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Chapter 4
Genes and Their Evolution: Population Genetics
Demes, Reproductive Isolation, and Species
Evolution is about groups of potentially reproducing organisms.
Deme refers to members of a species that produce offspring.
All the genetic material within a population is referred to as the gene pool.
“Species” = populations and members capable of breeding producing fertile offspring.
Species are defined on the basis of reproductive isolation.
Population genetics studies change over time (or the lack of it) in gene pools.
Change in gene frequency over a few generations
For example, Morgan’s fruit fly eye color changes
Substantial change in characteristics over many generations
Hardy-Weinberg Law: Testing the Conditions of Genetic Equilibrium
It is a method for studying genetic change in populations
If no change is occurring within the population, gene frequencies remain the same
If change is occurring, evolution is happening within the population
Mutation: The Only Source of New Alleles
Mutation is the only source of new genetic information
Mutation can be any heritable change in the structure or amount of genetic material
Mutations are only important to populations if found in sex cells
Two types of mutations:
– Spontaneous (no known cause)
– Induced (from human causes such as exposure to X-rays)
Most mutations are harmless
Mutations are important for populations ONLY if they result in some change that
(positive or negative) the organisms ability to fit in the environment
Positive changes passed on to successive generations by natural selection.
Negative changes result in extinctions
Natural Selection in Animals: The Case of the Peppered Moth and Industrial Melanism
The peppered moth is the best documented evidence of natural selection
Found in England
Two forms: light and dark
As pollution covered trees, lighter peppered moths were more easily preyed on by birds
and the darker form became more prevalent
1970s, stricter pollution laws again changed the moths’ habitat, and the darker form
became the easier prey; the lighter form became more common
Natural Selection in Humans: Abnormal Hemoglobins and Resistance to Malaria
The Hemoglobin S gene causes sickle-cell anemia in humans
Found in millions of humans, destroys red blood cells (thus anemia)
Cells become sickle-shaped and rigid, and plug capillaries
Life expectancy 42-48 yrs
The Geography of Sickle-Cell Anemia and a Possible Association with Malaria
25 % of people living in equatorial Africa have the S gene
(symptoms: blood/oxygen issues, death during strenuous events such as sports)
Frequencies overlap areas where malaria is endemic (constantly present).
(Turks, No. Africans, Iranians, Arabs)
This gene has 2 alleles. If you have one, you are malaria resistant and not highly anemic.
If you have both, you are sickle-cell anemic
One inherited from each parent, thus the 1:4 ratio observed in local populations
A relationship has been documented between possession of one S gene and higher
survival when exposed to malaria
Scientists were puzzled by such a high frequency of a bad adaptation (anemia)
The Biology of Sickle-Cell Anemia and Malarial Infection
It was discovered that people with one allele had lower oxygen levels in their blood (due
to anemia), which is where the malaria parasite finishes its life cycle
The parasite generally cannot survive and reproduce
History of Sickle-Cell Anemia and Malaria
Sickle-cell tied to spread of Bantu, who carried the S mutation into equatorial Africa
The Bantu introduced agriculture into the region; large, cleared areas were ideal
environments for mosquitoes carrying malaria
Genetic Drift: Genetic Change Due to Chance
Random change in allele frequency over time
Can lead to one allele being lost and the other fixed in a population
May occur in a group that is endogamous (reproducing only within the group)
Gene Flow: Spread of Genes across Population Boundaries
Gene flow often refers to migration, though influenced by culture, social structure
Effects of gene flow (exchange of genes between populations) have increased over time
All living humans are the same species, but can we all really interbreed? In theory yes, but
what factors prevent this?
Natural Causes for Breeding Isolates
– Land masses, islands, oceans, mountain ranges, canyons, rivers, etc.
– Deserts, jungles, flora and fauna distribution, etc.
Cultural Causes for Breeding Isolates
Humans control breeding by cultural factors including endogamy and exogamy.
– Endogamy is mating with others from inside the same group
– Exogamy is mating with others from outside the group
– Worldwide, there are many examples of both
The Adaptive Significance of Human Variation
Humans have a number of long and short term adaptations that allow them to live
diverse environments
Next chapter, we will learn how variation within the human species has helped humans adapt
to different environmental conditions including:
– Differences in altitude
– Differences in exposure to sun
– Differences in environmental temperatures
– Long-term exposure to infectious diseases