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NATURAL SELECTION
Ms. Muneer
Call website
http://bloor-sbi3u.wikispaces.com
[email protected]
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Descent with Modification
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Darwin proposed that the descendants of the
earliest organisms spread into various habitats very
millions of years.
In these habitats, they accumulated different
modifications, or adaptations, to diverse ways of
life.
Darwin called this process descent with modification.
Darwin never actually used the term “evolution”.
“Evolution” came into use later and replaces
“descent with modification.”
Hardy-Weinberg Principle
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Evolution can also be described mathematically
Explained the relationship between allele
frequencies with in a population using a
mathematical equation
In large populations in which only random chance is
at work, allele frequencies are expected to remain
constant from generation to generation
Hardy-Weinberg Principle
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5 conditions: we talked about them last class
Small population
Non-random mating opportunities
Genetic mutation
Migration
Natural selection
Types of Selection
Many factors influence how selection can
operate on individual phenotypes
Stabilizing Selection
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Environment selects the most common phenotype
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Eg. Bill length in hummingbirds fits food source
Directional Selection
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Environment favours individuals with more extreme
variations of a trait
Eg. Hummingbird population moves to new habitat
with longer flowers
Disruptive Selection
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Favours individuals with variations at opposite
extremes of a traits over those with intermediate
variations
Eg. Hummingbird population in a habitat with both
short and long flowers, birds with longer and shorter
bills will be more successful
Sexual Selection
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Favours the selection of any trait that influences the
mating success of the individual (usually male)
Results in sexual dimorphism (striking differences
between males and females)
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Mating game is risky for male tungara frogs. When
calling for a mate in the dark, they run the risk of
giving away their location to the dearly frog-eating
bat
Selection and Antibiotic
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An example of Natural Selection in action can be
found in the resurgence of tuberculosis (TB)
TB is caused by rod shaped bacterium
Mycobacterium tuberculosis
TB causes severe lung infections, cough and chest
pain
Selection and Antibiotic
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Streptomycin, an antibiotic, was developed for TB
treatment
By 1970, TB had been almost completely wiped out
2006, there was an outbreak of TB in South Africa and
by 2008 it spread to 49 countries.
World Health Organization (WHO) reported that 1/3
of the population has TB and 5% does not respond of
Streptomycin
How did this resistance occur?
Cumulative Selection
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Evolution of a complex structure such as the eye is a
cumulative process
Rare, beneficial mutations may be separated by vast
amounts of time
Natural selection favors them and the adaptations they
produce accumulate one by one
Cumulative selection is the accumulation of many small
evolutionary changes over long periods of time and many
generations, resulting in a significant new adaptation
relative to the ancestral species
Activity
Modeling Genetic Drift