Genetic Drift
All the genes people have is called the gene pool for their population.
Each different gene type within the pool occurs with a certain frequency.
For example, the frequency of type AB blood in the white USA population is 3%.
But gene frequencies change and that can cause a population to change.
Genetic drift is the random change in the frequency of a gene's
occurrence in a population over many generations of reproduction and cycles of
life and death. The change is random because of two factors:
- During reproduction, not all of a person's genes get reproduced to pass to
the child — only half do, says biochemist
Laurence Moran of the University of Toronto. Sex is a
random event. Chance alone picks the half of the parent's genes for
passing on. The parent, however, is not exactly reproduced (that's
cloning). The child is different from the parent in a
random way and, therefore, the gene frequency within the population's gene
pool changes in a random way when the child is born.
If a parent has many children, the chances go up that more of the
parent's genes get passed along
with each successive child since more random samples of the parent's genes are
selected.
- Chance also rules whether an individual survives and reproduces (or
chooses to reproduce). If an individual dies without having children, then
the pool loses his or her genes.
Genetic drift affects small populations
Genetic drift effect, is particularly strong
- "in small populations (e.g., 100 breeding pairs or fewer);
- when the gene is neutral; that is, is neither helpful nor deleterious,"
says biologist
John W. Kimball author of
Biology.
Back to our previous example to illustrate how genetic drift can drastically
affect small populations ― to the extent they become extinct. Consider
the neutral gene for determining blood type, a characteristic which neither
helps or hurts a population.
Envision a situation where Mary is a member of a small population of 10
intrepid adventurers colonizing a remote island in the Atlantic Ocean. She
has blood type B. The white colonists, let's assume, originally came from
the USA, where the frequency for type B blood is 10%. So, in a population
of only 10 people, as luck would have it, Mary is one of two people on the island
with type B blood (the other person has type AB blood with the
alleles for both A and B).
Unfortunately, a shark kills Mary before she has had a chance to reproduce.
So the frequency count for genes producing type B blood in the gene pool drops
to zero. This loss of genetic diversity probably won't hurt the surviving
population but it might.
Maybe some disease-causing bacterium about then changes to attack blood type O, but is still harmless for
the types A an B. Then, the resulting
disease could kill off about 50% of the population ― leaving only five people
to carry on. Without Mary, the population is doomed because all four other
survivors are male.
In any such small populations, eventually the entire population may end up
all with the same gene determining a certain characteristic (say blue eyes) or a
gene type may disappear, Kimball says. Indeed, the blood type AB does not
exist among the Blackfoot Indian population of northern US and Canada or the
Navajo population, writes biologist
Wayne P.
Armstrong of Palomar College in San Marcos, California.
Wikipedia illustrates random drift with a clever bottle experiment:
Marbles in a jar
Twenty marbles in the first jar represent an original population of 20
people. Half of the people have a "red" and half a "blue" gene type. The next
jar represents the offspring the marble people produce for the next
generation. In each new generation the people reproduce at random.
(To represent this reproduction, randomly select any marble of either color
from the original jar and deposit a new marble with the selected color as its
"offspring" in the second jar. Repeat the process until there are 20 new marbles in
the second jar.)
The second jar will then contain a second generation of "offspring" – one
child for each parent marble and 20
marbles total of various colors. Unless the second jar contains exactly 10 red and 10
blue marbles, there will have been a purely random shift in the frequencies of
the red and blue gene types.
Repeat this process a number of times, randomly reproducing each generation
of marbles to form the next. The numbers of red and blue marbles picked each
generation will fluctuate: sometimes more red, sometimes more blue. This
fluctuation is genetic drift – a change in the population's gene-type frequency
resulting from a random variation in the distribution of gene types from one
generation to the next.
It is even possible that, in any one generation, no marbles of a particular
color will be chosen, meaning they have no offspring. In this example, if no red
marbles are selected, the jar representing the new generation will contain only
blue offspring. If this happens, the red gene type has been lost permanently in
the population, while the remaining blue gene type has become fixed: all future
generations will be entirely blue.
In small populations, fixation to a single surviving gene type can occur in
just a few generations. Given enough time, this outcome is nearly inevitable for
populations of any size.
In this simulation, there is fixation in the blue "gene type" within
five generations.
Further Reading:
Polymorphisms, founder effect and genetic drift, 20 Sep. 2007, John W. Kimball, Kimball's
Biology Page.
Random Genetic Drift, 22 January 1993, Lawrence Moran, University of Toronto
and TalkOrigins.org
Genetic
Drift In Blood Type Populations, Palomar College
(Answered 25 July 2009)
Return to
Space
Colonization
Comment
|
