Download Selection and Speciation

Factors that affect allele
frequencies
Genetic Variation
• Most traits in a population vary from one extreme to
another (eg. Height, weight)
Allele Frequency
• How often a
particular allele
appears in a
population.
30% of us have the
brown phenotype
Homozygous Individuals
carry two copies of the
allele, while heterozygous
individuals carry one.
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:
Population:
15 blue
blue (65%)
(75%)
13
Green (35%)
(25%)
75 Green
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 genetic drift
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
- 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%)
35 Green (18%)
(25%)
Some
individuals
migrate
Genetic
makeup of
that group
populate
new habitat
Population
82 Blue (50%)
82 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
An 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.