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11,12,13
Each one of the conditions required to maintain HardyWeinberg equilibrium is violated!
• Are all populations large?
– Although many populations are, some are small enough
for random events to change gene frequencies.
• This microevolutionary phenomena is called genetic
drift.
• Bottleneck Effect:
• In this case, disasters such as floods and fires can
drastically reduce the size of the population, leaving
by chance, individuals that are not necessarily
representative of the original population.
By chance the surviving population is made up mostly of pink
Greater Prairie chicken had major
populations removed due to farming.
By 1993 only 2 populations totaling 50
birds remained
• Founder Effect
• This occurs whenever a few individuals colonize a
new habitat.
• The founding population is usually small.
• Their gene pool may not be representative of the
entire gene pool they left.
• In the Amish, in fact, Ellis-
van Creveld syndrome has
been traced back to one
couple, Samuel King and his
wife, who came to the area
in 1744. The mutated gene
that causes the syndrome
was passed along from the
Kings and their offspring, and
today it is many times more
common in the Amish
population than in the
American population at
large.
Description
This is a rare autosomal recessive genetic resulting in short stature (average:
3-5'1"), postaxial polydactyly, short arms and legs in relation to the trunk,
short ribs and small lungs, and 50-60% of individuals have congenital heart
defects.
So people with EvC have smaller than average, or absent nails, teeth can be
oddly shaped, have very thin enamel and make an appearance far too early
(Angus cut his first tooth when he was 7 weeks old), and brittle hair (this is
just a name. The hair looks normal but each hair is quite thin). There are
other conditions grouped under the term ectodermal dysplasias which affect
the sweat glands, but I haven't heard of anyone with EvC having these
problems.
It is extremely rare for intelligence to be affected but a very small percentage
(less than 1%) of babies born with EvC can have structural abnormalities in
their brains.
The most upsetting aspect of EvC as a parent is finding out that your child has
a heart defect. Short stature and pretty unique hands are fine because they
don't affect your child's independence and survival, but a heart defect is a
huge blow. The majority of children with a common atrium or ASD (atrial
septal defect) will be quite happy up until they reach a year or two and then
they can have planned surgery which will fix the problem. Recovery can be
slower in kids with EvC because of their smaller lungs, but once they get over
it they can carry on as normal. Some children deteriorate before this time
Polydactyly
Gene flow occurs
• Populations may gain or lose alleles by the
migration or immigration of individuals, seeds,
pollen etc.
• For example, a wind storm may blow pollen from
an aa population into a population consisting of just
AA individuals.
Mutations occur……
• A new mutation that is transmitted in gametes
can immediately change a gene pool of a
population.
• In fact, rates of one mutation per locus per 105 to
106 gametes is typical for most species.
3 types of selection:
Stabilizing, Directional, Disruptive
ch 13 (12,13)
Three modes of natural selection
• Stabilizing Selection
• Directional Selection
• Disruptive Selection
Stabilizing
Directional
Disruptive
• Stabilizing selection
– Favors the intermediate phenotype out of a range of
phenotypes.
– The extremes in variation are selected against.
– For example, infants weighing significantly less or more than
7.5 pounds have higher rates of infant mortality.
– Selection works against both extremes.
– Tends to maintain status quo and reduce variation
• Directional selection
– Favors phenotypes at one extreme of the range of variation.
– Insecticide resistance is an example. DDT was a widely used
insecticide. After a few years of extensive use, DDT lost its
effectiveness on insects. Resistance to DDT is a genetic trait
that the presence of DDT in the environment made into a
favored trait. Only those insects resistant to DDT survived,
leading over time to populations largely resistant to DDT.
• Disruptive selection
– favors individuals at both extremes of variation: selection is
against the middle of the curve.
– This causes a discontinuity of the variations, causing two or
more morphs or distinct phenotypes.
– The African swallowtail butterfly (Papilo dardanus) produces
two distinct morphs, both of which resemble brightly colored
but distasteful butterflies of other species. Each morph gains
protection from predation although it is in fact quite edible.
So what about sexual selection
section 14
• Darwin was the first to investigate the idea
that individuals with certain inherited
characteristics are more likely than others to
obtain mates.
• Sexual selection can result in sexual
dimorphism
Sexual selection and sexual differences
• Intrasexual – selection within the same sex.
Individuals of one sex are competing for mates of
the opposite sex. (Usually this is between males
competing for a large group of females…..male
deer)
• Intersexual – selection of mate choice.
Individuals of one sex (usually females) are
choosy in selecting their mates. ….males with the
largest or most colorful feather showing
So what’s the purpose of the “choosy
female?”
• It appears the females are searching for the
males with the “good genes”
• A study was done on a group of tree frogs .
Females usually chose the male with the
longest mating call
A study was done with one females
eggs
• ½ of eggs were fertilized with long calling
males
• ½ of eggs were fertilized with short calling
males
• Results………………..
The offspring from the long calling
males grew bigger, faster and survived
better
Pheromones!
The sweaty T –shirt Experiment
• Experiment: sniff test….
• http://www.pbs.org/wgbh/evolution/library/0
1/6/l_016_08.html
• Result: Women favor the smell of the sweaty T
shirts worn by men with immune response
genes (IRG) different from their own.
Are Humans Exempt from Natural Selection?
• It has been argued that advances in medicine,
sanitation, etc. have removed humans from the
rigors of natural selection. There is probably some
truth to this, but consider:
– Of all the human eggs that are fertilized, only one-third
or so will ever reproduce themselves. The others are
eliminated as follows:
• Mortality selection
– Some 30% of pregnancies end by spontaneous abortion of
embryos and fetuses.
– 5% by stillbirths and infant deaths.
– 3% by childhood deaths.
• Sexual selection
– Another 20% will survive to adulthood but never marry.
• Fecundity selection
– Of those that do marry, 10% will have no children.