Exam II – Study Guide
The Red Queen Hypothesis
: sex is needed to keep “one step ahead” of parasites that cause disease. Need to be able
to recombine and maintain a set of genes that code for defenses, e.g. extra mucous, more effective T-cells.
Sexual species carry a storehouse of “defenses,” This backup comes in 2 forms:
Heterozygosity - when an organism carries two different forms of a gene, Polymorphism - when a population carries
many different forms of a gene.
Normally, mucus works to move microbes out of lungs, CF causes
thick mucus, and clogs air passages, Interferes with pancreas.
Heterozygosity and cystic fibrosis:
two alleles for a gene, when alleles combine, it determines what trait will develop, Mendelian – Dominant alleles
produce the traits they carry even if recessive allele is present, but 2 recessive alleles will produce recessive trait.
Heterozygosity, sickle cell anemia and malaria:
Human sickle gene - may be defense against malaria,
Hemoglobin is a molecule that transports oxygen in the blood, single allele mutation changes a small part of the
molecule. Human sickle gene - may be defense against malaria. Heterozygous infants: 1 normal and 1 sickle allele
(AS) = more resistant to malaria, Homozygous people with no sickle allele are very susceptible to malaria (AA)
Homozygous people with both alleles “sickle” are anemic (SS) and do not live long.
Resistance genes & computer
Modeling sex and disease
which is more “fit” in terms of
evolution: sexual or asexual
with a population of
100 sexual; 100 asexual
Sexual population always died
out - sexual reproduction is
more difficult, conclusion? Made model more complex, SIM-pathogens with “virulence” genes could kill SIM-
hosts, SIM-hosts given “resistance” genes; Sexual population won game when disease was present.
Usually sexual, but may
reproduce asexually, Get blackspot disease,
caused by a parasitic
much more blackspot
than the sexual ones.
Rabbits overrun Australia:
introduced to Australia (1859) Overran rangeland - major pest. Looked for solution, European rabbits in South
America, Native rabbits in South America, virus.
Virus deliberately introduced to Australia in 1950,
Highly lethal (ca. 99.9%), Next year mortality at 90%, In 7 yrs, mortality dropped to 25%, 1965 - 1/5 as many
rabbits lived in Australia as had been there prior to 1950.
Co-evolution of host and pathogen:
New pathogen encounters a naïve population
Disease highly lethal
In small, disbursed population
quickly die out
In large, dense populations, epidemic may
come in waves
Stable disease pattern can
develop only if
both host pop. and pathogen pop. survive the
can evolve much more quickly than humans - generation times so short, Microbes adjust much more quickly in new