Download 3. Genetic Drift

3. Genetic Drift
• The change in allele frequencies as a result of chance
processes.
• Directly related to the population numbers.
• These changes are much more pronounced in small
populations.
• Smaller population sizes are more susceptible to genetic
drift than larger populations because there is a greater
chance that a rare allele will be lost.
Imagine that in one generation, two brown beetles
happened to have four offspring survive to reproduce.
Several green beetles were killed when someone stepped
on them and had no offspring. The next generation would
have a few more brown beetles than the previous
generation—but just by chance. These chance changes from
generation to generation are known as genetic drift.
In a population of 100 bears, suppose there are
two alleles for fur color: A1 (black) and A2 (brown).
A1 has a frequency of .9, A2 a frequency of .1
(1.0 = 100%). The number of individuals carrying
A2 is very small compared to the number of
individuals carrying A1, and if only fifty percent of
the population survives to breed that year, there's
a good chance that the A2s will be wiped out.
Examples of Genetic Drift
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.
E.g., 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.
Ellis - van Creveld Syndrome
Bottleneck = any kind of event that reduces the
population significantly..... earthquake....flood.....
disease.....etc.…
E.g. 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.
4. Mutations
• Are inheritable changes in the genotype.
• Provide the variation that can be acted upon by natural
selection.
• Mutations provide the raw material on which natural
selection can act.
 Only source of additional genetic material and new
alleles.
 Can be neutral, harmful or beneficial( give an individual
 a better chance for survival).
 Antibiotic resistance in bacteria is one form.
• Mutation is a change in DNA the hereditary material of
life. An organism’s DNA affects how it looks, how it
behaves, and its physiology—all aspects of its life. So a
change in an organism’s DNA can cause changes in all
aspects of its life.
• Somatic mutations occur in non-reproductive cells and
won’t be passed onto offspring.
 The only mutations that matter to largescale evolution are those that can be
passed on to offspring. These occur in
reproductive cells like eggs and sperm and
are called germ line mutations.
 A single germ line mutation can have a
range of effects:
No change occurs in phenotype.
2. Small change occurs in phenotype.
3. Big change occurs in phenotype.
Some really important phenotypic
changes, like DDT resistance in insects are
sometimes caused by single mutations1.
A single mutation can also have strong
negative effects for the organism.
Mutations that cause the death of an
organism are called lethal — and it doesn't get
more negative than that.
Causes of Mutations
 DNA fails to copy accurately.
 External influences can create mutations.
 Mutations can also be caused by exposure
to specific chemicals or radiation.
5. Non-Random Mating
• In animals, non-random mating can change allele
frequencies as the choice of mates is often an
important part of behavior.
• 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.
There are two types of sexual
selection.
• intrasexual selection: competition among
males
• intersexual selection: males display certain
traits to females