13.9 – 13.10
3 causes of evolutionary change:
1. Natural selection
2. Genetic Drift
3. Gene Flow

Change in the gene
pool of a population
due to chance
Genetic Drift

Genetic drift resulting
from a drastic
reduction in population
size
Bottleneck Effect

Northern elephant seal
population reduced to
about 20 individuals in
the population due to
human predation.
Population allowed to
recover. The DNA of
24 genes studied –
found no variation /
only one allele for each
gene.
Example

Random change in the
gene pool that occurs
in a small colony of a
population
Founder Effect

1814 – 15 people
founded a British Colony
on Tristan da Cunha.
One person carried a
recessive allele for
retinitis pigmentosa
(progressive form of
blindness). In the 1960’s
the island descendants
were samples – of 240
individuals, 4 had the
disease. The frequency
for the disease is much
higher among islanders
than the area from which
the founders came.
Example

Gain or loss of alleles
from a population by
the movement of
individuals or gametes
into or out of the
population
Gene Flow

Human movement
today promotes gene
flow; no
Example

Which type of event explains the high
incidence of polydactylism found in the
Amish population?
Founder Effect

Flower population: ww = white flower; WW
and Ww = purple flower; a mud slide wipes
out most of the white flowered plants. What
type of event occurred?
Bottleneck

Genetic modifications from transgenic plants
passed to neighboring populations of wild
relatives. What type of event occurred?
Gene Flow

Millions of greater prairie chickens once lived
on the prairies of Illinois. As these prairies
were converted to farmland and other uses
during the 19th and 20th century, the number
of greater prairie chickens plummeted to less
than 50 individuals. The few survivors have
low levels of genetic
variation. What type of event
occurred?
Bottleneck

This event resulted in the world wide spread
of several insecitcide-resistance alleles in the
mosquito Cluex pipiens, a vector for West
Nile virus and Malaria. What type of event
occurred?
Gene Flow

Reduction in geographic variation in the
appearance of humans is caused by what
type of event?
Gene Flow

A few weeks after two major hurricanes swept
through the Caribbean islands, fishermen on the
island of Anguilla observed a large raft float to the
shore. Green iguanas were seen on the raft. These
iguanas are thought to have come from the islands of
Guadeloupe. The allele frequencies for the founding
population on Anguilla differed from those on
Guadeloupe. What type of event occurred?
Founder Effect

Humans on the island of Pingelap Atoll in the South
Pacific suffered the effect of typhoon followed by a
famine around 1775. Only about 20 people out a
population of several thousand managed to
survive. One of the survivors carried a color
blindness allele. In today’s population on this
island, over 1 in 20 people is afflicted with color
blindness – well over 20%. In the original
population about 2.5% of the people had this form
of color blindness. What type of event occurred?
Bottleneck