Morehouse College BIO 112 General Biology Peer-Lead Team-Learning (PLTL) Workshop Evolution and Natural Selection Introduction Biological evolution and the process of natural selection are among the most fundamental and essential concepts in biology. In this workshop, you will review the features of genetics, population biology, and ecology that are required for understanding evolution and natural selection. Common misconceptions will be challenged by the workshop activities and you will consider the implications of life that has been molded by the process of natural selection. Pre-Workshop Assignment Prior to your laboratory class meeting you should complete the questions given below and bring your completed work to your laboratory class meeting. Activity A. Concept and Relationships. 1. Write a brief definition for the following terms, in complete sentences: Phenotype Genotype Environment Phenotypic variation Differential survival and reproduction Heritability Hardy-Weinberg population 1 Morehouse College BIO 112 General Biology Natural selection Stabilizing selection Adaptation Mutation Genetic Drift Migration Darwin’s Challenge Inclusive Fitness Effects Activity B. Variation in a Population Despite our tendency to think of individuals within a given species as uniform, because we describe just a few key characteristics that make species different from each other, individual differences characterize all living things. This individual variation (differences between individuals within one species, within one population) is in biologically significant traits and such variation is always present. What are the causes for differences between individuals in their characteristics? What is responsible for the constant production of variation in a population? Imagine a population of rabbits that vary in their reaction time to the scent of a coyote. Some individuals immediately run to their burrow when they smell this predator, while others are slower in their response. Draw a frequency histogram (on the graph below) showing this response variation assuming a normal distribution in the response times of rabbits. Label the graph axes and draw a smooth curve of the response frequencies. 2 Morehouse College BIO 112 General Biology Activity C. Evolution is Change Over Time We sometimes casually use the terms “evolution” and “adaptation” to refer to very shortterm changes that occur in the lifetime of an individual (an evolving personality or an adaptable salesman). However, these casual uses are not biologically accurate and do not correctly reflect the biological meaning of evolution and the adaptations that are the consequence of evolutionary change. Biological evolution does not result from learning or physiological changes that can be transferred directly from one generation to the next. What is the time scale for biological evolution? What must change for biological evolution to have occurred? Biologically, what is actually transferred from parent to offspring, from one generation to another? How do we measure evolutionary change? Is it ever possible for individuals to evolve? 3 Morehouse College BIO 112 General Biology PLTL Workshop Problems Activity 1. Causes for Natural Selection The cause for natural selection can be any factor that results in some individuals experiencing poorer survival and reproduction than other individuals in a population. Charles Darwin called the causes for natural selection “The Hostile Forces of Nature”, because they are the factors that prevent populations from growing infinitely large and individuals must constantly confront these factors as they attempt to live and reproduce. Ecologists often describe the causes for natural selection limiting factors, limiting conditions and limiting resources. a. Name and describe Darwin’s Hostile Forces of Nature (there are five): b. Define the following: Environmental Limiting Factor Environmental Condition Environmental Resource c. Conduct a round-robin in your group identifying each potential cause for natural selection as either an environmental condition or a resource (and justify your answer): Sunlight Water Calcium The opposite sex Air temperature Wind velocity Glucose Carbon dioxide gas Oxygen gas pH of water shelter food 4 Morehouse College BIO 112 General Biology d. Can a given factor be limiting in one environment and non-limiting in another? Each member of your group should choose one of the following factors and describe for the other members in the group two situations or examples. Each member of your group will pull an organism card from the “Species” Box and both situations or examples must involve this organism. In one example, this factor will be limiting and a second it will be non-limiting: Sunlight Water Calcium The opposite sex Air temperature Wind velocity Activity 2. Natural Selection It is convenient to assume that variation in a population (phenotypic variation) has a normal distribution (as in the frequency histogram below), but any distribution pattern of variation is sufficient for natural selection to occur. a. List and describe the requirements for natural selection? (Let’s not be concerned about whether natural selection results in evolutionary change at this point.) b. Work in pairs on this section and present your results to the other members of your group. Each student pair should choose one case below. In each case, label the axes and mark the a frequency histogram showing which individuals experience the negative effects of natural selection, then redraw the frequency histogram showing how the population changed (evolve) as a consequence of selection. Case 1. Penicillin is an antibiotic (an anti-bacterial) that can be used to treat the sexually transmitted disease known as Gonorrhea, caused by the bacterium Neisseria gonorrhoeae. There is natural variation in the resistance of Neisseria strains to penicillin and prior to 1970, strains of Neisseria that were resistant to this drug were rare. During the 1970s penicillin was distributed to sex workers in the Phillipines and SE Asia (locations of US military R&R during the Vietnam War) and used as a gonorrhea prophylactic treatment. The frequency of penicillin resistant gonorrhea infections increased during the 1970s and is as high as 40% in some Asian countries today. Case 2. The application of air quality regulations in much of the industrial world has dramatically reduced the emissions of sulfur dioxide and soot (dust) in both the United 5 Morehouse College BIO 112 General Biology States and Western Europe. In the 1950s and 1960s, species of peppered moths (genus Biston) on both sides of the Atlantic were predominantly dark winged individuals (95% melanic individuals) that were well camouflaged from their predators (birds) when perched on the dark branches of trees in polluted environments. The uncontrolled combustion of fossil fuels (mainly coal) releases sulfur dioxide that kills lichens that would normally grown on tree trunks and branches. Light winged peppered moths are best camouflaged when perched on trees covered with light colored lichens. Starting in the mid-1960s, serious efforts to reduce the emission of soot and sulfur dioxide were initiated. The population frequencies of dark winged peppered moths decreased as pollution levels decreased. In the 1990s, the frequency of dark winged moths was only 30% of individuals. Case 3. Average human birth weight is 7.0 pounds. In the recent past (1950), the probability of early childhood mortality was approximately 1.5% for babies weighing 7 – 7.5 lbs. Newborns weighing less than 7.0 lbs had an increasing risk of death with smaller birth weight (due to premature birth). Early mortality was 5% for newborns weighing 5.0 lbs, 20% for those weighing 4.0 lbs and 60% for those weighing 3.0 lbs. Newborns weighing more than 7.5 lbs also experienced increasing risk of death with greater birth weight (due to increased birth trauma). Early mortality was 3% for newborns weighing 9.0 lbs, 6% for those weighing 10.0 lbs and 15% for those weighing 11.0 lbs. Initial Frequency Histogram Population Mean Future Frequency Histogram 6 Morehouse College BIO 112 General Biology Activity 3. When Natural Selection Causes Evolution Natural Selection occurs whenever there is phenotypic variation and a cause for selection that “acts on” those differences between individuals. Are there any differences between isogenic and polygenic population in the process of natural selection? Natural selection does not necessarily cause evolution. Why? Each pair of students who worked on a particular case in Activity 2 should now explain to your group the requirements for natural selection to cause the evolutionary change that was observed. Draw a “Future Frequency Histogram” for each case if the population were isogenic. What causes genetic differences between individuals in a population? When are these causes actively inducing genetic variation? Do the causes for natural selection cause genetic change in an individual during their lifetime? What is the relationship between mutation and natural selection? What is heritability and what makes it important in the process of evolution by natural selection? What are the three types of natural selection and resulting evolutionary change? Work in pairs. Assign each pair one of the types of natural selection and draw “before” and “after” frequency histograms for that type of selection and evolution. Mark the “before” histogram in red showing that part of the population experiencing the negative effects of selection and mark the same histogram in green to indicate that part of the population experiencing the positive effects of selection. Each pair should present their histograms to the group and explain the process and any assumption made. 7 Morehouse College BIO 112 General Biology We define adaptations as products of evolution by natural selection. Again, for each of the cases in Activity 2, describe the adaptation that resulted from evolution by natural selection. For a given species, is it possible for a particular trait (bill length in a bird species) to be an adaptation in one environment but not in another environment? What is the purpose of an adaptation? Activity 4. Causes for Evolution Natural selection is the most potent cause for evolutionary change but it is not the only cause for evolution. What do we mean when we say that a population has evolved? What would we measure to determine whether evolution had occurred? What are the potential causes for evolutionary change in addition to natural selection? How do these other causes for evolution differ from natural selection? Activity 5. Are Phenotypes Altruistic or Selfish? Charles Darwin recognized that his mechanism for evolutionary change had logical limits such that some phenotypes could not evolve as a consequence natural selection. Darwin’s statement of this logical limitation is often called Darwin’s Challenge: "If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection." (Charles Darwin, 1859, The Origin of Species) 8 Morehouse College BIO 112 General Biology Define the following terms (in a round-robin) with respect to Darwin’s Challenge: Selfish phenotype Altruistic phenotype Inclusive Fitness Effects Mutualism Kin Selection Reciprocal altruism Each member of your group should pull a card from the “Mutualism” box which lists an example of a mutualistic interaction. Take turns describing the nature of this interaction by listing the assistance each of the two species in the interaction provides to the other species. Within a given species, individuals may assist each other, but this assistance is typically directed to help individuals who carry genes identical by descent. Often an evaluation of the coefficient of relationship between two individuals is enough to understand the benefit an aid giver receives by assisting another individual. On the pedigree diagram below, take turns estimating the coefficient of relationship between individuals assuming sexual reproduction and both males and females having diploid genomes. Describe all the relationships that are possible in this pedigree. Relatives Parent-offspring Full siblings Grandparent-grandchild Aunt (uncle)-niece (nephew) Cousins Coefficient of Relationship (r) 9 Morehouse College BIO 112 General Biology What factors, other than the coefficient of relationship, influences the evolution of inclusive fitness effects? Activity 6. What is Natural Selection Actually Selecting? The level of organization at which natural selection occurs is not limited to that of the individual organism and in fact may occur at multiple levels simultaneously. None-the-less, it is important to identify the level at which selection is most potent because that will determine the function of evolutionary changes and thus the function of adaptations. In your small groups, list the units of organization of life (on file cards) and organize the cards to create a hierarchy of these units. You need not go down to the level of atoms, but include all units that can reproduce, exhibit variation, and heritability. After your group has prepared a hierarchy of the units of life, complete the table below indicating the relative values for variability, reproduction rate and heritability. Units of Life Variability Reproduction Rate Heritability How is the rate of evolutionary change influenced by variability, reproduction rate and heritability? 10 Morehouse College BIO 112 General Biology Explain how group selection (at the level of population or species) can occur but is unlikely to be as potent as natural selection at the level of the individual organism. Activity 7. Sexual Selection Sexual selection is a special form of natural selection in which selection is different in one sex than the other, and consequently evolution is different between the sexes. Sexual selection may occur as a consequence of either male-male competition or as a consequence of female choice between potential mates. Male-male competition can result in selection for characteristics in males that improve their ability to compete with other males for access to reproductive females. Examples sexual selection by male-male competition are larger body size in male than females and male weapons that are absent or reduced in females (such as horns and antlers). Female choice results in some male being more successful in reproduction than others because they had characteristics that were favored by females. Bright coloration in males, and elaborated structures used in courtship displays by males are examples of sexual selection by female choice. The requirements for evolution by sexual selection are the same as the requirements for evolution by natural selection. In your small group, list and explain the requirements for evolution by sexual selection. Work in pairs, work on one case, then report to your group. Draw a frequency histogram of the trait in question and show how selection and evolution would be expected to occur. Is sexual selection is occurring? What additional information do you need to determine whether sexual selection is occurring? What is the cause of sexual selection in each case? Case 1. In a species of stream fish, the mottled sculpin (Cottus bairdi), males build nests each spring under rocks and females visit males and deposit eggs on the ceiling of their rock nests. There is considerable variation in the body size of males that are found in rock nests and the largest males in a given population receive the majority of the eggs 11 Morehouse College BIO 112 General Biology that are deposited in nests. Although there is overlap in the size distributions of males and females, the largest males are always larger than the largest females. Case 2. The northern elephant seals, Mirounga angustirostris, is a marine mammal that only comes on land to reproduce. Adult females form a group on a sandy beach and are guarded by one male who vigorously and violently attempts to exclude other males from his harem. The largest males are the most successful in excluding other males and successfully copulating with many females. In fact, small young males may have little or no reproductive opportunities. Adult males can be as large as 20 feet long and weigh 8000 lbs. while adult females are only as large as 11 feet long and weigh 2000 lbs. Case 3. Red-winged blackbirds, Agelaius phoeniceus, are freshwater marsh birds. Males are black and have bright orange-red and yellow patches on each shoulder. This is in strong contrast to females and immature birds that are smaller than mature males and are a streaked reddish-brown color. Case 4. Sage grouse, Centrocercus urophasianus, is a pheasant-size bird of the western United States. Males are larger than females and have a black throat and bib not seen in females. Each year males and females visit a courtship site, a lek, where males perform a distinctive courtship dance with elaborate tail feather displays and inflate bright yellow breast air sac. Females watch males display before mating. Some males are much more successful in attracting females than others. 12 Morehouse College BIO 112 General Biology Post-Workshop Activities Review your understanding of evolution and natural selection by working the following problems: 1. How do mutation and recombination yield phenotypic variation in organisms? 2. Does mutation only occur when natural selection has a negative effect on a population? How about recombination? 3. Explain how truly altruistic phenotypes would be expected to disappear from a population as a consequence of natural selection? 4. Does the absence of true altruism in nature mean that contributing to a hurricane relief effort is unnatural? How about helping your brother with his homework? 5. Describing the thick winter fur of mountain sheep as “good for the species” may be true but it is likely to be a misinterpretation of how natural selection has yielded thick fur in this species. Explain. 6. The use of antibiotic drugs in animal feeds is used to minimize disease and increase growth rates. How might this use of antibiotics backfire? 7. What determines how rapidly evolutionary change may occur? 8. What would happen to a population in which there was very strong negative selection so on average each individual produced only one-half an offspring? 9. A population of dandelions, Taraxicum officinale, varies in the height of the flowering stalk. Rabbits eat these flowers but prefer to consume those on the tallest stalks. Is natural selection occurring? Is evolution occurring? Explain. 10. Plans to stockpile the antiviral medications Tamiflu as a fail-safe against the possibility that H5N1 bird flu becomes pandemic in humans have been criticized by some scientists. Some rare strains of H5N1 bird flu are resistant to Tamiflu. Biologists predict that H5N1 will “become invincible” to Tamiflu and more common with exposure to this drug. Explain the logic behind these predictions. 13 Morehouse College BIO 112 General Biology Resources Evolution Library. 2001. WGBH Educational Foundation and Clear Blue Sky Productions, Inc. http://www.pbs.org/wgbh/evolution/library/01/index.html Evolution Resources From the National Academies, National Academy of Sciences. 2005. http://nationalacademies.org/evolution/?referrer=google#research Understanding Evolution. 2005. The University of California Museum of Paleontology, Berkeley. http://evolution.berkeley.edu/ 11/2005 by L. Blumer. 14