GN 301 Module 7: Population Genetics and Evolution

Define Evolution:

changes in the genetic composition of a population including the emergence of species, divergence of species and extinction of species, over a long period of time

List forces that affect changes in allele and genotypic frequencies and affect diversity within and between populations

• Mutation, migration, selection, small population size

• Affects of mating not at random: alters genotype frequency, but not allele frequency

Importance of mutation from an evolutionary point of view:

the source of new alleles, or the source of variability.

Why does mutation not have a significant effect on allele frequency over a short period of time?

The mutation rate is low and is too slow to have a significant effect on allele frequency. Other evolutionary forces are needed to keep the mutant alleles in the population and allow them to increase in frequency.

Migration effects:

Migration is the movement of individuals from one population to another. Assuming that the frequencies of the immigrants differ from that in the recipient population, this decreases diversity between the two populations and increases diversity within the recipient population.

Directional selection effects

• favors one extreme or the other.

• Population mean increases or decreases depending on which extreme is favored.

Disruptive selection effects

• Advantage for both extremes. Heterozygote least fit.

• Leads toward bimodal population

Stabilizing selection effects

• Heterozygotes favored

• Decreases variance

• Leads to polymorphisms (both alleles remain in population)

Sickle Cell and Malaria: an example of _________ selection

stabilizing

**Balanced Polymorphism:**

Heterozygote is the most fit genotype so both alleles are maintained in population.

**Cystic Fibrosis:** 

• homozygotes lack functioning chloride channels so fluids accumulate

• Thought that about 6% of people of European origin carry allele

• unclear reason why so many Europeans have the allele since it doesn’t protect them against any diseases (like Cholera and Typhoid)

Lactose Tolerance

• Individuals with low lactase levels develop gas, cramps, nausea, etc after eating lactose containing foods.

• Often see increase in lactose intolerance with age

• Adults mammals originally intolerant

• Selection for tolerance as dairy products introduced into diet (advantage to be able to use as nutrition source)

• Turkey and the nearby areas in Russia seem to be original region for lactose tolerance

• % Intolerance:

  • Asian-Americans 90%

  • African- Americans 75%

  • Native Americans 75%

  • European-Americans 10%

Tolerance to high altitude

• Athletes often train at high altitudes to make more hemoglobin so that they increase their oxygen carrying ability

• Physical problems at high altitude for average person include breathing difficulties and altered sleep patterns and too much hemoglobin

• Too much hemoglobin can cause thick blood and high blood pressure (unhealthy)

• Tibetans’ bodies have adapted to low oxygen concentration in high altitudes

**Assortative Mating**:

mate based on phenotype

Positive assortative mating

mating like individuals together, results in similar situations as inbreeding (increased homozygotes) but only for loci in mate selection

Negative assortative mating

“opposites attract” would keep diversity in the population and tends to increase the frequency of heterozygous individuals for the loci in mate selection.

Inbreeding

mating of related individuals changes frequency of genotypes, but not allele frequency. Leads to more homozygous individuals in population over time. Affects all loci in the organism.

Non-random mating changes frequency of **genotypes** but not ____

allele frequency

Define and describe effects of Founder Population

• A founder population occurs when a small group of individuals from one population leaves the big population and colonizes a new area.

• The founder population is typically small so it is likely to undergo genetic drift

• Allele frequencies in the founder population may differ from those in the original population

• Selection pressures on founder population will probably be different from those on original population since they are in a different environment

  • Often this is a harsher environment causing more selection pressure and a more rapid change due to selection

Define and describe effects of Bottle-neck

• A disaster (or something) wipes out a large portion of the population

• The survivors rebuild, but the frequencies of alleles in the survivors may differ from that of the population before the disaster

• Bottleneck occurs when the population size decreases dramatically due to some type of disaster.

Define and describe effects of Genetic Drift: 

• Genetic drift is the random variation in gene frequency from generation to generation due to small population size and sampling error

• leads to random fixation or loss of alleles over time.

How We Can Evaluate Evolutionary Changes?

• Fossil records

  • Lower layers – simpler life forms

  • Upper layers – more complex life  and more similar to today’s forms

• Geographic Distribution

  • Similar organisms in different places suggests common ancestors, migration and selection

• Comparative Embryology, Anatomy and Biochemistry

  • Skeletal, respiratory, digestive systems

  • Chromosome number, structure and organization

  • Protein structure similarities

• Molecular Evolution

  • Comparing DNA or protein sequences and chromosome banding patterns

**Banding Patterns**: eg:

__Robertsonian__ translocation between two ancestral chromosomes in great apes resulted in human chromosome #2. 

How many chromosomes are expected in a normal somatic cell from a normal chimpanzee? Why?

48, because Chromosome #2 in humans contains the approximate information from 2 chromosomes in the chimp, so the chimp must have an extra pair

Which 2 types of DNA sequences are used to evaluate divergence in human populations? 

Mitochondrial DNA and Y chromosome DNA

Mitochondrial DNA

The mitochondrial DNA comes only from the female line and allows that line of inheritance to be traced. Mitochondrial DNA has other advantages as well. It mutates faster than nuclear DNA with mutation estimates of 2-3% per million years. The mutations are not repaired nearly as often as nuclear mutations. It is also easy to sample an abundance of mitochondrial DNA since each cell has many mitochondria.

Y chromosome

The Y chromosome is passed only from father to son so it allows a way to trace paternal inheritance through many generations. Another advantage of the Y chromosome is that it does not cross over with other chromosomes so that other than the effect of mutation, the haplotypes remain consistent from generation to generation. This makes it easier to trace relationships.

Phylogenetic trees

Phylogenetic trees can be used to show relationships between species.

The **nodes** represent the

common ancestors that were present prior to divergence of the two groups.

**Branches** are the

evolutionary connections between the organism.

**Terminal nodes** are the

nodes at the end of the tree closest to the pictures.

**Internal nodes** are

those nodes located within the middle of the tree (Not at either end).