preguntas Flashcards
A. Molecular evolutionary change at the level of a single nucleotide.
An example of molecular evolutionary change at the level of a single nucleotide is a point mutation, where one nucleotide in the DNA sequence is replaced by another. For instance, a cytosine (C) base might mutate into a thymine (T) base due to a replication error or exposure to mutagens.
B. Molecular evolutionary change at the level of structure.
One example is the evolution of protein structure through amino acid substitutions. Amino acid substitutions can alter the three-dimensional structure of proteins, affecting their function. For instance, a single amino acid change in a protein can lead to conformational changes that affect enzyme activity or ligand binding specificity.
D. Molecular evolutionary change at the level of genome size.
Genome size is affected by:
Segmental mutations (duplications and large deletions). Gene duplication events can contribute to an increase in genome size. When a gene is duplicated, an additional copy of the gene is inserted into the genome. Accumulation of duplicated genes can result in an increase in genome size.
Transposable elements (TEs) are DNA sequences that can move or copy themselves within the genome. The activity of TEs can lead to the expansion of genome size by inserting copies of themselves into new locations. Also, while less common, loss of genes or genomic regions can lead to a reduction in genome size.
Whole genome duplication events, where an organism’s entire genome is duplicated, can lead to a doubling of genome size. Polyploidy is relatively common in plants and some animal groups.
Indels - small insertions or deletions
C. Molecular evolutionary change at the level of karyotype.
An example is chromosomal rearrangements, such as inversions or translocations, which alter the structure of chromosomes within the genome. These rearrangements can lead to changes in gene order, gene expression patterns, and ultimately, phenotypic diversity.
The Wright-Fisher Model of Genetic Drift provides a framework to model the role of random genetic drift in causing changes in allele frequency from one generation to the next. List the seven assumptions of this mathematical model. (3.5 p)
haploid
finite population
no mutation
no selection
no migration
no overlapping families
random mating
A. What is the major tenet of Kimura’s (1968) Neutral Theory of Molecular Evolution? (3.5p)
The major tenet of Kimura’s Neutral Theory of Molecular Evolution is that the majority of molecular changes at the genetic level, specifically in DNA sequences, are caused by random genetic drift acting on neutral mutations, rather than by natural selection.
In other words, most genetic variation observed within populations and species is due to the accumulation of mutations that do not confer either a selective advantage or disadvantage to the organism.
B. Kimura used two main arguments to support his Neutral Theory:
1, Haldanes cost of selection argument = rate of evolution for protein sequences too high to be explained by selection
2. Levels of genetic variation at a given point in time were too high (segregation load)
Natural selection cannot account for these observations but random genetic drift can!
A. What is an homologous sequence?
A sequence inherited in two species from a common ancestor.
B. Name and distinguish between the two classes of homologous sequences. (4p)
Homologous sequences can be either: Orthologous or Paralogs
Orthologs are homologs where the sequence diverges after a speciation event, but the sequence and its main function is conserved. Genes separated by speciation are called orthologs.
If a sequence is duplicated in a species, the resulting duplicated sequences are paralogs of each other, even though they can become different over time. Genes separated by gene duplication events are called paralogs.
mk:
sumas todo
sacas frequencias que es la suma de la derecha entre el total
luego expected que es la multiplicacion de lo de abajo por la frequencia
luego x2 que es (O-E)**2/E
Explain how the allele frequency spectrum differs from standard neutral expectations
when Tajima’s D is negative.
on neutral expectations we would expect to see in the sfs, an increase on the rare alleles, the ones with the lowest frequencies, so a peak on the left side of the graph and it lowers as we move away from it. When taijimas D is negative we would see an increase of the common alleles, the ones with the highest frequencies. So when Tajima’s D is D<0 and negative it means that Θw is larger than π and positive selection is very likely happening there.
Example of mutations that can affect gene order and not the number of genes in the genome are
INVERSIONS AND TRANSLOCATIONS
mutations that can affect the number of genes ar
duplication, deletions, INSERTIONS
The rate of substitutions for neutral mutations doesn’t depend on the population size because.
The rate of gene substitutions is defined as the number of substitutions or fixations per unit time. For neutral mutations, the rate of substitution equals the mutation rate, so it’s independent of the population size.
What is the site frequency spectrum (SFS) and how can it be used to tell us something about how selection has operated on the genome? Illustrate with a sketch how the SFS is expected to look like for different evolutionary scenarios
In a neutral scenario we would have an increase in singletons while the other more common alleles are not as present here our D = 0. Here Tajima’s estimator = Watterson’s estimator and Tajima’s D = 0.
In a positive selection (selective sweep), we have an increase in the most common allele and a decrease of the intermediate frequent alleles, here our D < 0.
Last, we have a balancing selection where we have an increase on the intermediate frequencies and a D > 0.
