mutation– immigration or emigration– nonrandom mating
• If none of the previous conditions exist in a large population, the population’s genetic make up should stay the same.
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Steps of evolution by natural selection
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