Download Factors that affect Natural Selection

AP Environmental 1.4: Evolution
9/6/2016
Natural Selection Notes:
• Natural selection: individuals with certain
traits are more likely to survive and reproduce
under a certain set of environmental
conditions
– Survival of the Fittest
Focus on Key Vocabulary!
• Natural selection: individuals with certain
traits are more likely to survive and reproduce
under a certain set of environmental
conditions
– Survival of the Fittest
• Biological evolution: how earth’s life
changes over time through changes in the
genetic characteristics of populations
– Darwin: Origin of Species
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Factors that affect Natural Selection
• Genetic variations
– Occurs through mutations (changes in DNA) in
reproductive cells
– Genetic drift unequal numbers of a given
genome, could lead to decreased variation
FACT: Natural selection does not happen
unless there is variation in a population.
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1. Environmental Change
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2. Competition
Example: A population of giraffes feeds on leaves of
small bushes on the ground and short trees.
Suddenly, their environment changes! After a
drought, all of the bushes on the ground die. The
giraffes are left with only taller trees to eat from.
Though some giraffes have necks that can reach
those leaves, others do not.
Example: A population of birds uses their beaks to
dig into the soil and find worms to eat. The birds
have a variety of beak sizes, but since there was
always an abundant supply of worms all of the birds
were able to feed. One spring the worm population
decreased dramatically due to a disease and the
birds had to compete over the worms. The birds
with the longest beaks were able to dig deepest
into the soil and find enough worms to survive.
How did variation aid in survival?
How did variation aid in survival?
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AP Environmental 1.4: Evolution
9/6/2016
Bottleneck effect- population is
reduced which reduces gene pool
How Do New Species Evolve?
• Speciation: one species splits into two or more species
• Geographic isolation: happens first; physical
isolation of populations for a long period
• Reproductive isolation: mutations and natural
selection in geographically isolated populations
lead to inability to produce viable offspring when
members of two different populations mate
• Example: Northern elephant seals have
reduced genetic variation probably because
of a population bottleneck humans inflicted
on them in the 1890s. Hunting reduced their
population size to as few as 20 individuals at
the end of the 19th century. Their population
has since rebounded to over 30,000—but
their genes still carry the marks of this
bottleneck: they have much less genetic
variation than a population of southern
elephant seals that was not so intensely
hunted.
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Founders effect- when a new colony is started
by a few members of the original population.
This small population size means that the
colony may have:
– reduced genetic variation from the original population.
– a non-random sample of the genes in the original population.
• For example, the Afrikaner population of Dutch settlers in
South Africa is descended mainly from a few colonists. Today,
the Afrikaner population has an unusually high frequency of
the gene that causes Huntington’s disease, because those
original Dutch colonists just happened to carry that gene
with unusually high frequency. This effect is easy to
recognize in genetic diseases, but of course, the frequencies
of all sorts of genes are affected by founder events.
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Extinction is Forever
• Extinction
– Biological extinction
– Local extinction
• Endemic species
Golden Toad of Costa Rica, Extinct
– Found only in one area
– Particularly vulnerable
• Background extinction: typical low rate of
extinction
• Mass extinction: 3-5 over 500 million years
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Geologic Processes Affect Natural Selection
Climate Change and Catastrophes Affect Natural Selection
• Tectonic plate movement affected evolution and
the location of life on earth
• Ice ages followed by warming temperatures
• Earthquakes
• Collisions between the earth and large asteroids
– New species & Extinctions
• Volcanic eruptions
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