Flashcards in Week 2 Deck (21):
What are the three patterns of selection? Explain them in terms of average trait value, population variability and fitness, and graphically. Provide an example of each.
Directional selection = form of selection in which one tail of the phenotypic bell curve is favored, leads to a shift in average trait value, does not affect population variability and fitness. Example is medium ground finch beak size during droughts (need long beaks to crack open big hard seeds); Stabilizing selection = form of selection in which the central portion of the phenotypic bell curve is favored, maintains the mean trait value and reduces the variability in the trait. Example is the Goldenrod plant gall size - parasitoid wasp oviposits eggs developing within larger goldenrod galls to hijack gall/eat pupa, but birds will peck into larger galls to reach fly larvae inside; Disruptive selection = form of selection in which the two tails of the phenotypic bell curve are favored, which produces a bimodal distribution of the trait, maintains the mean trait value and maintains variability.
How can rainfall patterns on the Galapagos Islands act as a selective pressure of finch beak morphology? What are the rainfall effects on beak phenotype?
Medium ground finches eat seeds of many sizes but prefer smaller seeds. Droughts in Galapagos reduced the overall seed production, small seeds became depleted. The proportion of finches with deeper, more massive bills increased. Small softer seeds became available again during abundant rainfall. Large hard seeds require large-billed finches to crack.
What are the ways, other than selection, that can lead to microevolutionary changes? How do they relate to selection?
Genetic drift - random changes in allele frequency (small populations); Gene flow - net gain/loss of certain alleles by movement of individuals (can introduce maladaptive traits, may reinforce or counteract selection or drift); mutation pressure- evolutionary change resulting from new mutants, usually deleterious but sometimes adaptive, in the case of resistance.
How do mutations relate to evolution? Examples include: antibiotic, insecticide or herbicide resistance.
New alleles introduced into populations due to changes in DNA sequences. Usually deleterious but new variation may be adaptive, in the case of resistance. i.e. sickle cell mutation, leads to sickle-shaped hemoglobin, carriers of sickle cell Hgb allele seem to have increased resistance to malaria.
Explain how parasitoid wasp and bird foraging behavior act as selective pressure on the size of fly-induced goldenrod galls.
Goldenrod plant (grows tumor (Gall) - developed by a pest goldenrod gall fly, deposits eggs). Parasitoid wasp oviposits eggs into fly pupae developing within a goldenrod gall, larger galls are more difficult to breach. Wasp larvae hatch inside pupa and kill pupa by eating it. Birds (woodpeckers) will peck through larger galls to reach flies inside.
Why are small populations more vulnerable to genetic drift?
In small populations the laws of probability do not operate as expected and random changes in allele frequencies are more apt to occur.
What does it mean for an allele to go to fixation?
It means that one allele has a frequency of 1.0 and another allele has disappeared from the population.
Can selection and drift act on the same individual or population? Explain why.
Genetic drift provides movement between peaks of an adaptive landscape, it can provide new allelic combinations that allow selection to act on, generating new fitness peaks.
What is the relationship between genetic drift and natural selection? Which is more likely to be more directional? Why?
Natural selection is more likely to be directional because genetic drift occurs randomly whereas natural selection works to push up the adaptive landscape.
What is the adaptive landscape? On what axes is it plotted? Who proposed it?
The adaptive landscape is a 3D representation of fitness of different genotypic combinations. It was proposed by Sewell Wright.
What is the Modern Synthesis and what is its importance to Darwinian evolution?
Reconciled the 19th century ideas of Darwin's natural selection and Mendelian genetics with population genetics. Malthusian competition (limited resources) + Variation --> Natural Selection
Mutation --> Genetic variation + Mendelian inheritance
Natural selection, Genetic variation, and mendelian inheritance --> Modern Synthesis
In addition to mutation, how can an organism acquire new genetic variation?
Random mating, random fertilization, recombination between homologous chromosomes during meiosis, HGT
What is the Malaria Hypothesis? Is it supported?
Sickle cell gene is most commonly found in areas with historically high levels of malaria, adding geographical support to the hypothesis that the gene, while potentially deadly, avoids disappearance through natural selection by providing protection against malaria.
Why are influenza strains characterized by a letter (A,B, or C) and H and N values? Why are vaccines targeted to different strains each year?
H = hemagglutinin
N = neuraminidase
What is the most prevalent flu strain affecting humans this flu season?
What is horizontal gene transfer and how may it occur?
Genes from one species are introduced into another organism (prokaryote/eukaryotes) compared to vertical (parent --> offspring); allows rapid acquisition of traits; more common among bacteria
What are scale-eating cichlid fish and why are they a good example of frequency dependent selection?
Have 2 morphs; mouthparts twisted left or right; parasitized fish learn to look left/right to avoid. Frequency dependent selection is when the fitness of genotype varies with its frequency in populations. It maintains variation by favoring rare genes.
Distinguish between positive and negative frequency-dependent selection and provide an example of each.
Positive frequency dependent selection = fitness increases with prevalence of genotype in population. Example = Batesian Mimicry - nonharmful species look similar to actually harmful species, but toxic species need to be at high enough population density for mimicry to be beneficial. Negative-frequency dependent selection = fitness increases with rareness of genotype in a population. Example = Elderflower orchid, pollinated by bees but provides no nectar reward. Bees sample the flowers to learn which produce rewards, they do not get reward from either purple or yellow flowers as they rotate through, but the less frequent color morph will be favored by the bess, so as a result both colors are pollinated.
In terms of the Hardy-Weinberg theorem, what do p and q represent? What do p^2, 2pq, and q^2 represent?
p = frequency of dominant allele in population; q = frequency of recessive allele in population; p-squared = frequency of homozygous dominant genotype; q-squared = frequency of homozygous recessive genotype; 2pq = frequency of heterozygous genotype
What does the Hardy-Weinberg theorem predict? What does it assume? What does it mean if the predictions are not upheld?
Serves as a null hypothesis for whether evolution is occuring in a population. H0 = evolution is not occuring in the population. It assumes: (1) large population size; (2) random mating - every genotype has equal probability of mating; (3) no immigration/emigration from population; (4) all genotypes are equally fit for survival/reproduction; (5) no mutations.