Lecture 6 - Forces of Evolution
Population Genetics
The study of genetic variation within and between populations.
Emphasis is on microevolution, which occurs generation to generation within a species.
Population
Members of a species that regularly mate with each other and exchange alleles.
Share a gene pool.
Biological Species Concept
Defines a species as a group of interbreeding organisms reproductively isolated from other organisms.
Allopatric Speciation
Geographic isolation occurs.
Gene flow is interrupted.
Populations evolve independently.
After many generations, interbreeding is no longer possible or does not occur.
Forces of Evolution
Mutation: Produces new variation.
Gene Flow: Redistributes variation.
Genetic Drift: Distributes variation.
Natural Selection: Acts on existing variation.
Mutation
A change in the sequence of bases in a gene.
The only source of completely new genetic variation (creates new alleles).
Can have a range of possible effects, from none to lethal.
Example: Point Mutation
A change in a single DNA base.
An altered sequence converts a codon for one amino acid to a codon for a different amino acid.
Example: Sickle Cell Disease
A mutation that codes for abnormal hemoglobin.
Autosomal recessive.
A single base is changed out of 438 in the gene.
CTC \rightarrow CAC (glutamic acid replaced by valine).
Point Mutations can be “Silent” / “Synonymous”
The DNA code for an amino acid is converted to a sequence that specifies the same amino acid.
CCAGTT \rightarrow CCAGTC (no change in amino acid).
Point Mutation: Nonsense
The DNA code for an amino acid is converted to a code for a chain-terminating codon.
Frameshift Mutation
An insertion or deletion of a base changes all codons “downstream” from the mutation.
Chromosomal Mutations
Deletions
Duplications
Inversions
Translocations
Mutation as an Evolutionary Mechanism
Mutation is a relatively weak evolutionary mechanism because:
Mutation rates are generally low.
Mutations can be neutral.
They must occur in gametes to have evolutionary consequences.
They become common in a population only through the action of other evolutionary forces.
Gene Flow
The exchange of genes between populations.
Influenced by physical distance and social structures (e.g., migration, exogamy).
Tends to decrease variation between populations.
Genetic Drift
Changes in the frequency of alleles due to chance.
Has the greatest impact in small populations (e.g., complete loss of alleles is more likely).
Example: Type O Blood Distribution
The frequency of type O blood is very high among Native Americans, suggesting a genetic drift event.
“Founder Effect”
Occurs when a subset of a larger population migrates and establishes its own gene pool.
The subset carries only a portion of the alleles present in the original population, leading to different allele frequencies.
Genetic Bottleneck
A sharp reduction in the size of a population due to environmental events or human activities.
Results in a loss of genetic diversity, as the surviving population may have different allele frequencies than the original.
Implications of Genetic Drift
Alleles can become established (or even fixed) or lost within a population without reference to their selective value.
Natural Selection
Removes unfavorable variations and preserves favorable ones.
Operates on the organism’s phenotype.
The environment acts as a filter.
A change in the environment leads to a change in selective pressures, which in turn leads to a change in allele frequencies.
Can be directional (favoring one extreme) or stabilizing (favoring the intermediate).
Example of Natural Selection in Humans: Sickle Cell Anemia
Why doesn’t this harmful genetic disorder disappear?
AA - normal hemoglobin (susceptible to malaria)
AS - heterozygous (carrier, resistant to malaria, mild or no sickle cell symptoms)
SS - sickle cell anemia (likely to die before reproducing)
In regions where malaria is prevalent, the heterozygous (AS) genotype has a selective advantage, maintaining the allele in the population.
Polymorphism
A trait for which two or more distinct phenotypes exist within a population.
Frequency and distribution are shaped by:
Gene flow
Genetic drift
Environments and natural selection
Culture
Example of Natural Selection in Humans: Lactase Persistence
Milk consumption is universal in infant mammals.
A decrease in lactase (the enzyme that digests lactose) with age is typical in mammals, leading to a loss of ability to digest lactose.
Lactose accumulates in the small and large intestine, causing digestive discomfort.
Why is lactase persistence common in some human populations?
It offered a selective advantage after the domestication of dairy animals (approximately \sim9,000 years