Download Selection and Speciation

Section 9.1 (Part 1)
SBI 3U1
pp. 350 - 356
FACTORS THAT AFFECT ALLELE
FREQUENCIES
GENETIC VARIATION
 Most
traits in a population vary from one
extreme to another (eg. Height, weight)
RECALL: MUTATION & NATURAL SELECTION
 Mutations:
introduce new alleles into a
population
 Natural
Selection: environment selects
individuals within a population with certain
traits that make them better suited to survive
& reproduce
 Result
– change in allele frequencies
GENE FLOW
 Movement
of individuals between populations
moves the alleles carried by the individuals
 Can
change the allele frequency in both
populations
 Can
also spread new alleles that arise in one
population
GENE FLOW
Migration
between
habitats
Population:
15 blue
blue (65%)
(75%)
13
(25%)
75 Green (35%)
Population:
5 blue (35%)
(25%)
7
15 Green (65%)
(75%)
13
GENETIC DRIFT

Genetic Drift – The gradual change in allele
frequencies due to chance. Especially strong in
small populations
GENETIC DRIFT
Population
17 blue (85%)
3 green (15%)
Random events
cause the death of
some individuals
Population
15 blue (93.75%)
1 green (6.25%)
BOTTLENECK
•
•
Bottleneck - occurs
when a population’s size
is reduced.
Because a bottleneck
acts more quickly to
reduce genetic variation
in small populations,
undergoing a bottleneck
can reduce a population’s
genetic variation by a
great deal
Cheetah
Cheetahs are so closely related to each other that they do not reject skin grafts
www.petsdoc.com/pics/funpages/ wildlifephotos/cheeta
NORTHERN ELEPHANT SEAL
Bottleneck
Effect
http://www.icsi.berkeley.edu/~dbailey/gallery/image/elephant.jpg
Elephant Seal
- Reduced to
20
individuals
in 1896
- Now
30,000
individuals,
with no
detectable
genetic
diversity
FOUNDER EFFECT
 Founder
Effect - If a population began with a
few individuals, one or more of whom carried a
particular allele, that allele may come to be
represented in many of the descendants.
FOUNDER EFFECT
Access to
new
habitat
Population
13
15 Blue (82%)
(75%)
3
5 Green (18%)
(25%)
Some
individual
s migrate
Genetic
makeup of
that group
populate
new habitat
Population
8
2 Blue (50%)
8
2 Green (50%)
FOUNDER EFFECT


In the 1680s Ariaantje and Gerrit Jansz
emigrated from Holland to South Africa,
one of them bringing along an allele for
the mild metabolic disease porphyria.
Today more than 30000 South Africans
carry this allele and, in every case
examined, can trace it back to this couple
— a remarkable example of the founder
effect
TRISTAN DE CUNHA ISLANDS
In 1814, 15 British
colonists founded a
settlement on Tristan da
Cunha, a group of small
islands in the Atlantic
Ocean, midway between
Africa and South America
One of the early colonists
apparently carried a
recessive allele
for retinitis pigmentosa, a
progressive form of
blindness that afflicts
homozygous individuals.
Of the
founding
colonists' 240
descendants
on the island
in the late
1960s, 4 had
retinitis
pigmentosa.
The frequency of
the allele that
causes this
disease is ten
times higher on
Tristan da Cunha
than in the
populations from
which the
founders came.
TO DO LIST

Section 9.1 p. 352 #1 – 3, p. 356 #7 – 12


Topic Approval Due Today

Meet in the Library Tomorrow
Section 9.1 (Part 2)
SBI 3U1
pp. 350 - 356
NATURAL SELECTION:
STABILIZING, DISRUPTIVE &
DIRECTIONAL
TYPES OF SELECTION
 Directional
 Stabilizing
 Disruptive
STABILIZING
 selective
Natural selection that favours
intermediate phenotypes and
acts against extreme variants
e.g. medium sized beaks
pressures select against the two
extremes of a trait, the population experiences
stabilizing selection
DIRECTIONAL
 one
Natural selection that favours
phenotypes at one extreme over
another, resulting in the distribution
curve of phenotypes shifting in the
direction of that extreme
E.g. large sized beaks
extreme of the trait distribution
experiences selection against it
DISRUPTIVE
 selection
Natural selection that favours the extremes of a range
of phenotypes rather than intermediate phenotype,
type of selection can result in the elimination of the
intermediate type. e.g. large and small sized beaks
pressures act against individuals in
the middle of the trait distribution. The result
is a two-peaked curve in which the two
extremes of the curve create their own smaller
curves
SEXUAL SELECTION
 Sexual
selection occurs in two ways:
through contests and through choice
 Contests are competitions between
members of the same sex for access to the
other sex

Video
 Choice
involves competition for attention
from the opposite sex
Video 1
 Video 2

TO DO LIST

Section 9.1 p. 359 #1-11