Population Dynamics

Humans and malaria

Science (2010), v.328:841

Global Map of Malaria

“…a far more urgent task: reducing the horrific toll of malariain central Africa, where five countries account for 50% of allglobal 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

– The inescapable conclusion:

Unequal reproductive successUnequal reproductive success• Darwin called this process natural selection.

• The result of natural selection is evolution when a population has adapted to its environment.

Evidence for humans?

Human Population GrowthThe 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 successUnequal reproductive success

• Darwin called this process natural selection.• The result of natural selection is evolution when a

population has adapted to its environment.

Evidence for humans?

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

from The Human Evolution Coloring Book, 2nd Ed.

Malaria infection causescells to sickle. Sickled cells, along with parasite, are killed.

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

– Sexual reproduction depends on• Production of gametes through Meiosis.• Fertilization.

Meiosis, The Basis of Sexual Reproduction

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.

Figure 8.14

– Humans are diploid organisms.

• Their cells contain two sets of chromosomes

• Their gametes are haploid, having only one set of chromosomes

– 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 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.

Homologous Chromosomes

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 TelophaseICytokin.htmlInterphase.html ProphaseII.htmlProphaseI.html MetaphaseII.htmlMetaphaseI.html AnaphaseII.htmlAnaphaseI.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

– What happens when errors occur in meiosis?

When Meiosis Goes Awry

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.