Bio 300 final- short answers

oiv01's version from 2015-12-09 05:25


Question Answer
Give examples of situations in which you would expect to see negative frequency dependent selection and examples of situations in which you would expect to see positive frequency dependent selection. What is the effect of each kind of frequency dependent selection on the expected amount of genetic variation in a characteristic?Negative frequency-dependent selection occurs when a trait has a higher fitness when it is rare. Examples of negative frequency-dependent selection can be found in the evolution of pathogens and immunity - pathogens evolve to attack the most common form of immunological defense in a population. These common forms of immunity have a lower fitness and become rare as most are wiped out by pathogens and alleles for immune defenses that were previously rare (so the pathogens were not able to evolve mechanisms by which to attack them) now have higher fitness, until eventually pathogens evolve the means to attack them successfully. Positive frequency-dependent selection occurs when a trait has a higher fitness when it’s common. For example, warning coloration in a poisonous species is likely to have a higher fitness when it is common because if more individuals are brightly colored, predators will learn more quickly to avoid brightly colored individuals and not attack them. This decreases genetic variability over time.
Why is it important to understand both the environmental and genetic components of quantitative traits in terms of a species’ ability to adapt to its environment? Describe an example.It is important to understand the genetic component of quantitative traits (heritability) vs. environmental components in order to predict the response of organisms to environmental change. If a trait is genetically controlled, populations must have enough variability for the expression of the trait to change with the environment. If all alleles contributing to the trait are fixed, then populations cannot adapt to changing environments. (If a trait is controlled mainly by the environment, then it should change as the environment changes with no fitness consequences). An example is migration times in birds. Early or late migration can have extreme fitness consequences and a warming environment affects food sources available during migration. Timing of migration is independent of temperature - this has been demonstrated to be under genetic control and in artificial selection experiments, blackcaps with genes for ealier migration departures were strongly selected for. Similarly, plants have been shown to change phenotypes over time in response to warming, although not as fast as some global climate change models predict temperatures will increase. Another example is AZT resistance in HIV virus populations - virions with genes for an altered reverse transcriptase survive the changing environment (AZT treatment) and pass on this mutation.
In a species of salamander you find that individuals vary in both the size of their toes and in how much webbing there is between the toes. You discover that the two characters are correlated in that the more webbing there is between toes the shorter the toes are. Suppose that the environment of these salamanders becomes more aquatic and that individuals with webbing between the toes can swim better and therefore survive better in an aquatic environment. Toe length has no direct effect on survival or reproduction. How would you expect toe length to change in the population over time? Explain why.Natural selection would increase toe webbing over time because salamanders with more webbing have higher fitness in a more aquatic environment. Toe length would become shorter because short toes are correlated with more webbing and evolution of the webbing character will drive the evolution of toe length (decrease) even though toe length itself has no fitness consequences.
Stabilizing selection and overdominance are similar in that in both cases, an intermediate phenotype is favored. Why is it that overdominance maintains genetic variation but stabilizing selection does not?Overdominance applies to single gene traits and the intermediate phenotype is the result of two alleles in an individual, so variation is maintained in the heterozygotes if they have the highest fitness. Stabilizing selection favors average phenotypes that are not necessarily the result of heterozygosity. This ultimately results in the loss of extreme phenotypes and a decrease in total genetic variation.
If mutation provides the variation upon which natural selection has acted through time to create diversity, what is the importance of gene duplication in evolution?Gene duplication is responsible for the great diversity of genes in most genomes. For example, over 2/3 of the genes in the human genome appear to have originated though gene duplication. When a gene is duplicated, one copy must continue to code for a protein that performs a particular physiological function – it is functionally constrained and substitutions that affect its function will be strongly selected against. The other copy is redundant, so substitutions will not necessarily have an impact on the organism’s fitness.
What is the fate of a duplicated gene?The duplicate copy will accumulate mutations because of relaxed constraints (the original copy performs the gene’s function). As mutations accumulate, it is likely that eventually a stop codon will get inserted and the gene will become totally non-functional (a pseudogene). With continued substitutions, this second copy will eventually become unrecognizable – it will disappear. This is the fate of most duplicated genes. On rare occasions, mutations in the duplicate copy will be beneficial to the organism’s fitness and when this happens, the second copy can be recruited to a new function by positive natural selection. This kind of recruitment to new function is how the large gene families we see in modern genomes have evolved.
Be able to explain why different regions of DNA (for example, introns, third codon positions, pseudogenes, second codon positions, etc.) have different rates of evolution, and discuss the fate of a new DNA mutation if you are given some information about how it affects the fitness of an organism.The fate of a new mutation depends on the effect it has on an organism’s phenotype, which determines how natural selection will act. According to the Neutral Theory, most mutations are selectively neutral (for example, any mutation in a degenerate codon position that does not change the amino acid, mutations in non-coding regions, mutations in redundant gene copies, and even mutations that change the amino acid but have little effect on the function of the protein). If a mutation changes the phenotype, then selection will act according to how the mutation affects fitness. Since most mutations decrease fitness, then selection will eliminate it from the population. Amino acids in positions critical to fitness (where any change would result in a severe reduction in fitness) are said to have functional constraint. The higher the functional constraint of a particular region of DNA, the more slowly mutations will accumulate through time. When a mutation results in an increase in an organism’s fitness, then selection will maintain the new mutation in the population.
When researchers compare a gene in closely related species, why is it logical to infer that positive natural selection has taken place if replacement substitutions outnumber silent substitutions?Replacement substitutions change the amino acid coded for, and therefore the entire protein. In most cases, a change to the protein’s primary structure will affect its function negatively, so most replacement substitutions are selected against and rapidly lost from the population. Therefore, replacement substitutions are much less common than silent substitutions, which do not affect the protein. If a gene shows a higher rate of replacement substitution than silent substitution when compared between two closely related species, that means that the replacement substitutions are somehow advantageous to the organism and natural selection is increasing the frequency of alleles containing the substitution – perhaps the protein is evolving a new function that benefits the organism.
Are silent substitutions always unaffected by natural selection? Explain your answer and give examples.Silent substitutions can be selected against through 1) codon bias, where certain codons are used more than others and if a silent substitution creates a codon that is rare in a pool of tRNAs, it will be selected against, especially in genes that are highly expressed (because a rare tRNA will slow down translation efficiency). 2) Neutral silent substitutions can also be indirectly selected for through genetic hitchhiking – this is when they are physically linked (in linkage disequilibrium) to traits that are under strong positive selection and they will increase in frequency along with the beneficial gene they are linked to.
In what types of traits have researchers found the most evidence for positive selection in human populations?Broadly, in genes for traits related to diet, metabolism, and response to pathogens/immunity. There is also evidence for genes that are involved in the physical aspects of speech, cognition, fine motor skills, and additional diet factors (olfaction, protein/carbohydrate/fatty acid/phosphate metabolism, skeletal muscle vs. fat).Use specific examples from class.
Sympatric speciation is generally considered to be less likely than is allopatric speciation. Why might this be?Because if two species are occurring in the same area, it seems likely that gene flow will occur between them and this will tend to prevent speciation.