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Population Dynamics
Humans and malaria
Science (2010), v.328:841
Global Map of Malaria
“…a far more urgent task: reducing the horrific toll of malaria
in central Africa, where five countries account for 50% of all
global deaths from the disease and elimination is not possible.”
Science (2010), v.328:849
Malaria
Malaria life cycle
http://www.cdc.gov/malaria/about/index.html
http://www.who.int/topics/malaria/en/
Malaria is large threat to
human health in equatorial
regions of the Earth
But some human populations
show resistance to the disease.
How??
Natural Selection
• Overproduction
• Environmental
pressure/competition
• Pre-existing individual
variation
• Heritable traits
• Happens over generations
(time)
• Happens in populations (not
single individuals)
• Offspring must be viable and
fertile
Darwin’s Inescapable
Conclusion
– Fact 1: Potential for overproduction
and competition for existence
– Fact 2: Individual variation
Evidence
for
humans?
– The inescapable conclusion:
Unequal reproductive success
• Darwin called this process natural selection.
• The result of natural selection is evolution when a population
has adapted to its environment.
Human Population Growth
The History of Global Population
Growth
Darwin’s Inescapable
Conclusion
– Fact 1: Potential for overproduction and
competition for existence
– Fact 2: Individual variation
– The inescapable conclusion:
Unequal reproductive success
Evidence
for
humans?
• Darwin called this process natural selection.
• The result of natural selection is evolution when a
population has adapted to its environment.
The Modern Synthesis:
Darwinism Meets Genetics
– The modern synthesis is the fusion of
genetics with evolutionary biology.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Populations as the Units of
Evolution
– A population
• Is a group of individuals of the same species
living in the same place at the same time.
• Is the smallest biological unit that can evolve.
– Population genetics
• Focuses on populations as the evolutionary
units.
• Tracks the genetic makeup of populations over
time.
Genetic Variation in
Populations
– Individual variation abounds in populations.
• Not all of this variation is heritable.
• Only the genetic component of variation is
relevant to natural selection.
Sources of Genetic Variation
in sexually reproducing
organisms
– Mutations
• Are changes in the DNA of an organism.
– Sexual recombination
• Crossing over
• Independent assortment of chromosomes
• Random fertilization
– Hemophilia
• Is a blood-clotting disease.
Intergenerational Mutation
Rate
• By how many mutations does your
genome differ from your parents
genome?
• Roach et al (2010) found about 60
mutations, 30 from each parent, that
occurred during the formation of egg
and sperm (meiosis).
Sickle-cell allele:
genotype, phenotype, and
resistance to malaria
Malaria infection causes
cells to sickle. Sickled cells,
along with parasite, are killed.
from The Human Evolution Coloring Book, 2nd Ed.
Sickle-cell Allele
Biochemistry of
sickle-cell disease
from Steinberg, M.H., (2006),
Trends Pharm. Sci., 27(4):204-210.
Sources of Genetic Variation
in sexually reproducing
organisms
– Mutations
• Are changes in the DNA of an organism.
– Sexual recombination
• Crossing over
• Independent assortment of chromosomes
• Random fertilization
Meiosis, The Basis of Sexual
Reproduction
– Sexual reproduction depends on
• Production of gametes through Meiosis.
• Fertilization.
Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings
Gametes and the Life Cycle of
a Sexual Organism
– The life cycle of a multicellular organism is
the sequence of stages leading from the
adults of one generation to the adults of
the next.
– Humans are
diploid
organisms.
• Their cells
contain two
sets of
chromosomes
• Their gametes
are haploid,
having only
one set of
chromosomes
Figure 8.14
– Fertilization
• Is the fusion
of sperm
and egg.
• Creates a
zygote, or
fertilized
egg.
– Sexual life
cycles
involve an
alternation of
diploid and
haploid
stages.
Eukaryotic Chromosomes
– Each eukaryotic
chromosome
contains one very
long DNA molecule,
• Typically bearing
thousands of genes.
– The number of
chromosomes in a
eukaryotic cell
• Depends on the
species.
Homologous Chromosomes
– Homologous chromosomes are matching
pairs of chromosomes.
–Humans have
•Two different sex
chromosomes, X
and Y.
•A female has XX
•A male has XY
•Twenty-two pairs
of matching
chromosomes,
called autosomes.
Meiosis is the process where the two sets of homologous
chromosomes are separated into different cells - egg or sperm.
Start with one cell with
2 sets of homologous chromosomes
End with 4 cells with
1 set of homologous chromosomes
How does this happen?
Meiosis is the process where the two sets of homologous
chromosomes are separated into different cells - egg or sperm.
The Process of Meiosis
– In meiosis,
•
•
•
Haploid daughter cells are produced in diploid organisms.
Two consecutive divisions occur, meiosis I and meiosis II,
preceded by interphase.
Crossing over occurs.
From movie folder play:
MeiosisOverview.html
Interphase.html
ProphaseI.html
MetaphaseI.html
AnaphaseI.html
TelophaseICytokin.html
ProphaseII.html
MetaphaseII.html
AnaphaseII.html
TelophaseIICytokin.html
Figure 8.16.1
Figure 8.16.2
Figure 8.16.3
Review: Comparing Mitosis
and Meiosis
– All the events unique to meiosis occur
during meiosis I.
Figure 8.17
The Origins of Genetic
Variation
– Offspring of sexual reproduction are
genetically different from their parents and
from one another.
– Independent assortment of chromosomes
– Random fertilization
– Crossing over
– Random mutations
Independent Assortment of
Chromosomes
– In independent assortment, every
chromosome pair orients independently of
the others during meiosis.
Random Fertilization
– The human egg cell is fertilized randomly
by one sperm, leading to genetic variety in
the zygote.
Crossing Over
– In crossing over,
• Homologous
chromosomes
exchange genetic
information.
• Genetic
recombination
occurs.
From the movie folder play:
- CrossingOver.html
When Meiosis Goes Awry
– What happens when errors occur in meiosis?
How Accidents During Meiosis
Can Alter Chromosome Number
– In nondisjunction,
• The members of a chromosome pair fail to separate
during anaphase.
• Gametes with an incorrect number of chromosomes
are produced.
Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings
Figure 8.20
– The result of nondisjunction
Down Syndrome: An Extra
Chromosome 21
– Down Syndrome
• Is a condition where an individual has an extra
chromosome 21.
• Is also called trisomy 21.
– The incidence of Down Syndrome
increases with the age of the mother.
Abnormal Numbers of Sex
Chromosomes
– Nondisjunction
• Also affects the sex chromosomes.