Download Variation 03.24.04

Variation and Natural Selection
(Chap. 18)
Two basic points of variation
1. Variation can occur in a wide array of
traits (physical features, color, size....)
2. We may not be aware of the variation
(enzymes, disease susceptibility, blood
type…)
Variation comes from:
Sexual reproduction
 genetic recombination in meiosis continually
produces variation by shuffling existing genes
and alleles
 fertilization allows the alleles from two diff.
individuals to be brought together in one new
individual
Mutations
 provides a steady input of new genes and alleles
Maintaining Genetic Variation
• Recessive traits tend to be maintained in a population
because of heterozygous individuals
– natural selection may not operate on the recessive
alleles.
• Often results in balanced polymorphism, a process
that inhibits the loss of recessive (even detrimental)
alleles
– the population’s ratios of alleles do not change
much from one generation to the next
– beneficial for a population to maintain two
different alleles creating different phenotypes
within the population.
Types/examples of balanced polymorphisms
1. Heterozygote advantage
• heterozygous genotype has greater success than
either homozygote
• ex) sickle cell anemia:
– heterozygous individuals do not develop
anemia and are resistant to malaria. In some
areas, 40% of a population are carriers.
–
Their red blood cells, containing some abnormal hemoglobin, tend to sickle when they are infected by the
malaria parasite. Those infected cells flow through the spleen, which culls them out because of their sickle
shape -- and the parasite is eliminated along with them.
• SS – normal, get malaria
• Ss – resistant to malaria and carrier of
sickle cell
Sickle Cell
• ss – get sickle cell disease
Video clip
a. The distribution of the falciparum malaria (Plasmodium falciparum) in the Old World before
extensive eradication of mosquitoes begun in the '20s. b. The distribution of the sickle-cell allele
within the same area. Intensity of color refers to increasing allele frequencies. Note the
correlation between the two distributions.
2. Frequency-dependent selection
• occurs when the reproductive success of a
genotype depends on its frequency in the
population
• Negative
– a trait has advantage when rare but
disadvantage when common which results in
a balanced polymorphism
•
•
Fish example in text
Positive
– a trait has higher fitness when it is common
than when it is rare
•
bright colored and poisonous animals
3. Transient polymorphism
• one allele is gradually replacing the other
– ex) sickle cell anemia
• If a population with Ss moves to a location
where malaria is no longer present, the s
allele will begin to disappear because the
selective pressure in its favor has
disappeared but the pressure against it (s)
continues.
– ex) industrial melanism
• Black moths slowly replace white moths in
dirty coal burning areas of England
Three observed examples of Natural Selection
in response to environmental change that
lead to the evolution of organisms
1. 1970’s Evolution of the beaks of Galapagos
Island Finches (Geospiza fortis)
• Periods of draught caused plants to produce
large, tough seeds.
• Sampling the G. fortis that died as well as those
that survived showed that:
– the larger birds were favored over the
smaller ones
– birds with larger beaks were favored over
those with smaller ones.
But did it produce evolution?
The answer turned out to be yes. As the population of G. fortis recovered after the
rains returned, the average body size and beak depth of their offspring was greater
than before (an increase of 4–5% for beak depth). The bell-shaped curve had been
shifted to the right — directional selection.
2. Multiple antibiotic resistance in bacteria
• Case study of one bacterial species, Staphylococcus aureus:
• 1943 - Penicillin becomes commercially available and
initially is seen as a "magic bullet" against staph infections
1947 - First staph resistance to penicillin reported
1980s-1990s - 90% of clinical cases are now resistant
1960s - Switch by hospitals to a new drug, methicillin, for
serious staph aureus
1961 - Methicillin resistance reported in Cairo
1980s - Methicillin resistance rising
1992 - 15% resistant to methicillin.
1996 - 35% resistant to methicillin
1990s - Switch to another drug, vancomycin, as last resort
1996 - First resistance to vancomycin reported
3. Peppered Moth (Fig 18.10 in text)
• Study took place in England (1800’s)
• Often referred to as Industrial Melanism
• Two morphs – peppered/light color (recessive)
and dark (dominant)
• Black morph became more common in dirty,
industrialized area.
• As England improved air pollution, the black
morph declined and peppered/white returned.
Fitness
• A measure of an organism’s contribution
to the gene pool of the next generation—
reproductive success
• Natural selection determines, not favors,
the fittest individuals.
• Can be quantified between 0-1.