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Bell Ringer
1. Silently Write the learning the targets off the
board, into your notebook.
1. Take out your homework and have it ready to
be turned in.
2. What is your understanding of natural
selection?
3. What are the 3 requirements for natural
selection to occur?
Evolution
How do we know it
happens?
Evidence?
How
do we know
it happens?
How does it
happen??????
How do organisms evolve?
Natural Selection
“survival of the fittest”
Microevolution
and
Microevolution
• Changes a gene pool from generation to
generation.
– Gene pool: the total of all genes in the population
Types of Microevolution
1. Gene Flow
2. Genetic Drift
3. Nonrandom Mating
2
1. Gene Flow: the exchange
of genes due to
migration
•
Mutations will occur over time. They are natural and
produce genetic diversity that is needed for survival.
How would this lead to the evolution of the population?
Gene Flow in Plants
Gene Flow in Humans
2. Genetic drift: the change in the gene pool of a
population due to a random occurrence.
Ex: Bottleneck Effect and Founder Effect
Bottleneck = any kind of event that reduces the population
significantly..... earthquake....flood.....disease.....etc.…
Examples of Genetic Drift
 A) The Founder Effect:
A founder effect occurs when a new colony is started by a
few members of original population.
 Small population that branches off from a larger
one may or may not be genetically representative of
the larger population from which it was derived.
 Only a fraction of the total genetic diversity of the
original gene pool is represented in these few
individuals.
• 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.
Examples of Genetic Drift
 B) Population Bottleneck:
 Occurs when a population undergoes an event in which a
significant percentage of a population or species is killed or
otherwise prevented from reproducing.
•The event may
eliminate alleles
entirely or also
cause other
alleles to be overrepresented in a
gene pool.
EX. Cheetahs
http://www.nytimes.com/1985/09/17/science/loss-of-gene-diversity-is-threat-to-cheetahs.htm
l
• An example of a bottleneck: 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.
3. Non-Random Mating
 A situation where mates are chosen for one
another or by another pre-determined factor.
 Leads to a loss of gene variability!
 Many plants self-pollinate, which is also a form of
non-random mating (inbreeding).
Sexual reproduction results
in variation of traits in offspring
as a result of crossing over in
meiosis and mutations
Genetic shuffling is a source
of variation.
Sexual selection occurs when certain
traits increase mating success.