Instructions: This review is worth and will be due in class Wednesday, 3/19/03. No exceptions – late reviews will result in at least a 10% deduction. You may not consult any references or any other person while working on this review. Your signature at the bottom of the last page signifies that the work is yours alone and is pledged under the Honor Code. When you break the seal on the envelope you will have five hours to complete the review! Please print legibly; I grade only what I can read! You may type your answers, but use an equivalent amount of space. If you do this, turn in both the review and the typed answers. For each question or part to a question, limit your answers to the space below each question, unless otherwise specified. Any part of your answer outside of the space provided will not be graded.
1. Identify the mechanism of evolution proposed by Charles Darwin. Describe the facts and inferences made by Darwin in his argument, and how the mechanism can affect biological evolution of a population. Be specific! (10 points)
Natural Selection: 5 facts & 3 inferences
The mechanism can affect biological evolution by favoring certain characteristics over others. The gene pool of the population changes over time to accumulate more and more copies of the alleles that code for adaptations that give their possessors higher fitness. That is, those individuals have a better ability to survive and they produce more offspring that themselves survive to reproduce.
2. What major piece of evidence did Darwin lack that was used by his critics as a major flaw in his hypothesis? (4 points)
Darwin hypothesized that there was a mechanism to pass traits on in a discrete way (not by blending as thought by others), but did not know what the genetic material was nor how it was passed on. Mendel’s work was unknown to him, even though he was a contemporary.
3. LIST two ways that biologists can measure variation in a population. (4 points)
Variation can be measured by direct observation of phenotypic variation, or by using molecular biology techniques such as protein gel electrophoresis and DNA sequencing.
4. List three assumptions of the Hardy-Weinberg equilibrium and provide examples (one for each assumption) of the consequences to genetic diversity of a single population when these three assumptions are violated (9 pts).
The five assumptions are: large population size, no natural selection, random mating, no gene flow, and no mutation. You can select any three.
The consequence for violation of large population size is a greater chance of genetic drift, which may lead to loss of genetic variation, even to the point of allele fixation.
The consequence for violation of no natural selection is natural selection, which may lead to a reduction in variation, as alleles coding for traits with a poor fit to the environment are eliminated by selection. Alternatively, disruptive selection may increase phenotypic variation by selecting for the extremes; this may or may not alter allele frequencies.
The consequence for violation of random mating is assortative mating. Positive assortative mating causes like to mate with like and tends to increase variation in the population. Negative assortative mating increases proportion of intermediate phenotypes in the population and thus reduces phenotypic variation. It may have no effect on allele frequencies.
The consequences for violation of no gene flow is an increase in genetic variation within populations and a decrease in genetic varation between populations. Two populations that are connected by gene flow become more similar in their gene pools. From the perspective of a single population, new alleles could be introduced by migrants that enter that population and mate with individuals already there.
The consequence for violation of no mutation is an increase in genetic variation. Mutation is the ultimate source of all variation in gene pools.
5. In a population of the plant Phlox the alcohol dehydrogenase (ADH) locus shows two alleles “a” and “b”; neither is dominant to the other. The following genotypic frequencies were found in a population; aa = 0.10, ab = 0.20, bb = 0.70. (SHOW WORK – 10 points)
a. If the population consists of 60 individuals, determine the actual number in the population showing each genotype.
b. Calculate the allele frequencies (set a = p and b = q).
c. Calculate the expected genotypic frequencies under Hardy-Weinberg equilibrium assumptions.
d. If there is there any evidence for evolution occurring in this population, what is a simple experiment we can perform to either confirm or eliminate the existence or action of one microevolutionary mechanism? (If not, state that evolution is not occurring)
There seems to be some evidence for selection against the heterozygotes (or selection for the two homozygotes); it could also be nonrandom mating (positive assortative mating). We’d have to run a chi-squared test to determine the probability that the distribution of observed and expected frequencies are the same (or not), but based on the values here it seems there might be some evolutionary mechanism in operation.
We can perform experiments to determine the survivability of all three genotypes in natural field conditions – if selection is occurring we’d expect to see lower survival of the heterozygotes.
We could perform mating experiments to determine the viability of pollen of one genotype on another genotype’s flower. Pollen viability of aa genotypes might be reduced on ab and bb flower types, for example. Alternatively, there might be some mechanism that prevents pollen from one genotype from even getting to another flower type, and we could test that using observational experiments.
6. Discuss one means by which speciation may occur, using theory and examples. Describe how two mechanisms of microevolution may be involved in such cases of macroevolution. (10 points).
Speciation may occur allopatrically, sympatrically, or parapatrically.
Allopatric speciation occurs when two populations of the same species are separated by a geographic barrier. This mechanism is thought to be the most common mode of speciation. In order for these two populations to become two separate species, however, they must ultimately be reproductively incompatible. If the barrier breaks down and the two populations merge, there must be no reproduction of viable offspring. Examples of incipient, potential speciation include squirrels on either side of the Grand Canyon, and fritillary butterflies in Europe, and an example of actual speciation caused by allopatry would be freshwater fish (and other aquatic animals) in North American rivers. Two mechanisms that might be involved are genetic drift and selection. If the geographic separation results in at least one small population, then it is easy to imagine how a founder effect might result in altered allele frequencies when comparing the two populations. Consequent selection on the two populations with different allele frequencies living in different habitats could result in evolution by natural selection.
Sympatric speciation occurs when two populations of the same species become reproductively incompatible while living in the same place at the same time. At least one reproductive isolating mechanism must have evolved to prevent the two populations from interbreeding. The most common way for this to occur is polyploidy (auto or allo). One non- polyploid example is the Apple Maggot Fruit Fly, where populations appear to be separating based on host preferences. Two mechanisms that might be involved are mutation and selection. Mutation may cause differences in host preference, and success of those mutants would lead to selection for those individuals bearing the mutation.
Parapatric speciation occurs when two populations of the same species are separated geographically, but not by a barrier. In these populations, speciation may be slower due to hybrid zones and gene flow, and the geographic separation may be related to a cline in some environmental condition. Mechanisms that would facilitate speciation via parapatry include genetic drift, selection for conditions along the cline, and mutation.
7. Refer to the figure below to answer the next three questions. (9 points total)
a. A taxonomic group (such as a Family) that included species B, C, and D would be ________________ when referring to the common ancestor of B and C.
POLYPHYLETIC
b. Could a taxonomic group that includes species A, C, and F ever be monophyletic? What else would need to be true?
YES, if the taxonomic group also included species B, D, and E.
c. The statement “species E and F evolved from species D” is TRUE/FALSE (circle one). Why?
FALSE, E and F evolved from the common ancestor of species D, E, and F. D may retain more primitive traits, but it also evolved from the same common ancestor. (It’s like saying humans evolved from chimps – we did not evolve from chimps, but we evolved from the common ancestor of chimps and humans)
8. Refer to the figure. Which species is an appropriate outgroup to develop a phylogeny of the Great Apes? (5 points total)
The Rhesus monkey or the spider monkey would both be appropriate outgroups to better understand the phylogeny of Great Apes.
9. Discuss one of the following two questions (9 points):
a. Describe two major evolutionary events in the history of life on Earth. For each, indicate EITHER how it may have played a role in one of history’s mass extinctions, OR its lasting importance to present day life.
Major evolutionary events in the history of life include the evolution of photosynthesis, which dramatically increased oxygen concentrations, the evolution of sexual reproduction, which caused an increase in genetic variation in populations, the evolution of the eukaryotic cell and multicellularity, both of which lead to more complex forms of life.
To receive full credit, one must describe two of these, and then indicate the consequences of the adaptation to subsequent history. For instance, when cyanobacteria evolved photosynthesis, the oxygen revolution occurred. This negatively affected anaerobic forms of life, in that oxygen was poisonous to them. Ultimately, it may have facilitated evolution of aerobic cellular respiration, which was much more efficient in terms of ATP generation. This may have speeded up increases in biodiversity and led to more complex forms of life – multicellularity is likely not possible for anaerobic forms of life.
b. Discuss the metabolic and habitat diversity of prokaryotes, using specific examples that correlate the two types of diversity.
Prokaryotes, especially in Kingdom Eubacteria, show the full range of metabolic diversity – members of this kingdom exhibit chemoautotrophy, chemoheterotrophy, photoheterotrophy, and photoautotrophy. This is the only kingdom to exhibit such a range. Some of these metabolic modes can only occur in specific habitats. For instance, chemoautotrophs usually occur in extreme habitats, such as hot springs or deep sea hydrothermal vents. Some modes are more likely to found in anaerobic habitats, and some can exist with or without oxygen. Ultimately, prokaryotes have a wider range of metabolic diversity and are found in many habitats in which eukaryotes don't or can't exist.
10. Use the comparative approach to explain variation in ichthyosaurs (from the Scientific American paper by R. Motani) for two of the following traits: caudal fins, vertebrae structure, eyes, body size/shape/mass. Describe the ecological selective factors that different ichthyosaurs faced that may have lead to the variation (10 pts).
Variation in ichthyosaurs includes the three different types of ichthyosaurs discussed in the reading. The three are lizard-like, fish-like, and intermediate types. The full range of ichthyosaur diversity fell within this continuum. Each of the traits above differed in the different types. For instance, body shape was more reptilian and vertebrae were longer and narrower in lizard-like ichthyosaurs, than in fish-like ichthyosaurs, where the body was more like a deep sea fish and the vertebrae were shaped more like hockey pucks. Eyes were much bigger, relative to body size, in fishlike ichthyosaurs than in lizard-like ichthyosaurs. Ecological selective factors were primarily the different habitats in which the types lived and prey that they hunted. For instance, living near the coast in shallow water where prey was dense and plentiful, lizard-like ichthyosaurs retained some ancestral reptilian features. However, when ichthyosaurs evolved to live in the open ocean and hunt scattered prey that often lived deep and swam fast, the features discussed above led to their success.
11. I recently discovered two populations of one species of flesh fly, one that lived in a forest and one that was in an urbanized area directly across a highway from the forest. The flies can cross the highway, but they often don’t, and when they do, they fly at low altitudes, traffic is heavy and constant, and most individuals don’t make it across. I want to know whether the two populations had diverged due to parapatry. I can’t think of a way to study this question, but you might be able to help me. Briefly describe a study that could investigate such a question. Include a population genetics hypothesis (null and research), a prediction (assuming the research hypothesis is correct), and the type of method you would employ to test your hypothesis. (10 points)
Null Hypothesis: the two populations are similar in allele frequencies and have not diverged genetically.
Research Hypothesis: the two populations have diverged in allele frequencies due to geographic separation (I call this parapatry because the barrier is passable by the flies, so there is the potential for gene flow). This will be tested by examining specific genetic loci within the flies (we could study them all, but at great cost). Also note that parapatry is NOT the same as parapatric speciation.
In the hypotheses, you must mention allele or genotype frequencies as the dependent variable that you will study - otherwise it's not a population genetics study.
I predict that there is very little gene flow between these populations due to the high mortality experienced by flies attempting to cross the highway. This will result in some divergence in allele frequencies at the loci studied, and I can test that using chi-square statistics.
The methods I will employ could be either protein gel electrophoresis, DNA sequencing, or some other method to assess genotypes for a sample of each population. I will then calculate allele frequencies and compare observed genotype frequencies between the two populations and allele frequencies between the two populations.
12. Discuss one of the following two questions (10 points):
a. Discuss major evolutionary advances in the vertebrates. To address this, identify two major evolutionary advances, what groups of vertebrates have them, and how those adaptations facilitated the success of those groups. BE specific.
Major evolutionary advances in the vertebrates include the evolution of the jaw, the shelled egg, lungs, legs from fins, the four-chambered heart, and others. For each, you must address where the adaptation is found, for instance, jaws are found in all vertebrates except the jawless fishes, and how the adaptation led to success, for instance, jaws allowed for more efficient prey capture and feeding than in groups without jaws. Another example is the shelled egg - it evolved in early reptiles and reptiles and their descendants (including birds and mammals) have this trait. It led to their success in that they were able to colonize dry, terrestrial habitats.
b. Outline the endosymbiotic theory of the evolution of eukaryotes. Include at least one other evolutionary event that may have facilitated endosymbiosis, and two pieces of evidence that support the endosymbiotic theory.
Endosymbiosis is a symbiotic relationship in which one individual (of a species) lives inside of another (of a different species). The endosymbiotic theory of the evolution of eukaryotes posits that endosymbiotic relationships between species of early eukaryotes and prokaryotes (or between two different prokaryotes) were so tight that ultimately the two species (or two individuals if considering just one cell) could not live without one another and they became one organism. These endosymbiotic events (we call them events, but they took many generations of living endosymbiotically to occur - they did not occur instantaneously) occurred 2-3 times, according to theory, leading to the evolution of mitochondria, chloroplasts, and peroxysomes. Evidence that supports this hypothesis includes genetic material of mitochondria and chloroplasts, prokaryote-like ribosomes in mitochondria and chloroplasts, phylogenetic relationships between mitochondria and chloroplasts and prokaryotes, the fact that antibiotic affects both types of organelle, and the endosymbiotic relationships between species existing today, such as corals and algae (zooxanthellae), and termites and their gut protozoan.
13. Explain the adaptive value of two of the following three advancements in animal body plans: triploblasty, segmentation, bilateral symmetry. In addition, explain how evolution of one of these advancements may have lead to the evolution of the other (i.e., how they may be linked evolutionarily). (10 points)
Triploblasty increased the ability of animals to compartmentalize and led to evolution of the coelom, which has many advantages. Bilateral symmetry facilitates movement through the environment; animals with this type of symmetry are more mobile and can actively hunt. It likely led to evolution of cephalization. Segmentation further increases the ability of animals to compartmentalize and have regional specialization within the body. It also facilitates movement and support for animals with hydrostatic skeletons. All three of these features may be linked evolutionarily. Triploblasty and bilateral symmetry evolved earlier than segmentation, and may have been linked in the sense that, as animals increased in complexity and became more active foragers, both led to increased success for those types of animals. As animals got bigger over evolutionary time, more systems evolved, which required compartmentalization and protection (obtained from triploblasty), and at the same time, bilateral symmetry allowed those bigger animals to actively hunt prey. Segmentation evolved in fewer animals (relatively), but is linked to the other two traits of interest in that it those other traits may have facilitated the evolution of segmentation - further compartmentalization was easier to obtain in triploblastic animals.
14. Discuss one of the following two questions (10 points):
a. Explain the concept of negative feedback as it relates to homeostasis and regulation. Contrast that with positive feedback using real examples.
Negative feedback is information that is used by a regulatory or control system to reduce or reverse a change that moves a condition or composition of a body fluid away from the desired set point. The feedback information is compared to the set point. Examples include temperature regulation, water balance, osmotic potential in cells, blood glucose concentration, and many others. Positive feedback amplifies a response away from a set point. It is usually a temporary movement away from the set point, and ultimately, the system returns to the set point by negative feedback. Positive feedback examples include voiding of body cavities and the sexual response.
b. Endotherms and ectotherms use some similar strategies for regulating body temperature; describe two strategies that both use.
Two strategies that both use are regulation of blood flow and heartrate, and behavior that moves the individual into a warmer or cooler environment. For the former, regulation of blood flow would probably include opening or closing arterioles leading to superficial capillary beds. Blood flow to the exterior parts of the body or to the extremeties is increased if the body is too hot, and this will allow for increased convection, evaporation, and conduction. Blood flow to the exterior is restricted if the environment is too cold and the individual is trying to retain heat - this way heat loss will be minimized. Slowing down heartrate in those situations will decrease the overall amount of blood flow to all parts of the body, including extremities, which is another mechanism for retaining heat. Finally, behavior is one of the first lines of defense for thermoregulation in both ecto and endotherms. Animals move to different microclimates to find temperatures at which they can minimize or maximize heat loss or gain.
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